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1
Pei, Wei, Wei Liu, Yue Zhang, Rongjian Qie, and Aimin Zhao. "Study on Kinetics of Transformation in Medium Carbon Steel Bainite at Different Isothermal Temperatures." Materials 14, no. 11 (2021): 2721. http://dx.doi.org/10.3390/ma14112721.
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Ultra-fine carbide-free bainitic (UCFB) steel, also known as nano-bainite (NB) steel, is composed of bainitic ferrite laths with nanoscale thickness and carbon-rich film-like retained austenite located between laths. The bainite transformation kinetic model can accurately describe the bainite transformation kinetics in conventional austempering (CA) processes based on the shear mechanism combined with the dilatometer test. UCFB steels with medium and high carbon composition are designed in this work to systematically study the transformation kinetics of bainite, and the evolution of its microstructure and properties, and reveal the influence of heat treatment processes on the microstructure and properties the UCFB steels. The results show that the activation energy for BF nucleation decreases during the CA process and isothermal transformation temperature decreases. The bainite transformation is first nucleated at the grain boundaries, and then nucleated at the newly formed bainitic ferrite/austenite interface.
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Zajac, Stanislaw, Volker Schwinn, and K. H. Tacke. "Characterisation and Quantification of Complex Bainitic Microstructures in High and Ultra-High Strength Linepipe Steels." Materials Science Forum 500-501 (November 2005): 387–94. http://dx.doi.org/10.4028/www.scientific.net/msf.500-501.387.
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This paper provides a detailed description of complex bainitic microstructures obtained during the recent development of low carbon linepipe steels with strengths in the range of X100 to X120. New experimental techniques based on a high resolution FEG-SEM and EBSD have been used to characterise and quantify the mixture of ultrafine bainitic ferrite and nanosize second phases in these steels. It was found that the occurrence of incomplete transformation generates new, previously unexplored bainitic microstructures with a wealth of microstructural features that is beyond classification based on conventional concepts. Clear differences in distributions of boundary misorientations and effective grain size were noted between upper, lower and granular bainites. Based on these results a new classification scheme and definition of bainite is proposed.
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Bevilaqua, William Lemos, Jérémy Epp, Heiner Meyer, et al. "Revealing the Dynamic Transformation of Austenite to Bainite during Uniaxial Warm Compression through In-Situ Synchrotron X-ray Diffraction." Metals 11, no. 3 (2021): 467. http://dx.doi.org/10.3390/met11030467.
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In this work, the microstructural evolution during the dynamic transformation of austenite to bainite was directly observed by in-situ high energy synchrotron X-ray diffraction measurements during warm uniaxial compression performed at the P07 beamline of PETRA III, DESY (Deutsches Elektronen-Synchrotron). Plastic deformation triggers the phase transformation, which is continuously stimulated by the introduction of dynamic dislocations into the austenite. This scenario accelerates the kinetics of bainite formation in comparison with conventional isothermal treatment. No mechanical stabilization of austenite was observed during dynamic transformation. Evidence of carbon partitioning between phases during plastic deformation was obtained. Further post-process investigations suggest that the bainitic microstructure developed during compression is oriented perpendicular to the loading direction. The findings open up new possibilities to design carbide-free bainitic microstructures directly via thermomechanical processing.
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Bhadeshia, H. K. D. H. "Nanostructured bainite." Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 466, no. 2113 (2009): 3–18. http://dx.doi.org/10.1098/rspa.2009.0407.
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An alloy system based on iron is described in which it has been possible to create a high density of interfaces by heat treatment alone. The resulting structure consists of a mixture of slender platelets of bainitic ferrite, just 20–40 nm in thickness, embedded in a matrix of carbon-enriched austenite. The rate at which this structure evolves is slow by conventional standards, but this permits components to be made which are large in all three dimensions, with uniform properties throughout. The fundamental mechanisms behind this novel nanostructured steel are reviewed, along with the factors determining its strength, ductility and fracture toughness. It is argued that, although reasonable toughness can be achieved in the context of strength levels exceeding 2000 MPa, the impact toughness remains poor and that it may not be possible to improve this particular parameter.
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Mao, Xing Feng, Kai Ming Wu, Lian Deng Yao, and Zi Gang Li. "Effect of Process Parameters on Microstructural Evolution and Grain Refinement in a Low Carbon High Strength Microalloyed Dual Phase Steel." Advanced Materials Research 284-286 (July 2011): 1244–52. http://dx.doi.org/10.4028/www.scientific.net/amr.284-286.1244.
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The effects of process parameters on microstructural evolution and grain refinement are determined in a Nb-Ti microalloyed high strength dual phase steel. With the increase of cooling rate, final microstructures change from a mixture of acicular ferrite (AF)+martensite/retained austenite (M/A) to conventional bainite (CB)+M/A. Accordingly, the morphology of M/A constituent changes from an equiaxed island in AF to an elongated interlath in CB. The length and width of bainite packets become smaller with the increase of cooling rate and the decrease of deformation temperature. The length of individual bainitic ferrite plates within the packets become smaller with the increase of cooling rate and the decrease of deformation temperature, whereas the thickness of them varies slightly with them. The optimized relaxing time on grain refinement is 60 s. The reheating temperature, reduction ratio and interrupt temperature has no obvious effect on the formation of dual phase of acicular ferrite or bainite and M/A.
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Vuorinen, Esa, Jesper Vang, Malo Carradot, Pernilla Johansson, and Erik Navara. "Powder Metallurgically Produced Carbide Free Bainite." Materials Science Forum 782 (April 2014): 480–86. http://dx.doi.org/10.4028/www.scientific.net/msf.782.480.
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Steels with carbide free bainitic (CFB) microstructures show excellent strength, toughness and wear resistance. Cast or wrought products produced by conventional metallurgy have become gradually introduced in manufacturing of numerous machine components. The required silicon addition of more than 1.5wt% in CFB-steels limits the possibilities to produce components of these steels by P/M methods. The aim of this work has been to investigate the possibilites to produce CFB-steels by pressing and sintering. Four different powder mixtures based on Distaloy DC powder have been pressed to a relative density of 90 % and sintered in a N2-H2 atmosphere at 1150 °C. The sintered components were then austenitized followed by austempering at a temperature above the martensite start temperature. Tensile and impact testing together with microhardness measurements have been performed. The microstructures were studied by optical microscopy as well as SEM and XRD-methods. The tensile strength values achieved varied from 313 to 737 MPa, the elongation after fracture were between 0.1 and 0.2%. The impact toughness values varied between 4 and 11 J. The hardness of the bainite after short sintering time varied between 630 and 710 HV and the hardness of the CFB was 350 HV after short sintering time but reached 573 after prolonged sintering. The microstructure consisted mainly of bainite, small amount of CFB mixed with austenite but also of ferrite and retained austenite after short sintering time. A longer sintering time created a structure consiting of mainly CFB with bainite and a small amount of ferrite. The most interesting applications for P/M produced CFB-containing steels should be components subjected to sliding or rolling-sliding wear loads, as gears. The hardness and strenght values achieved in the present work indicate that P/M produced CFB-steels can prove superior to conventional P/M steels in many applications.
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Pashangeh, Shima, Hamid Reza Karimi Zarchi, Seyyed Sadegh Ghasemi Banadkouki, and Mahesh C. Somani. "Detection and Estimation of Retained Austenite in a High Strength Si-Bearing Bainite-Martensite-Retained Austenite Micro-Composite Steel after Quenching and Bainitic Holding (Q&B)." Metals 9, no. 5 (2019): 492. http://dx.doi.org/10.3390/met9050492.
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To develop an advanced high strength steel with reasonable ductility based on low alloying concept as well as micro-composite microstructure essentially consisting of bainite, martensite and retained austenite, a Si-bearing, low alloy medium carbon sheet steel (DIN1.5025 grade) was subjected to typical quenching and bainitic holding (Q&B) type isothermal treatment in the bainitic region close to martensite start temperature (Ms) for different durations in the range 5s to 1h. While the low temperature bainite has the potential to provide the required high strength, a small fraction of finely divided austenite stabilized between the bainitic laths is expected to provide the desired elongation and improved work hardening. Various materials characterization techniques including conventional light metallography, field emission scanning electron microscopy FE-SEM, electron backscatter diffraction (EBSD), differential thermal analysis, X-ray diffraction (XRD) and vibrating sample magnetometry (VSM), were used to detect and estimate the volume fraction, size and morphology and distribution of retained austenite in the micro-composite samples. The results showed that the color light metallography technique using LePera’s etching reagent could clearly reveal the retained austenite in the microstructures of the samples isothermally held for shorter than 30s, beyond which an unambiguous distinction between the retained austenite and martensite was imprecise. On the contrary, the electron microscopy using a FE-SEM was not capable of identifying clearly the retained austenite from bainite and martensite. However, the EBSD images could successfully distinguish between bainite, martensite and retained austenite microphases with good contrast. Although the volume fractions of retained austenite measured by EBSD are in accord with those obtained by XRD and color light metallography, the XRD measurements showed somewhat higher fractions owing to its ability to acquisition and analyze the diffracted X-rays from very finely divided retained austenite, too. The differential thermal analysis and vibrating sample magnetometry techniques also confirmed the stabilization of retained austenite finely divided in bainite/martensite micro-composite microstructures. In addition, the peak temperatures and intensities corresponding to the decomposition of retained austenite were correlated with the related volume fractions and carbon contents measured by the XRD analysis.
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Sharma, Sathyashankara, B. M. Gurumurthy, U. Achutha Kini, Ananda Hegde, and Ajinkya Patil. "Mechanical characteristics evaluation of dual phase and related hardening techniques on AISI 4340 steel." Journal of Mechanical Engineering and Sciences 12, no. 4 (2018): 4018–29. http://dx.doi.org/10.15282/jmes.12.4.2018.03.0349.
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Steel has wide range of applications and is used in various machinery and general metallic components. Depending on the particular application, steels with tailorable and appropriate properties are used. This requires various methods which can be used to alter the properties based on the requirements. Generally, mechanical properties of the steel are improved by conducting the heat treatment processes. The aim of the present work is to experimentally investigate the effects of conventional heat treatments and special hardening techniques for dual phase structure on mechanical properties of AISI 4340 steel. The test specimens are machined as per ASTM standards and hardness, tensile, impact and microstructure analysis were carried out after the heat treatment processes. Dual phase heat treatment to obtain ferrite-bainite structure is performed by heating the as-bought specimen to the intercritical temperature for two hours followed by isothermal holding in fusible salt bath containing sodium nitrate and sodium nitrite at subcritical temperature for 30 minutes and cooling in air to room temperature. Similarly, ferrite-martensite structure is obtained by air cooling after holding isothermally in the salt bath for 10 seconds. Ferrite-bainite steel was observed to be soft, whereas ferrite-martensite steel was relatively harder. Austempered steel has high toughness with optimum hardness and conventionally hardened steel is the hardest among all. Microstructure shows colony of bainite and martensite in ferrite matrix of ferrite-bainite and ferrite-martensite dual phase structures respectively. An increase in brittleness was observed with the increase in hardness due to the conventional hardening to display lesser impact strength compared to austempered steel.
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Maisuradze, Mikhail V., and Maksim A. Ryzhkov. "Microstructure and Mechanical Properties of the Heat Treated Hy-TUF Steel." Materials Science Forum 989 (May 2020): 324–28. http://dx.doi.org/10.4028/www.scientific.net/msf.989.324.
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A study of the high-strength HY-TUF steel applied for the manufacturing of heavy loaded parts was carried out. The mechanical properties of the austempered HY-TUF steel were compared to the characteristics obtained after the conventional oil quenching and tempering. The upper bainite with low impact strength was formed during the austempering at 400 °C and higher. Conventional oil quenching and tempering at temperature 400...500 °С also led to the embrittlement of the steel under consideration. The best combination of toughness and strength of the HY-TUF steel was achieved after the austempering at the temperature of lower bainite formation.
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Meng, Jiang Ying, Zhi Geng Jia, Tong Liang Wang, Kai Fang Li, and Li He Qian. "Microstructure and Mechanical Properties of a Lamellar-Structured Low-Alloy TRIP Steel." Materials Science Forum 1016 (January 2021): 1188–92. http://dx.doi.org/10.4028/www.scientific.net/msf.1016.1188.
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In this paper, we report a lamellar-structured low-alloy transformation-induced plasticity (TRIP) steel; the microstructure of the steel consists of alternate lamellae of intercritical ferrite and reverted austenite on microscale, with the latter consisting of bainitic ferrite laths and retained austenite films on nanoscale. Such a microstructure was produced by a heat treatment process similar to that for producing conventional TRIP-assisted steels, i.e. intercritical annealing followed by austempering. Nevertheless, quenched martensite rather than a mixture of ferrite and pearlite was used as the starting structure for intercritical annealing to form austenite, and the resulting austenite was then transformed to bainite by austempering treatment. This steel exhibits much enhanced strength-ductility combinations as compared with those conventional polygonal-structured low-alloy TRIP steels.
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Li, Fan, Hai Long Yi, Zhen Yu Liu, and Guo Dong Wang. "Influence of Cooling Rate on the Microstructures and Properties of Q550." Applied Mechanics and Materials 117-119 (October 2011): 1705–7. http://dx.doi.org/10.4028/www.scientific.net/amm.117-119.1705.
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A Q550 high strength steel was selected at two different cooling rates through ultra-fast cooling process, and its microstructures and strengthening mechanisms were analyzed. The results show the bainite transformation temperature of the steel decreased with the increasing of cooling rate.The ultra-fast cooling process can improve the performance of Q550 compared with the conventional cooling process, and the yield strength, tensile strength and elongation are 600MPa, 755MPa and 19%, and - 20 °Cimpact energy is 253J, and good strength and toughness are obtained under ultra-fast cooling process. The microstructure of this steel is bainite and good strength and toughness are caused by the refinement of bainite and fine precipitates. Ultra-fast cooling technology improves the strength and toughness of this steel effectively.
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Tomita, Y., K. Tanabe, and K. Koyama. "Improving Toughness of Electron Beam Welds of Heavy Mn-Mo-Ni Steel Plates for Pressure Vessels." Journal of Pressure Vessel Technology 115, no. 3 (1993): 242–48. http://dx.doi.org/10.1115/1.2929523.
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Electron beam welding melts and solidifies steel plate without using any welding material, unlike the conventional welding. Therefore, the toughness at the weld metal can decrease, depending on the chemical composition of the steel plate. Toughness at the electron beam weld can be increased by turning the microstructure from upper bainite into lower bainite and making the effective grain size finer. The microstructure can be controlled by the addition of alloy elements and optimization of impurity elements. In case the chemical compositions cannot be varied, largely because of the specification for their ranges, and the weld metal microstructure remains as upper bainite even after the application of microstructure control, methods to improve the toughness of electron beam weld itself, regardless of steel grades, becomes necessary. Phenomena peculiar to the electron beam weld are segregation during solidification and intergranular segregation over the dendrite surface. The fracture initiation is accelerated by the microcracks caused by the segregations during solidification. The fracture propagation is promoted by intergranular cracking caused by the intergranular segregation. By reducing these segregations, the fracture initiation and propagation are restrained and toughness increases despite the upper bainite microstructure. This can be achieved by the higher purification of steel. Through the foregoing investigations, ASTM A533 Type B Class 2 steel plate of 100 mm in thickness for electron beam welds has been developed for pressure vessels. Various welding tests as pressure vessels have been conducted, and it becomes clear that the developed steel plate has excellent toughness at the weld superior to those obtainable by conventional welding. The use of this steel greatly reduces the welding period compared to the conventional welding method.
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Kučerová, L., K. Opatová, J. Káňa, and H. Jirková. "High Versatility of Niobium Alloyed AHSS." Archives of Metallurgy and Materials 62, no. 3 (2017): 1485–91. http://dx.doi.org/10.1515/amm-2017-0230.
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AbstractThe effect of processing parameters on the final microstructure and properties of advanced high strength CMnSiNb steel was investigated. Several processing strategies with various numbers of deformation steps and various cooling schedules were carried out, namely heat treatment without deformation, conventional quenching and TRIP steel processing with bainitic hold or continuous cooling. Obtained multiphase microstructures consisted of the mixture of ferrite, bainite, retained austenite and M-A constituent. They possessed ultimate tensile strength in the range of 780-970 MPa with high ductility A5mmabove 30%. Volume fraction of retained austenite was for all the samples around 13%. The only exception was reference quenched sample with the highest strength 1186 MPa, lowest ductility A5mm= 20% and only 4% of retained austenite.
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Maetz, Jean-Yves, Matthias Militzer, Yu Chen, et al. "Modeling of Precipitation Hardening during Coiling of Nb–Mo Steels." Metals 8, no. 10 (2018): 758. http://dx.doi.org/10.3390/met8100758.
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Nb–Mo low-alloyed steels are promising advanced high strength steels (AHSS) because of the highly dislocated bainitic ferrite microstructure conferring an excellent combination of strength and toughness. In this study, the potential of precipitation strengthening during coiling for hot-strip Nb–Mo-bearing low-carbon steels has been investigated using hot-torsion and aging tests to simulate the hot-rolling process including coiling. The obtained microstructures were characterized using electron backscatter diffraction (EBSD), highlighting the effects of Nb and Mo additions on formation and tempering of the bainitic ferrite microstructures. Further, the evolution of nanometer-sized precipitates was quantified with high-resolution transmission electron microscopy (HR-TEM). The resulting age hardening kinetics have been modelled by combining a phenomenological precipitation strengthening model with a tempering model. Analysis of the model suggests a narrower coiling temperature window to maximize the precipitation strengthening potential in bainite/ferrite high strength low-alloyed (HSLA) steels than that for conventional HSLA steels with polygonal ferrite/pearlite microstructures.
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Tressia, Gustavo, Luis H. D. Alves, Amilton Sinatora, Helio Goldenstein, and Mohammad Masoumi. "Effect of bainitic transformation on the microstructure and wear resistance of pearlitic rail steel." Industrial Lubrication and Tribology 72, no. 9 (2020): 1095–102. http://dx.doi.org/10.1108/ilt-07-2019-0282.
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Purpose The purpose of this study is to develop a lower bainite structure consists of a dispersion of fine carbide inside plates of bainitic ferrite from chemical composition unmodified conventional pearlitic steel under bainitic transformation and to investigate its effect on tensile properties and wear resistance. Design/methodology/approach A commercial hypereutectoid pearlitic rail steel was subjected to three different bainitic transformation treatments followed by tempering to develop a desirable microstructure with a DIL805 BÄHR dilatometer. A comprehensive microstructural study was performed by scanning electron microscopy and energy dispersive x-ray spectroscopy. Finally, the mechanical properties and wear resistance were evaluated by tensile, microhardness, and pin-on-disc tests. Findings The results showed that the best combination of mechanical properties and sliding wear resistance was obtained in the sample subjected to bainitic transformation at 300°C for 600 s followed by tempering at 400°C for 300 s. This sample, which contained a bainitic ferrite structure, exhibited approximately 20% higher hardness and approximately 53% less mass loss than the as-received pearlitic sample due to the mechanically induced transformation in the contact surface. Originality/value Although pearlitic steel is widely used in the construction of railways, recent studies have revealed that bainitic transformation at the same rail steels exhibited higher wear resistance and fatigue strengths than conventional pearlitic rail at the same hardness values. Such a bainitic microstructure can improve the mechanical properties and wear resistance, which is a great interest in the railway industry. Peer review The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-07-2019-0282/
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Napadłek, Wojciech. "Influence Environment and Parameters Ablative Laser Texturing on Selected Properties Surface Layer Steel 100CrMnSi6-4." Advanced Materials Research 874 (January 2014): 17–22. http://dx.doi.org/10.4028/www.scientific.net/amr.874.17.
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This paper presents results of laboratory investigation microstructure bearing steel 100CrMnSi6-4 after laser hardening in the air and cryogenic environment. Usied high-power laser CO2 (4 kW) were selected the best parameters for hardening process (power density, scanning speed, the overlap surface hardened zones). As a result, laser hardening of the surface layer steel 100CrMnSi6-4 with selected process parameters obtained highdispersing martensitic microstructure of microhardness to 900HV0.1 with isolated fragmented chromium carbides. In the heat affected zone found martensitic- bainitic and bainite microstructure. In comparison with conventional hardening (e.g. inductive) were significant microstructure fragmentation and increase hardness about 15%. This paper presents ablative laser texturing surface layer above steel hardened before by laser. Texturing process was carried out in two environments and used pulsed iterbium fiber laser radiation Nd: YAG with a wavelength λ = 1064 nm.
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Tioguem, Frank, Matthieu Maziere, Franck Tankoua, André Galtier, and Anne-Françoise Gourgues-Lorenzon. "Identification of ductile to brittle transition temperature by using plane strain specimen in tensile test and correlation with instrumented Charpy impact test: experimental and numerical study." Mechanics & Industry 19, no. 1 (2018): 107. http://dx.doi.org/10.1051/meca/2017034.
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This study addresses the correlation between the ductile-to-brittle transition temperature ranges of high strength 4140 steel obtained respectively from tensile tests under plane strain (PS) conditions and from conventional Charpy impact tests. Specimens were taken respectively at 25 mm (P) and at 55 mm (M) from skin of a cylindrical 90-mm-radius hot rolled bar water quenched from 875 °C, tempered at 600 °C and air cooled. P and M samples respectively showed a fully martensitic and a martensite-bainite microstructure. Fracture surface observations showed good agreement for physical fracture mechanisms (cleavage facet size, mixed ductile + brittle fracture in the transition region, ductile fracture at higher temperatures) between PS and Charpy, in particular sensitivity of upper bainite to cleavage fracture that reduces fracture energy in the lower self-energy on Charpy tests.
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Kiatdherarat, Wasita, Pinai Mungsantisuk, Ruangdaj Tongsri, et al. "Effects of Cooling Rate and Carbon Content on Mechanical Property of Sintered Fe-Cr-Mo Alloys." Key Engineering Materials 658 (July 2015): 69–75. http://dx.doi.org/10.4028/www.scientific.net/kem.658.69.
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Advanced high strength steel (AHSS) was prepared using the conventional ‘press and sinter’ process.The compacts of ultralow carbon Fe-Cr-Mo powder with carbon additions (base metal powder admixed with 0.1,0.2 and 0.3 wt.% graphite) and without carbon addition (plain base powder) were sintered in a vacuum furnace at pressure of 1.28 x 10-5MPa at 1280 °C for 45 min. After sintering, the sintered specimens were continuously cooled with different nitrogen gas pressures of 0, 2500 and 5000 mbars (or 0, 0.25, 0.5 MPa). Mechanical properties of the sintered alloys were strongly controlled by carbon contents and cooling rates after sintering. The sintered specimens, with 0.3 wt.% carbon and cooled by nitrogen of 5000 mbars, showing superior tensile strengths and good ductility, had microstructures dominated by carbide-free bainitic structures and some retained austenite. The sintered specimens with lower carbon contents and cooled under slower cooling rates, having lower tensile strengths but slightly higher ductility, had microstructures with lower bainite volume fractions and even without bainitic structures. The dominant phase in the sintered specimens with low strength but high ductility was ferrite.
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Maisuradze, Mikhail V., Maksim Ryzhkov, and O. Surnaeva. "Phase Transformations in Novel Medium Carbon High Hardenability Steels." Solid State Phenomena 265 (September 2017): 717–22. http://dx.doi.org/10.4028/www.scientific.net/ssp.265.717.
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Novel steels with high hardenability were proposed to replace the conventional HY-TUF steels for the large parts manufacturing. The chemical composition of the steels under consideration was, mass. %: C – 0.16...0.18; Cr – 2.35...2.55; Mn – 0.67...1.99; Si – 0.76...1.03; Ni – 1.17...2.31; Mo – 0.34...0.47; S. P < 0.025. The dilatometer experiments revealed that during the continuous cooling of the steels with the constant rates 0.1... 30 °C/s only martensite and bainite transformations occurred. for conventional HY-TUF steel ferrite and pearlite formed after cooling with the rates 0.1...0.3 °C/s.
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Majid Safi, Seyed, and M. K. Besharati Givi. "A New Modified Austempering to Increase Strength and Ductility Simultaneously for UHS Steels." Defect and Diffusion Forum 297-301 (April 2010): 1109–15. http://dx.doi.org/10.4028/www.scientific.net/ddf.297-301.1109.
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In this paper, a modified up-quenching heat treatment method to the ASSAB 705M steel (ultra high strength steel) is proposed. A low alloy steel (0.33%C), was used to study the effect of isothermal austempering, successive austempering and modified up-quenching austempering heat treatment on the mechanical properties. The specimens, were cut from a bar with 25mm diameter and after achieving the best temperature and time of austenitizing, austenitized at for 60 min and followed by quenching at for the high austempering temperature to achieve the upper bainite morphology and at for the lower austempering temperature to achieve the lower bainite morphology. In the case of successive austempering, the specimens were first austempered at for different periods (500 sec and 60 sec) and then austempered at for 1000 sec to achieve the mixed structure of upper bainite and lower bainite morphology. The specimens selected for up-quenching, after austenitization were quenched to below ( ) for 120 sec. followed by heating at to achieve the mixed structure of tempered martensite and lower bainite and to achieve the mixed structure of tempered martensite and upper bainite for 1000 sec. All of the processes were performed in the salt bath furnaces. Experimental results are presented and the advantages of the modified method are discussed. As well, it is shown that the best combination of strength and ductility can be achieved by the proposed heat treatment method. This modified method, can offer techniques that simultaneously improve not only strength 12 %( compare with results of strength after other heat treatment methods), but also ductility 38 %( compare with results of ductility after other heat treatment methods). While, conventional heat treatment of ultra high strength steels (UHSS) cannot always meet the strict engineering requirements for improved strength and ductility simultaneously. It has been shown that the mixed structure of tempered martensite and lower bainite that has been suggested in this investigation offers a good combination of strength and ductility. The technical reason for this superiority returns back to the fact that it has increased dislocation density. As a result of the increment of the dislocation density, in the morphology, the inter lath carbide (e.g. cementite) decreases, and the intra lath carbide increases. This modified austempering is applicable to all the ultra high strength steels, has noticeable economic advantages because it is simple. Use of this modified austempering for heavy parts, leads to the lightness of heavy parts and combination of the thermomechanical methods with this modified austempering can yield even much more improvements.
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Vercruysse, Florian, Felipe M. Castro Cerda, Roumen Petrov, and Patricia Verleysen. "Static and dynamic response of ultra-fast annealed advanced high strength steels." EPJ Web of Conferences 183 (2018): 03017. http://dx.doi.org/10.1051/epjconf/201818303017.
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Ultra-fast annealing (UFA) is a viable alternative for processing of 3rd generation advanced high strength steels (AHSS). Use of heating rates up to 1000°C/s shows a significant grain refinement effect in low carbon steel (0.1 wt.%), and creates multiphase structures containing ferrite, martensite, bainite and retained austenite. This mixture of structural constituents is attributed to carbon gradients in the steel due to limited diffusional time during UFA treatment. Quasi-static (strain rate of 0.0033s-1) and dynamic (stain rate 600s-1) tensile tests showed that tensile strength of both conventional and UFA sample increases at high strain rates, whereas the elongation at fracture decreases. The ultrafast heated samples are less sensitive to deterioration of elongation at high strain rates then the conventionally heat treated ones. Based on metallographic studies was concluded that the presence of up to 5% of retained austenite together with a lower carbon martensite/bainite fraction are the main reason for the improved tensile properties. An extended stability of retained austenite towards higher strain values was observed in the high strain rate tests which is attributed to adiabatic heating. The extension of the transformation induced plasticity (TRIP) effect towards higher strain values allowed the UFA-samples to better preserve their deformation capacity resulting in expected better crashworthiness.
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Kuklina, Aleksandra A., Mikhail V. Maisuradze, and Yury V. Yudin. "Analytical Description of the Bainite Transformation Kinetics in Steels 300M and D6AC." Materials Science Forum 907 (September 2017): 31–37. http://dx.doi.org/10.4028/www.scientific.net/msf.907.31.
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The most widely used equation for analytical description of the transformation kinetics of the metastable solid solutions (the steel austenite in particular) is Kolmogorov-Johnson-Mehl-Avrami (KJMA) equation [1]. However the practical analysis of the experimental isothermal bainite transformation kinetics often gives significant deviation from the conventional theory [2]. This problem can be solved by the derivation of an analytical function which would provide the best fit of the experimental results. Two analytical approaches describing the kinetics of bainite transformation in steels 300M and D6AC are proposed. The first one is based on an approximation of the experimental ln (-ln (1-Р)) vs. ln τ dependence by a second order polynomial function. The second approach is based on the solution of the differential equation y(x) = ay’(x)+b, where x= ln τ, y(x) = ln(-ln(1-P)). A comparison between the proposed approaches and Kolmogorov - Johnson - Mehl – Avrami equation is conducted. The adequacy of the two analytical models is estimated using Fisher ratio test.
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Perkins, Jeff, M. H. Wu, and K. Adachi. "Interfaces and substructures in copper-based shape memory alloys." Proceedings, annual meeting, Electron Microscopy Society of America 46 (1988): 786–87. http://dx.doi.org/10.1017/s0424820100105990.
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The delineation of interfaces and substructures by conventional transmission electron microscopy is reviewed for three different transformations of interest in copper-based shape memory alloys.The first case considers the interfacial structure of martensite plate boundaries in a Cu-14.6Zn-16.1Al (atomic %) alloy. The detailed structure of such interfaces is important to their relative mobility in shape memory alloys. One-dimensional lattice images such as seen in Fig. 1 help to model the effect of substructural features in the martensite on the atomistic structure and so the mobility of such boundaries1. In the example shown, the presence of very thin structural faults with a 2H structure are detected within an otherwise 18R martensite plate.The second case describes the interaction of the state of parent phase order with the structure of bainite plates formed upon isothermal aging at 150-350 C in a Cu-27Zn-4Al (weight %) alloy . Two types of atomic ordering occur in the parent phase and tend to be inherited by the bainite transformation product.
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Liang, Xiao Jun, Ming Jian Hua, C. Isaac Garcia, and Anthony J. DeArdo. "The Thermomechanical Controlled Processing of High-Strength Steel Plate: A New View of Toughness Based on Modern Metallography." Materials Science Forum 762 (July 2013): 38–46. http://dx.doi.org/10.4028/www.scientific.net/msf.762.38.
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Modern thermomechanical controlled processing (TMCP) of advanced steels is now an important processing route in the production of engineering structures and products that are of value to society. The principles of TMCP are now practiced in the hot mill, cold mill and press forming shops around the world. Successful TMCP means that the proper metallurgical microstructure has been obtained in the required areas of the steel. The ideal microstructure is often defined by the correct phase balance and dimensions either of the parent austenite or final ferritic phase. Technological and economic demands have led to ever increasing levels of strength, especially for applications such as large diameter linepipe. The operative yield strengths in 18mm hot rolled plate have increased from X52(ferrite pearlite) in 1970 to X80(ferrite-bainite) today. The next frontier is the X100-X120 strength range, where bainitic or martensitic microstructures are required. It is clear that achieving a high-strength bainitic microstructure in heavy plate requires a high Carbon Equivalent value (C. E. II or Pcm), a rapid cooling rate, and a low water-end temperature. The requirement of high toughness and good weldability also means a low carbon content. This paper will describe the results of a research program where a steel of C. E. 0.56 and Pcm 0.23 was reheated, rough rolled for grain refinement, finish rolled for austenite pancaking, and direct quenched to below the Bs temperature. It was found that the strength and especially the toughness of the fully processed plates could not be explained using conventional metallographic techniques in conjunction with known structure-property relationships. However, the application of modern metallographic techniques based on FEG-SEM incorporating OIM led to microstructural characterization that more fully explained the observed mechanical properties. Of particular importance were the amount of MA micro-constituent, the crystallographic packet size of the bainite, and the high angle boundary character, especially the CSL boundaries, found in the microstructure. In the future, improved modeling of microstructural evolution and attendant mechanical properties will incorporate these important features.
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Skołek, E., K. Dudzińska, J. Kamiński, and W. A. Świątnicki. "Corrosion Resistance of The Bearing Steel 67SiMnCr6-6-4 with Nanobainitic Structure." Archives of Metallurgy and Materials 60, no. 1 (2015): 503–9. http://dx.doi.org/10.1515/amm-2015-0081.
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AbstractThe paper describes a comparative study of the corrosion resistance of bearing steel 67SiMnCr6-6-4 after two kinds of nanostructuring treatments and two kinds of conventional quenching and tempering treatments. The nanostructuring treatment consisted of austempering with an isothermal quenching at 240°C and 300°C. The conventional heat treatment consisted on quenching and tempering at 350°C for 1 h and quenching and tempering at 550°C for 1 h. Time and temperature of tempering was chosen so that the hardness of both samples (nanostructured as well as quenched and tempered) was similar. The microstructure of steel after each heat treatment was described with the use of transmission electron microscopy (TEM). It was shown, that the austempering conducted at 240°C produced homogenous nanobainitic structure consisting of carbide-free bainite plates with nanometric thickness separated by the layers of retained austenite. The austempering at 300°C produced a sub-micrometric carbide-free bainite with retained austenite in form of layers and small blocks. The conventional heat treatments led to a tempered martensite microstructure. The corrosion resistance study was carried out in Na2SO4acidic and neutral environment using potentiodynamic and electrochemical impedance spectroscopy (EIS) methods. The corrosion resistance of nanostructured steel samples were compared to the steel samples with tempered martensite. The obtained results indicate, that the corrosion resistance of bearing steel with nanobainitic structure is similar to steel with tempered martensite in both acidic and neutral environment. This means that the high density of intercrystalline boundaries in nanobinite does not deteriorate the corrosion properties of the bearing steel.
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Choi, Sungki, Junsang Lee, Jae-Yik Lee, et al. "Effect of Low Transformation Temperature Welding Consumable on Microstructure, Mechanical Properties and Residual Stress in Welded Joint of A516 Carbon Steel." Korean Journal of Metals and Materials 59, no. 8 (2021): 524–32. http://dx.doi.org/10.3365/kjmm.2021.59.8.524.
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The microstructure, mechanical properties and residual stress of flux-cored arc welded ASTM A516-70N carbon steel using a Mn-based low-temperature transformation (LTT) welding consumable were investigated. Microstructural analysis with X-ray diffraction, an electron backscattered diffractometer and a field-emission scanning electron microscope showed that the LTT weld metal was made up of ferrite, austenite, martensite, and bainite with phase fractions 50.5%, 0.2%, 40.2% and 9.1%, respectively. The increase in hardness and the decrease in absorbed impact energy of the LTT weld metal compared with conventional consumable welds were confirmed to be due to the relatively high fraction of martensite phase in the weld metal. The welding residual stress distributions in three coupons (LTT, conventional and postweld heat-treated conventional weld) were compared by the results using instrumented indentation testing. The LTT weld coupon showed compressive residual stress distributed in the weld metal and heat-affected zone (HAZ), confirming previous studies in which this residual stress was attributed to a martensitic phase transformation at relatively low temperature. PWHT in the conventionally welded coupon considerably reduced the tensile residual stress distributed in the weld metal and HAZ. The LTT consumable, however, showed a significant advantage in welding residual stress, even compared with the heat-treated conventional consumable.
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Ławrynowicz, Zdzisław. "Diagnostics of the bainite transformation mechanism and the effect of normalizing and tempering on the hardness and microstructure of the new Cr-Mo-V-Ti steel for operation at elevated temperature." MATEC Web of Conferences 182 (2018): 02006. http://dx.doi.org/10.1051/matecconf/201818202006.
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The Cr-Mo-V-Ti based low alloy steels are widely used in thermal power plants because of their ability to withstand elevated temperatures and high pressure under continuous service. In the present work conventional heat treatment like normalizing and tempering of the alloys has been performed. The material used in this study was the laboratory prepared experimental low alloy Cr-Mo-V-Ti steel. Samples were austenitized at 980oC for 0.5 hour air cooled and tempered at 500, 550, 600, 650, 700 and 750oC for 1 hour. Mechanism of bainite transformation has been studied in Fe-C-Cr-Mo-V-Ti steel using high speed dilatometry. These experimental data indicate that bainitic ferrite forms by a displacive transformation mechanism, but soon afterwards, excess of carbon is partitioned into the residual austenite. The changes observed in the microstructure of the steel tempered at the higher temperature, i.e. 750°C were more advanced than those observed at the temperature of 500°C. Performed microstructural investigations have shown that the degradation of the microstructure of the examined steel was mostly connected with the processes of recovery and polygonization of the matrix, disappearance of lath bainitic microstructure and the growth of the carbides. The magnitude of these changes depended on the temperature of tempering.
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Wang, Hongcai, Lijie Cao, Yujiao Li, Mike Schneider, Eric Detemple, and Gunther Eggeler. "Effect of cooling rate on the microstructure and mechanical properties of a low-carbon low-alloyed steel." Journal of Materials Science 56, no. 18 (2021): 11098–113. http://dx.doi.org/10.1007/s10853-021-05974-3.
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AbstractHeavy plate steels with bainitic microstructures are widely used in industry due to their good combination of strength and toughness. However, obtaining optimal mechanical properties is often challenging due to the complex bainitic microstructures and multiple phase constitutions caused by different cooling rates through the plate thickness. Here, both conventional and advanced microstructural characterization techniques which bridge the meso- and atomic-scales were applied to investigate how microstructure/mechanical property-relationships of a low-carbon low-alloyed steel are affected by phase transformations during continuous cooling. Mechanical tests show that the yield strength increases monotonically when cooling rates increase up to 90 K/s. The present study shows that this is associated with a decrease in the volume fraction of polygonal ferrite (PF) and a refinement of the substructure of degenerated upper bainite (DUB). The fine DUB substructures feature C-rich retained austenite/martensite-austenite (RA/M-A) constitutes which decorate the elongated micrograin boundaries in ferrite. A further increase in strength is observed when needle-shaped cementite precipitates form during water quenching within elongated micrograins. Pure martensite islands on the elongated micrograin boundaries lead to a decreased ductility. The implications for thick section plate processing are discussed based on the findings of the present work.
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Wei, Zhan Shan, Zhuang Li, Wei Lv, and Zhen Yao Shao. "The Influence of the Substitution of Si by Al on the Properties of Hot Rolled C-Mn-Si TRIP Steel." Materials Science Forum 896 (March 2017): 198–201. http://dx.doi.org/10.4028/www.scientific.net/msf.896.198.
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The influence of the substitution of Si by Al on the properties of hot rolled C-Mn-Si TRIP steel was investigated by TMCP. The results have shown that the microstructures of the present steels consist of polygonal ferrite, granular bainite and retained austenite. The Al substitution of Si in a conventional C-Mn-Si TRIP steel leads to excellent mechanical properties (UTS>714MPa, A50>31%). TMCP led to the stability of the remaining austenite and a satisfactory TRIP effect. Excellent mechanical properties were obtained through tmcp for the hot rolled TRIP steel.
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Pashangeh, Shima, Seyyed Sadegh Ghasemi Banadkouki, Fatemeh Besharati, Fatemeh Mehrabi, Mahesh Somani, and Jukka Kömi. "Color Light Metallography Versus Electron Microscopy for Detecting and Estimating Various Phases in a High-Strength Multiphase Steel." Metals 11, no. 6 (2021): 855. http://dx.doi.org/10.3390/met11060855.
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In this study, fresh attempts have been made to identify and estimate the phase constituents of a high-silicon, medium carbon multiphase steel (DIN 1.5025 grade) subjected to austenitization at 900 °C for 5 min, followed by quenching and low-temperature bainitizing (Q&B) at 350 °C for 200 s. Several techniques were employed using different chemical etching reagents either individually (single-step) or in combination of two or more etchants in succession (multiple-step) for conducting color metallography. The results showed that the complex multiphase microstructures comprising a fine mixture of bainite, martensite and retained austenite phase constituents were selectivity stained/tinted with good contrasting resolution, as observed via conventional light optical microscopy observations. While the carbon-enriched martensite-retained austenite (M/RA) islands were revealed as cream-colored areas by using a double-step etching technique comprising etching with 10% ammonium persulfate followed by etching with Marble’s reagent, the dark gray-colored bainite packets were easily distinguishable from the brown-colored martensite regions. However, the high-carbon martensite and retained austenite in M/RA islands could be differentiated only after resorting to a triple-step etching technique comprising etching in succession with 2% nital, 10% ammonium persulfate solution and then warm Marble’s reagent at 30 °C. This revealed orange-colored martensite in contrast to cream-colored retained austenite in M/RA constituents, besides the presence of brown-colored martensite laths in the dark gray-colored bainitic matrix. A quadruple-step technique involving successive etching with 2% nital, 10% ammonium persulfate solution, Marble’s reagent and finally Klemm’s Ι reagent at 40 °C revealed even better contrast in comparison to the triple-step etching technique, particularly in distinguishing the RA from martensite. Observations using advanced techniques like field emission scanning electron microscopy (FE-SEM) and electron back scatter diffraction (EBSD) failed to differentiate untempered, high-carbon martensite from retained austenite in the M/RA islands and martensite laths from bainitic matrix, respectively. Transmission electron microscopy (TEM) studies successfully distinguished the RA from high-carbon martensite, as noticed in M/RA islands. The volume fraction of retained austenite estimated by EBSD, XRD and a point counting method on color micrographs of quadruple-step etched samples showed good agreement.
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Dudrová, Eva, Marco Actis Grande, Mario Rosso, Margita Kabátová, Róbert Bidulský, and Eduard Hryha. "Improvement of Mechanical Properties of Fe-Cr-Mo-[Cu-Ni]-C Sintered Sintered Steels by Sinter Hardening." Materials Science Forum 672 (January 2011): 31–38. http://dx.doi.org/10.4028/www.scientific.net/msf.672.31.
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The effect of high temperature sintering and high cooling rate on shifting the microstructural composition to the favourably of martensite-bainite structures and thus effective improvement of mechanical properties of sintered steels based on Astaloy CrL powder with an addition of 1 and 2% Cu or 50% Distaloy AB powder and 0.65% C was investigated. All the systems were processed by both sinter-hardening and conventional sintering. The vacuum sintering at high-temperature of 1240 0C and at common temperature of 1180 0C were integrated with high (6 0C/s), medium (3 0C/s) and slow (0.1 0C/s) cooling rates; conventional sintering at 1180 0C with cooling rate of ~0.17 0C/s was carried out in a N2+10%H2 atmosphere. In dependence on chemical composition, the yield and tensile strengths of 890-1150 MPa and 913-1230 MPa respectively and impact energy of 10-15 J were achieved by sinter-hardening. The yield and tensile strengths are approximately double than those resulting from conventional sintering.
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Koyama, Toshiyuki, and Hidehiro Onodera. "Calculation of Stress-Strain Curve of Two-Phase Microstructure on the Basis of the Extended Secant Method." Materials Science Forum 638-642 (January 2010): 3325–30. http://dx.doi.org/10.4028/www.scientific.net/msf.638-642.3325.
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The secant method proposed by Weng [1] is a practical calculation method to evaluate the stress-strain curve of the two-phase materials, but the shape of the inclusion phase has been often assumed to be a sphere or an ellipsoid in the calculation. In this study, we modified the secant method by utilizing the phase-field micro-elasticity theory [2,3] so as to be able to calculate the SS-curve of the materials consisting arbitrary morphology of microstructure, and applied this method to the conventional microstructures in steels, i.e., the ferrite-bainite two-phase microstructure.
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Yi, Hai Long, Yang Xu, Zhen Yu Liu, and Guo Dong Wang. "Influence of Cooling Rate on the Microstructures and Properties of a Nb-Ti-Mo Steel." Applied Mechanics and Materials 152-154 (January 2012): 14–17. http://dx.doi.org/10.4028/www.scientific.net/amm.152-154.14.
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A Nb-Ti-Mo high strength steel was selected at two different cooling rates through ultra-fast cooling process, and its microstructures and strengthening mechanisms were analyzed. The results show the size of ferrite was decreased and the amount of bainite and micro-hardness were increased with increasing of cooling rate through thermal simulation. The UFC technology can improve the yield and tensile strength 25MPa and 35MPa, respectively, compared with conventional TMCP. The microstructure of this steel is mainly ferrite and good strength and toughness are caused by the refinement of ferrite and fine precipitates. Ultra-fast cooling technology improves the strength and toughness of this steel effectively.
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Ławrynowicz, Z. "Bainitic reaction and microstructure evolution in two normalized and tempered steels designed for service at elevated temperatures." Advances in Materials Science 17, no. 4 (2017): 22–36. http://dx.doi.org/10.1515/adms-2017-0019.
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Abstract In the present work conventional heat treatment like normalizing (bainitic microstructure) and tempering of the alloys has been performed. The materials used in this study were two steels, one the laboratory prepared experimental low alloy Cr-Mo steel in comparison to typical commercial 10CrMo9-10 steel. The determined carbon concentrations of the residual austenite at the different temperatures of bainite transformation supports the hypothesis that the growth of bainitic ferrite occurs without any diffusion with carbon being partitioned subsequently into the residual austenite. It was found that bainitic reaction has stopped when average carbon concentration of the untransformed austenite is close to the T0 line and supports formation of bainitic ferrite by a shear mechanism, since diffusionless transformation is not possible beyond the T0 curve. Normalized samples were air cooled down to room temperature before tempering at various temperatures in the range of 500-750°C. Samples have been austenitized at 980°C for 0.5 hour air cooled and tempered at 500, 550, 600, 650, 700 and 750°C for 1 hour. After heat treatment, the assessment in the microstructure and phase precipitation was made using the samples prepared for metallographic and transmission electron microscope (TEM) on thin foils analysis. Quantitative X-ray analysis was used to determine the retained austenite content after heat treatment like normalizing and tempering and the total volume fraction of the retained austenite was measured from the integral intensity of the (111)γ and (011)α peaks. The changes observed in the microstructure of the steel tempered at the higher temperature, i.e. 750°C were more advanced than those observed at the temperature of 500°C. Performed microstructural investigations have shown that the degradation of the microstructure of the examined steel was mostly connected with the processes of recovery and polygonization of the matrix, disappearance of lath bainitic microstructure, the growth of the size of M23C6 carbides, and precipitation of the secondary M2C precipitates. The magnitude of these changes depended on the temperature of tempering.
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Dias, Erica, Laís Horimoto, and Marcelo dos Santos Pereira. "Microstructural Characterization of CP Steel Used in Automotive Industry." Materials Science Forum 775-776 (January 2014): 141–45. http://dx.doi.org/10.4028/www.scientific.net/msf.775-776.141.
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This study aims to characterize the microstructure of the complex phase steel (CP). Using the conventional and colored metallographic analysis with 3% Nital etchant, sodium metabisulfite 10% and LePera. Techniques were applied in this work of optical microscopy, using, besides the lighting in bright field, dark field illumination of the reverse contrast in bright field illumination, the method of polarized light, which generates colorful contrast, providing a complementary identification phases present in the microstructure, and the system by differential interference contrast (DIC). The results obtained by metallography CP indicates that the steel has a microstructure composed of ferrite, retained austenite, bainite and martensite and precipitates arranged in a refined and complex morphology. Besides bright field illumination others optical microscopys techniques such as dark field illumination were applied.
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Kwon, Ohjoon, Kyoo Young Lee, Gyo Sung Kim, and Kwang Geun Chin. "New Trends in Advanced High Strength Steel Developments for Automotive Application." Materials Science Forum 638-642 (January 2010): 136–41. http://dx.doi.org/10.4028/www.scientific.net/msf.638-642.136.
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The body design with light weight and enhanced safety is a key issue in the car industry. Corresponding to this trend, POSCO is developing various automotive steel products with advanced performance. Conventional advanced high strength steels such as DP and TRIP steels are now expanding their application since the steels exhibit higher strength and ductility than those of conventional solution and precipitation strengthened high strength steels. Efforts have been made to enhance the mechanical performance of these steels such as ductility, hole expansion ratio, deep drawability, etc. Current research is focused on development of extra- and ultra-AHSS. Extra-AHSS are designed to utilize nano-scale retained austenite embedded in fine bainite and martensite. Ultra-AHSS are designed to have austenite as the major phase, and the ductility is enhanced primarily by continuous strain hardening generated during forming. These steels including extra- and ultra-AHSS are believed to be the next generation automotive steels which will replace the existing high strength steels due to their extremely high strength and ductility combinations.
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Hernandez-Duran, Eliseo, Luca Corallo, Tanya Ros-Yanez, Felipe Castro-Cerda, and Roumen H. Petrov. "The Effect of Different Annealing Strategies on the Microstructure Development and Mechanical Response of Austempered Steels." Metals 11, no. 7 (2021): 1041. http://dx.doi.org/10.3390/met11071041.
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This study focuses on the effect of non-conventional annealing strategies on the microstructure and related mechanical properties of austempered steels. Multistep thermo-cycling (TC) and ultrafast heating (UFH) annealing were carried out and compared with the outcome obtained from a conventionally annealed (CA) 0.3C-2Mn-1.5Si steel. After the annealing path, steel samples were fast cooled and isothermally treated at 400 °C employing the same parameters. It was found that TC and UFH strategies produce an equivalent level of microstructural refinement. Nevertheless, the obtained microstructure via TC has not led to an improvement in the mechanical properties in comparison with the CA steel. On the other hand, the steel grade produced via a combination of ultrafast heating annealing and austempering exhibits enhanced ductility without decreasing the strength level with respect to TC and CA, giving the best strength–ductility balance among the studied steels. The outstanding mechanical response exhibited by the UFH steel is related to the formation of heterogeneous distribution of ferrite, bainite and retained austenite in proportions 0.09–0.78–0.14. The microstructural formation after UFH is discussed in terms of chemical heterogeneities in the parent austenite.
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Choi, Byoung-Wook, Dong-Han Seo, Jang-Yong Yoo, and Jae-il Jang. "Predicting macroscopic plastic flow of high-performance, dual-phase steel through spherical nanoindentation on each microphase." Journal of Materials Research 24, no. 3 (2009): 816–22. http://dx.doi.org/10.1557/jmr.2009.0109.
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An attempt was made to predict the macroscopic plastic flow of a high-performance pipeline steel, consisting of dual constituent phases (soft ferrite and hard bainite), by performing nanoindentation experiments on each microphase with two spherical indenters that have different radii (550 nm and 3.3 μm). The procedure is based on the well known concepts of indentation stress-strain and constraint factor, which make it possible to relate indentation hardness to the plastic flow of the phases. Additional consideration of the indentation size effect for sphere and application of a simple “rule-of-mixture” led us to a reasonably successful estimation of the macroscopic plastic flow of the steel from the microphases properties, which was verified by comparing the predicted stress-strain curve with that directly measured from the conventional tensile test of a bulky sample.
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Lim, Bokkyu, and Young Woo Choi. "Effect of Semi Austempering Treatment on the Fatigue Properties of Ductile Cast Iron." Key Engineering Materials 345-346 (August 2007): 295–98. http://dx.doi.org/10.4028/www.scientific.net/kem.345-346.295.
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Single phase bainite structure which is obtained by the conventional austempering treatment reduces the ductility of ductile cast iron. Because of the reduction of ductility it is possible to worsen the fatigue properties. Therefore, semi austempered ductile iron which is treated from +ϒ is prepared to investigate the static strength and fatigue properties in comparison with fully austempered ductile iron (is treated from ϒ). In spite of semi austempered ductile iron shows the 86% increase of ductility. Also, semi austempered ductile iron shows the higher fatigue limit and lower fatigue crack growth rate as compared with fully austempered ductile iron. By the fractographical analysis, it is revealed that the ferrite obtained by semi austempering process brings about the plastic deformation(ductile striation) of crack tip and gives the prior path of crack propagation. The relatively low crack growth rate in semi austempered specimen is caused by above fractographical reasons
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Bigg, Timothy D., and David Edmonds. "Microstructural Evolution during the Novel Quenching and Partitioning (Q&P) Heat Treatment of Steel." Materials Science Forum 654-656 (June 2010): 33–36. http://dx.doi.org/10.4028/www.scientific.net/msf.654-656.33.
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The novel non-equilibrium heat treatment procedure known as Quenching and Partitioning (Q&P) may offer the prospect of higher strength steel products with enhanced formability based upon martensitic microstructures containing controlled quantities of carbon-enriched retained austenite. The Q&P process requires an interrupted quench and isothermal annealing (partitioning) step at intermediate temperatures, whereby untransformed austenite can be thermodynamically stabilised by carbon migration from supersaturated martensite regions. The concept is comparable to that producing carbide-free bainite, for example, in TRIP-assisted steel, although Q&P allows separation of the ferrite formation and austenite enrichment stages of the process. However, although the Q&P concept is readily understood, evolution of the microstructure during interrupted quenching and partitioning has been inferred indirectly from dilatometer studies and metallographic examination after final quenching to room temperature. Consequently, a model alloy was developed in which the sequential steps of heat treatment could be separated for direct inspection by conventional metallography, X-ray diffraction and neutron diffraction techniques.
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Xiong, Zhiping, Andrii G. Kostryzhev, Yanjun Zhao, and Elena V. Pereloma. "Microstructure Evolution during the Production of Dual Phase and Transformation Induced Plasticity Steels Using Modified Strip Casting Simulated in The Laboratory." Metals 9, no. 4 (2019): 449. http://dx.doi.org/10.3390/met9040449.
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Instead of conventional steel making and continuous casting followed by hot and cold rolling, strip casting technology modified with the addition of a continuous annealing stage (namely, modified strip casting) is a promising short-route for producing ferrite-martensite dual-phase (DP) and multi-phase transformation-induced plasticity (TRIP) steels. However, at present, the multi-phase steels are not manufactured by the modified strip casting, due to insufficient knowledge about phase transformations occurring during in-line heat treatment. This study analysed the phase transformations, particularly the formation of ferrite, bainite and martensite and the retention of austenite, in one 0.17C-1.52Si-1.61Mn-0.195Cr (wt. %) steel subjected to the modified strip casting simulated in the laboratory. Through the adjustment of temperature and holding time, the characteristic microstructures for DP and TRIP steels have been obtained. The DP steel showed comparable tensile properties with industrial DP 590 and the TRIP steel had a lower strength but a higher ductility than those industrially produced TRIP steels. The strength could be further enhanced by the application of deformation and/or the addition of alloying elements. This study indicates that the modified strip casting technology is a promising new route to produce steels with multi-phase microstructures in the future.
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Abreu, H. F. G., W. S. Tavares, Hélio Cordeiro de Miranda, Maria P. Cindra Fonseca, H. N. Virgens-Neto, and M. Béreš. "Effect of Multipass Welding Using a Low Transformation Temperature Filler Metal on Residual Stress and Toughness." Materials Science Forum 783-786 (May 2014): 627–32. http://dx.doi.org/10.4028/www.scientific.net/msf.783-786.627.
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Residual stresses in welded components are consequence of stress and/or thermal gradients and influenced by factors such as joint geometry, variation in strength of the material, preheat temperature, heat input, post-weld heat treatment and phase transformation strains. During the 70’s, it was observed that the level of residual stress accumulated in a constrained sample during cooling from austenite could be reduced after transformation to martensite or bainite. Some works have evaluated effect of welding using a low transformation temperature martensitic filler metal on the level of residual stress in single pass joints. According to these studies, martensite start temperature in the range 200–250°C can be extremely effective for mitigation of tensile residual stresses. The outcome of most of these works was on one hand increase of fatigue life due to the mitigation of tensile residual stresses via transformation strains, on the other a significant reduction of the fracture toughness.In the present study, sections of API 5L class B steel tubes were multipass welded using a 12Cr-5Ni low transformation temperature filler metal in addition to a conventional filler metal. Residual stresses in the inner and outer surfaces were measured by X-ray diffraction. Aspects related to the improvement of toughness in the weld metal due to the tempering of one pass by the subsequent were also discussed.
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López, N. M., and A. Salinas R. "Effects of Austenitizing Temperature and Cooling Rate on the Phase Transformation Texture in Hot Rolled Steels." MRS Proceedings 1485 (2012): 131–36. http://dx.doi.org/10.1557/opl.2013.281.
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ABSTRACTThe effects of austenitizing temperature and cooling rate on the microstructures and textures produced by phase transformations in high strength hot rolled Fe-C-Mn steel plates are investigated using orientation imagining microscopy. Samples machined from the plates are austenitized at temperatures between 820-950°C during 30 minutes and quenched in either iced-water, water or oil. Finally, the quenched samples are tempered at 450°C during 30 minutes. Characterization of microstructure and textures produced by these heat treatments was performed by conventional metallography using a reflected light microscope and orientation imaging microscopy using backscattered-electron diffraction patterns in a scanning electron microscope with thermo-ionic electron source.The results show that the microstructure and texture produced under a given combination of austenitizing temperature and cooling rate are strongly dependent on the mechanism involved in the phase transformation of the austenite (γ). High austenitizing temperatures and cooling rates promote martensitic transformation and development of textures containing significant volume fractions of Br, Cu, transformed-Cu and transformed-Br orientation components. In this case, the austenite and martensite phases are clearly related through the Kurdjumov-Sachs orientation relationship. In contrast, low temperatures and low cooling rates result in a complex mixture of transformation products, such as polygonal ferrite, Widmanstäten ferrite, martensite, bainite and pearlite. The textures formed under these conditions are quite different and contain significant volume fractions of cube, rotated-cube, Goss and rotated-Goss components, following the Bain orientation relationship.
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Nagao, A., K. Hayashi, K. Oi, S. Mitao, and N. Shikanai. "Refinement of Cementite in High Strength Steel Plates by Rapid Heating and Tempering." Materials Science Forum 539-543 (March 2007): 4720–25. http://dx.doi.org/10.4028/www.scientific.net/msf.539-543.4720.
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The precipitation behavior of cementite in low carbon steels at various heating rates from 0.3 to 100 K/s has been studied using a high-frequency induction heating apparatus. The materials used in this study were steel platesfor welded structures: 610 and 780 MPa class steel plates with a mixed microstructure of bainite and martensite.Cementite was observed using a carbon extraction replica method and the hardness and toughness were also examined. When heated at the conventional slow rate of 0.3 K/s, relatively large cementite particles with an average diameter of 72 nm precipitated at the lath boundaries, whereas when heated at a rapid rate over 3.0 K/s, cementite precipitated both within the laths and at the lath boundaries, and the cementite was refined down to an average diameter of 54 nm. With such refinement of the cementite, the toughness was improved. On the other hand, the hardness was irrespective of the heating rate and was dependent on the tempering parameter. TEM observations of the cementite precipitation behavior during the rapid heating process revealed that cementite begins to precipitate at the lath boundaries at about 773 K and within the laths at about 873 K. It is concluded that rapid heating especially from 773 to 873 K contributes to the cementite refinement and consequently the improvement in toughness. The effect of alloying elements such as chromium, molybdenum or silicon on the cementite growth during the rapid heating and tempering treatment is also discussed.
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Mraz, Lubos, Leif Karlsson, Pavol Mikula, and Miroslav Vrána. "Identification of Weld Residual Stresses Using Diffraction Methods and their Effect on Fatigue Strength of High Strength Steels Welds." Materials Science Forum 768-769 (September 2013): 668–74. http://dx.doi.org/10.4028/www.scientific.net/msf.768-769.668.
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It is well known that fatigue strength of welded joints does not depend on steel strength. Better fatigue strength of welded joints, e.g. longer life time of fatigue loaded weld structures, can be achieved with a smooth transition between the weld and the base material to minimize stress concentration. It has also been recognized that residual stresses play a critical role in the fatigue behaviour of welds. In the last decade an extensive research has been performed in order to increase the fatigue strength of high strength steel weldments. The martensite and bainite transformation start temperatures of weld metals have been shown to have a large effect on fatigue life time of high strength steel welds. This is of particular importance if the full potential of high strength steels is to be used in fatigue loaded constructions. A detailed investigation of the effect of phase transformation temperature on residual stress distribution in the vicinity high strength steel welds and its effect on fatigue life time has been performed. The transformation temperature of the weld metal was varied by changing the chemical composition of the filler material. Residual stress distributions have been measured by neutron as well as by X-ray diffraction and fatigue tests have been performed on the fillet welds. A strong effect of weld metal phase transformation temperature on residual stress level was observed. Fatigue strength increased approximately three times when an optimised low transformation temperature filler material was used in comparison to the application of conventional filler material.
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Papaefthymiou, Spyros, Vassilios Karamitros, and Marianthi Bouzouni. "Ultrafast Heating and Initial Microstructure Effect on Phase Transformation Evolution of a CrMo Steel." Metals 9, no. 1 (2019): 72. http://dx.doi.org/10.3390/met9010072.
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Main target of the present work is to elucidate the effect of both initial microstructure and heating rate on phase transformations that occur during ultrafast processing. For this purpose, two initial microstructures, a ferritic-pearlitic and a soft-annealed microstructure were considered. We applied different heating rates (10 °C/s, 200 °C/s, 300 °C/s) up to the peak austenitization temperature, θ ≅ 900 °C. The evolving microstructure is analysed via SEM and EBSD, whereas the carbide dissolution and austenite formation is simulated with Thermocalc® and DICTRA software. Data obtained in this research proves that, when the heating rate increases, the carbide dissolution rate is disseminated. Compared to a conventional heating rate, where the local chemical composition homogenizes as a result of diffusion, rapid reheating leads to intense segregation of the substitutional atoms at the cementite/austenite interface and turns diffusion to a sluggish process. This fact, combined to the infinitesimal time for diffusion, forms an inhomogeneous carbon distribution along the microstructure. This inhomogeneity is further enhanced by the presence of increased carbides’ size present in the initial microstructure. Due to rapid heating, these carbides cannot be decomposed since the diffusion distance of alloying elements increases and the diffusion of alloying elements is impeded during ultrafast heating, thus, remain undissolved at peak austenitization temperature. Their presence and effect in heterogeneous ferrite nucleation restrict austenite grain growth. Consequently, fine austenite grains in conjunction with their chemical heterogeneity lead to the coexistence of fine martensite, bainite laths and undissolved carbides in the final microstructure after quenching.
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Rubešová, Kateřina, Ivan Vorel, Hana Jirková, and Štěpán Jeníček. "EFFECTS OF Q&P PROCESS PARAMETERS ON PROPERTIES OF 42SiCr STEEL." Acta Metallurgica Slovaca 24, no. 2 (2018): 126. http://dx.doi.org/10.12776/ams.v24i2.1063.
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<p class="AMSmaintext"><span lang="EN-GB">The requirement for high strength and good ductility poses problems in today’s advanced steels. This problem can be tackled by appropriate heat treatment which produces suitable microstructures. By this means, ultimate strengths of about 2000 MPa and elongations of more than 10% can be obtained. One of such advanced heat treatment techniques is the Q&amp;P (Quenching and Partitioning) process. It produces a mixture of martensite and retained austenite, where the latter is an important agent in raising the ductility of steel. </span></p><p class="AMSmaintext"><span lang="EN-GB">In this experiment, a low-alloy steel with 0.41% carbon and manganese, silicon and chromium was used. An air furnace and a salt bath were employed for heat treatment and quenching, respectively. In order to obtain the best ultimate strength and elongation levels, partitioning temperatures of 250°C and 300°C were applied. Partitioning involves carbon diffusion from super-saturated martensite into retained austenite, and tempering of hardening microstructure. Effects of the quenching temperatures of 200°C and 150°C were studied as well. To map the impact of the Q&amp;P process on mechanical properties, an additional schedule with conventional quenching and tempering was carried out. Upon optimization of the parameters, the process produced martensite with a small amount of bainite and retained austenite. The ultimate strength was between 1930 and 2080 MPa and the elongation levels were from 9 to 16%.</span></p><p class="AMSmaintext"><span lang="EN-GB"> </span></p>
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Kucukomeroglu, T., and S. M. Aktarer. "Microstructure, microhardness and tensile properties of FSWed DP 800 steel." Journal of Achievements in Materials and Manufacturing Engineering 2, no. 81 (2017): 56–60. http://dx.doi.org/10.5604/01.3001.0010.2038.
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Purpose: Dual phase (DP) steels are widely used in the automotive industry due to their properties of a high balance of strength and formability. However, it is known that conventional welding of high strength steel leads to some undesirable results such as hardness decrease in the heat affected zone. Friction stir welding (FSW) is a new solid state joining method, which is used to join these steels due to its advantage of low heat input. The aim of this study is to evaluate the microstructural change and mechanical properties of friction stir welded DP800 steel. Design/methodology/approach: DP 800 steels with 1.5 mm thickness were subjected to friction stir welding, by using a tungsten carbide (WC) tool. The tool was tilted 2°, and downforce of the tool was kept constant at 6 kN. During processing, the tool rotation and traverse speed were fixed at 1600 rpm and 170 mm∙min-1, respectively. Findings: The friction stir welded region comprises martensite, bainite, refined ferrite. The average microhardness of stir zone has increased from 260 HV0.2 to about 450 HV0.2. The tensile sample shows a decrease in the ultimate tensile strength (σUTS) about 3%, from 827 MPa to 806 MPa for the joint. The yield strength (YS) of the joint is about 566 MPa and the value is near that of DP800. Research limitations/implications: The tungsten carbide tool used for the friction stir welding has suffered deterioration in the pin profile after 1 meter welding operation. It may be advisable to drill a pre-hole in the specimens for a longer tool life. Practical implications: Tool wear for industrial applications will be a major problem. Therefore, the use of tools with high wear resistance such as polycrystalline cubic boron nitride may be recommended. Originality/value: Works on friction stir welding of dual phase steels are limited and they mostly focus on spot welding. Also, this study systematically investigates the microstructure and mechanical properties of dual-phase 800 steels after the friction stir welding.
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Bhattacharya, Debanshu. "Niobium Containing Advanced High Strength Steels for Automotive Applications – Processing, Microstructure, and Properties." Materials Science Forum 773-774 (November 2013): 325–35. http://dx.doi.org/10.4028/www.scientific.net/msf.773-774.325.
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Two major drivers for the use of advanced steels in the automotive industry are fuel efficiency and increased safety performance. Fuel efficiency is mainly a function of weight of steel parts, which in turn, is controlled by gauge and design. Safety is determined by the energy absorbing capacity of the steel used to make the part. All of these factors are incentives for the automobile manufacturers to use Advanced High Strength Steels (AHSS) to replace the conventional steels used to manufacture automotive parts in the past. AHSS is a general term used to describe various families of steels. The most common AHSS is the dual-phase steel that consists of a ferrite-martensite microstructure. These steels are characterized by high strength, good ductility, low tensile to yield strength ratio and high bake-hardenability. Another class of AHSS is the complex-phase or multi-phase steel which has a complex microstructure consisting of various phase constituents and a high yield to tensile strength ratio. Transformation Induced Plasticity (TRIP) steels is another class of AHSS steels finding interest among the U.S. automakers. These steels consist of a ferrite-bainite microstructure with significant amount of retained austenite phase and show the highest combination of strength and elongation, so far, among the AHSS in use. High level of energy absorbing capacity combined with a sustained level of high n value up to the limit of uniform elongation as well as high bake hardenability make these steels particularly attractive for safety critical parts and parts needing complex forming. A relatively new class of AHSS is the Quenching and Partitioning (Q&P) steels. These steels seem to offer higher ductility than the dual-phase steels of similar strengths or similar ductility as the TRIP steels at higher strengths. Finally, martensitic steels with very high strengths are also in use for certain parts. The most recent initiative in the area of AHSS is the so-called 3rd Generation AHSS. These steels are designed to fill the region between the dual-phase/TRIP and the Twin Induced Plasticity (TWIP) steels with very high ductility at strength levels comparable to the conventional AHSS. Enhanced Q&P steels may be one method to achieve this target. Other ideas include TRIP assisted dual phase steels, high manganese steels and higher carbon TRIP type steels. In this paper, some of the above families of advanced high strength steels for the automotive industry will be discussed with particular emphasis on the role of niobium.
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Lee, Ki Myung, and Andreas A. Polycarpou. "Wear of conventional pearlitic and improved bainitic rail steels." Wear 259, no. 1-6 (2005): 391–99. http://dx.doi.org/10.1016/j.wear.2005.02.058.
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