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1
Abudaia, Fouad Belgassem. "Microstructure and fatigue strength of high performance gear steels." Thesis, University of Newcastle Upon Tyne, 2003. http://hdl.handle.net/10443/176.
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Observations on some steels used in high performance gears are presented in this thesis. The object was to understand how microstructure and residual stress influenced mechanical properties, particularly fatigue strength. The investigations were carried out using fatigue testing, metallographic techniques, shot peening, hardness testing and Xray diffraction to determine residual stress and the amount of retained austenite. The work is divided into two main parts. In the first part, the opportunity was taken of investigating a batch of case-carburised gears manufactured from 17CrNiMo6 steel that contained an abnormally high level of retained austenite. . The benefits or otherwise of retained austenite is a matter of some controversy in the literature and this was an opportunity of testing the effect of high retained austenite in gears. In the second part, the properties of a recently formulated through hardening steel was investigated. The steel is inexpensive and capable of being fully hardened by air-cooling. It is believed that use of the material could reduce the cost of gear manufacture by eliminating the carburisation process, oil quenching and the subsequent need to correct distortions that arise from quenching. Gears were manufactured using l7CrNiMo6 steel. After carburization, one batch was found to have an abnormally high level of retained austenite (40 to 60%). Normally, gears with this level of retained austenite would be rejected. Nevertheless, gears from this batch were found to have surface and bending fatigue strengths that were not very different from those with much lower retained austenite contents. Despite the fact that the material is relatively soft, it was found that a very thin surface layer had hardened during back-to-back gear testing, probably by strain-induced transformation of the retained austenite. It was deduced that this thin layer of hardened material is sufficient to maintain a good level of surface contact fatigue strength. The second part of the work includes a basic study of the through-hardening steel. The study includes heat treatment and hardness, estimation of fracture toughness and the development of residual stress during heat treatment. Specialised standing contact fatigue (SCF) tests were also done using this material. The SCF test consists of cyclic loading of a hard ball in contact with the surface of the specimen, which is meant to simulate asperity contact in surface contact fatigue. Radial cracks or ring cracks or both are nucleated just outside the indentation circle in these tests, depending on the load and the development of plasticity. The stresses near the indentation were modelled using Finite Element analysis and were found to be consistent with the experimental results. Finally, the effect of residual compressive stress induced by shot peeing was studied using the SCF test. It was found that shot peening suppressed the formation of radial cracks.
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Vale, Ellen. "Interval timing and dopamine : effects of attention deficit hyperactivity disorder and caffeine on the reproduction of short intervals." Thesis, University of Newcastle Upon Tyne, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.273680.
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Saha, Podder Arijit. "Tempering of a mixture of bainite and retained austenite." Thesis, University of Cambridge, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.609405.
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Yalci, Hasan Kayhan. "The effect of retained austenite on hydrogen enbrittlement in high strength steels." Thesis, University of Oxford, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.315785.
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Mark, Alison Fiona Lockie. "Microstructural effects on the stability of retained austenite in transformation induced plasticity steels." Thesis, Kingston, Ont. : [s.n.], 2007. http://hdl.handle.net/1974/960.
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Adams, Derrik David. "Characterization of the Factors Influencing Retained Austenite Transformation in Q&P Steels." BYU ScholarsArchive, 2020. https://scholarsarchive.byu.edu/etd/8425.
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Formable Advanced High-Strength Steels (AHSS) have a unique combination of strength and ductility, making them ideal in the effort to lightweight vehicles. The AHSS in this study, Quenched and Partitioned 1180, rely on the Transformation Induced Plasticity (TRIP) effect, in which retained austenite (RA) grains transform to martensite during plastic deformation, providing extra ductility via the transformation event. Understanding the factors involved in RA transformation, such as local strain and grain attributes, is therefore key to optimizing the microstructure of these steels. This research seeks to increase understanding of those attributes and the correlations between microstructure and RA transformation in TRIP steels. To measure local strain, the viability of using forescatter detector (FSD) images as the basis for DIC study is investigated. Standard FSD techniques, along with an integrated EBSD / FSD approach (Pattern Region of Interest Analysis System), are both analyzed. Simultaneous strain and microstructure maps are obtained for tensile deformation up to around 6% strain. The method does not give sub-grain resolution, and surface feature evolution prevents DIC analysis across large strain steps; however, the data is easy to obtain and provides a natural set of complementary information for the EBSD analysis. In-situ tensile tests combined with EBSD allow RA grain and neighboring attributes to be characterized and corresponding transformation data to be obtained. However, pseudo-symmetry of the ferrite (BCC) and martensite (BCT) phases prevents EBSD from accurately identifying all phases. Measuring the relative distortion of the crystal lattice, tetragonality, is one approach to identifying the phases. Unfortunately, small errors in the pattern center can cause significant errors in tetragonality measurement. Therefore, this research utilizes a new approach for accurate pattern center determination using a strain minimization routine and applies it to tetragonality maps for phase identification. Tetragonality maps based on dynamically simulated patterns result in the most accurate maps and can also be used to predict approximate local carbon content. Machine learning is then used on the collected data to isolate key attributes of RA grains and provide a decision tree model to predict transformation based on those attributes. Among the most relevant attributes found, RA grain area, RA grain shape aspect ratio, a “hardness” factor, and major axis orientation are included. Possible correlations between these factors and transformation improve understanding of relevant attributes and show the advantage that machine learning can have in unravelling complex material behavior.
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Malmberg, Andreas. "The influence of carbonitriding on hardness, retained austenite and residual stress in 52100 steel." Thesis, KTH, Materialvetenskap, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-173804.
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High rolling contact fatigue parts are vital for the long service life of fuel pumps. Cummins Fuel Systems are currently using an M2 tool steel for one of the most important roller bearing application in their pumps, namely the cam follower. The future design of the cam follower is a pin-less tappet roller. The wear and fatigue properties of the roller is vital to ensure reliability of the fuel system. M2 tool steel is an expensive material and becomes even more so if diamond like coating (DLC) is needed to decrease the friction coefficients. To cut costs of the fuel pump it might be possible to replace the M2 tool steel with 52100 steel (100Cr6). Competitive methods have proven that carbonitrided 52100 can reach excellent wear and fatigue properties making it a candidate to replace M2 tool steel. How the properties of hardness, toughness and compressive residual stresses are developed in 52100 and how they affect the fatigue and wear resistance has been researched from the literature. A big part of this project was to do an extensive analysis of a roller bearing that was believed to have gone through one of these competitive methods that produce excellent wear and fatigue resistance. The analysis was done with background to the knowledge gathered from the literature. Finally process trials were set up to carbonitride 52100 steel samples. The trials were done to develop a better understanding of how adding carbon together with nitrogen to the surface of 52100 steel will influence the metallurgical parameters that results in good wear and fatigue resistance. From this analysis Cummins hope to create a process recipe that can be used for carbonitriding the cam follower and maybe other components in their fuel systems.
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Björklund, Erik. "The Influence of Hardness and Retained Austenite on the Fatigue Limit after Shot Peening." Thesis, Uppsala universitet, Institutionen för kemi - Ångström, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-221610.
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This thesis was performed at Scania CV AB and aimed to optimize the effect of shot peening in order to increase the bending fatigue limit of planetary gears. In this project three parameters affecting the performance of the shot peening were examined in more detail, specifically the hardness of the shot peening media, the hardness of the material and the amount retained austenite. Shot peening and measurements were performed on carburized gears consisting of ss 92506 steel. In order to find out how the fatigue limit is affected by the material hardness, a number of gears were tempered whereas other remained untempered. Similarly the retained austenite content was altered in some of the gears with a cryotreatment. The shot peening of these gears was performed by dual shot peening using media of two different hardness. The fatigue limit was evaluated by a servo hydraulic material testing machine. The results show that the fatigue limit is enhanced when the shot peening was performed with hard media. This was expected since the harder media can cause larger plastic deformation in the material giving rise to larger compressive residual stresses which is known to increase the fatigue limit. The fatigue limit was also improved for samples with lower surface hardness. The amount of retained austenite does not seem to affect the fatigue limit for samples shot peened with media of sufficient hardness in order to cause adequate plastic deformation. The surface roughness remained unchanged for samples shot peened by different media, which indicates that this parameter should not hinder a change of media.
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Qu, Hao. "Advanced High Strength Steel Through Paraequilibrium Carbon Partitioning and Austenite Stabilization." Case Western Reserve University School of Graduate Studies / OhioLINK, 2011. http://rave.ohiolink.edu/etdc/view?acc_num=case1283353953.
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Huamán, Raúl Minaya. "Avaliação do tratamento criogênico na desestabilização da austenita retida no aço AISI D2." Universidade de São Paulo, 2017. http://www.teses.usp.br/teses/disponiveis/18/18158/tde-06122017-110813/.
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O processo de tratamento térmico à baixa temperatura é um dos métodos mais promissores para melhorar o desempenho dos materiais. O tratamento criogênico promove a transformação de austenita retida do aço em martensita, o que é atribuída para melhorar a dureza e resistência ao desgaste. Neste trabalho foram analisados os efeitos dos diferentes ciclos de tratamentos térmicos, comparando-se à tempera convencional (têmpera 1050°C + revenido simples e duplo a 200/530°C) respeito à adição do tratamento criogênico, (têmpera 1050°C + criogenia a -125°C + revenido simples e duplo a 200/530°C) com e sem tempo de espera de 24 horas, com a finalidade de avaliar a estabilização térmica da austenita retida no aço em relação a quantidade presente na microestrutura e consequentemente na influencia nas propriedades mecânicas do aço ferramenta para trabalho a frio AISI D2. As análises foram conduzidas através de testes de dureza, impacto, microscopia óptica, microscopia eletrônica de varredura e difração de raios-X. Os resultados encontrados foram uma variação pouco significativa na dureza entre 57 e 58 HRC. Foi evidenciada a baixa tenacidade ao impacto do aço AISI D2, independente das rotas dos ciclos de tratamento térmico, resultado da alta percentagem de carbonetos dispostos na microestrutura. A resistência ao impacto no aço em estudo após o tratamento criogênico, esses resultados foram relacionados à microestrutura do material.The heat treatment process at low temperature is one of the most promising methods to improve the performance of materials. The cryogenic treatment promotes the transformation of retained austenite from the steel into martensite, which is attributed to improved hardness and wear resistance. In this work the effects of the different cycles of thermal treatments were analyzed, comparing to conventional tempering (tempering 1050°C + single and double annealing at 200/530°C) with respect to the addition of the cryogenic treatment (tempera 1050°C + cryogenics to - 125°C + single and double tempering at 200/530°C) with and without waiting time of 24 hours, in order to evaluate the thermal stabilization of the austenite retained in the steel in relation to the amount present in the microstructure and consequently in the influence on the mechanical properties of cold working tool steel AISI D2. The analyzes were conducted through tests of hardness, impact, optical microscopy, scanning electron microscopy and X-ray diffraction. The results found were a minor variation in hardness between 57 and 58 HRC. It was evidenced the low impact toughness of the AISI D2 steel, independent of the thermal treatment cycle routes, as a result of the high percentage of carbides disposed in the microstructure. The impact resistance in the steel studied after the cryogenic treatment, these results was related to the microstructure of the material.
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Chelladurai, Isaac. "Characterization of Phase Transformation and Twin Formation in Automotive Sheet Metal Alloys to Quantify and Understand Their Impact on Ductility." BYU ScholarsArchive, 2019. https://scholarsarchive.byu.edu/etd/8628.
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The motivation to use lightweight materials in the construction of the automotive structure is the resultant increased fuel efficiency. However, these materials possess certain drawbacks that make it challenging to adopt them into current automobile manufacturing processes. In this dissertation the microstructural response observed in a magnesium alloy, AZ31, and an advanced high strength steel alloy, QP1180, to uniaxial deformation is analyzed and the results are presented. In AZ31 the required slip modes are not activated at room temperature leading to its low ductility at room temperature. The resulting activity of these twins in response to uniaxial tension is analyzed and its correlations with the microstructure features is reported. Additionally, a neighborhood viscoplastic self-consistent model is developed that will allow more accurate simulation of twin response to outside deformation. Furthermore, activity of slip modes that are usually observed at high temperatures (>200°C) are also observed at lower temperatures (<125°C) and they are compared to the relative twin activity at these temperatures. It is observed that larger grains, with high schmid factors, longer grain boundaries and have misorientation with its neighboring grain greater than 27° are more favorable for twin formation and transmission in the AZ31 microstructure in response to uniaxial tension. The nature of retained austenite (RA) transformation into martensite that gives QP1180 its enhanced ductility, is not clearly understood primarily because of challenges present in characterization of these metastable RA. Further, a 2 dimensional characterization method does not provide the complete information of the RA grain. These challenges are overcome by characterization of a 3 dimensional volume element using serial sectioning and EBSD followed by reconstruction using DREAM3D. The influence of 3d morphology and orientation direction on RA transformation is studied using as-is and uniaxially deformed samples. A novel shear affinity factor is introduced as a metric to describe the ease of RA transformation under uniaxial tension. The 3d nature of the information collected allows a new classification of disk shape in addition to globular and lamellar shapes for RA. It is found that RA that are low volume laths and have low shear affinity factor transform later compared to disk shaped RA’s. Through these guidelines the preparation of a microstructure that is conducive to RA transformation under uniaxial tension is possible.
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Gibbs, Parker Kenneth. "Strain Path Effect on Austenite Transformation and Ductility in TBF 1180 Steel." BYU ScholarsArchive, 2019. https://scholarsarchive.byu.edu/etd/7127.
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TBF 1180 steel was studied under various conditions focusing on the correlation of ductility and amount of retained austenite. Samples were prepared from sheet stock and then strained using limiting dome height tooling (LDH), a standard uniaxial test frame, and a tensile stage for use in an electron microscope. The steel was observed in plane, biaxial, and uniaxial strain to determine its effect on retained austenite transformation and ultimately, ductility. Retained austenite was observed using a scanning electron microscope (SEM) equipped with an electron backscatter detector (EBSD) to distinguish the different phases present. Initial austenite levels were around 5% by volume and was quickly reduced as the sample was strained. The biaxial samples were the slowest to transform, having about 2.5% austenite at .05 effective strain, which allowed the specimen to reach an effective strain of .3 with 1.1% austenite remaining. In contrast, the plane strain samples had the fastest rate of transformation having only 1.2% austenite at .05 effective strain and .7% austenite at a final effective strain of .18. Both forms of uniaxial, (in-situ and ex-situ), were near identical, as expected, and exhibited an austenite transformation curve between that of the plane and biaxial curves. The uniaxial austenite level at .05 strain was 2.1% and was able to reach about .15 strain with a final austenite percentage around 1%. It was concluded that the biaxial strain path had the greatest ductility due in part to its slower austenite transformation rate while plane and uniaxial strain paths were not as ductile with their faster austenite transformation rates.
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Pešina, Zbyněk. "Analýza deformačně indukovaných změn fázového složení oceli TRIP metodou EBSD." Master's thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2008. http://www.nusl.cz/ntk/nusl-228272.
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The diploma thesis deals with phase composition measurement of the TRIP steel, using EBSD method. The steel was delivered as thermo-mechanically treated via two different routes. The phase composition of the steel was examined during gradual plastic deformation in the range 0 to10.99%. One route of thermo-mechanical treatment exhibited good agreement with the literature in terms of measured fraction of the retained austenite (15.6%) as well as its decrease during the deformation (to 8.9% at the maximum imposed strain). The samples of the second route did not show any agreement in either of the parameters spoken.
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Cramer, Jeffrey Grant. "Strain Path Effect on Austenite Transformation and Ductility in Q&P 1180 Steel." BYU ScholarsArchive, 2017. https://scholarsarchive.byu.edu/etd/6650.
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The ductility of Q&P 1180 steel was studied with regard to retained austenite transformation under different strain paths. Specimens were tested in uniaxial tension in a standard test frame as well as in situ in the scanning electron microscope (SEM). Then digital image correlation (DIC) was used to compute the effective strain at the level of the individual phases in the microstructure. Stretching experiments were also performed using limiting dome height (LDH) tooling, where specimens were strained in both biaxial and plane strain tension. The experiments were done incrementally, for each strain path, and the retained austenite at each level of strain was measured using electron backscatter diffraction (EBSD). Retained austenite levels in the uniaxial tension case dropped from an initial measured level of about 8% to about 2% during an initial strain increment of 0.02, but then stabilized as the specimen was strained to 0.1. In the plane strain and biaxial tension cases retained austenite also dropped significantly during an initial strain increment of about 0.04, but then continued to decrease as the specimens were strained to failure. Biaxial tension, in particular, was the most effective strain path for transforming retained austenite to martensite, resulting in a final volume fraction of 0.3% at an effective strain of 0.3. Retained austenite in the plane-strain tension case dropped at a faster rate than in the biaxial tension case, but finished at about 1% at a strain of 0.1. The greatest limit strains were seen in the biaxial tension case, which may be partly explained by the more effective conversion of retained austenite than was seen in the uniaxial tension case.
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Silva, Valdinei Ferreira da. "Influência da austenita retida no crescimento de trincas curtas superficiais por fadiga em camada cementada de aço SAE 8620." Universidade de São Paulo, 1997. http://www.teses.usp.br/teses/disponiveis/88/88131/tde-25072012-113812/.
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A austenita retida está sempre presente na microestrutura de camada cementada de aços, em maior ou menor quantidade. Como é uma fase dúctil comparada à martensita, sua presença tem sido alvo de muita controvérsia. Este trabalho apresenta um estudo sobre a influência da austenita retida na propagação de trincas curtas por fadiga em camada cementada de aço SAE 8620. Foram feitos ensaios de fadiga por flexão em quatro pontos, a temperatura ambiente, em corpos de prova sem entalhe com três níveis de amplitude de tensão e razão de tensões de 0,1. Através de diferentes ciclos de cementação e tratamentos térmicos, foram obtidas camadas cementadas com quatro níveis de austenita retida na microestrutura. O teor de austenita retida foi medido através da técnica de difração de Raios-X. Trincas superficiais foram monitoradas por meio da técnica de réplicas de acetato. Como resultados foram obtidos tamanho de trinca em função do número de ciclos e taxa de crescimento de trincas curtas. Corpos de prova com maiores níveis de austenita retida apresentaram maior vida em fadiga.The retained austenite is always present in case carburized steel microstructure in small or high percentages. Since it is a ductile phase, its presence has long been a controversial subject. The influence of retained austenite on short fatigue crack propagation in case carburized SAE 8620 steel was studied in this work. Four-point-bend fatigue tests were carried out at room temperature in specimens without notch using three levels of stress range and a stress ratio of 0.1. Four different amount of retained austenite in the case carburized microstructure were obtained through different cycles of carburizing and heat treating. The retained austenite content was measured by X-ray technique, and the surface short crack growth was monitored by means of acetate replication technique. Crack length versus number of cycles and crack growth rate versus mean crack length were obtained as results. Specimens with higher levels of retained austenite in the carburized case showed longer fatigue life.
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Huyghe, Pierre. "ON THE RELATIONSHIP BETWEEN MICROSTRUCTURE DEVELOPMENT AND MECHANICAL PROPERTIES IN Q&P STEELS." Doctoral thesis, Universite Libre de Bruxelles, 2018. https://dipot.ulb.ac.be/dspace/bitstream/2013/278740/5/contratPH.pdf.
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The Quenching and Partitioning (Q&P) heat treatment has been proposed in the early 2000s to produce cold-rolled sheets combining high-strength and formability for the automotive market. Q&P consists, first, of an interrupted quench between the martensite-start temperature (Ms) and the martensite-finish temperature (Mf) from intercritical annealing or full austenitization in order to form controlled fractions of martensite. This is followed by a partitioning step in order to stabilize the untransformed austenite through carbon enrichment. In order to maximize the carbon transfer from martensite to austenite, the use of specific alloying elements and the design of appropriate Q&P parameters are required to eliminate or minimize competing phenomena such as carbide formation and austenite decomposition. The final quenched and partitioned microstructure, using full austenitization, ideally consists of carbon-depleted lath martensite and significant fractions of retained austenite providing an improved combination of strength and ductility. Hence, the transformation of retained austenite upon straining at room temperature (TRIP effect) provides supplementary work-hardening and eventually improves the ductility. In the present work, Quenching and Partitioning (Q & P) heat treatments were carried out in a quench dilatometeron a 0.2 wt% carbon steel. The microstructure evolution of the Q & P steels was characterized usingdilatometry, SEM, EBSD and XRD. The martensitic transformation profile was analyzed in order to estimate thefraction of martensite formed at a given temperature below the martensite start temperature Ms. Q & P wasshown to be an effective way to stabilize retained austenite at room temperature. However, the measuredaustenite fractions after Q & P treatments showed significant differences when compared to the calculated valuesconsidering ideal partitioning conditions. Indeed, the measured austenite fractions were found to be less sensitiveto the quench temperature and were never larger than the ideal predicted maximum fraction. Competitivereactions such as austenite decomposition into bainite and carbide precipitation were found to occur in thepresent work.Furthermore, a broad range of mechanical properties was obtained when varying the quenching temperaturesand partitioning times. The direct contributions between Q & P microstructural constituents -such as retainedaustenite as well as tempered/fresh martensite- and resulting mechanical properties were scrutinized. This wascritically discussed and compared to quenching and austempering (QAT) which is a more conventional processingroute of stabilizing retained austenite at room temperature. Finally, Q & P steels were shown to exhibit aninteresting balance between strength and ductility. The achievement of this interesting combination of mechanicalproperties was reached for much shorter processing times compared to QAT steels.Doctorat en Sciences de l'ingénieur et technologie
info:eu-repo/semantics/nonPublished
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Katemi, Richard Jackson [Verfasser], Franz [Akademischer Betreuer] Hoffmann, Franz [Gutachter] Hoffmann, and Udo [Gutachter] Fritsching. "Influence of Carbonitriding Process on Phase Transformation during Case Hardening, Retained Austenite and Residual Stresses / Richard Jackson Katemi ; Gutachter: Franz Hoffmann, Udo Fritsching ; Betreuer: Franz Hoffmann." Bremen : Staats- und Universitätsbibliothek Bremen, 2019. http://d-nb.info/1199003603/34.
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Georges, Cédric. "Improvement of the mechanical properties of TRIP-assisted multiphase steels by application of innovative thermal or thermomechanical processes." Université catholique de Louvain, 2008. http://edoc.bib.ucl.ac.be:81/ETD-db/collection/available/BelnUcetd-08232008-100716/.
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For ecological reasons, the current main challenge of the automotive industry is to reduce the fuel consumption of vehicles and then emissions of greenhouse gas. In this context, steelmakers and automotive manufacturers decided for some years now to join their efforts to promote the development and use of advanced high strength steels such as TRIP steels. A combination of high strength and large elongation is obtained thanks to the TRansformation Induced Plasticity (TRIP) effect. However, improvement of the mechanical properties is still possible, especially by the refinement of the matrix. In this work, two main ways were followed in order to reach improved properties. The classical way consisting of the annealing of cold-rolled samples and an innovative way consisting of obtaining the desired microstructure by direct hot rolling of the samples. In the classical way, this refinement can be obtained by acting on the chemical composition (with such alloying elements like Cu and Nb). It was observed that complete recrystallisation of the ferrite matrix is quite impossible in presence of Cu precipitates. In addition, if the ferrite recrystallisation is not completed before reaching the eutectoid temperature, the recrystallisation will be slowed down by a large way. An innovative heat treatment consisting in keeping the copper in solid solution in the high-Cu steel was developed. Therefore, ferrite recrystallises quite easily and very fine ferrite grains (~1µm) were obtained. In the innovative way, the effects of hot-rolling conditions on TRIP-assisted multiphase steels are of major importance for industrial practice and could open new dimensions for the TRIP steels (i.e. thanks to precipitation mechanisms leading to additive strengthening). Impressive mechanical properties (true stress at maximum load of 1500 MPa and true strain at uniform elongation of 0.22) were obtained with a relatively easy thermomechanical process, the role played by Nb being essential.
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Bedekar, Vikram. "Nanostructural Evolution of Hard Turning Layers in Carburized Steel." The Ohio State University, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=osu1366195383.
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Boneti, Ludiere Lucas Toldo. "INFLUÊNCIA DAS TEMPERATURAS DE AUSTENITIZAÇÃO E AUSTÊMPERA NA MICROESTRUTURA E PROPRIEDADES DE TRAÇÃO DE UM FERRO FUNDIDO NODULAR." UNIVERSIDADE ESTADUAL DE PONTA GROSSA, 2014. http://tede2.uepg.br/jspui/handle/prefix/1471.
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Previous issue date: 2014-08-29The present study aimed to evaluate the influence of the austempering heat treatment parameters on microstructure and mechanical properties of a ductile cast iron produced in industrial conditions, containing 3.59% carbon, 2.68% silicon, 0.46% copper (in wt%) and a carbon equivalent of 4.50%. The material was produced in a foundry located in the southwest region of Parana state, Brazil. The specimens were obtained by casting the alloy into Y-block molds. The austempering heat treatments consisted of pre-heating at 500°C, followed by austenitizing step at 870°C, 900°C and 930°C during 60 minutes. Austempering was carried out in molten metal baths at temperatures of 300°C and 370°C for 30 minutes. Microstructural characterization was carried out by light optical microscopy (LOM) with image analysis, scanning electron microscopy (SEM-FEG) and X-ray diffraction with Rietveld refinement. The mechanical properties were evaluated by tensile and Vickers hardness tests. The as-cast microstructure displayed a very heterogeneous microstructure, characterized by the presence of regions with graphite flotation and carbide containing intercellular regions. The graphite nodules showed low nodularity, of 85%, which was attributed to the fading effect of magnesium alloy. Austempered samples were characterized by the presence of bainitic ferrite, interspersed with retained austenite in films and blocks. The austempering at 300°C resulted in a finer microstructure containing smaller volume fractions of retained austenite. All heat treated samples displayed transformation gradients between graphite flotation and intercellular regions. These gradients affected the mechanical properties, as well as the fracture characteristics. The best results of mechanical properties were obtained in the specimen austenitized at 900°C followed by austempering at 300°C, allowing the ADI produced to fit into a high strength class, according to ASTM A897. The study of fracture surfaces showed a sharp transition between graphite flotation and intercellular regions. The fracture at the vicinity of graphite nodules have occurred by a ductile mechanism, characterized by dimples. It was observed a rapid transition to the cleavage mode at intercellular regions, containing solidification carbides.
O presente trabalho teve como objetivo avaliar a influência dos parâmetros de tratamento térmico de austêmpera na microestrutura e nas propriedades mecânicas de tração e dureza, em uma liga de ferro fundido nodular produzida em condições industriais. A liga em estudo possui de teor de carbono de 3,59%, silício de 2,68% e adições de 0,46% de cobre, possuindo ainda um carbono equivalente de 4,50%. O material foi produzido em condições industriais, em uma fundição localizada no sudoeste do estado do Paraná. Os corpos de prova para retirada de amostras foram obtidos pelo vazamento da liga em moldes de blocos Y. Os tratamentos térmicos de austêmpera consistiram de pré-aquecimento a 500°C, seguido da etapa de austenitização a 870°C, 900°C e 930°C, por um tempo fixo de 60 minutos. A etapa de austêmpera foi realizada em banhos de metais fundidos, em temperaturas de 300°C e 370°C, durante 30 minutos. A caracterização microestrutural de amostras foi realizada pelas técnicas de microscopia ótica, com análise de imagens, microscopia eletrônica de varredura e difração de raios X, com refinamento pelo método de Rietveld. As propriedades mecânicas foram avaliadas por meio de ensaios de tração e dureza Vickers. A liga no estado bruto de fundição apresentou microestrutura bastante heterogênea, caracterizada pela presença de regiões com flotação de grafita e regiões intercelulares contendo carbetos de solidificação. Os nódulos de grafita apresentaram baixo grau de nodularização, de 85%, resultado que foi atribuído ao efeito de fadiga térmica da liga nodularizante. A microestrutura de amostras austemperadas foi caracterizada pela presença de agrupamentos de feixes de ripas de ferrita bainítica, entremeadas por austenita retida, na forma de filmes e blocos. A austêmpera a 300°C produziu microestruturas mais refinadas e com menores frações volumétricas de austenita retida. Em todas as amostras tratadas termicamente, foi observado um gradiente de transformação entre as regiões de flotação de grafita e regiões intercelulares. Estes gradientes afetaram tanto os resultados de propriedades mecânicas, como as características de fratura dos corpos de prova. Os melhores resultados de propriedades mecânicas foram obtidos para a condição de austenitização a 900°C seguida de austêmpera, permitindo enquadrar o ADI em uma classe de alta resistência, segundo a norma ASTM A897. As análises das superfícies de fratura dos corpos de prova de tração mostraram uma mudança nas características de fratura entre as regiões de flotação de grafita e as regiões intercelulares. Nas proximidades dos nódulos de grafita houve predominância do mecanismo de fratura dúctil, caracterizada pela presença de cavidades alveolares (“dimples”), com rápida transição para o modo de fratura por clivagem nas regiões com baixos números de nódulos de grafita, contendo carbetos de solidificação.
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Irshad, Muhammad Aatif. "The effect of prior austenite grain size on the machinability of a pre-hardened mold steel. : Measurement of average grain size using experimental methods and empirical models." Thesis, Karlstads universitet, Avdelningen för maskin- och materialteknik, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:kau:diva-8777.
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The use of pre-hardened mold steels has increased appreciably over the years; more than 80% of the plastic mold steels are used in pre-hardened condition. These steels are delivered to the customer in finished state i.e. there is no need of any post treatment. With hardness around ~40HRC, they have properties such as good polishability, good weldability, corrosion resistance and thermal conductivity. Machinability is a very important parameter in pre-hardened mold steels as it has a direct impact on the cost of the mold. In normal machining operations involving intricate or near net shapes, machining constitutes around 60% of the total mold cost. Efforts are underway to explore every possible way to reduce costs associated with machining and to make production more economical. All the possible parameters which are considered to affect the machinability are being investigated by the researchers. This thesis work focuses on the effect of prior austenite grain size on the machinability of pre-hardened mold steel (Uddeholm Nimax). Austenitizing temperatures and holding times were varied to obtain varying grain sized microstructures in different samples of the same material. As it was difficult to delineate prior-austenite grain boundaries, experimental and empirical methods were employed to obtain reference values. These different grain sized samples were thereafter subjected to machining tests, using two sets of cutting parameters. Maximum flank wear depth=0.2mm was defined for one series of test which were more akin to rough machining, and machining length of 43200mm or maximum wear depth=0.2mm were defined for second series of tests which were similar to finishing machining. The results were obtained after careful quantative and qualitative analysis of cutting tools. The results obtained for Uddeholm Nimax seemed to indicate that larger grain sized material was easier to machine. However, factors such as retained austenite content and work hardening on machined surface, which lead to degradation of machining operations were also taken into consideration. Uddeholm Nimax showed better machinability in large grained samples as retained austenite(less than 2%) content was minimal in the large grained sample. Small grained sample in Uddeholm Nimax had a higher retained austenite (7+2%) which resulted in degradation of machining operation and a lesser cutting tool life.
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Korpala, Grzegorz. "Gefügeausbildung und mechanische Eigenschaften von unlegiertem bainitischem Warmband mit Restaustenit." Doctoral thesis, Technische Universitaet Bergakademie Freiberg Universitaetsbibliothek "Georgius Agricola", 2017. http://nbn-resolving.de/urn:nbn:de:bsz:105-qucosa-229501.
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Seit vielen Jahren wächst die Nachfrage bezüglich sparsamer Fahrzeuge; die Autohersteller konkurrieren miteinander und werben mit neuen Fahrzeugkonzepten, in denen hochmoderne Werkstoffe ihre Anwendung finden. In dieser Arbeit werden Legierungskonzepte und entsprechende Warmwalztechnologien einer ultrahochfesten bainitischen Stahlsorte mit Restaustenit vorgestellt, die der genannten Anwendung angepasst werden können. Der gewählte Werkstoff gehört zu den Stählen mit mittleren Kohlenstoffgehalten, die sich nach der - im Rahmen dieser Arbeit entwickelten - Behandlung durch hohe Zugfestigkeit bei vergleichsweise hoher Bruchdehnung auszeichnen. Es werden erweiterte Modelle zur Beschreibung der Phasenumwandlung von Stählen im Bainitgebiet vorgestellt. Die Ergebnisse aus den Experimenten wurden genutzt, um die Modelle zu ergänzen und zu evaluieren. Dabei wird nicht nur der Warmwalzprozess, sondern auch die chemische Zusammensetzung der Stähle selbst optimiert. Die hier präsentierte Arbeit erstreckt sich über die gesamte Produktionskette und zeigt geeignete Herstellungsbedingungen, die in Betriebsanlagen leicht realisierbar sind und umgesetzt wurden.
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Ferrer, Modesto Hurtado. "Estudo das transformações de fase de aços TRIP ao Si-Mn microligados com Nb." Universidade de São Paulo, 2003. http://www.teses.usp.br/teses/disponiveis/3/3133/tde-05082003-115928/.
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Estudou-se a cinética das transformações de fase em resfriamento contínuo e em tratamentos isotérmicos de cinco ligas de aços TRIP microligados com Nb, contendo teores variáveis de Mn e Si, através de ensaios dilatométricos, de caracterização morfológica dos produtos de transformação e de cálculos termodinâmicos e simulações numéricas usando os programas Thermocalc ® e Dictra®. Foram determinados os diagramas RC para a transformação da austenita, e foi estudada a influência da precipitação de ferrita pró-eutetóide e de bainita na fração volumétrica de austenita retida. Através dos diagramas de resfriamento contínuo foi possível delimitar a extensão do campo intercrítico dos cinco aços analisados, com determinação da janela de resfriamento e seus intervalos de temperaturas. Isso permitiu projetar os ciclos de resfriamento controlado a serem aplicados durante o processamento termomecânico dos Aços TRIP-D, TRIP-E e TRIP-H. Os cálculos pelo modelo numérico de redistribuição de carbono e de elementos substitucionais na interface ferrita/austenita, bem como as medidas de microanálise química por WDS e EDS permitiram verificar que a taxa de crescimento da ferrita pró-eutetóide é controlada pela difusão do carbono na austenita. Para tempos curtos de tratamento, o modelo de crescimento que melhor se ajusta é o do equilíbrio local com partição negligível de soluto. Verificou-se através de tratamentos isotérmicos no campo bainítico, que o silício atrasa a precipitação de carbonetos durante a reação bainítica, o que justifica o aumento da estabilidade da austenita retida no aço de maior Si (TRIP-H), quando comparado com o aço de menor Si (TRIP-E). Baseado nos resultados dos estudos das transformações de fase por resfriamento contínuo foram selecionadas as ligas TRIP-D, TRIP-E e TRIP-H, para simular dois esquemas de laminação controlada por meio de ensaios de torção a quente. Nesses ensaios foram variados o grau de deformação e a temperatura de acabamento, de modo a estudar os efeitos dos parâmetros de deformação mecânica na fração transformada dos diferentes constituintes microestruturais, e em particular na fração volumétrica de austenita retida. O primeiro ensaio refere-se à laminação controlada por recristalização estática (LCRE) e o segundo à laminação convencional (LCC), com temperatura de acabamento de 1030°C e 850°C, respectivamente. O resfriamento consistiu em dois tratamentos isotérmicos consecutivos: o primeiro no campo intercrítico (austenita + ferrita), e o segundo no campo bainítico. O aumento do grau de deformação na simulação por torção a quente da laminação controlada por recristalização estática, levou a um aumento da porcentagem de austenita retida obtida durante o resfriamento controlado (de 9 a 14,0 %). O acúmulo de energia de deformação abaixo da TNR na simulação do processo de laminação controlada convencional provocou uma diminuição da fração volumétrica de austenita retida bem como da concentração de carbono contido nela. Os perfis de Mn e C obtidos a partir de análises químicas com EDS e WDS em amostras do aço TRIP-E, deformadas com deformação total de 2,1 e deformação total de 2,8, mostram a contribuição do refinamento de grão para a difusão destes elementos na frente da interface ferrita/austenita, durante a precipitação de ferrita pró-eutetóide.The phase transformation kinetics of five Nb microalloyed Si-Mn TRIP steels was studied under continuous cooling and isothermal treatments, using dilatometric techniques, morphologic characterization, Thermocalc computational thermodynamics and Dictra numerical simulation. WDS and EDS X-ray microanalysis and Dictra numerical modeling of C, Mn and Si distribution during transformation showed that the reaction is carbon diffusion controlled and growth occurs under local equilibrium with negligible partition. CCT diagrams for austenite transformation were determined and the effect of the amount of proeutectoid ferrite and bainite precipitation on the volume fraction of retained austenite was also estimated. The CCT diagrams allowed determining the boundaries of the critical zone and the processing window to obtain bainite plus austenite microstructures. Based on this information cooling cycles were selected to perform thermomechanical treatments. Three TRIP steels were selected to simulate, in a hot torsion testing machine, two different controlled rolling sequences: Recrystallization Controlled Rolling and Conventional Controlled Rolling. The influence of the degree of deformation and the finishing temperature on the amount of retained austenite was studied. After rolling the cooling cycle comprised two isothermal treatments, one in the austenite + ferrite field and the other in the bainitic field. Increasing the strain during simulation of Recrystallization Controlled Rolling led to an increase in the volume fraction of retained austenite to the range 9 to 14 %. The energy stored during simulation bellow TNR of the Conventional Controlled Rolling led to a decrease in the volume fraction and in the carbon content of retained austenite. The Mn and C contents measured by EDS and WDS analysis of TRIP-E steel, showed that grain refinement due to recrystallization contributes to diffusion of these elements in front of the ferrite/austenite interface during precipitation.
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Rehan, Arbab. "Microstructure and mechanical properties of a 5 wt.% Cr cold work tool steel : Influence of heat treatment procedure." Licentiate thesis, Högskolan Väst, Forskningsmiljön produktionsteknik(PTW), 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:hv:diva-10915.
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The demand for Advanced High Strength Steel (AHSS) in the automotive industry is increasing day by day. It is mainly motivated by the fact that AHSS can be used as thin sheets while having high strengths. It enables weight reduction of the automobiles which consequently increases the fuel efficiency and has proven to be less harmful to the environment. It is also expected that AHSS will have even higher strength in the near future. Cold work tools steels with 5 wt.% Cr are commonly used to process AHSS. Therefore, the tool steel must meet the challenges in the future, i.e. have even higher hardness, compressive strength and toughness. One way of increasing the mechanical properties of the tool steel is by improving the heat treatment parameters. However, it is not possible without a deeper understanding of the heat treatment process. Therefore, this work presents investigations related to phase transformations occurring in a 5 wt.% Cr cold work tool steel during heat treatment. Furthermore, the influence of austenitisation and tempering temperatures on the microstructure and mechanical properties were investigated. The studies revealed that a higher austenitisation temperature can be used to achieve a higher hardness, good compressive strength and adequate toughnessof the steel. However, too high austenitisation temperature may result inexcessive coarsening of prior austenite grains which reduced the impact toughness. It was also found that retained austenite can transform during tempering by two different mechanisms. Firstly, when tempering at 525°C, carbides precipitate in retained austenite lowering its stability and permitting a transformation to marten site on cooling. Secondly, when tempering at 600°Cfor extended holding time retained austenite isothermally transforms to ferrite and carbides. This occurs by precipitation of carbides in retained austenite followed by a final transformation to ferrite and carbides.These results were used to understand the standard tempering procedure of the 5 wt.% Cr cold work tool steel. Furthermore, alternative heat treatment procedures are discussed based on the important findings presented in this thesis.
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Yahia, Mohamed Salah. "Contribution à l'étude de l'influence de l'azote dans les traitements thermochimiques de surface des aciers en phase austénitique." Vandoeuvre-les-Nancy, INPL, 1995. http://docnum.univ-lorraine.fr/public/INPL_T_1995_YAHIA_M_S.pdf.
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Nous avons exploité un procédé nouveau de carbonitruration, ou les enrichissements en carbone et en azote sont effectués successivement (cémentation suivie de nitruration), pour déterminer le rôle de l'azote dans les traitements thermochimiques de surface des aciers en phase austénitique. L’étude thermodynamique de l'interaction du carbone et de l'azote dans l'austénite, validée par des essais de thermogravimétrie et des mesures de profils de diffusion de l'azote et du carbone, montre que l'influence de l'azote sur le coefficient de diffusion du carbone dans l'austénite est très faible. La carbonitruration des aciers de nuance 27CD4 provoque la précipitation de nitrures de chrome CrN et des carbures de type cémentite. Lors de la carbonitruration de l’acier 27MC5 seuls des carbures de type cémentite précipitent pour une concentration en carbone supérieure à 0,65%. Cependant la densité de ces précipités étant très faible, ils n'ont aucune influence sur la dureté des couches carbonitrurées. Dans l'intervalle des concentrations superficielles en carbone et en azote visées dans les traitements de carbonitruration (0,65 à 0,7% en carbone et inferieur à 0,5% en azote) l'influence de l'azote sur la dureté des couches carbonitrurées est entièrement masquée par le carbone et ce quelle que soit la nuance de l'acier utilisé (Xc, 27MC5 ou 27CD4). Au delà d'une teneur en azote de 0,5% et en fonction du temps de traitement et de la teneur en carbone, il y a risque de formation de porosité. Finalement, nous avons déterminé l'influence de l'azote sur le volume de l'austénite retenue après trempe ([gamma]r) et établi, dans le système Fe-N, une expression qui relie le point Ms à la concentration en azote (pour 0,6 <% N < 1,4 en % poids). Dans le système ternaire Fe-C-N, l'influence de l'azote sur le volume de [gamma]r dépend de la teneur en carbone. De même que pour la dureté, plus la concentration en carbone augmente, plus l'effet de l'azote sur [gamma]r est masqué
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Almeida, Alan Barros de. "Detecção da transformação da austenita retida por deformação plástica em aços para gasodutos classe API 5L X80 através de medidas magnéticas." Universidade de São Paulo, 2013. http://www.teses.usp.br/teses/disponiveis/3/3133/tde-17102014-120751/.
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O presente trabalho avaliou o efeito de tratamentos térmicos ou diferentes graus de deformação plástica na transformação da austenita do microconstituinte AM de uma chapa de aço alta resistência baixa liga (ARBL) classe API 5L X80 usada para gasodutos. A chapa tem espessura de 19 mm e passaria pelo processo de conformação UOE, mas a deformação foi realizada por laminação a frio, a temperatura ambiente, com reduções de 5 a 20%. O propósito foi compreender melhor o microconstituinte AM, explorar a transformação martensítica induzida por deformação (SIMT) e a decomposição austenítica por tratamento térmico, com ênfase em seu comportamento magnético. A transformação da austenita foi acompanhada através de medidas de polarização magnética, comparada com a densidade de massa e difração de raios X. A deformação plástica e os tratamentos térmicos alteraram a polarização magnética de saturação e a densidade de massa da amostra de aço de forma compatível com a eliminação da austenita retida metaestável. O método de densidade hidrostática foi considerado sensível para mensurar transformações de fase. Os dados obtidos revelam expansão volumétrica de aproximadamente 0,13%, correspondendo a 3,2% a quantidade de austenita retida original do material, enquanto os valores obtidos por polarização magnética de saturação são 2,8% pelo histeresígrafo e 2,1% por MAV. A difração de raios X nas amostras sob deformação ou tratamentos térmicos resultaram em queda nos primeiros picos da austenita quando comparadas com a amostra como recebida.This study evaluated the effect of different degrees of plastic deformation or heat treatment on the transformation of austenite into martensite of an HSLA steel plate API 5L X80 for pipelines. A 19 mm thickness plate would be submitted to UOE forming process, but the cold work instead occurred by cold rolling at room temperature, with reductions of 5 up to 20%. The purpose was to better understand the MA constituent, explore the strain-induced martensitic transformation (SIMT) and austenitic decomposition by heat treatment with emphasis on its magnetic behavior. The transformation was accompanied by saturation magnetization measurements, compared with the mass density and X-ray diffraction. The plastic deformation or the heat treatment altered the saturation magnetization and the mass density in a manner consistent with the elimination of metastable retained austenite. The density method is sensible to measure phase transformations induced by strain. The data obtained shows a volumetric expansion of about 0.13%, corresponding to an amount of retained austenite of the original material of 3.2%, while the values obtained by magnetization saturation are 2.8% by hysteresigraph and 2.1% by VSM. By X-ray diffraction there is a clear drop in first peaks of austenite of the samples under deformation or heat treatment compared with the sample as-received.
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Calcinelli, Luca. "Ottimizzazione del trattamento termico di acciai inossidabili martensitici per stampi." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2017.
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L’acciaio inossidabile martensitico AISI 420 viene impiegato per la realizzazione di stampi per la formatura di materie plastiche grazie alle sue elevate proprietà di resistenza all'usura e stabilità dimensionale. Esse sono funzione del trattamento termico che esso subisce e che può compromettere proprietà meccaniche e corrosive tipiche di questo acciaio. Il presente studio prende avvio proprio da queste considerazioni e dagli esiti di alcune failure analysis su stampi per bottiglie in PET, in cui sono state evidenziate rotture per fatica innescate da pitting corrosivo con propagazione intergranulare.
Nell’ambito della sperimentazione riportata si è cercato di ottimizzare il trattamento termico di bonifica in modo da massimizzare resistenza a corrosione, a fatica e resilienza pur garantendo una sufficiente stabilità dimensionale. A seguito di un approfondimento bibliografico, si è definita una microstruttura obiettivo caratterizzata dalla presenza di carburi M23C6 globulizzati ed uniformemente distribuiti nella matrice martensitica e si sono testate differenti condizioni di trattamento termico.
L'esito della sperimentazione, che si è avvalsa di tecniche di microscopia ottica ed elettronica, ha indicato come trattamento ottimale quello costituito da una austenitizzazione di 30 minuti a 1020°C seguito da una tempra in azoto a 10 bar ed un ciclo di tre rinvenimenti a 250°C. La ridotta temperatura di austenitizzazione ha permesso la limitazione dei tenori di austenite residua mentre elevata velocità di raffreddamento impiegata e ridotte temperature di rinvenimento hanno permesso di evitare la precipitazione di carburi fini infragilenti e causa di sensibilizzazione.
Sono state inoltre eseguite numerose analisi che hanno permesso di accertare una certa variabilità microstrutturale del materiale allo stato di fornitura evidenziando come la microstruttura di quest'ultimo sia fondamentale per ottenere l'esito desiderato dal trattamento termico.
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Kroll, Martin, Peter Birnbaum, Josephine Zeisig, Verena Kraeusel, and Martin Franz-Xaver Wagner. "Manufacturing of 42SiCr-Pipes for Quenching and Partitioning by Longitudinal HFI-Welding." MDPI AG, 2019. https://monarch.qucosa.de/id/qucosa%3A34778.
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In the pipe manufacturing and pipe processing industry, the demand for cost-effective pipes with high strength and good ductility is increasing. In the present study, the inductive longitudinal welding process was combined with a Quenching and Partitioning (Q&P) treatment to manufacture pipes with enhanced mechanical properties. The aim of the Q&P process is to establish a martensitic structure with increased retained austenite content. This allows for the beneficial use of both phases: the strength of martensite as well as the ductility of retained austenite. A 42SiCr steel, developed for Q&P processes, was joined at the longitudinal seam by a high-frequency induction (HFI) welding process and was subsequently heat-treated. The applied heat treatments included normalizing, austenitizing, quenching, and two Q&P strategies (Q&P-A/Q&P-B) with distinct quenching (Tq = 200/150 °C) and partitioning temperatures (Tp = 300/250 °C). Investigations of the microstructures revealed that Q&P tubes exhibit increased amounts of retained austenite in the martensitic matrix. Differences between the weld junction and the base material occurred, especially regarding the morphology of the martensite; the martensite found in the weld junction is finer and corresponds more to the lath-type morphology, compared to the base material in the circumference. In all zones of the welded tube circumference, retained austenite has been found in similar distributions. The mechanical testing of the individual tubes demonstrated that the Q&P treatments offer increased strength compared to all other states and significantly improved ductility compared to the quenched condition. Therefore, the approach of Q&P treatment of HFI-welded tubes represents a route for the mass production of high-strength tubular products with improved ductility.
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Godin, Hélène. "Effet de la microstructure sur la transition ductile-fragile d'aciers inoxydables martensitiques emboutissables à chaud." Thesis, Paris Sciences et Lettres (ComUE), 2018. http://www.theses.fr/2018PSLEM036.
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Les aciers emboutissables à chaud sont utilisés pour alléger les pièces de structure automobile. Ilsprésentent une excellente combinaison entre aptitude à la mise en forme, résistance mécanique et ductilité. De nouvelles nuances d’aciers inoxydables martensitiques ont été développées pour cette application. Parmi les propriétés requises, la résilience est un indicateur de la capacité à absorber l’énergie d’un crash. Cependant, le lien entre la microstructure et la résistance au clivage de ces aciers restait à établir. Ces travaux traitent de l’influence de la composition chimique (teneur en niobium), du traitement d’austénitisation et du refroidissement après l’emboutissage à chaud, sur la microstructure et la résilience de ces aciers. Pour ce faire, une analyse microstructurale détaillée est effectuée après différents traitements thermiques. Les microstructures obtenues sont complexes, avec des lattes de martensite plus ou moins auto-revenues, de la ferrite résiduelle, des films d’austénite retenue et des carbures. Les transitions ductile-fragile ont été caractérisées par des essais Charpy sur une large gamme de températures. La contrainte critique de clivage a été déterminée en adoptant l’approche locale de la rupture et vaut 2400 MPa quelle que soit la microstructure. Le niobium a pour principal effet d’affiner la microstructure, ce qui permet d’augmenter la résistance à la propagation des fissures de clivage et d’améliorer significativement la transition ductile-fragile. De plus, l’austénite retenue, dont la fraction dépend du traitement thermique, fait varier l’écrouissage au début de la déformation plastique donc la déformation nécessaire pour atteindre localement la contrainte critique de fissuration par clivage. Ainsi, l’austénite résiduelle permet de diminuer considérablement la température de transition ductile-fragileHot stamping steels are widely used for lightweight automotive structural parts, because of their excellent combination of formability, strength and ductility. New hot stamping martensitic stainless steel grades have been developed for this application. Among key properties required for automotive structural parts, impact toughness is a useful indicator of crash worthiness. However, the link between microstructure and the brittle fracture resistance of these steels had still to be established. The present work examined the effect of the chemical composition (niobium addition), austenitization heat treatment, and cooling conditions after hot stamping, on the microstructure and impact toughness of these steels. In order to do this, a detailed analysis of the microstructures obtained after various heat treatments was done. The microstructures are complex, with more or less auto-tempered martensitic laths, untransformed ferrite, retained austenite, and fine alloy carbides. The ductile-to-brittle transition behavior was characterized using Charpy tests over a large temperature range. The critical cleavage fracture stress was determined to be around 2400 MPa whatever the microstructure, by applying the local approach to fracture. The main effect of niobium was torefine the grain size, resulting in a strong improvement of the ductile-to-brittle transition behavior by increasing the resistance to cleavage crack propagation. Moreover, the heat treatment impacts the retained austenite fraction and thus modifies incipient plasticity and the strain necessary to reach locally the criticalstress required to trigger cleavage fracture. In this way, retained austenite plays a determining role to decrease the ductile-to-brittle transition temperature
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Arlazarov, Artem. "Évolution des microstructures et lien avec les propriétés mécaniques dans les aciers 'Médium Mn'." Thesis, Université de Lorraine, 2015. http://www.theses.fr/2015LORR0086/document.
Full textAbstract:
Lors d’un recuit inter-critique d’un acier dit « Medium Manganèse », dont la teneur en Mn est située entre 4 et 12 %, avec une microstructure initiale complètement martensitique, la formation de l’austénite obéit à un mécanisme spécifique qui porte le nom d'ART - « Austenite Reverted Transformation » (transformation inverse de l’austénite). L’objectif de ce travail de thèse était d’étudier et de modéliser les évolutions microstructurales en lien avec les propriétés mécaniques lors d’un recuit ART. Il a été déterminé que la microstructure finale se compose de phases de nature (ferrite, austénite résiduelle et martensite de trempe) et morphologie (en forme d’aiguille et polygonale) différentes. Une attention particulière a été accordée aux cinétiques de dissolution des carbures et de formation de l’austénite. Une vision complète de ces processus a été construite. En outre, le mécanisme de stabilisation de l’austénite résiduelle à la température ambiante a été étudié et discuté. Enfin, des essais de traction ont été réalisés afin d’évaluer le comportement mécanique de l’acier après différents recuits ART et établir le lien avec la microstructure. Une analyse plus détaillée du comportement de chaque constituant de la microstructure a été effectuée. A l'issue de cette thèse, un modèle complet est disponible pour calculer les courbes de contrainte vraie - déformation vraie d’un acier Medium MnDuring the intercritical annealing of fully martensitic Medium Mn steel, containing from 4 to 12 wt.% Mn, the formation of austenite happens through the so-called “Austenite Reverted Transformation” (ART) mechanism. In this PhD work, the evolution of both microstructure and tensile properties was studied as a function of holding time in the intercritical domain. The microstructure evolution was studied using a double experimental and modeling approach. The final microstructure contained phases of different natures (ferrite (annealed martensite), retained austenite and fresh martensite) and of different morphologies (lath-like and polygonal). A particular attention was paid to the kinetics of austenite formation in connection with cementite dissolution and to the morphology of the phases. A mechanism was proposed to describe the formation of such microstructure. The critical factors controlling thermal austenite stability, including both chemical and size effects, were determined and discussed, based on the analysis of the retained austenite time-evolution. At last, tensile properties of the steel were measured as a function of holding time and the relation between microstructure and mechanical behavior was analyzed. Advanced analysis of the individual behavior of the three major constituents was performed. As a final output of this work, a complete model for predicting the true-stress versus true-strain curves of medium Mn steels was proposed
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Bertilsson, Anders. "Alternative welding methods for nitrogen alloyed steel." Thesis, Örebro universitet, Institutionen för naturvetenskap och teknik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:oru:diva-62142.
Full textAbstract:
This project explores the feasibility of the solid-state welding method direct-drive friction welding to be used as a joining method for the nitrogen alloyed steel Uddeholm Vanax SuperClean, produced via processes based on powder metallurgy. Vanax SuperClean cannot be welded using fusion welding methods where the base material melts, due to nitrogen escaping the material, resulting in inferior quality welds. The cost of the material motivates the use of Vanax SuperClean for critical parts in applications, combined with a less costly material for the remaining parts, causing alternative joining methods to be examined. Vanax SuperClean is friction welded to itself and to Uddeholm steel types Stavax ESR and UHB 11. Samples are prepared for a number of examinations. Microstructures of the samples are examined using microscopy, microhardness testing is carried out per the Vickers principle, retained austenite is measured using X-ray diffraction and tensile testing of the welded samples is performed. Defect-free welds are produced in all examined samples, showing that the method is suitable for Vanax SuperClean and that no preheating or slow cooling of workpieces are necessary. The possibility of using friction stir welding as a joining method for Vanax SuperClean is discussed.Detta projekt undersöker möjligheten att använda trycksvetsningsmetoden friktionssvetsning som sammanfogningsmetod för det kvävelegerade pulvermetallurgiskt framställda stålet Uddeholm Vanax SuperClean. Vanax SuperClean kan inte svetsas med smältsvetsmetoder där grundmaterialet smälter, på grund av kvävgasbildning som resulterar i undermåliga svetsfogar. Kostnaden för materialet motiverar användandet av Vanax SuperClean för kritiska delar i applikationer, kombinerat med ett mindre kostsamt material till övriga delar, vilket föranleder undersökning av alternativa sammanfogningsmetoder. Vanax SuperClean friktionssvetsas mot sig själv, såväl som mot Uddeholmsstålen Stavax ESR och UHB 11. Prov tas fram för ett antal undersökningar. Mikrostruktur undersöks med mikroskopi, mikrohårdhetsprovning utförs enligt Vickersprincipen, restaustenitnivåer mäts med röntgendiffraktion och dragprovning utförs. Lyckade svetsfogar fås i alla undersökta prover, vilket visar att svetsmetoden är lämplig för Vanax SuperClean och att varken förvärmning eller långsamt svalnande av arbetsstycken krävs. Möjligheten att använda friktionsomrörningssvetsning som sammanfogningsmetod för Vanax SuperClean diskuteras.
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Lin, Xiu, and 林秀. "Effect of Retained Austenite on the Wear Behavior of Bearing Steels." Thesis, 2019. http://ndltd.ncl.edu.tw/handle/49fb56.
Full textAbstract:
碩士大同大學
材料工程學系(所)
107
The bearing steels, SUJ 2 and SUJ 3, obtain retained austenite was changed by different heat treatments, and the influence of the wear behavior was discussed. SUJ 3 via 840℃, 940℃ and 980℃ in a salt bath for 30 minutes and followed by oil quench at 90℃, Ms quench at 210℃ and mareqenching at 240℃ and then tempered. The microstructure obtained by quenched and tempered at 840 ℃ is spheroidized cementite dispersed in the quenched and tempered martensite matrix. The microstructure obtained by quenched and tempered at 940 ℃ and 980 ℃ is quenched and tempered martensite + retained austenite. The specimens via quenched at 980 ℃ can obtain the highest retained austenite by mareqenching. Also changing the alloy composition by carburized to increases the retained austenite of the specimens. The hardness of the SUJ2and SUJ 3 specimens were controlled to a hardness value of 60 ± 0.5 HRC. After quenched and tempered and tempered after carburized, the volume of retained austenite of SUJ 3 is more than SUJ 2. Specimens were subjected to a block-on-roller type wear testing with a load of 92N at a speed of 200 rpm. The result after the wear test of 6,000-cycle and 10,000-cycle loading indicates that the specimens after quenched and tempered have the less weight loss.
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Liu, Chin-Hung, and 劉錦鴻. "A study on the determination of retained austenite in tool steels by magnetic measurements." Thesis, 2004. http://ndltd.ncl.edu.tw/handle/68713133712624075541.
Full textAbstract:
碩士國立高雄第一科技大學
機械與自動化工程所
92
The high quality product highly relies on the good equipment. Especially the high precision and stability of machine parts will be the most important factors. Therefore how to treat the selected material to acquire the appropriate properties such as high hardness、wear resistance and good toughness, the right heat treatment process is necessary. It is well known that when the tool steel was quenched, the composition will transform to martensite and usually accompanied with certain amount of meta-stable retained austenite. The retained austenite is an unstable phase, which will transform to martensite when energy applied and unfortunately the volume will expand simultaneously when austenite transform to martensite, which will lead to dimension expansion of machines part. So how to eliminate the amount of the retained austenite within the martensite in the quenched steel is an important matters. The basic elements in steel are Fe and C with some other alloying element such as Ni、Co、Cr and etc. Through heat treatment the final composition mostly are magnetic martensite and non-magnetic retained austenite. In this study we use high carbon tool steel SK3 as the experimental specimen. During studying, we heat the steel to Austenite temperature and soaked at this temperature in an appropriate time and then quench in the water. After quenching, we continue to treat in the following subzero temp, －40℃,－80℃ and －196℃, respectively. And then we measure the amount content of retained austenite and the martensite saturation magnetization. Through our study the amount of retained austenite in quench steel can be determine by magnetic strength. As the amount of the retained austenite decrease the magnetic strength will increase. We also found in this study the magnetic strength will highly affect by heat treatment composition as well as some precipitated phase.
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Tsai, Yuan-Chieh, and 蔡元捷. "A Stydy on Transforming Retained Austenite with High Temperature Rapid Heating for Caburized Parts." Thesis, 1994. http://ndltd.ncl.edu.tw/handle/62516665657361325859.
Full textAbstract:
碩士大同工學院
材料科學(工程)研究所
82
The retained austenite is inevitably existed in the outer layer of a carburized part. The subzero trea- tment is ordinary applied for transforming the exc- essive amount of retained austenite into a more st- able structure of martensite, however, it is costly for it consumes large amount of dry ice, alcohol or liquid nitrogen. This study is aimed to investigate the feasibility of a new alternative process by dipping the carbur- ized part into a 300-450℃metal bath for seconds to decompose the retained austenite to a practically acceptable amount, and due to the rapid heating of the work in such a short time, the mechanical prope- rties (hardness, fatigue limit, toughness) are not deteriorated and which was proved to be equivalent to that of the conventional subzero treatment.
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CHIANG, JASMINE SHEREE. "Effect of Microstructure on Retained Austenite Stability and Tensile Behaviour in an Aluminum-Alloyed TRIP Steel." Thesis, 2012. http://hdl.handle.net/1974/7499.
Full textAbstract:
Transformation-induced plasticity (TRIP) steels have excellent strength, ductility and work hardening behaviour, which can be attributed to a phenomenon known as the TRIP effect. The TRIP effect involves a metastable phase, retained austenite (RA), transforming into martensite as a result of applied stress or strain. This transformation absorbs energy and improves the work hardening rate of the steel, delaying the onset of necking.
This work describes two distinct TRIP steel microstructures and focuses on how microstructure affects the RA-to-martensite transformation and the uniaxial tensile behaviour. A two-step heat treatment was applied to an aluminum-alloyed TRIP steel to obtain a microstructure consisting of equiaxed grains of ferrite surrounded by bainite, martensite and RA -- the equiaxed microstructure. The second microstructure was produced by first austenitizing and quenching the steel to produce martensite, followed by the two-step heat treatment. The resulting microstructure (labelled the lamellar microstructure) consisted of elongated grains of ferrite with bainite, martensite and RA grains. Both microstructural variants had similar initial volume fractions of RA. A series of interrupted tensile tests and ex-situ magnetic measurements were conducted to examine the RA transformation during uniform elongation. Similar tests were also conducted on an equiaxed microstructure and a lamellar microstructure with similar ultimate tensile strengths.
Results show that the work hardening rate is directly related to the RA transformation rate. The slower transformation rate, or higher RA stability, that was observed in the lamellar microstructure enables sustained work hardening at high strains. In contrast, the equiaxed microstructure has a lower RA stability and thus exhibits high values of work hardening at low strains, but the effect is quickly exhausted. Several microstructural factors that affect RA stability were examined, including RA grain size, aspect ratio, carbon content and spatial distribution of the phases. Two of these factors were characteristic of only the lamellar microstructures and led to higher RA stabilities: elongated RA grains and RA grains being primarily surrounded by bainite. The results were also compared with previous work on a silicon-alloyed TRIP steel to show that the aluminum-alloyed compositions could achieve similar, if not better, combinations of strength and ductility.Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2012-09-24 16:52:28.032
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Hu, Long-Yu, and 胡隆羽. "The Relationship Between Alloying Elements and the Amount of Retained Austenite in SUS420 Stainless Steel Welds." Thesis, 1998. http://ndltd.ncl.edu.tw/handle/14462185843363078590.
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Barlow, Lilian Debra. "The effect of austenitising and tempering parameters on the microstructure and hardness of martensitic stainless steel AISI 420." Diss., 2009. http://hdl.handle.net/2263/29839.
Full textAbstract:
The effect of austenitising and tempering practice on the microstructure and mechanical properties of two martensitic stainless steels was examined with the aim of supplying heat treatment guidelines to the consumer or fabricator that, if followed, would result in a martensitic structure with minimal retained austenite, evenly dispersed carbides and a hardness of between 610 HV and 740 HV (hardness on the Vickers scale) after quenching and tempering. The steels examined during the course of this examination conform in composition to medium-carbon AISI type 420 martensitic stainless steel, except for the addition of 0.13% vanadium and 0.62% molybdenum to one of the alloys. The effect of various austenitising and tempering heat treatments was examined. Steel samples were austenitised at temperatures between 1000°C and 1200°C, followed by quenching in oil. The as-quenched microstructures were found to range from almost fully martensitic structures to martensite with up to 35% retained austenite after quenching, with varying amounts of carbide precipitates. The influence of tempering, double tempering, and sub-zero treatment was investigated. Optical and scanning electron microscopy was used to characterise the as-quenched microstructures, and X-ray diffraction analysis was employed to identify the carbide present in the as-quenched structures and to quantify the retained austenite contents. Hardness tests were performed to determine the effect of heat treatment on mechanical properties. As-quenched hardness values ranged from 700 HV to 270 HV, depending on the amount of retained austenite. Thermodynamic predictions (using the CALPHAD™ model) were used to explain these microstructures based on the solubility of the carbide particles in the matrix at various austenitising temperatures. The carbide particles were found to be mainly in the form of M7C3 at elevated temperatures, transforming to M23C6 on cooling.Dissertation (MSc)--University of Pretoria, 2010.
Materials Science and Metallurgical Engineering
unrestricted
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Lawrence, Benjamin. "The Effect of Phase Morphology and Volume Fraction of Retained Austenite on the Formability of Transformation Induced Plasticity Steels." Thesis, 2010. http://hdl.handle.net/1974/5402.
Full textAbstract:
Transformation induced plasticity (TRIP) steels are a class of steels with exceptional formability properties, due mainly to the presence of meta-stable retained austenite which transforms to martensite under loading, locally hardening the steel. The volume fraction and mechanical stability of the retained austenite play an important role in producing the high formabilities of TRIP steels. In this thesis, two separate morphologies of retained austenite, equiaxed versus lamellar, have been produced through thermo-mechanical processing of a single common TRIP steel chemistry. The sheet formability characteristics of these two microstructures were examined, with varying volume fractions of retained austenite, through uniaxial tensile and in-plane plane-strain (IPPS) testing.
It was found that higher levels of retained austenite produced better formability properties for both microstructures and strain paths. In uniaxial tension it was seen that the the lamellar microstructure attained higher strains at maximum load, and exhibited more sustained instantaneous n values than the equiaxed structure, despite having a lower volume fraction of retained austenite.
IPPS testing was performed using an optical measurement of local strain and a comparative forming limit based on differences in strain rate between a developing neck and the surrounding material. It was found that the lamellar microstructure performed better than the equiaxed microstructure for this strain path, achieving higher strains before reaching the comparative forming limit.Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2010-01-25 16:36:07.598
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Camerino, Nester H. "Effect of prior warm rolling on the retained austenite content and hardening response of (VIM-VAR)--AISI M-50 steel." Thesis, 1985. http://hdl.handle.net/10945/21293.
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(11185158), Dallin S. Morris. "Investigation of Microstructural Effects in Rolling Contact Fatigue." Thesis, 2021.
Find full textAbstract:
Rolling contact fatigue (RCF) is a common cause of failure in tribological machine components such as rolling-element bearings (REBs). Steels selected for RCF applications are subject to various material processes in order to produce martensitic microstructures. An effect of such material processing is the retention of the austenitic phase within the steel microstructure. Retained austenite (RA) transformation in martensitic steels subjected to RCF is a well-established phenomenon. In this investigation, a novel approach is developed to predict martensitic transformations of RA in steels subjected to RCF. A criteria for phase transformations is developed by comparing the required thermodynamic driving force for transformations to the energy dissipation in the microstructure. The method combines principles from phase transformations in solids with a damage mechanics framework to calculate energy availability for transformations. The modeling is then extended to incorporate material alterations as a result of RA transforming within the material. A continuum damage mechanics (CDM) FEM simulation is used to capture material deterioration, phase transformations, and the formation of internal stresses as a result of RCF. Crystal lattice orientation is included to modify energy requirements for RA transformation. Damage laws are modified to consider residual stresses and different components of the stress state as the drivers of energy dissipation. The resulting model is capable of capturing microstructural evolution during RCF.
The development and stability of internal stresses caused by RA transformation in bearing steel material was experimentally investigated. Specimens of 8620 case carburized steel were subjected to torsional fatigue at specific stress levels for a prescribed number of cycles. X-ray diffraction techniques were used to measure residual stress and RA volume fraction as a function of depth in the material. A model is set forth to predict compressive residual stress in the material as a function of RA transformation and material relaxation. Modeling results are corroborated with experimental data. In addition, varying levels of retained austenite (RA) were achieved through varying undercooling severity in uniformly treated case carburized 8620 steel. Specimens were characterized via XRD and EBSD techniques to determine RA volume fraction and material characteristics prior to rolling contact fatigue (RCF). Higher RA volume fractions did not lead to improvement in RCF lives. XRD measurements after RCF testing indicated that little RA decomposition had occurred during RCF. The previously established RCF simulations were modified to investigate the effects of RA stability on RCF. The results obtained from the CDM FEM captured similar behavior observed in the experimental results. Utilizing the developed model, a parametric study was undertaken to examine the effects of RA quantity, RA stability, and applied pressure on RCF performance. The study demonstrates that the energy requirements to transform the RA phase is critical to RCF performance.
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Kasonde, Maweja. "Optimising the mechanical properties and microstructure of armoured steel plate in quenched and tempered condition." Diss., 2006. http://hdl.handle.net/2263/23576.
Full textAbstract:
The effect of the chemical composition, austenitisation temperature and tempering temperature and time on the mechanical properties and on the ballistic performance of martensitic steel armour plates was studied. It was established in this study that the mechanical properties and the ballistic performance of martensitic steels can be optimised by controlling the chemical composition and the heat treatment parameters. However, it was observed that for a given chemical composition of the steel the heat treatment parameters to be applied to advanced ballistic performance armour plates were different from those required for higher mechanical properties. Such a contradiction rendered the relationship between mechanical properties and ballistic performance questionable. Systematic analysis of the microstructure and the fracture mechanism of some martensitic armour plate steels was carried out to explain the improved ballistic performance of steels whose mechanical properties were below that specificied for military and security applications. It was inferred from phase analysis and its quantification by X-ray diffraction, characterisation of the martensite using scanning electron microscopy, transmission electron microscopy and atomic force microscopy that the retained austenite located in the plate interfaces and on grain boundaries of the martensite was the main constituent resisting localised yielding during ballistic impact on thin steel plates. A part of the kinetic energy is transformed into adiabatic heat where a reaustenitisation of the plate martensite and the formation of new lath martensite was observed. Another part is used to elastically and plastically deform the ballistic impact affected region around the incidence point. Dislocation pile-ups at twinned plate interfaces suggest that the twin interfaces act as barriers to dislocation movement upon high velocity impact loading. The diameter of the affected regions, that determines the volume of the material deforming plastically upon impact, was found to vary as a function of the volume fraction of retained austenite in the martensitic steel. Upon impact, retained austenite transforms to martensite by Transformation Induced Plasticity, the “ TRIP ” effect. High volume fractions of retained austenite in the martensitic steel were found to yield low values of the ratio yield strength to ultimate tensile strength (YS/UTS) and a high resistance against localised yielding and, therefore, against ballistic perforation. A Ballistic Parameter was proposed for the prediction of ballistic performance using the volume fraction of retained austenite and the thickness of the armour plate as variables. Based on the martensite structure and the results of the ballistic testing of 13 armour plate steels a design methodology comprising new specifications was proposed for the manufacture of armour plates whose thicknesses may be thinner than 6mm.Dissertation (MSc (Metallurgical Engineering))--University of Pretoria, 2007.
Materials Science and Metallurgical Engineering
unrestricted
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Li, Xiujun. "Quantitative characterization of microstructure in high strength microalloyed steels." Master's thesis, 2009. http://hdl.handle.net/10048/532.
Full textAbstract:
Thesis (M. Sc.)--University of Alberta, 2009.Title from pdf file main screen (viewed on July 16, 2009). "A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfillment of the requirements for the degree of Master of Science in Materials Engineering, Department of Chemical and Materials Engineering, University of Alberta." Includes bibliographical references.