Dissertations / Theses: 'Ultra high strenght steels'

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Tolf, Erik. "Challenges in Resistance Welding of Ultra High Strength Steels." Licentiate thesis, KTH, Svetsteknologi, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-167985.

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Increasing the use of Ultra High Strength Steels (UHSS) in vehicle bodystructures is important for reducing weight and cutting CO2 emissions. This thesis investigates challenges in resistance welding that can be a barrier to implementing UHSS as a replacement for low strength steels in vehicle structures. Empirical research has been performed to offer new approaches for improved joint strength and to increase knowledge on cracking mechanisms in resistance projection welding and resistance spot welding of UHSS. By optimising the current build-up phase and peak current during the first milliseconds of weld time, it was shown that the strength could be improved by up to two-fold for projection welded joints. An approach to improve the ductility and strength of resistance spotwelds in UHSS using reduced cooling time was unsuccessful. The reduced cooling rate after weld metal solidification did not fully create the desired softened microstructure. The study on the surface cracking mechanism in resistance spot welded dual-phase UHSS showed that cracking is linked to the galvanization method. It is proposed that formation of aluminium oxide layers on the electrode tips increases the surface temperature and thereby increases the probability for liquid metal embrittlement and surface cracking.

QC 20150526

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Ali, Ashraf. "Widmanstätten ferrite and bainite in ultra high strength steels." Thesis, University of Cambridge, 1991. https://www.repository.cam.ac.uk/handle/1810/221885.

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Figueroa-Gordon, Douglas J. "Hydrogen re-embrittlement susceptibility of ultra high strength steels." Thesis, Cranfield University, 2005. http://dspace.lib.cranfield.ac.uk/handle/1826/1433.

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300M ultra high strength steel has been widely used for over forty years as a structural material in aerospace applications where a high strength is required. These parts are generally protected from corrosion by electroplated cadmium sacrificial coatings. However, there are concerns over this coating material due to its high toxicity and alternative coatings including Zinc-14%Nickel and SermeTel®1140/962 have been considered. It is known that applying electrodeposited coatings causes atomic hydrogen to be absorbed by the steel substrate producing delayed failure by direct hydrogen embrittlement. Hydrogen is also absorbed when a sacrificial coating undergoes corrosion in service and this process is known as re-embrittlement. The effect of electroplated Zinc-14%Nickel and aluminium based SermeTel®1140/962 sacrificial coatings in causing hydrogen embrittlement and re-embrittlement of 300M steel have been compared to that of conventional electroplated cadmium. AerMet®100 ultra high strength steel has been also considered as alternative replacement for the conventional 300M. Hence, the hydrogen embrittlement and re-embrittlement susceptibilities of AerMet®100 were studied when coated with cadmium, Zinc-14%Nickel and SermeTel®1140/962. In addition, two alternative alloys GifloM2000 and CSS-42LTM were also taken into consideration and only the extent of hydrogen re-embrittlement was assessed when coated with cadmium and SermeTel®1140/962, respectively. Slow strain rate tests, SSRT, were carried out for plated, plated and baked as well as plated, baked and corroded tensile specimens. The time to failure values were compared using a Weibull distribution, statistical ttests and embrittlement indices. Differences in hydrogen susceptibility of the high strength steels considered might depend upon their intrinsic hydrogen transport characteristics. These properties were studied and compared in terms of hydrogen diffusivity and solubility.

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Haglund, Adam. "Reduction of hydrogen embrittlement on Electrogalvanized Ultra High Strength Steels." Thesis, Uppsala universitet, Institutionen för kemi - Ångström, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-236603.

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Ultra-high strength steels is known to be susceptible for hydrogen embrittlement at very low concentrations of hydrogen. In this thesis three methods to prevent or reduce the hydrogen embrittlement in martensitic steel, with tensile strength of 1500 MPa, were studied. First, a barrier layer of aluminium designed to prevent hydrogen to enter the steel, which were deposited by vacuum evaporation. Second, a decarburization process of the steels surface designed to mitigate the induced stresses from cutting. Last, a hydrogen relief treatment at 150°C for 11 days and 200°C for 4 days, to reduce the hydrogen concentration in the steel. The effect of the hydrogen embrittlement was analyzed by manual measurements of the elongations after a slow strain rate testing at 5*10-6 mm/s, and the time to fracture in an in-situ constant load test with a current density of 1.92 mA/cm2 in a 0.5 M Na2SO4 solution. The barrier layer showed an increase in time to fracture, but also a decrease in elongations. The decarburized steel had a small increase in the time to fracture, but not enough to make it a feasible process. The hydrogen relief treatment showed a general decrease in hydrogen concentrations, but the elongation measurements was irregular although with a tendency for improvement. The simplicity of the hydrogen relief treatment makes it an interesting process to reduce the influence of hydrogen embrittlement. However, more investigations are necessary.

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5

Pan, Xin. "Development of lean maraging steels for ultra high strength applications." Thesis, University of Sheffield, 2017. http://etheses.whiterose.ac.uk/20511/.

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Lean maraging steels were designed for several application sectors by providing very high strength and ductility, with the addition of relatively cheaper elements like manganese. In this work, the microstructural and mechanical properties of four niobium-containing (0.035 wt. %) and vanadium-containing (0.02 wt. %) Fe-7Mn-2Ni-1Ti-1Mo-0.03C (in wt. %) with different content of aluminium (1~2 wt. %) aged at different temperatures between 420 °C and 570 °C were investigated. As-quenched Fe-7Mn alloys exhibited a good combination of high strength (~700 MPa of 0.2 proof strength, ~ 850 MPa of UTS) and ductility (~ 10 % tensile elongation). The as-quenched microstructure consisted of lath martensite and a small amount (~ 0.3 vol. %) of micronsized (Ti, Mo, Nb/V)C carbides. The aging process significantly strengthened/hardened the Fe-7Mn alloys which is due to the formation of nano-sized Nix(Ti, Mn, Al) precipitates. Nix(Ti, Mn, Al) precipitates exhibit a very high number density (52.9×1014/m2 in the peak-aged state of Alloy 2, aged at 500 °C for 24 h) and fine size (average diameter was 17.4 ± 4.2 nm after aged at 500 °C for 168 h). The Vickers hardness increased with aging time in the under-aged stage which was due to the precipitate growth and the alloy was strengthened by Orowan bypassing mechanism. The hardness decreased with aging time after the peak hardness as the precipitate coarsened. There were two types of the crystal structure observed for Nix(Ti, Mn, Al) precipitates: The L21-Ni2(Ti, Mn, Al) phase (lattice parameter, a = 0.5863 ~ 0.5895 nm, which is co-planar with martensite matrix, with only 1.72 % of lattice misfit. And the L12-Ni3(Ti, Mn, Al) structure ( a = 0.3598 ~ 0.3613 nm). A short time-aging resulted in a yield strength above 1 GPa but led to embrittlement of Fe-7Mn alloys, which was believed to be due to the segregation of Mn to the grain boundaries. Both carbides and nano-precipitates formed along grain boundaries were likely to reduce the cohesion across the boundary plane, as well as resulted in stress-strain incompatibilities. However, the prolonged aging resulted in the formation of reverted austenite (RA) in the over-aged stage, which led to the recovery of ductility when aged at 570 °C as the austenite reversion removed the Mn solute from the grain boundaries. Reverted austenite exhibited lath-like shape with the length between 50 and 2000 nm. Both the size and volume fraction of RA increased with the increasing aging time and aging temperature. RA was formed with the diffusion-controlled mechanism, and it was observed exhibiting a Kurdjumov-Sachs (K-S) orientation relationship with the neighbouring aged martensite grains with an enrichment of Mn and Ni. Higher content of Al addition resulted in ~ 25 vol. % of δ-ferrite in the as-quenched microstructure, which was stable during aging. 2 wt. % of Al also resulted in higher volume fraction of nano-precipitates and increased the dissolution temperature of precipitates, however, it delayed the peak-aging time and austenite reversion. Nb-containing alloys exhibited relatively finer size of prior austenite grains and (Ti, Mo, V)C carbides, larger size and higher number density of Ni(Ti, Mn, Al) precipitates, but slightly lower austenite fraction, compared to V-containing alloys. Based on the results, it is suggested that Alloy 3 aged at 570 °C for 2~6 h gives the optimized mechanical properties.

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Kim, Bij-Na. "Design and modelling of ultra-high strength steels : nanoprecipitation and plasticity." Thesis, University of Cambridge, 2014. https://www.repository.cam.ac.uk/handle/1810/245234.

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Understanding the changes occurring in the mechanical properties during martensite tempering is essential in the development of new industrial grades. The aim of this research was to develop new ultra-high strength steels via nanoprecipitation control, which requires an understanding of the processing-microstructure-property relationship in medium carbon (0.5-0.6 wt.%) steels throughout tempering. Much of the work has been centred in understanding the role of silicon at the precipitation level and in the recovery of martensite. By using an existing spring steel grade, the effect of interrupted ageing (IA) in tempered martensite has been studied. In IA, an intermediate step between quenching and tempering is introduced, where quenched martensite is left to rest at room temperature for a defined period of time. By allowing carbon segregation into dislocation cores, the incorporation of IA resulted in a more stable microstructure and hardness improvement. The effect of silicon in the epsilon to cementite carbide transition has also been studied. The classical nucleation theory was applied in order to model cementite formation under paraequilibrium conditions, thus incorporating silicon during nucleation. Characterisation using high energy X-rays showed the inhibiting effect of silicon in the overall cementite precipitation. The second effect of silicon was observed in the martensite recovery. A series of experiments were carried out in order to capture the various microstructural changes taking place during tempering: precipitation, grain size and dislocation density evolution. It was observed that the addition of silicon reduces the rate of martensite recovery, owing to the reduced cross-slip in the ferrite lattice. A plasticity model based on irreversible thermodynamics and EBSD characterisation was applied to identify the effective grain size. The results from these two techniques require further research. Nevertheless, based on the post-failure analysis by TEM, it appears that at relatively early tempering stages, even low angle lath boundaries can contribute to strengthening, where piled-up dislocations have been observed at lath boundaries.

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Ratanathavorn, Wallop. "Dissimilar joining of aluminium to ultra-high strength steels by friction stir welding." Doctoral thesis, KTH, Svetsteknologi, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-207356.

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Multi-material structures are increasingly used in vehicle bodies to reduce weight of cars. The use of these lightweight structures is driven by requirements to improve fuel economy and reduce CO2 emissions. The automotive industry has replaced conventional steel components by lighter metals such as aluminium alloy. This is done together with cutting weight of structures using more advanced strength steels. However, sound joining is still difficult to achieve due to differences in chemical and thermal properties. This research aims to develop a new innovative welding technique for joining aluminium alloy to ultra-high strength steels. The technique is based on friction stir welding process while the non-consumable tool is made of an ordinary tool steel. Welding was done by penetrating the rotating tool from the aluminium side without penetrating into the steel surface. One grade of Al-Mg aluminium alloy was welded to ultra-high strength steels under lap joint configuration. Different types of steel surface coatings including uncoated, hot-dipped galvanised and electrogalvanised coating have been studied in order to investigate the influence of zinc on the joint properties. The correlation among welding parameters, microstructures, intermetallic formation and mechanical properties are demonstrated in this thesis. Results have shown that friction stir welding can deliver fully strong joints between aluminium alloy and ultra-high strength steels. Two intermetallic phases, Al5Fe2 and Al13Fe4, were formed at the interface of Al to Fe regardless of surface coating conditions. The presence of zinc can improve joint strength especially at low heat input welding due to an increased atomic bonding at Al-Fe interface. The formation of intermetallic phases as well as their characteristics has been demonstrated in this thesis. The proposed welding mechanisms are given based on metallography investigations and related literature.

QC 20170519

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8

Chamisa, Alfonce. "Development of ultra high strength steels for reduced carbon emissions in automotive vehicles." Thesis, University of Sheffield, 2014. http://etheses.whiterose.ac.uk/6274/.

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Automotive steels with enhanced strength and ductility beyond the current bounds can be engineered through microstructural strategies that take into account the benefits brought about by nanoprecipitates formed during the transformation from austenite to ferrite. Three multiphase steel compositions were initially studied. A patented Ti-Al-Mo steel composition was selected as the baseline for comparison with the other two steels. It is claimed that this steel has exceptional mechanical properties. The Ti and V microalloyed steels were selected to check whether interphase precipitates (IP), which can yield a high degree of precipitation strengthening, could be produced. Results showed that Ti-Al-Mo had superior microstructure and properties as compared to the other two. The microstructure was composed of ferrite, martensite, bainite and retained austenite. Unlike the other two steels, IP was also observed and the UTS of 780MPa and uniform elongation of 21% previously reported by other authors were also confirmed. The V microalloyed steel composition was selected for the next part of the project since it would be commercially viable to produce for Tata Steel. The time/temperature/transformation behaviours of the V microalloyed steel were extensively studied. The microstructures developed were analysed and high precipitate number densities averaging 394 particles/m2 were recorded in the sample transformed at 700oC for 1200s. A high uniform elongation of 30.8% and the highest UTS of 627MPa were also reported on the same sample. The UTS value was attributed to the high precipitate number density which made an overall contribution to the yield strength of 270MPa. However, further studies need to be carried out, since the properties were not optimised and were inferior to some of the steels in current use for automotive applications. Questions were asked as to why IP was not observed. The low austenising temperature of 950oC was cited as the possible reason. Thermodynamic calculations using Thermo-Cal software had predicted that the optimum should have been 1050oC. As a result, 950oC was believed to be inadequate to effectively dissolve the carbides present to allow effective formation of interphase precipitates during the temperature hold in the α + γ temperature region. The high N content was cited as another possible reason, but this was not conclusive and shown in itself not to be true by work in the next stage of the project. It has since been established that Mo retards precipitate growth in both Nb and Ti alloyed steels. However, nothing has been reported on the effects of Mo on V microalloyed steels. As a result, the next stage of the project studied the effects of Mo on V microalloyed steels. Predominantly ferritic steels with Nb-V-Mo microalloying additions were produced and coiled at different temperatures. Samples microalloyed with Ti-Mo, Ti, V-Mo, V, Nb-Mo and Nb were also produced for comparison purposes. IP was observed in most of the Nb-V-Mo steels. IP with average interparticle distances of 8 ± 2nm and row spacing of 22 ± 3nm were observed in sample 10-630Nb+VMo. High YS of 925MPa, UTS of 1023MPa and total elongation of 16.8% were recorded for this sample. The exceptional mechanical properties were attributed to high number densities of fine IP averaging 1766 particles/m2. 82% of the precipitates had average sizes below 7nm and these made a contribution to YS of approximately 546MPa. It was then concluded that Mo additions were likely to have influenced the formation of fine precipitates that strengthened the ferritic steels. Hence Mo is likely to influence the high nucleation rate and slow precipitate growth in the same way that it influences Ti and Nb microalloyed steels. Since one of the steels studied at this stage had high N additions, it was also confirmed that the precipitate number densities in the previous V microalloyed steels batch had nothing to do with the N content; instead, it all had to do with the low austenising temperature which failed to put the carbides into solution.

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Ardehali, Barani Araz. "Optimization of the critical content of tramp elements in ultra-high strength silicon chromium spring steels through thermomechanical treatment." Göttingen Cuvillier, 2007. http://d-nb.info/988382563/04.

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Ardehali, Barani Araz. "Optimization of the critical content of tramp elements in ultra-high strength silicon chromium spring steels through thermomechanical treatment /." Göttingen : Cuvillier, 2008. http://bvbr.bib-bvb.de:8991/F?func=service&doc_library=BVB01&doc_number=017079000&line_number=0001&func_code=DB_RECORDS&service_type=MEDIA.

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Ravindran, Deepak. "Finite Element Simulation of Hot Stamping." The Ohio State University, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=osu1307540892.

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12

Abosbaia, Alhadi A. S. "Design and processing of low alloy high carbon steels by powder metallurgy. P/M processing and liquid phase sintering of newly designed low-alloy high carbon steels based on Fe-0.85Mo-C-Si-Mn with high toughness and strength." Thesis, University of Bradford, 2010. http://hdl.handle.net/10454/4857.

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The work presented has the ultimate aim to increase dynamic mechanical properties by improvements in density and optimisation of microstructure of ultra high carbon PM steels by careful selection of processes, i.e. mixing, binding, alloying, heating profile and intelligent heat treatment. ThermoCalc modelling was employed to predict liquid phase amounts for two different powder grades, Astaloy 85Mo or Astaloy CrL with additive elements such as (0.4-0.6wt%)Si, (1.2-1.4wt%)C and (1-1.5wt%)Mn, in the sintering temperature range 1285-1300ºC and such powder mixes were pressed and liquid phase sintered. In high-C steels carbide networks form at the prior particle boundaries, leading to brittleness, unless the steel is heat-treated. To assist the breaking up of these continuous carbide networks, 0.4-0.6% silicon, in the form of silicon carbide, was added. The water gas shift reaction (C + H2O = CO + H2, start from ~500ºC) and Boudouard reaction (from ~500ºC complete ~930ºC) form CO gas in the early part of sintering and can lead to large porosity, which lowers mechanical properties. With the use of careful powder drying, low dew point atmospheres and optimisation of heating profiles, densities in excess of 7.70g/cm3 were attained. The brittle microstructure, containing carbide networks and free of cracks, is transformed by intelligent heat treatment to a tougher one of ferrite plus sub-micron spheroidised carbides. This gives the potential for production of components, which are both tough and suitable for sizing to improve dimensional tolerance. Yield strengths up to 410 MPa, fracture strengths up to 950 MPa and strains of up to 16 % were attained. Forging experiments were subsequently carried out for spheroidised specimens of Fe-0.85Mo+06Si+1.4C, for different strain rates of 10-3, 10-2, 10-1 and 1sec-1 and heated in argon to 700¿C, density ~7.8g/cm3 and 769 MPa yield strength were obtained.
Libyan Education Ministry Office

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13

Abosbaia, Alhadi Amar Salem. "Design and processing of low alloy high carbon steels by powder metallurgy : P/M processing and liquid phase sintering of newly designed low-alloy high carbon steels based on Fe-0.85Mo-C-Si-Mn with high toughness and strength." Thesis, University of Bradford, 2010. http://hdl.handle.net/10454/4857.

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The work presented has the ultimate aim to increase dynamic mechanical properties by improvements in density and optimisation of microstructure of ultra high carbon PM steels by careful selection of processes, i.e. mixing, binding, alloying, heating profile and intelligent heat treatment. ThermoCalc modelling was employed to predict liquid phase amounts for two different powder grades, Astaloy 85Mo or Astaloy CrL with additive elements such as (0.4-0.6wt%)Si, (1.2-1.4wt%)C and (1-1.5wt%)Mn, in the sintering temperature range 1285-1300ºC and such powder mixes were pressed and liquid phase sintered. In high-C steels carbide networks form at the prior particle boundaries, leading to brittleness, unless the steel is heat-treated. To assist the breaking up of these continuous carbide networks, 0.4-0.6% silicon, in the form of silicon carbide, was added. The water gas shift reaction (C + H2O = CO + H2, start from ~500ºC) and Boudouard reaction (from ~500ºC complete ~930ºC) form CO gas in the early part of sintering and can lead to large porosity, which lowers mechanical properties. With the use of careful powder drying, low dew point atmospheres and optimisation of heating profiles, densities in excess of 7.70g/cm3 were attained. The brittle microstructure, containing carbide networks and free of cracks, is transformed by intelligent heat treatment to a tougher one of ferrite plus sub-micron spheroidised carbides. This gives the potential for production of components, which are both tough and suitable for sizing to improve dimensional tolerance. Yield strengths up to 410 MPa, fracture strengths up to 950 MPa and strains of up to 16 % were attained. Forging experiments were subsequently carried out for spheroidised specimens of Fe-0.85Mo+06Si+1.4C, for different strain rates of 10-3, 10-2, 10-1 and 1sec-1 and heated in argon to 700ºC, density ~7.8g/cm3 and 769 MPa yield strength were obtained.

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14

Picas, Anfruns Ingrid. "Mechanical behaviour of tools for shearing Ultra High-Strength Steels: influence of the microstructure on fracture and fatigue micro-mechanisms of tool steels and evaluation of micro-mechanical damage in tools." Doctoral thesis, Universitat Politècnica de Catalunya, 2012. http://hdl.handle.net/10803/112059.

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On account of environmental and safety related requirements, the majority of the most popular automotive manufacturers convey to introduce Ultra High Strength Steels (UHSS) and Press Hardened Steels (PHS) in the vehicle body-in-white. Nevertheless, the real success of UHSS and PHS parts implementation into the upcoming automobiles is being so far restricted by their harsh cold forming and post forming operations, e.g. shearing. Their high yield stress leads to accelerated damage and premature fracture of forming and cutting tools and this stands as great handicap of their manufacturing process. Tool materials require the maximum hardness ¿ toughness relationship so as to face up to such severe mechanical solicitations. The performance of tool steels during cold forming and shearing applications depends on the mechanical properties resulting from the complex interaction between their main microstructural constituents; i.e. primary alloy carbides and tempered martensite matrix (which consists in a fine distribution of nanometer-sized alloy carbides, the so-called secondary hardening carbides, precipitated in a martensite matrix).In this regard, the present Thesis has analysed the microstructure, macro- and micro-mechanical properties, crack nucleation and growth under monotonic loads, R-curve behaviour and fatigue crack nucleation and propagation mechanisms of four different cold work tool steels: 1.2379, UNIVERSAL, K360 and HWS. The mechanical and tribological behaviour of industrial and laboratory tools made of these steels have been studied, as well as the effects of shearing process parameters on their performance and quality of sheet sheared edges. The suitability of Linear Elastic Fracture Mechanics (LEFM) based models to estimate the performance of tools has been evaluated in order to predict their service lives. Rationalisation of damage on tool steels from a micro-mechanical point of view in laboratory samples as well as in real tools has permitted to identify the aspects that a model should consider in order to successfully predict tool lives. In addition, it has permitted to propose a new approach to estimate tool performances based on the reduction of tool steels mechanical properties with the increase of the number of strokes. 1.2379, UNIVERSAL and K360 are produced following conventional ingot metallurgy routes while HWS is obtained by Powder Metallurgy (PM). As follows from their different chemical compositions and manufacturing routes, these steels show different primary alloy carbide features and distributions in the microstructure. Nucleation of cracks in tool steels is due to the failure of primary carbides by cleavage (and inclusion particles in case of PM steels). Small cracks nucleated from primary carbides in ingot cast steels are initially very shallow, but they tend to grow to semi-circular shapes as they propagate through the matrix bands. Ingot cast tool steels present R-curve behaviour, i.e. small cracks nucleated in the microstructure have lower toughness values than those determined for long cracks in standard measurement tests. The main failure mechanism of tools for punching and slitting UHSS is chipping at the cutting edge. Plastic deformation and nucleation of cracks at the surface of tools occurs as a result of the high frictional forces during sliding against the sheet. Due to the high contact pressures at the surface, carbides within a narrow range of depth beneath the surface are broken and cracks are also nucleated from surface asperities. Chipping is then explained by the nucleation and coalescence of these cracks. In presence of marked grinding grooves, failure of tools is significantly accelerated. Models based on LEFM have proved to predict with rather poor accuracy the life of tools. A new approach to predict tool performances is proposed which holds the idea that the micro-mechanical properties of the metallic matrix in tool steels decrease due to the application of repetitive load
Per tal de satisfer les creixents demandes de seguretat i medi ambient, la major part d’empreses del sector de l’automoció han convingut en la introducció d’acers d’ultra alta resistència mecànica (UHSS) i acers d’estampació en calent (PHS) per la fabricació de peces estructurals en vehicles . Tanmateix, l’èxit de la implementació d’aquestes peces es veu afectat per les dificultats que deriven de les seves operacions de conformació i tall. L’elevat límit elàstic dels acers UHSS i PHS condueix al dany accelerat i fractura prematura de les eines, el que esdevé un impediment important a l’hora de dur a terme aquestes operacions a escala industrial.Els acers d’eina han de presentar alts valors de duresa i tenacitat per tal de resistir les elevades sol•licitacions mecàniques que reben. El seu rendiment ve determinat per les propietats micro-mecàniques que deriven de la complexa interacció entre els principals constituents microestructurals, és a dir els carburs primaris i la matriu de martensita revinguda.En aquesta Tesi s’han analitzat la microestructura, propietats macro- i micro-mecàniques, nucleació i creixement d’esquerdes a nivell monotònic, comportament de corba-R i mecanismes de nucleació i propagació d’esquerdes a fatiga en quatre acers d’eina diferents: 1.2379, UNIVERSAL, K360 i HWS. S’ha estudiat el comportament mecànic i tribològic d’eines fabricades a partir d’aquests acers treballant a escala industrial i de laboratori, així com els efectes dels paràmetres de tall en el rendiment de les eines i la qualitat dels marges tallats. La idoneïtat dels models basats en la Mecànica de la Fractura Elàstica Lineal (LEFM) a l’hora de estimar el rendiment de les eines ha estat avaluada per tal de predir la vida d’aquestes. La racionalització del dany en acers d’eina des d’un punt de vista micro-mecànic, en provetes de laboratori així com en eines reals, ha permès identificar els aspectes que un model hauria de considerar per predir de forma satisfactòria la vida d’eines. A més, també ha permès proposar una nova aproximació en aquest sentit, basada en la reducció de les propietats mecàniques de la matriu metàl•lica com a conseqüència de l’aplicació de càrregues repetitives. Els acers 1.2379, UNIVERSAL i K360 es fabriquen seguint processos de fosa i forja convencionals mentre que l’HWS s’obté per pulvimetal•lúrgia. Les diferents composicions químiques i processos de fabricació d’aquests acers expliquen que presentin microestructures amb carburs primaris de diferents característiques i distribucions. La nucleació d’esquerdes es deu a la fractura de carburs primaris per clivatge (i d’inclusions no metàl•liques en l’HWS). Les esquerdes petites nucleades a partir de carburs primaris en els acers fosos i forjats són inicialment molt poc profundes, però tendeixen a adquirir formes semi-circulars al créixer a través de la matriu metàl•lica. Els acers fosos i forjats presenten comportament de corba-R, és a dir que les esquerdes petites nucleades a la microestructura tenen una tenacitat inferior al valor determinat per esquerdes grans en els assajos estàndard. El principal mecanisme de dany observat en eines de tall d’acers UHSS és la fractura per fatiga a baix nombre de cicles. La deformació plàstica i nucleació d’esquerdes a la superfície de les eines s’explica per les elevades forces de fricció que es desenvolupen pel lliscament de la xapa. A més, les altes pressions de contacte a la superfície provoquen que els carburs primaris trenquin lleugerament per sota d’aquesta, i que d’altres esquerdes s’iniciïn a partir d’aspreses superficials. Les fractures es poden entendre doncs, per la nucleació i coalescència de les esquerdes esmentades. La presència de línies de mecanitzat afecta negativament la vida de les eines ja que accelera de forma molt significativa els processos de fractura. Els models basats en LEFM no s’han mostrat prou adients per predir la vida de les eines de tall i per això, una nova aproximació ha estat proposada.

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Baglieri, Giulia. "Caratterizzazione a fatica con intaglio di acciai innovativi ultra-alto resistenziali." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2017.

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Lo scopo della presente ricerca è valutare l’idoneità dell’acciaio da utensili innovativo K890, prodotto da Böhler con metallurgia delle polveri, ad essere impiegato per la realizzazione di componenti motore di moto da corsa, quali alberi a camme e a gomiti, in sostituzione dell’acciaio da nitrurazione GKHW, prodotto da Aubert&Duval con processo ESR (Electro Slag Remelting). Lo studio è stato condotto a valle di una precedente sperimentazione condotta su provini non intagliati ed ha previsto l'esecuzione di prove di trazione e prove di fatica a flessione rotante con intaglio oltre a prove di durezza ed analisi microstrutturali e frattografiche. Sono stati testati due lotti di acciaio K890, aventi durezza di 752 e 821 HV rispettivamente. Durante i test di fatica il lotto in K890 a durezza maggiore ha mostrato un limite di fatica con intaglio superiore al GKHW nitrurato. I risultati ottenuti sui due lotti di K890 sono stati impiegati per valutare l’applicabilità di relazioni empiriche per la stima del coefficiente di concentrazione a fatica delle tensioni ad acciai ultra-alto resistenziali. E’ emerso che le relazioni più tradizionali sovrastimano l’effetto dell’intaglio per questa tipologia di acciaio. A conclusione dello studio, considerando l'esito delle stesse prove condotte sull'acciaio GKHW nitrurato attualmente in uso, si può affermare che l'acciaio K890 è un possibile candidato per la produzione di alberi in ambito racing. La disamina dei risultati sperimentali è preceduta da tre capitoli incentrati su un inquadramento teorico, derivanti da ricerche bibliografiche: il primo riporta un’introduzione al funzionamento ed alle caratteristiche degli alberi a camme e a gomiti; il secondo è relativo alle proprietà degli acciai da utensili tradizionali ed innovativi; infine il terzo riguarda una trattazione sul danneggiamento da fatica, con particolare riguardo all’effetto dell’intaglio ed alle caratteristiche del fenomeno negli acciai ultra-alto resistenziali.

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16

He, Junjing. "High temperature performance of materials for future power plants." Doctoral thesis, KTH, Materialvetenskap, 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-191547.

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Increasing energy demand leads to two crucial problems for the whole society. One is the economic cost and the other is the pollution of the environment, especially CO2 emissions. Despite efforts to adopt renewable energy sources, fossil fuels will continue to dominate. The temperature and stress are planned to be raised to 700 °C and 35 MPa respectively in the advanced ultra-supercritical (AUSC) power plants to improve the operating efficiency. However, the life of the components is limited by the properties of the materials. The aim of this thesis is to investigate the high temperature properties of materials used for future power plants. This thesis contains two parts. The first part is about developing creep rupture models for austenitic stainless steels. Grain boundary sliding (GBS) models have been proposed that can predict experimental results. Creep cavities are assumed to be generated at intersection of subboundaries with subboundary corners or particles on a sliding grain boundary, the so called double ledge model. For the first time a quantitative prediction of cavity nucleation for different types of commercial austenitic stainless steels has been made. For growth of creep cavities a new model for the interaction between the shape change of cavities and creep deformation has been proposed. In this constrained growth model, the affected zone around the cavities has been calculated with the help of FEM simulation. The new growth model can reproduce experimental cavity growth behavior quantitatively for different kinds of austenitic stainless steels. Based on the cavity nucleation models and the new growth models, the brittle creep rupture of austenitic stainless steels has been determined. By combing the brittle creep rupture with the ductile creep rupture models, the creep rupture strength of austenitic stainless steels has been predicted quantitatively. The accuracy of the creep rupture prediction can be improved significantly with combination of the two models. The second part of the thesis is on the fatigue properties of austenitic stainless steels and nickel based superalloys. Firstly, creep, low cycle fatigue (LCF) and creep-fatigue tests have been conducted for a modified HR3C (25Cr20NiNbN) austenitic stainless steel. The modified HR3C shows good LCF properties, but lower creep and creep-fatigue properties which may due to the low ductility of the material. Secondly, LCF properties of a nickel based superalloy Haynes 282 have been studied. Tests have been performed for a large ingot. The LCF properties of the core and rim positions did not show evident differences. Better LCF properties were observed when compared with two other low γ’ volume fraction nickel based superalloys. Metallography study results demonstrated that the failure mode of the material was transgranular. Both the initiation and growth of the fatigue cracks were transgranular.

QC 20160905

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17

Anderson, Cheryl Marie. "The weldability of high and ultra-high strength steel." Thesis, Swansea University, 2003. https://cronfa.swan.ac.uk/Record/cronfa42947.

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Weight reduction in body-in-white structures is necessary to make automobiles more fuel-efficient. A range of high and ultra-high strength strip steels have been developed, that will play a key role in achieving lower weights since the steels have the potential to achieve equivalent strength and crashworthiness at thinner gauges. However, the full potential of these advanced alloys can only be realised if they can be integrated into production facilities that rely on resistance spot welding as the predominant means of component joining. In particular, spot welds manufactured in these modern high strength steels will need to meet the strength and fracture resistance requirements that are based on automotive manufacturers' familiarity with low alloy steels. Dual phase steels are a range of modern alloys causing considerable excitement due to their combination of high strength, high ductility and improved crashworthiness in automotive components, compared to mild steel. Their commercial production routes rely on a metallurgical understanding of how chemical composition and thermomechanical treatments interrelate to produce appropriate microstructures. Their often complex alloy compositions mean that there is potential for significant changes to take place in the microstructure on resistance welding. This research programme has considered the important relationships from which resistance spot-welds, produced in high strength steels, derive their properties. This includes an investigation into the continuous cooling transformation behaviour of four dual phase alloys, in comparison to low alloy grades, and measurement of the mechanical properties associated with their microstructures. The thermal profiles generated within spot welds have been measured using a thermocouple technique. Advanced resistance spot welding processes, that can modify the metallurgical condition of a spot-weld, have been investigated with some success, both in terms of reductions in weld hardness following pulsed welding schedules, and an understanding of the effect of such schedules on the thermal cycle.

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18

Sederstrom, Jack Hunter. "Spot friction welding of ultra high-strength automotive sheet steel / /." Diss., CLICK HERE for online access, 2007. http://contentdm.lib.byu.edu/ETD/image/etd1724.pdf.

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19

Hartman, Trent J. "Friction Stir Spot Welding of Ultra-High Strength Steel." BYU ScholarsArchive, 2012. https://scholarsarchive.byu.edu/etd/3302.

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Friction stir spot welding (FSSW) is quickly becoming a method of interest for welding of high strength steel (HSS) and ultra high strength steel (UHSS). FSSW has been shown to produce high quality welds in these materials, without the drawbacks associated with fusion welding. Tool grade for polycrystalline cubic boron nitride (PCBN) tools has a significant impact on wear resistance, weld quality, and tool failure in FSSW of DP 980 steel sheet. More specifically, for a nominal composition of 90% CBN, the grain size has a significant impact on the wear resistance of the tool. A-type tools performed the best, of the three grades that were tested in this work, because the grain size of this grade was the finest, measuring from 3-6 microns. The effect of fine grain size was less adhesion of DP 980 on the tool surface over time, less abrasive wear, and better lap shear fracture loads of the welds that were produced, compared to the other grades. This is explained by less exposure of the binder phase to wear by both adhesion and abrasion during welding of DP 980. A-type tools were the most consistent in both the number of welds per tool, and the number of welds that reached acceptable lap shear fracture loads. B-type tools, with a bimodal grain size distribution (grain size of 4 – 40 microns) did a little bit better than C-type tools (grain size of 12-15 microns) in terms of wear, but neither of them were able to achieve consistent acceptable lap shear fracture load values after the first 200 welds. In fact only one out of five C-type tools was able to produce acceptable lap shear fracture loads after the first 100 welds.

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Sederstrom, Jack H. "Spot Friction Welding of Ultra High-Strength Automotive Sheet Steel." BYU ScholarsArchive, 2007. https://scholarsarchive.byu.edu/etd/842.

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Spot friction welding (SFW) was performed on ultra high strength steel (UHSS) steel sheet commonly used in automobile manufacturing. Alloys studied included DP780, DP780EG, DP980, and DF140T sheet steel of varying thickness from 1.2 mm to 1.4 mm. Welding was accomplished using a PCBN standard tool. Weld strengths were then compared to a proposed AWS standard. Initial hardness readings were taken in cross sectioned samples. Grain structure in a SFW is presented. Resistance spot welds were created in three steels. This study focuses on the strength of SFW joints as compared to traditional resistance spot welding (RSW) in welding like materials to one another. Cycle times of SFW were also evaluated and compared to production rate cycle times of RSW.

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21

Lord, Michael. "Design and modelling of ultra-high strength steel weld deposits." Thesis, University of Cambridge, 1999. https://www.repository.cam.ac.uk/handle/1810/221873.

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22

De, Carufel Sarah. "Effect of High-Performance Steel Materials on the Blast Behaviour of Ultra-High Performance Concrete Columns." Thesis, Université d'Ottawa / University of Ottawa, 2016. http://hdl.handle.net/10393/35380.

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Previous events have demonstrated the vulnerability of reinforced concrete infrastructure to blast loading. In buildings, ground-story columns are key structural components, and their failure can lead to extensive damages which can cause progressive collapse. To prevent such disasters, the steel reinforcement in such columns must be properly detailed to ensure sufficient strength and ductility. The use of modern concrete materials such ultra-high performance concrete (UHPC) is one potential solution to improve the blast performance of columns. UHPC shows high compressive strength, high tensile resistance and superior toughness, properties which make it ideal for use in the blast-resistant design of columns. The combined use of UHPC and high-performance steels can potentially be used to further enhance the blast resistance of columns. This thesis presents an experimental and analytical study which investigated the use of high-performance materials to increase the blast capacity and ductility of reinforced concrete columns. As part of the experimental study, a total of seventeen columns were tested under simulated blast loading using the University of Ottawa Shock-Tube. Parameters investigated included the effect of concrete type (NSC and UHPC), steel reinforcement type (normal-strength, high-strength or highly ductile), longitudinal reinforcement ratio, seismic detailing and fiber properties. The test program included two control specimens built with normal-strength concrete, five specimens built with UHPC in combination with high-strength steel, and ten columns built with highly ductile stainless steel reinforcement. Each column was subjected to a series of increasing blast pressures until failure. The performance of the columns is investigated by comparing the displacements, impulse capacity and secondary fragmentation resistance of the columns. The results show that using high-performance steels increases the blast performance of UHPC columns. The use of sufficient amounts of high-strength steel in combination with UHPC led to important increases in column blast capacity. The use of ductile stainless steel reinforcement allowed for important enhancements in column ductility, with an ability to prevent rupture of tension steel reinforcement. The study also shows that increasing the longitudinal reinforcement ratio is an effective means of increasing the blast resistance of UHPC columns The thesis also presents an extensive analytical study which aimed at predicting the response of the test columns using dynamic inelastic, single-degree-of-freedom (SDOF) analysis. A sensitivity analysis was also performed to examine the effect of various modelling parameters on the analytical predictions. Overall, it was shown that SDOF analysis could be used to predict the blast response of UHPC columns with reasonable accuracy. To further corroborate the results from the experimental study, the thesis also presents an analytical parametric study examining the blast performance of larger-scale columns. The results further demonstrate the benefits of using UHPC and high-performance steel reinforcement in columns subjected to blast loading.

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23

Ma, Ning [Verfasser]. "Prediction of springback for ultra high strength steel sheets / Ning Ma." Aachen : Hochschulbibliothek der Rheinisch-Westfälischen Technischen Hochschule Aachen, 2013. http://d-nb.info/1043521739/34.

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24

Aldén, Rickard. "Innovative Methods for Welding Ultra High Strength Steel with Resistance Spot Welding." Thesis, KTH, Industriell produktion, 2018. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-245224.

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Resistance spot welding (RSW) is the most frequently used method for welding thin sheets in manufacturing industries such as the automotive industry, because of the high productivity of RSW. In order to reduce CO2 emissions the automotive industry strives towards creating structures of light weight, this is partly achieved by the use of lightweight materials such as Aluminum and composite materials. In parts of the car body designed to protect the driver and passengers in case of a collision High strength steel is used due to its high strength and relative high ductility. High strength steels are called Ultra High Strength Steels (UHSS) with typical ultimate tensile strength of 700 up to 2000 MPa and elongation of 10-40%. Because of the strive against lighter structures and great safety demands UHSS materials is of great interest for the automotive industry in order to create strong structures of light weight. In welding of modern materials such as UHSS with RSW, achieving adequate weld quality is a challenge. Hence this thesis aims to investigate new innovative ways to broaden the area of use and include modern materials such as UHSS for the traditional method of welding such as RSW. In RSW elliptical shaped welds are created between two or more faying metal sheets by passing current through the sheets. The current is applied to the sheets by copper electrodes in contact with the sheets on each side. The geometrical shape of these electrodes will affect multiple welding parameters such as applied stress, current density, electromagnetic stirring, temperature gradients and the possibility for the welded material to thermally expand during welding. Hence the geometrical shape of the electrodes will affect the final shape and size of the weld nugget. In this thesis RSW electrode geometries are modified and tested. The weld properties from modified electrodes are compared to the weld properties from standard RSW electrodes with respect to process robustness, weld nugget shape and size, micro hardness and weld tensile strength. Various modified geometries are used, all modified geometries are designed in order to allow the welded material to expand more, compared to standard electrodes. Previous work has been done and shown that hollow electrodes that allow the welded material to expand can improve the weld quality and process robustness. However, this has been to the cost of nugget growth in the normal direction to the welded sheet, leaving a non-uniform surface. Hence the aim of this thesis is to investigate if it is possible to widen the current range in the weld lobe diagram when welding UHSS material combinations with RSW by the use of hollow electrodes without affecting weld quality negatively compared to standard electrodes. Weld quality in this thesis will be evaluated based on surface condition, mechanical strength, micro-hardness and weld nugget size. The modified electrodes have shown better weld properties with respect of current range in the weld lobe curves in most cases tested but not all of the material combinations tested compared with standard electrodes. The surface conditions of the welded specimens have been controlled by measuring any indent and raise by line laser scanning. Modified RSW electrodes has showed improved welding properties with respect to current range in the weld lobe curves compared to standard RSW electrodes when welding UHSS material combinations. However modified electrodes have shown a higher sensitivity to misalignment and angle fault. Several material combinations of UHSS that has shown non-weldable behavior with standard RSW electrodes have shown improved current range. In the best case the current range was increased to 3,9 kA for an UHSS material combination that is non-weldable with standard electrodes.
Motståndsvetsning är den vanligaste metoden för svetsning av tunna plåtar i tillverkningsindustrier som bilindustrin på grund av den höga produktiviteten hos punktsvetsning. För att minska koldioxidutsläppen strävar bilindustrin efter att skapa lättviktskonstruktioner, vilket delvis uppnås genom användning av lätta material såsom aluminium och kompositmaterial. I delar av bilkroppen konstruerad för att skydda föraren och passagerare vid kollision används höghållfast stål på grund av sin höga hållfasthet och relativt höga duktilitet. Höghållfasta stål kallas Ultra High Strength Steels (UHSS) med typisk draghållfasthet på 700 upp till 2000 MPa och förlängning av 10-40%. På grund av bilindustrins strävan mot lättare strukturer och höga säkerhetskrav är UHSS-material av stort intresse för bilindustrin för att skapa starka strukturer av lätt vikt. Vid svetsning av moderna material som UHSS med punktsvetsning är det en utmaning att uppnå tillräcklig svetskvalitet. Därför syftar denna avhandling till att undersöka nya innovativa sätt att bredda användningsområdet och inkludera moderna material som UHSS för den traditionella svetsmetoden punktsvetsning. Under punktsvetsning skapas elliptiskt formade svetsar mellan två eller flera metallplåtar genom att ström passerar genom plåtarna. Strömmen appliceras till plåtarna genom kopparelektroder i kontakt med plåtarna på var sida. Den geometriska formen av dessa elektroder kommer att påverka flera svetsegenskaper såsom applicerad tryck, strömtäthet, elektromagnetisk omröring, temperaturgradienter och möjligheten för det svetsade materialet att termiskt expandera under svetsning. Följaktligen kommer den geometriska formen av elektroderna att påverka den slutliga formen och storleken hos svetslinsen. I denna avhandling modifieras och testas elektrodgeometrier som används vid punktsvetsning. Svetsegenskaperna från modifierade elektroder jämförs med svetsegenskaperna från standardelektroder med avseende på processens robusthet, svetslinsform-och storlek, mikrohårdhet och svetshållfasthet. Olika modifierade geometrier används, alla modifierade geometrier är utformade för att låta det svetsade materialet expandera mer jämfört med standardelektroder. Tidigare arbete har gjorts och visat att ihåliga elektroder som tillåter det svetsade materialet att expandera kan förbättra svetskvaliteten och processens robusthet. Detta har dock varit till kostnaden av svetslinstillväxt i riktning vinkelrätt till den svetsade plåten, vilket lämnar en ojämn yta. Avsikten med denna avhandling är därför att undersöka om det är möjligt att bredda användingen av punktsvetsning till svetsning av UHSS-materialkombinationer genom användning av ihåliga elektroder utan att påverka svetskvaliteten negativt jämfört med standardelektroder. Svetskvaliteten i denna avhandling kommer att utvärderas baserat på yttillstånd, mekanisk hållfasthet, mikrohårdhet och svetslinsstorlek. De modifierade elektroderna har visat bättre svetsegenskaper med avseende på svetsbarhet i de flesta fall testade men inte alla materialkombinationer som testats jämfört med standardelektroder. Ytförhållandena för de svetsade exemplen har kontrollerats genom att mäta intryck och upphöjnad på den svetsade ytan genom laserskanning. Flera materialkombinationer av UHSS som har visat sig osvetsbara med standard elektroder har visat förbättrad svetsbarhet med modifierade elektroder. I bästa fall ökade strömintervallet med godkänd svets till 3,9 kA för en UHSS-materialkombination som inte är svetsbar med standardelektroder.

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25

Eman, Jesper. "Study and characterization of localization and failure behaviour of ultra high strength steel." Licentiate thesis, Luleå : Luleå tekniska universitet/Tillämpad fysik, maskin- och materialteknik/Hållfasthetslära, 2007. http://epubl.ltu.se/1402-1757/2007/23/.

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26

Hjelmtorp, Kristofer. "Resistance Spot Welding of AlSi-coated Ultra High Strength Steel : An experimental study." Thesis, KTH, Materialvetenskap, 2019. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-241644.

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The automotive industry of today faces ever harder requirements from regulatory bodies to increase the fuel efficiency, reduce the carbon footprint and increase the safety of their vehicles. The problem is being tackled in different ways; one of them being the use of innovative materials to reduce the overall weight while improving the crash safety of the vehicle. One such material is 22MnB5, an ultra-high strength (UHS) boron-alloyed steel, capable of reaching tensile strength of 1900 MPa. The weldability is a vital factor for applying boron steel in an efficient way into a vehicle construction. Resistance spot welding (RSW) is, among the different welding methods, the primary joining methods used within the automotive industry. The main challenges with RSW of UHS boron steel is the narrow welding window and increased risk of expulsion compared to conventional automotive steel. The aim of this thesis was evaluating how the weldability of three-sheet UHS boron steel combinations could be improved by applying different innovative welding methods. The methods investigated where; three-pulsed welding, two-pulsed welding with force profile and using hollow-cone electrodes instead of regular electrodes. The different methods where evaluated with welding experiments and analysis of the nugget diameter, vicker hardness comparison and tensile strength test of welding nugget. The results from this thesis shows that the current window of three-sheet combinations with UHS boron steel can be significantly improved by using hollow-cone electrodes in RSW. The results also showed that the width of the current window varied depending on the depth of the hole in the electrode, a deeper hole improved the current window but also increased the oxide build-up. Applying a force profile with lowered electrode force during the welding sequence provided an improved process window compared to the constant electrode force when welding a three-sheet combination containing AlSi-coated boron steel. A three-pulse welding sequence performed better than the reference two-pulse welding schedule but still not good enough to meet VCC acceptance criteria.
Bilindustrin står idag inför allt hårdare krav från tillsynsmyndigheter förbättra bränsleeffektiviteten, minska koldioxidavtrycket och öka säkerheten på deras fordon. Problemet angrips från ett flertal olika vinklar. varav en ökad användning av innovativa material för att minska den totala vikten samtidigt som fordonets kraschsäkerhet bibehålls eller ens förbättras. Ett sådant material är 22MnB5, ett höghållfast (UHS) borstål, kapabelt att uppnå brottgränser på 1900 MPa. Svetsbarheten är en vital faktor för att kunna applicera borstål på ett effektivt sätt i en fordonskonstruktion. Inom bilindustrin är motståndspunksvetsning (RSW) den dominanta svetsmetoden. De största utmaningarna med att punktsvetsa höghållfast AlSi-belagt borstål är det har ett generellt smalare svetsfönstren, samt den ökade risken för sprut under svetsprocessen, jämfört med konventionella stål. Målet med denna avhandling var att utvärdera hur svetsbarheten av tre-plåtskombinationer med höghållfast AlSi-belagt borstål kunde förbättras genom att applicera innovativa svetsmetoder. De utvärderade metoderna var; tre-pulsad svetsning, två-pulsad svetsning med applicerad kraftprofil, samt användning av ihåliga elektroder istället för vanliga elektroder. Metoderna utvärderades genom svetsexperiment och analys av svetslobens storlek, vicker hårdhets mätning samt brottgränsmätning av svetsloben. Resultaten från denna avhandling visar att svetsbarheten för tre-plåts kombinationer med UHS borstål kan förbättras avsevärt genom att använda ihåliga elektroder för punktsvetsning. Resultaten pekar också på att förbättringen beroende på hålets djup i elektroden. Ett djupare hål gav större förbättringar men ökade också uppbyggnaden av oxid och restmetall i elektroden. Genom att applicera en kraftprofil, där elektrodkraften sänktes under svetsprocessen kunde svetsbarheten förbättras för två-puls svetsning, jämfört med att ha konstant elektrodkraft, vid svetsning av en tre-plåtskombination innehållande höghållfast AlSi-belagt borstål. En tre-puls svetssekvens utförde bättre än referenspulssvetsschemat men fortfarande inte tillräckligt bra för att uppfylla VCC-acceptkriterierna.

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Seymour, Andrew Richard Ian. "Optimisation of the fracture toughness of a novel ultra-high strength maraging steel." Thesis, University of Birmingham, 2018. http://etheses.bham.ac.uk//id/eprint/8718/.

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This thesis details work that was carried out to optimise the fracture toughness and thermal stability of a new maraging steel called F1E. This steel was designed to precipitate Laves phase to improve the creep properties, and it was initially found that this was detrimental to toughness properties, and that further precipitation occurred during extended time at desired operating temperatures, embrittling the alloy. Initial work focussed on development of a heat treatment to stabilise the Laves phase, using a slow cool to the service temperature to fully precipitate the equilibrium volume fraction of Laves phase at this temperature without nucleating fresh particles, as it was believed that it was fresh particles forming during service which led to the loss of ductility after thermal exposure. This heat treatment process successfully stabilised the properties. Modifications were then made to the composition in an attempt to improve the ductility and toughness without losing strength or creep performance. Two changes were made - an increase in the nickel content, as increasing the concentration of nickel in the matrix of maraging steels has been shown to improve toughness; and a decrease in molybdenum and tungsten content to reduce the Laves phase volume fraction by 25%. These changes, along with further optimisations of the heat treatment used and refinement of the prior austenite grain size, were successful in improving the fracture toughness of F1E (or RR9922 as the modified composition is known) by a factor of 2, from 23 MPa m\(^0\)\(^.\)\(^5\) to 46.9 MPa m\(^0\)\(^.\)\(^5\).

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Tungala, Vedavyas. "Exceptional Properties in Friction Stir Processed Beta Titanium Alloys and an Ultra High Strength Steel." Thesis, University of North Texas, 2017. https://digital.library.unt.edu/ark:/67531/metadc984167/.

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The penchant towards development of high performance materials for light weighting engineering systems through various thermomechanical processing routes has been soaring vigorously. Friction stir processing (FSP) - a relatively new thermomechanical processing route had shown an excellent promise towards microstructural modification in many Al and Mg alloy systems. Nevertheless, the expansion of this process to high temperature materials like titanium alloys and steels is restricted by the limited availability of tool materials. Despite it challenges, the current thesis sets a tone for the usage of FSP to tailor the mechanical properties in titanium alloys and steels. FSP was carried out on three near beta titanium alloys, namely Ti6246, Ti185 and Tiβc with increasing β stability index, using various tool rotation rates and at a constant tool traverse speed. Microstructure and mechanical property relationship was studied using experimental techniques such as SEM, TEM, mini tensile testing and synchrotron x-ray diffraction. Two step aging on Ti6246 had resulted in an UTS of 2.2GPa and a specific strength around 500 MPa m3/mg, which is about 40% greater than any commercially available metallic material. Similarly, FSP on an ultra-high strength steel―Eglin steel had resulted in a strength greater than 2GPa with a ductility close to 10% at around 4mm from the top surface of stir zone (SZ). Experimental techniques such as microhardness, mini-tensile testing and SEM were used to correlate the microstructure and properties observed inside SZ and HAZ's of the processed region. A 3D temperature modeling was used to predict the peak temperature and cooling rates during FSP. The exceptional strength ductility combinations inside the SZ is believed to be because of mixed microstructure comprised of various volume fractions of phases such as martensite, bainite and retained austenite.

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Alatarvas, T. (Tuomas). "Evolution of inclusion population in calcium treated ultra-high strength steels:novel applications of sample data treatment." Doctoral thesis, Oulun yliopisto, 2018. http://urn.fi/urn:isbn:9789526220987.

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Abstract Non-metallic inclusions are unavoidable particles in steel and are often detrimental to the steelmaking process and the mechanical properties of the steel. While it is not feasible to remove all inclusions, in the well-established calcium treatment of aluminium-killed steel, solid aluminium oxides are transformed into less harmful liquid calcium aluminates. The main objective of this work was to develop a new inclusion identification and classification method. The presented method offers valuable information on phases of the inclusions. This data is often buried within simple inclusion classification criteria. The method offers the best approximation of the phases in each inclusion detected with minimal time, if no time-consuming elemental map analyses are available. In this work, applications for the inclusion phase identification method are presented. Utilising the method, the dispersion and evolution of inclusions during the sampling of liquid steel in lollipop samples is investigated, as well as the evolution of inclusions during continuous casting. New information is obtained on the elongation of inclusions and formation of oxide–sulphide stringers during hot rolling. The results for the investigated steels show that with sulphur contents higher than 10 ppm, calcium aluminates were back-modified to alumina and spinel inclusions during casting. However, with decreasing sulphur contents, and adequate simultaneous calcium treatment of oxides, Al₂O₃ and spinel formation is hindered or even avoided. The most elongated inclusions are often also found in stringers. According to the results of this study, low melting calcium aluminate inclusions are not the most elongated oxide inclusions in the hot rolled product. With moderate calcium treatment, an optimal oxide composition can be obtained, found within the C12A7–CA–MgO composition. These liquid inclusions ensure good castability, while they do not easily elongate or fragment into detrimental stringers after continuous casting
Tiivistelmä Epämetalliset sulkeumat ovat väistämättä osa terästä, ja ne aiheuttavat ongelmia prosessille ja teräksen mekaanisille ominaisuuksille. Sulkeumien poiston sijaan niiden koostumusta muokataan kalsiumkäsittelyllä. Tällöin kiinteät alumiinioksidit muuttuvat vähemmän ongelmallisiksi suliksi kalsiumaluminaateiksi. Tämän työn päätavoitteena oli kehittää uusi sulkeumien tunnistus- ja luokittelumenetelmä. Sulkeuma-analyysien data voi jäädä hyödyntämättä, jos luokitteluun käytetään yksinkertaisia kriteerejä. Työssä esitetty menetelmä antaa parhaan arvion sulkeumissa olevista faaseista, mikäli sulkeumista ei ole alkuainekarttoja, joiden muodostaminen vie huomattavasti aikaa. Väitöskirjassa esitetään sulkeumien tunnistus- ja luokittelumenetelmän soveltamiskohteita. Menetelmän avulla tutkitaan sulkeumien muutosta ja jakaantumista sulan teräksen näytteenotossa ja sulkeumakuvan muutosta jatkuvavalun aikana. Uutta tietoa esitetään oksidi-sulfidiketjujen muodostumisesta kuumavalssauksen aikana. Tutkituilla teräksillä rikkipitoisuuden noustessa yli 10 ppm:n sulat kalsiumaluminaatit takaisinmuokkautuvat kiinteiksi alumiinioksideiksi tai spinelleiksi jatkuvavalun aikana. Riittävän matalalla rikkipitoisuudella ja kohtuullisella kalsiumkäsittelyllä alumiinioksidien ja spinellien muodostumista voidaan hillitä tai jopa estää se. Tutkimuksessa esitetään, että kuumavalssauksessa venyvimmät sulkeumat muodostavat myös lukuisten sulkeumien oksidi-sulfidiketjuja, jotka ovat tuotteen ominaisuuksien kannalta haitallisia. Tulosten mukaan kalsiumaluminaatit, jotka sulavat matalimmissa lämpötiloissa, eivät kuitenkaan ole taipuvaisimpia venymään valssauksessa. Maltillisella kalsiumkäsittelyllä saavutetaan optimaalinen oksidikoostumus C12A7–CA–MgO-faasiseurueesta. Nämä sulkeumat ovat sulia jatkuvavalussa varmistaen teräksen valettavuuden. Toisaalta ne eivät helposti veny tai rikkoonnu ketjuiksi valssauksessa jatkuvavalun jälkeen

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Weickum, Britney. "Friction Bit Joining of 5754 Aluminum to DP980 Ultra-High Strength Steel: A Feasibility Study." BYU ScholarsArchive, 2011. https://scholarsarchive.byu.edu/etd/2789.

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In this study, the dissimilar metals 5754 aluminum and DP980 ultra-high strength steel were joined using the friction bit joining (FBJ) process. The friction bits were made using one of three steels: 4140, 4340, or H13. Experiments were performed in lap shear, T-peel, and cross tension configurations, with the 0.070" thick 5754 aluminum alloy as the top layer through which the friction bit cut, and the 0.065" thick DP980 as the bottom layer to which the friction bit welded. All experiments were performed using a computer controlled welding machine that was purpose-built and provided by MegaStir Technologies. Through a series of designed experiments (DOE), weld processing parameters were varied and controlled to determine which parameters had a significant effect on weld strength at a 95% confidence level. The parameters that were varied included spindle rotational speeds, Z-command depths, Z-velocity plunge rates, dwell times, and friction bit geometry. Maximum lap shear weld strengths were calculated to be 1425.4lbf and were to be obtained using a bit tip length at 0.175", tip diameter at 0.245", neck diameter at 0.198", cutting and welding z-velocities at 2.6"/min, cutting and welding RPMs at 550 and 2160 respectively, cutting and welding z-commands at -0.07" and -0.12" respectively, cooling dwell at 500 ms, and welding dwell at 1133.8 ms. These parameters were further refined to reduce the weld creation time to 1.66 seconds. These parameters also worked well in conjunction with an adhesive to form weld bonded samples. The uncured adhesive had no effect on the lap shear strengths of the samples. Using the parameters described above, it was discovered that cross tension and T-peel samples suffered from shearing within the bit that caused the samples to break underneath the flange of the bit during testing. Visual inspection of sectioned welds indicated the presence of cracking and void zones within the bit.

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Peer, Andrea J. "Performance Testing and Modeling of Ultra-High Strength Steel and Complex Stack-Up Resistance Spot Welds." The Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu1493403670252986.

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Lin, Xuchuan. "Development of Low-to Mid-rise Building Structures Using Weld-free Built-up Columns Made of Ultra-high Strength Steel." 京都大学 (Kyoto University), 2012. http://hdl.handle.net/2433/161010.

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Wood, Shane Forrest. "Manipulation and Automation of FBJ Short-Axis Fasteners." BYU ScholarsArchive, 2018. https://scholarsarchive.byu.edu/etd/7311.

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Legislative and market pressures are pushing automakers to achieve new fuel economy requirements in the coming years. To help achieve these goals automakers are reducing the overall weight of the vehicle by increasing the use of high-strength aluminum and advanced high-strength steels, and with this increased use comes the desire to quickly, and securely, join these materials within the vehicle. Friction bit joining is a process that lends itself well to joining these materials. This process uses consumable fasteners that need to be used in an automated production line. The geometry of these fasteners causes two main problems: the bits have a short longitudinal axis, which makes them difficult to orient, and the welding platform may be used at different angles; requiring a robust reloading system that is indifferent to its orientation.Our research explored ways that these short axis FBJ fasteners could be handled and transported using various automated methods. We tested the use of small mechanical carriages and magnetic tracks to test their viability for transporting FBJ fasteners. The two different types of fasteners that were used in the project are described. Blow feed tubes ended up being a reliable method of transportation given that the fastener has suitable geometry. The superior bit and feed system design were bench tested using a manually controlled feed system. The system was tested in various orientations to test the robustness of the system since the system was designed to be part of the end effector on a production line robot. The testing revealed that the feed tube is a reliable method of bit transportation and mechanical jaws are a suitable solution for FBJ fastener manipulation. These jaws have several key design features that dramatically increase their effectiveness. Suggestions for future work would be an optimized feed tube cross section, improved material properties in the bit jaw, and more air flow at a higher pressure through the feed tube.

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Serbino, Edison Marcelo. "Estudo dos mecanismos de fadiga e análise de desempenho em mola para válvula automotiva em regime de alto e ultra-alto ciclo." Universidade de São Paulo, 2012. http://www.teses.usp.br/teses/disponiveis/3/3133/tde-16112012-111233/.

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Este trabalho apresenta um estudo dos mecanismos de fadiga em mola para válvula automotiva, realizado através da análise do comportamento mecânico correlacionado ao aspecto microestrutural. As amostras foram fabricadas em aço de alta resistência Si-Cr, submetidas a tratamentos térmicos convencionais (têmpera e revenimento) e não convencionais (austêmpera), obtendo um mesmo nível de dureza (aproximadamente 610 HV). Os corpos de prova foram submetidos a ensaios de bancada com diversos níveis de carregamentos, controlados através de variação da amplitude de tensão cisalhante. Esta variação promoveu diferentes regimes de fadiga, conhecidos como alto e ultra-alto ciclo, para maiores e menores níveis de amplitudes de tensões, respectivamente. O desempenho foi medido através do cálculo da vida total em fadiga, probabilidade de sobrevivência, além da verificação do relaxamento da força elástica e da tensão residual. As amostras foram caracterizadas através de métodos metalográficos, determinação da composição química, ensaios de dureza e resistência à tração, além de técnicas difratométricas de raios-X. As superfícies de fratura foram investigadas utilizando técnicas de microscopia eletrônica de varredura Os mecanismos de início e propagação das trincas, de ambas as amostras, foram semelhantes, apesar do tratamento térmico proposto de austêmpera produzir uma microestrutura mista, com predominância de bainita inferior, contendo regiões esparsas de austenita retida. A microestrutura bainítica, resultante da austêmpera, quando comparada com as amostras martensíticas, geradas pela têmpera e revenimento, propiciou um aumento da vida total em fadiga nos maiores níveis de amplitudes das tensões, tendendo à equalização com a diminuição desta amplitude, além de apresentarem um maior relaxamento elástico, gerando uma discreta diminuição de magnitude da tensão média. Por outro lado, não houve relaxamento das tensões residuais, em ambas as amostras, após uma quantidade significativa de ciclos, portanto este fenômeno não influenciou em sua vida total.
This paper presents a study on the valve spring fatigue mechanisms, carried out through analysis of mechanical behavior correlated to microstructural aspect. The samples were made of high strength steel Si-Cr, obtained by conventional (quenching and tempering) and unconventional (austempering) heat treatments, generating a single level of hardness (approximately 610 HV). The samples were subjected to bench test with various loading levels, controlled through shear stress amplitude fluctuation. This variation has promoted distinct fatigue regimes known as high and ultra-high cycle, for greater and smaller levels of stress amplitudes, respectively. The performance was measured by calculating the fatigue total life, survival probability, verification of the elastic force and residual stress relaxation. The samples were characterized by means of metallographic methods, chemical composition analysis, hardness/tensile strength testing and X-ray diffractometry techniques. The fracture surfaces were investigated using scanning electron microscopy techniques. The cracks initiation and propagation mechanisms were similar in both samples; despite the proposed austempering heat treatment produced a mixed microstructure with predominance of lower bainite, containing sparse regions of retained austenite. The bainitic microstructure, resulting of austempering, when compared with martensitic samples, generated by quenching and tempering, led to an increase in fatigue total life under higher levels of stress amplitudes, but tending to be the same as the stress amplitude deacresases and the number of cycles increases. On the other hand, there was no residual stresses relaxation in both samples, after a significant amount of cycles, so this phenomenon did not influence the total life.

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Saunders, Nathan David. "High Speed Friction Stir Spot Welding on DP 980 Steel:Joint Properties and Tool Wear." BYU ScholarsArchive, 2012. https://scholarsarchive.byu.edu/etd/3003.

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With the desire to improve passenger safety and fuel efficiency, Ultra High Strength Steels (UHSS) have been developed for use in the automotive industry. UHSS are high strength steels with high ductility and strength. DP 980 is one of these UHSS being applied in automobile manufacturing. DP 980 is difficult to join with Resistance Spot Welding (RSW) because of the high carbon content and alloying in this material. The weld becomes brittle when it solidifies during the welding process. With the desire and motivation of widely using UHSS, new welding processes are needed to be developed in order to effectively join DP 980. Friction Stir Spot Welding (FSSW) is a developing welding process aimed to replace RSW in the automotive industry because of its ability to join materials at a lower temperature. Currently the welding loads of the tools are higher than 2000 pounds, ranging from 3,000 to 5,000 pounds, which exceeds the limit of the welding robots in the automotive factories. It is proposed that the welding loads can be reduced by increasing the spindle speed of the FSSW tool. Other focuses in the research include increasing the life of the tool and developing acceptable welding parameters for High Speed FSSW. The experimental work done for this thesis provided support that weld strength can be obtained at levels above the acceptable standard for DP 980 material (greater than 2400 pound lap shear fracture load for 1.2 mm material) while keeping the vertical load on the welding machine spindle below 2000 lbs.

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Amir, Sayyafi Ehssan. "Development of a Lightweight Hurricane-Resistant Roof System." FIU Digital Commons, 2017. http://digitalcommons.fiu.edu/etd/3364.

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Roofs are the most vulnerable part of the building envelope that often get damaged when subjected to hurricane winds. Damage to the roofs has a devastating impact on the entire structure, including interior losses and service interruptions. This study aimed at the development of a novel light-weight composite flat roof system for industrial, commercial and multi-story residential buildings to withstand Category 5 hurricane wind effects based on the Florida Building Code requirements for hurricane-prone regions, the strictest wind design code in the United States. The proposed roof system is designed as a combination of two advanced materials: ultra-high performance concrete (UHPC), reinforced with high strength steel (HSS). The novel combination of these two materials in a specially designed cross section led to a lightweight low-profile ultra-thin-walled composite roof deck, with only 17 pounds per square foot self-weight, 4-inch overall depth and only ¾-inch thick flanges and webs, with no shear reinforcement or stirrup. Two groups of specimens, single-cell and multi-cell, were fabricated and tested in four-point flexure to determine the ultimate bending capacity and ductility of the system. Each group of specimens included two short-span (9 ft.) samples (due to the laboratory constraints) -- one specimen subjected to positive bending and the other one subjected to negative bending, representing the critical loading conditions including the effects of wind pressures. All specimens exhibited pure flexural failure in a ductile behavior and with no sign of shear failure. Finite element models of laboratory specimens were also developed and calibrated based on experimental data in order to project the performance of the system for larger and more realistic spans. The experimental work and the finite element analyses showed that the proposed roof system with its given section has adequate flexural and shear strength, and also meets serviceability requirements for a 20-foot long span. Moreover, connections for the roof system were proposed, including panel-to-panel connections and roof-to-wall connections. In addition to safety, the other advantages of the proposed roof system in comparison to the equivalent reinforced concrete roofs include a three-fold reduction in self-weight, a three-fold reduction in overall profile height, and a five-fold reduction of steel reinforcement. Together, these advantages may lead to an increased span length beyond what is typically feasible for the conventional reinforced concrete slabs. All these features translate the proposed deck to a sustainable roof system.

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Cora, Omer Necati. "DEVELOPMENT OF RAPID DIE WEAR TEST METHOD FOR ASSESSMENT OF DIE LIFE AND PERFORMANCE IN STAMPING OF ADVANCED/ULTRA HIGH STRENGTH STEEL (A/UHSS) SHEET MATERIALS." VCU Scholars Compass, 2009. http://scholarscompass.vcu.edu/etd/2003.

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Automotive companies are actively pursuing to increase the use of high-strength-lightweight alloys such as aluminum, magnesium, and advanced/ultra high-strength steels (A/UHSS) in body panel and structural part applications to achieve fuel efficiency while satisfying several environmental and safety concerns. A/UHSS sheet materials with higher strength and crashworthiness capabilities, in comparison to mild steel alloys, are considered as a near-term (i.e., ~5 years) choice of material for body and structural components due to their relatively low cost when compared with other lightweight materials such as aluminum and magnesium. However, A/UHSS materials present an increased level of die wear and springback in stamping operations when compared to the currently used mild steel alloys due to their higher surface hardness and high yield strength levels. In order to prevent the excessive wear effect in stamping dies, various countermeasures have been proposed such as alternative coatings, modified surface enhancements in addition to the use of newer die materials including cast, cold work tool, and powder metallurgical tool steels. In this study, a new die wear test method was developed and tested to provide a cost-effective solution for evaluating various combinations of newly developed die materials, coatings and surfaces accurately and rapidly. A new slider type of test system was developed to replicate the actual stamping conditions including the contact pressure state, sliding velocity level and continuous and fresh contact pairs (blank-die surfaces). Several alternative die materials in coated or uncoated conditions were tested against different AHSS sheet blanks under varying load, sliding velocity circumstances. Prior to and after wear tests, several measurements and tribological examinations were performed to obtain a quantified performance evaluation using commonly adapted wear models. Analyses showed that (1) the rapid wear method is feasible and results in reasonable wear assessments, (2) uncoated die materials are prone to expose severe form wear (galling, scoring, etc.) problems; (3) coated samples are unlikely to experience such excessive wear problems, as expected; (4) almost all of the the recently developed die materials (DC 53, Vancron 40, Vanadis 4) performed better when compared to conventional tool steel material AISI D2, and (5) in terms of coating type, die materials coated with thermal diffusion (TD) and chemical vapor deposition (CVD) coatings performed relatively better compared to other tested coating types; (6) It was seen that wear resistance correlated with substrate hardness.

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38

Ghasemi, Sahar. "Innovative Modular High Performance Lightweight Decks for Accelerated Bridge Construction." FIU Digital Commons, 2015. http://digitalcommons.fiu.edu/etd/2248.

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At an average age of 42 years, 10% of the nation’s over 607,000 bridges are posted for load restrictions, with an additional 15% considered structurally deficient or functionally obsolete. While there are major concerns with decks in 75% of structurally deficient bridges, often weight and geometry of the deck further limit the load rating and functionality of the bridge. Traditional deck systems and construction methods usually lead to prolonged periods of traffic delays, limiting options for transportation agencies to replace or widen a bridge, especially in urban areas. The purpose of this study was to develop a new generation of ultra-lightweight super shallow solid deck systems to replace open grid steel decks on movable bridges and as well serve as a viable alternative in bridge deck replacements across the country. The study has led to a lightweight low-profile asymmetric waffle deck made with advanced materials. The asymmetry comes from the arrangement of primary and secondary ribs, respectively perpendicular and parallel to the direction of traffic. The waffle deck is made with ultrahigh performance concrete (UHPC) reinforced with either high-strength steel (HSS) or carbon fiber reinforced polymer (CFRP) reinforcement. With this combination, the deck weight was limited to below 21 psf and its overall depth to only 4 inch, while still meeting the strength and ductility demands for 4 ft. typical stringer spacing. It was further envisioned that the ultra-high strength of UHPC is best matched with the high strength of HSS or CFRP reinforcement for an efficient system and the ductile behavior of UHPC can help mask the linear elastic response of CFRP reinforcement and result in an overall ductile system. The issues of consideration from the design and constructability perspectives have included strength and stiffness, bond and development length for the reinforcement, punching shear and panel action. A series of experiments were conducted to help address these issues. Additionally full-size panels were made for testing under heavy vehicle simulator (HVS) at the accelerated pavement testing (APT) facility in Gainesville. Detailed finite element analyses were also carried out to help guide the design of this new generation of bridge decks. The research has confirmed the superior performance of the new deck system and its feasibility.

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39

Křemen, Jan. "Vliv žárového zinkování na vlastnosti vysokopevnostích ocelí." Master's thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2010. http://www.nusl.cz/ntk/nusl-229261.

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This thesis deals with the influence of hod dip galvanizing on properties of high-strenght steel. Galvanizing adversely affects the mechanical properties of high-strenght steel. This paper also examines the influence of hod dip galvanizing at hardness high-strenght steels. The task is to assess how the yield stress can galvanize steel.

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Panenková, Monika. "Reaktivní práškové kompozity a cementové kompozity bez makropórů." Master's thesis, Vysoké učení technické v Brně. Fakulta stavební, 2018. http://www.nusl.cz/ntk/nusl-372126.

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This diploma thesis deals with the design principles and manifacture of ultra performance concrete (UHPC), reactive powder composites (RPC) and other fine-grained materials, such as Macro defect free (MDF) or Densified systems with small particles (DSP). Theoretical part of this work is focused on the requirements of properties and composition UHPC and RPC and methodology of their design principles. Experimental part describes design principles RPC, manufacture of test specimens, testing of certain physical and mechanical properties, such as tensile strenght, bending strenght and compressive strenght and determination chemical character X-ray and thermal analysis.

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Liou, Kuen-Tai, and 劉琨泰. "Seismic Evaluation of High Rise Steel Buildings Using Ultra High Strength Steels." Thesis, 2014. http://ndltd.ncl.edu.tw/handle/69483258659711243774.

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碩士
國立高雄大學
土木與環境工程學系碩士班
102
Case studies were carried out to investigate the benefits of applying high strength steels to high-rise buildings in seismically active zones. In detail, two 40-story steel moment frames were first designed for the Taipei Microzonation II. One example building uses SN490B steel in the columns. The other uses SM570 steel to replace SN490B in the columns from the 1st to the 14th story. The buildings were designed in accordance with building codes and design guides. The stress ratios and story drift ratios were limited to 0.9 and 0.005. The use of SM570 steel was found to save steel materials up to 137 tons steel in total, approximately 2.7%. To limit the story drift ratio to 0.005, the beam sections need enlarging when using the SM570 steel. The seismic evaluation was then made using nonlinear time history analysis with 14 sets of ground motions scaled to have the return periods of 475 years and 2500 years. The example building using the SM570 steel was found to have smaller drift responses and variations. In addition, the analyzed 2.30% drift demand is smaller than the code requirement, i.e. 4%. By referencing FEMA 356, the fragility of example buildings was finally assessed using the drift ratios of 2.5% and 4% for the limit states of life safety and collapse prevention. For the limit state of life safety, the example buildings were found to have similar fragility. But for the limit state of collapse prevention, the example building using the SM570 steel was found to be superior to the other. The results of case studies have shown the benefits of applying high strength steels to high-rise buildings in seismically active zones.

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42

Veerababu, R. "Microstructural Studies on High Cr-Mo Secondary Hardening Ultra-High Strength Steels." Thesis, 2015. http://etd.iisc.ernet.in/2005/3687.

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Secondary hardening ultra-high strength (SHUHS) steels possess a unique combination of strength, fracture toughness and stress corrosion cracking resistance, which makes them candidate materials for aircraft landing gear and armour applications. There is a sustained drive to develop stronger and tougher materials for such applications. The objectives of this thesis are two-fold: first, to develop a new SHUHS alloy that is stronger than the existing SHUHS steel developed at Defence Metallurgical Research Laboratory (DMRL), Hyderabad and second, to establish processing-structure-property correlations for the new alloy. Empirical design and development of these complex steels involves enormous effort, cost, time and materials resources. To avoid this, a semi-empirical approach was espoused in this thesis wherein thermodynamic calculations using ThermoCalc were conducted to computationally design a series of alloys with varying levels of Cr and Mo. The design space was constrained by two objectives related to M2C carbides which are the primary cause of secondary hardening in these alloys. The first objective was to increase the amount of M2C to increase the peak strength, while the second objective was to lower the Cr/Mo ratio of the M2C to control its over-ageing behavior. Two new alloys C23 (with 2Cr-3Mo, wt. %) and C55 (with 5Cr-5Mo, wt. %) and a base alloy akin to the DMRL SHUHS steel, C21 were selected for experimental validation. These alloys were melted, rolled and subjected to a battery of heat treatments. Austenitization studies revealed that the new alloys required higher austenitization temperatures to dissolve primary carbides. However such a treatment also resulted in an austenite composition that was not conducive for obtaining a fully martensitic microstructure on quenching. Based on these studies, the design space was modified to include additional criteria related to the Ms and precipitate dissolution temperatures. C55 failed to clear either criteria, while C23 cleared both, and so tempering studies were limited to C23. Isochronal tempering studies revealed that C23 in the peak aged condition was >10% stronger than C21 indicating that the alloy design objective of strength enhancement was achieved successfully. Microstructural characterization revealed that the strength enhancement was due to the higher number density and volume fraction of the M2C-like solute clusters in C23, which resist shearing in the under-aged condition and strengthen by Orowan mechanism in the over-aged condition. This thesis has successfully demonstrated that the design paradigm of enhancing strength by increasing the amount of M2C is justified and that ThermoCalc can be used to as an objective-oriented alloy design tool in this class of the steels.

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43

Davies, Nicholus Paul Widdrington. "Strain ageing in ultra-high strength drawn pearlitic steels." Thesis, 2015. http://hdl.handle.net/10539/16599.

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44

Naderi, Malek [Verfasser]. "Hot stamping of ultra high strength steels / vorgelegt von Malek Naderi." 2007. http://d-nb.info/987526111/34.

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45

Yen, Hung-Wei, and 顏鴻威. "TEM Investigation on the Interphase Precipitation of Nanometer-sized Carbides in Advanced Ultra High-Strength Steels." Thesis, 2011. http://ndltd.ncl.edu.tw/handle/17303951222686101803.

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博士
國立臺灣大學
材料科學與工程學研究所
99
To reduce fuel consumption and CO2 emission in automobiles, the development of advanced high-strength steels (AHSS) has been the bull’s eye in recent years. The ultra high-strength hot-rolled steel strips have been developed with tensile strength of ~780MPa and excellent formability by JFE and China Steel Co. The strong ferrite in the steels has been achieved by nanometer-sized carbides which contribute about 300MPa to the total strength. These tiny carbides nucleate on moving γ/α interface during austenite-to-ferrite transformation. It has been well known as interphase precipitation. This steel strip is considered to be avatar of interphase precipitation in low-carbon steels. For the purpose to explore a complete scope for the mechanism of interphase precipitation, TEM techniques have been developed and discussed to characterize the nanometer-sized interphase-precipitated MX carbides in this study. This study initially utilized Moiré fringes in high resolution TEM (HRTEM) to characterize the crystal structure and orientation relationship (OR) of nanometer-sized carbide. It was found that TiC carbides in ferrite will transit from single variant of Baker-Nutting OR to multi variants of Nishiyama-Wessermann OR during isothermal holding at 755oC. HRTEM associated with NanoProbe EDS provided the evidence to suggestthat (Ti, Mo)C carbide is a NaCl structured MX-type carbide. The distribution of Mo has been revealed by high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM). Besides, this study provided the observation condition: hu+kv+lw = 0 to measure the sheet spacing. By estimating the sample thickness from electron energy loss spectrum (ELLS), the interparticle spacing in sheet could be calculated. The orientation of sheet plane has been identified by analyzing the convergent beam Kikuchi diffraction patterns. Using the developed TEM techniques, experiments and investigations were conducted in steels with different chemical compositions under different heat treated conditions. The planar sheets of carbides have been analyzed and found to be oriented close to ferrite planes {211}, {210} and {111}; transmission electron microscopy results provide strong evidence to suggest that the development of interphase-precipitated carbides can be associated with the growth of incoherent ferrite/austenite interface by the ledge mechanism. The sheet spacing corresponding to the ledge height can be predicted by Bhadeshia’s formula. And the variation of interparticle spacing in sheet is related to both the moving speed of ledges and carbide nucleation rate. Based on the new mechanism of interphase precipitation, the ultra high-strength hot-rolled steel strips have been developed in lab-level. The tensile strength of the strips can exceed 700 MPa and the total elongation can be over 20%. With measured microstructural parameters from TEM, an anisotropy-related Orowan equation was applied to estimate contribution of nanometer-sized carbides to the yield strength; the value is higher than 200 MPa. Furthermore, atom probe tomography (APT) has been used to study the thermal stability of interphase-precipitated (Ti, V)C complex carbides in atomic scale. It is found that the clusters of Ti, V and C can be classified into two groups: (1) tiny clusters with 2 to 30 atoms and (2) coarse clusters with 31 to 350 atoms. It is proposed that the tiny clusters with 2 to 30 atoms in the ferritic matrix retards the diffusion rates of carbide forming elements so that the coarsening rate of carbides could be suppressed. Besides, the density of coarse clusters with above 30 atoms is higher than the density of carbides estimated from TEM by one order. The distribution of clusters is also sheeted distribution and it seems that the clustering occurs during austenite-to-ferrite phase transformation. Since the discovery of interphase precipitation in 1964, the results of this work bring about a whole new perspective to the theory of interphase precipitation.

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46

Itay, Tamar Judith. "The development of ultra-high strength pearlitic steel wires." Thesis, 2015. http://hdl.handle.net/10539/18137.

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A dissertation s u b m i t t e d to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the Degree of Doctor of Philosophy. Department of Metallurgy University of the Witwatersrand November 1985
This thesis describes a systematic investigation of the influence of alloying additions on the microstructure and mechanical properties of eutectoid carbon steels. The goal of this work was to develop an alloy which would provide a tensile strength in excess of 2500 MPa in drawn wire. Alloys were fabricated using vacuum induction melting, casting and hot-rol 1 .‘.ng. Specimens of each material were heat treated to produce pearl J. tic structures. M i c r o structures were evaluated using light, scanning electron and transmission electron microscopy. Mechanical properties were determined from hardness and tensile tests. Suitable heat treated rods were subjected to d r awing trials, aid the structures and properties re-determined. In the initial part of the study, the influence of chrom i u m and manganese addition on the properties of a high purity carbon alloy were studied. This work demonstrated the benefits of these small alloying additions on strength, while maintaining adequate ductility. In subsequent work the additional effects of alloying with silicon were studied. Of the alloys investigated, the 2Si-lCr composition achieved a drawn strength in excess of 2500 MPa, This material also exhibited acceptable values in standard shear and torsion tests for commercial wires. Since the strength of steel is strongly dependent on carbon content, the further influence of this variable on properties was examined using the 2Si-lCr base composition. Increasing the carbon content to 0.9% did provide additional strength, but increased the ageing susceptibility during drawing. The work was extended towards commer c i a l compositions by preparing 2Si-lCr alloys using a commercial eutectoid carbonmanganese steel base. Again, in the as-drawn condition, the strength goal was exceeded. However, these materials exhibited inferior ductility compered with the high-purity alloys. This is thought to be due to ageing. A brief study was also conducted on the problem of strain ageing, but no evidence for precipitation of deleterious carbides could be found even using electron microscopy. In summary, small alloying additions are extremely effective in i n c r e a s i n g the s t r e n g t h of p e a r l i t i c wire, w h i l e maintaining adequate ductility. A 2Si-lCr alloy exhibited the best results of the materials studied. This composition has commercial potential, but may require more sophisticated p r o d u c t i o n m e t h o d s to p r o v i d e cleaner, h i g h e r p u r i t y material. Nevertheless, this alloy is capable of providing as-drawn tensile strengths in excess of the 2500 MPa goal

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47

Ye, Ming-Da, and 葉明達. "Study on the Form Tapping Process for Ultra High-Strength Steel." Thesis, 2011. http://ndltd.ncl.edu.tw/handle/28354371606727749828.

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碩士
國立高雄第一科技大學
機械與自動化工程研究所
99
The Ultra high-strength steel has the higher strength, good tenacity, better weldabailty, workability, fatigueability and weatherability. At present, the high tensity steel sheets are used below 0.5mm of thickness and above 390 MPa of tensile strength gradually in the automobile sheet metal, the 3C product casing and the large-scale casing of liquid crystal display TV. In this study is to investigate the process parameters of form-tapping with the metric tooth M6 × 1, M8 × 1.25, M10 × 1.5 on JSC440MPa, JSC590MPa, JSC780MPa and JSC980MPa Ultra high-strength steel. The study is consist of two tasks, First , “tensile strength of different grades of Ultra high-strength steel piercing parameter optimization study”. Second, “tensile strength of different grades of Ultra high-strength steel flanging and form-tapping process study”. The study of first task is to use the software of finite element method as DEFORM 2D for simulation piercing. Meanwhile, to develop experiment die to experiment and prove, and then to use the the best method for the best piercing parameters. The study of second task is to perform flanging and form-tapping on Ultra high-strength steel by the results from first task, and use the software of finite element method as DEFORM 2D for simulation flanging and DEFORM 3D for form-tapping. Meanwhile, develop experiment die to experiment and prove. The results show that the simulation can predict results of the piercing, flanging and form-tapping. The maximum rates of flanging on JSC440MPa, JSC590MPa, JSC780MPa and JSC980MPa Ultra high-strength steel are 45.65%, 40.22%, 27.27%, and 25.17% respectively.

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48

Fredriksson, Mikael, Elin Åkerlund, Jakob Åberg, Patrik Österberg, and Rebecka Havo. "High Performance Steel for Percussive Drilling." Thesis, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-323789.

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Atlas Copco Secoroc AB are searching after new bulk materials for drill heads that are used in percussive drilling in order to improve their strength and durability. The aim of this project is to assist Atlas Copco in this search and provide them with further information regarding material properties, alloying elements, suppliers, etc. A literary study was carried out in order to identify materials that had UTS and KIC more than or equal to 1700 MPa and 70 MPa*m^1/2, respectively. Materials that fulfilled these criteria were T250 grade maraging steel, Cobalt free maraging steel, High cobalt maraging steel, 300 grade maraging steel, AerMet 100, AF1410, S53, M54, 300M, 4340M and PremoMet. These were categorized into maraging steels, high alloy secondary hardened steels, and low alloy steels, and were then further researched. The material with the highest combination of UTS and KIC was M54 followed by AerMet 100; while AF1410 had the highest KIC but a low UTS, and PremoMet had the highest UTS but a low KIC. Maraging steels and HASH steels have a similar price range, while low alloy steels are much cheaper.

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49

Lee, W. C., and 李文志. "Notch Tensile Properties and Hydrogen Permeation in an Ultra-High-Strength Steel." Thesis, 2000. http://ndltd.ncl.edu.tw/handle/74490149673800377218.

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50

Hung, Tzu-Hao, and 洪梓豪. "A Study on Hot Stamping Process of Ultra High Strength Steel Sheets." Thesis, 2018. http://ndltd.ncl.edu.tw/handle/25279y.

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博士
國立臺灣大學
機械工程學研究所
107
In order to reach the target of lightweight vehicle, the hot stamping process of ultra high strength steel sheets has been widely used in automotive parts. By the hot stamping process, the product parts can have the ultra high strength, and it also have fewer springback defects in the forming process compared to the parts with advanced high strength steel sheets. However, the forming mechanism of hot stamping process is quite complex, it contains the material mechanical, thermal, and material metallurgy mechanisms in the process. And it has to consider the interface heat transfer mechanism and high temperature friction mechanism between the blank sheet and tools. Moreover, it also needs to design the tool with cooling system or tool with heating system to control the strength of product parts. The manufacture design and tool design are much more complex, and it also difficulty to predict the forming defects. For these reasons, it becomes an important topic about realizing the characteristics of the hot stamping process and establish the technology of forming analysis and tool design for the hot stamping process. This study is focus on the hot stamping process of ultra high strength steel sheets. The research topics include the introduction of hot stamping process and material properties of blank sheets and tools for the hot stamping process, the characteristics of the interface heat transfer coefficient between blank and tools, the characteristics of high temperature friction between blank and tools, the CAE analysis technology of hot stamping process, the tools cooling system design in hot stamping process, the analysis technology of the tailored die quenching hot stamping process, and the thermal deformation of stamping sheets in hot stamping process. In this study, the hot stamping process was firstly introduced and the material properties of blank sheet and die tool were also described. Then, the experimental platforms for measuring the interface heat transfer coefficient and high temperature friction coefficient were also applied to measure and calculate the interface heat transfer coefficient and high temperature friction coefficient. It also conducted the analysis to realize the effects of process parameters on the interface heat transfer coefficient and high temperature friction coefficient. The characteristics of interface heat transfer and high temperature friction coefficient were also studied, and the parameters of interface heat transfer coefficient and high temperature friction coefficient were also established for simulation analysis. The simulation analysis technology for hot stamping process was also established in this study, including the forming analysis and die quenching analysis. The analysis technology was also applied to conduct the formability analysis with different forming methods. In the study of the hot stamping tools cooling system design, the simulation technology was also established. The simulation analysis and the method of design of experiment were conducted to the cooling system design by flat plate tools. Then the empirical equations of cooling ability of hot stamping tools could be established for initial cooling system design. Moreover, the simulation analysis technology for the tailored die quenching hot stamping process was also established in this study. The technology can be applied to the forming analysis and tool design for the tailored die quenching hot stamping process. Finally, the behaviors of thermal deformation were also discussed in this research. By the simulation analysis, the deformation phenomenon of steel sheets was investigated.

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