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1
Del, Sant Ricardo. "Estudo da transformação da austenita retida em martensita induzida por deformação plástica em aços multifásicos /." Guaratinguetá : [s.n.], 2010. http://hdl.handle.net/11449/103753.
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Orientador: Tomaz Manabu HashimotoBanca: Marcelo dos Santos Pereira
Banca: Alfeu Saraiva Ramos
Banca: Jorge Otubo
Banca: Rosinei Batista Ribeiro
Resumo: Os aços multifásicos constituídos de ferrita, bainita, austenita retida e martensita apresentam combinações muito atrativas de resistência e tenacidade. Há ainda um potencial adicional de melhorias de propriedades mecânicas quando a fração de austenita retida for alta, conferindo alta conformabilidade pelo efeito TRIP. Neste contexto, é fundamental a análise qualitativa e quantitativa das fases, especialmente de austenita retida e sua transformação em martensita induzida por deformação. Este trabalho enfoca a transformação da austenita retida em martensita por deformação em tração monotônica em um aço AISI 4340 com estrutura multifásica. Os resultados confirmam a transformação da austenita retida em martensita atingindo cerca de 80% de transformação. As frações volumétricas de austenita retida antes e após a deformação foram estimadas por duas técnicas. A primeira foi feita por análise de imagens em microscopia óptica e a segunda por magnetização de saturação, tendo em vista o caráter paramagnético desta fase. As frações estimadas pelas duas técnicas foram comparadas, concluindo-se que o método magnético deve ser reavaliado, tendo sido proposto um fator de correção na equação básica deste processo e presenta na literatura.
Abstract: The multiphase steels made of ferrite, bainite, retained austenite and martensite present very attractive combinations and toughness. There is still an additional potential of improvement of the mechanical properties when the fraction of retained austenite is high allowing high conformability by the TRIP effect. In this context the qualitative and quantitative analysis of the phases is essentual specially from retained austenite and its transformation in martensite induced by deformation. This work focus on the transformation of retained austenite in martensite by the deformation in monotonic traction in one steel AISI 4340 with multiphasic structure. The results confirm the transformation of retained in martensite reaching about 80% of transformation. The volumetric of retained austenite before and after the deformation were estimated by two technique: the first was made by the anllysis of images in optic microscopy and the second by magnetization of saturation taking into consideration the paramagnetica character of this phase. The estimated fraction by the two technique were compared leading to the conclusion that the magnetic method must be evaluated again using a proposed factor of correction in the basic equation of this process and present in the literature.
Doutor
2
Singh, Shiv Brat. "Phase transformations from deformed austenite." Thesis, University of Cambridge, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.246513.
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Evteev, Alexander V., Elena V. Levchenko, Irina V. Belova, and Graeme E. Murch. "Carbon diffusion in austenite: computer simulation and theoretical analysis: Carbon diffusion in austenite: computer simulation andtheoretical analysis." Diffusion fundamentals 6 (2007) 16, S. 1-2, 2007. https://ul.qucosa.de/id/qucosa%3A14190.
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Leguen, Claire. "Prior Austenite Grain Size Controlled by Precipitates." Phd thesis, INSA de Lyon, 2010. http://tel.archives-ouvertes.fr/tel-00511322.
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During this study, the correlation between the evolution of the prior austenitic grain size and of the precipitation state during thermal treatment performed on steels is presented. To do this, the precipitation state has been finely characterized. Precipitate volume fractions were measured by plasma spectroscopy. Transmission Electron Microscopy (TEM) was used to determine the precipitate size distributions (HAADF images) and the precipitate chemical composition (EDX, EELS for carbon and nitrogen). In order to treat ELLS spectra obtained on complex carbonitrides (V,Nb,Ti)(C,N), a routine based on the Least Mean square Fitting have been developed. Results obtained with this method are in gopd agreement with those obtained by EDX analysis for metallic elements (Nb, V, Ti, ...). Then, grain size distributions were determined using a special etching called "Bechet-Beaujard", which reveals the prior austenite grain boundaries. Two alloys have been characterized in this study. (i) A model alloy, the FeVNbCN, which presents two precipitate types, NbC and VCN. This alloy was chosen to study the role of nitrogen on the precipitation state during reversion treatments. A model predicting the precipitation kinetics, coupled with a model for grain growth, give a good agreement with experimental results on grain sizes, precipitate sizes and on precipitate volume fraction. (ii) An industrial steel, the 16MnCr5+Nb was also studied. This alloy exhibits the presence of AlN and NbC precipitates. The correlation obtained between the Prior Austenite Grain Size and the evolution of the precipitation state shows that a large volume fraction of small precipitates allows a great pinning of grain boundaries. Finally, during thermo-mechanical treatments performed in the industry, some large grains may grow faster than smaller grains, leading to the so-called abnormal grain growth. This kind of growth can lead to undesirable mechanical instabilities. We have developed a criterium for abnormal grain growth which predicts the risk of such growth for a given precipitation state. This model presents a good agreement with all experimental results for both studied alloys.
5
Stormvinter, Albin. "Low Temperature Austenite Decomposition in Carbon Steels." Doctoral thesis, KTH, Metallografi, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-100993.
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Martensitic steels have become very important engineering materials in modern society. Crucial parts of everyday products are made of martensitic steels, from surgical needles and razor blades to car components and large-scale excavators. Martensite, which results from a rapid diffusionless phase transformation, has a complex nature that is challenging to characterize and to classify. Moreover the possibilities for modeling of this phase transformation have been limited, since its thermodynamics and kinetics are only reasonably well understood. However, the recent development of characterization capabilities and computational techniques, such as CALPHAD, and its applicability to ferrous martensite has not been fully explored yet. In the present work, a thermodynamic method for predicting the martensite start temperature (Ms) of commercial steels is developed. It is based mainly on information on Ms from binary Fe-X systems obtained from experiments using very rapid cooling, and Ms values for lath and plate martensite are treated separately. Comparison with the experimental Ms of several sets of commercial steels indicates that the predictive ability is comparable to models based on experimental information of Ms from commercial steels. A major part of the present work is dedicated to the effect of carbon content on the morphological transition from lath- to plate martensite in steels. A range of metallographic techniques were employed: (1) Optical microscopy to study the apparent morphology; (2) Transmission electron microscopy to study high-carbon plate martensite; (3) Electron backscattered diffraction to study the variant pairing tendency of martensite. The results indicate that a good understanding of the martensitic microstructure can be achieved by combining qualitative metallography with quantitative analysis, such as variant pairing analysis. This type of characterization methodology could easily be extended to any alloying system and may thus facilitate martensite characterization in general. Finally, a minor part addresses inverse bainite, which may form in high-carbon alloys. Its coupling to regular bainite is discussed on the basis of symmetry in the Fe-C phase diagram.QC 20120824
Hero-m
6
Tafteh, Reza. "Austenite decomposition in an X80 linepipe steel." Thesis, University of British Columbia, 2011. http://hdl.handle.net/2429/34583.
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The final microstructure and resulting mechanical properties in the heat-affected zone (HAZ) of welded linepipes are predominantly determined by austenite decomposition during cooling after welding processes. Thus, a full understanding of continuous cooling transformation of austenite is a key step toward improving the overall performance of linepipes. The main objective of the current study is to investigate the influence of cooling rate, prior austenite grain size and niobium content of austenite on austenite decomposition kinetics and the resulting microstructures for an X80 linepipe steel. To consider the significant effect of the niobium solid solution level on the transformation of austenite, two thermal histories were developed. For the first case, Nb was dissolved in solid solution prior to austenite decomposition. In contrast, the second scenario involved the formation of Nb(C,N) precipitates prior to austenite decomposition, i.e. leaving a low level of Nb in solid solution. Austenite grain growth studies were conducted to obtain grain sizes similar to those observed in the HAZ of the girth-welded steel, i.e. 5-80μm. Furthermore, employing appropriate thermal cycles, continuous cooling transformation (CCT) tests were conducted to examine the effect of niobium condition, austenite grain size and cooling rate on austenite decomposition behavior of the steel. Cooling rates varied in the range of 3−100ºC/s and dilation measurements were utilized to capture the transformation kinetics of austenite upon cooling. The resulting microstructures, which usually consist of ferrite, bainite and martensite-austenite (MA) constituents, were examined using optical microscopy. They were revealed using appropriate etchants and the corresponding phase volume fractions were subsequently measured in accordance with ASTM standards. Hardness measurements were also conducted on CCT samples.
7
Kaya, Ali Arslan. "Decomposition of austenite in high chromium steels." Thesis, University of Oxford, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.316869.
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Del, Sant Ricardo [UNESP]. "Estudo da transformação da austenita retida em martensita induzida por deformação plástica em aços multifásicos." Universidade Estadual Paulista (UNESP), 2010. http://hdl.handle.net/11449/103753.
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Os aços multifásicos constituídos de ferrita, bainita, austenita retida e martensita apresentam combinações muito atrativas de resistência e tenacidade. Há ainda um potencial adicional de melhorias de propriedades mecânicas quando a fração de austenita retida for alta, conferindo alta conformabilidade pelo efeito TRIP. Neste contexto, é fundamental a análise qualitativa e quantitativa das fases, especialmente de austenita retida e sua transformação em martensita induzida por deformação. Este trabalho enfoca a transformação da austenita retida em martensita por deformação em tração monotônica em um aço AISI 4340 com estrutura multifásica. Os resultados confirmam a transformação da austenita retida em martensita atingindo cerca de 80% de transformação. As frações volumétricas de austenita retida antes e após a deformação foram estimadas por duas técnicas. A primeira foi feita por análise de imagens em microscopia óptica e a segunda por magnetização de saturação, tendo em vista o caráter paramagnético desta fase. As frações estimadas pelas duas técnicas foram comparadas, concluindo-se que o método magnético deve ser reavaliado, tendo sido proposto um fator de correção na equação básica deste processo e presenta na literatura.
The multiphase steels made of ferrite, bainite, retained austenite and martensite present very attractive combinations and toughness. There is still an additional potential of improvement of the mechanical properties when the fraction of retained austenite is high allowing high conformability by the TRIP effect. In this context the qualitative and quantitative analysis of the phases is essentual specially from retained austenite and its transformation in martensite induced by deformation. This work focus on the transformation of retained austenite in martensite by the deformation in monotonic traction in one steel AISI 4340 with multiphasic structure. The results confirm the transformation of retained in martensite reaching about 80% of transformation. The volumetric of retained austenite before and after the deformation were estimated by two technique: the first was made by the anllysis of images in optic microscopy and the second by magnetization of saturation taking into consideration the paramagnetica character of this phase. The estimated fraction by the two technique were compared leading to the conclusion that the magnetic method must be evaluated again using a proposed factor of correction in the basic equation of this process and present in the literature.
9
Carvalho, Leandro Gomes de. "Estudo dilatométrico das transformações de fase em aços maraging M300 e M350." Universidade de São Paulo, 2011. http://www.teses.usp.br/teses/disponiveis/3/3133/tde-26032012-112344/.
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Os aços maraging são aços de baixo teor de carbono com estrutura martensítica (CCC), que são endurecidos pela precipitação de fases intermetálicas. O objetivo deste trabalho é estudar as transformações de fases desses aços: precipitação, reversão da martensita para a austenita e transformação martensítica. Nesse trabalho, foram caracterizadas uma corrida de aço maraging da série 300 e três corridas da série 350, usando diversas técnicas complementares: microscopia ótica, microscopia eletrônica de varredura com análise dispersiva de energia, microdurômetro, difração de raios-X, ferritoscópio e dilatometria. Os resultados obtidos mostraram que as corridas com maiores teores de cobalto e titânio apresentaram maiores valores de microdureza nos estados solubilizado e envelhecido. Por outro lado, medidas dilatométricas mostraram que há uma influência significativa tanto da composição química, quanto da taxa de aquecimento nas reações de precipitação e reversão da martensita para a austenita. No entanto, a transformação martensítica mostrou-se dependente apenas da taxa de aquecimento.Maraging steels are steels with a low carbon martensitic structure (BCC), which are hardened by precipitation of intermetallic phases. The aim of this work is to study the phase transformations of these steels: precipitation, martensite to austenite reversion and martensitic transformation. In this study, one cast of 300 grade and three casts of 350 grade were characterized using several complementary techniques: optical microscopy, scanning el ectron microscopy with energy dispersive analysis, microhardness, X-ray diffraction, ferritoscope and dilatometry. The results showed that the casts with higher concentrations of cobalt and titanium showed higher microhardness in the solution annealed and aged states. On the other hand, dilatometry measurements showed that there is a significant influence of both the chemical composition and the heating rate on the reactions of precipitation and reversion of martensite to austenite. However, the martensitic transformation was dependent solely on the heating rate.
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Riehm, Derek J. "Kinetics of the pearlite to austenite reversion transformation." Thesis, University of British Columbia, 1990. http://hdl.handle.net/2429/29739.
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The pearlite-to-austenite reversion transformation kinetics under isothermal and continuous
heating conditions in a eutectoid plain-carbon steel have been measured, using a dilatometric technique on a Gleeble 1500 Thermomechanical Simulator. The isothermal data was characterized in terms of the transformation start time at temperature for the onset of the P→ γ transformation, and in terms of the Avrami parameters n and b. Under the assumption that the P→ γ transformation was additive, the Scheil equation was applied to the measured isothermal transformation start data to predict the onset of the transformation on continuous heating, and the isothermal phase transformation kinetics were used to predict the continuous heating kinetics.
It was found that the kinetic model significantly underpredicted the transformation start time during continuous heating. This was attributed to the large experimental error inherent in the estimation of the isothermal transformation start time, t[formula omitted].
The model's continuous heating kinetic predictions were excellent at low heating rates, but it tended to overpredict the kinetics at higher heating rates. The problem was traced to an observed difference between the measured temperature and the programmed temperature
during the high heating rate tests. When the model was modified to incorporate the actual temperature profile, its prediction of the kinetics was considerably improved. Thus the austenite reversion transformation was concluded to be experimentally additive.
An average Avrami n value of 2.2 suggested that austenite was nucleating on pearlite colony corners and edges. This conclusion was verified with optical and scanning electron microscopy.
Previously published data, which indicated that the pearlite-to-austenite transformation
is isokinetic, was found to be based on questionable assumptions. Metallographic information suggests, however, that the nucleation sites are saturated early in the reaction.
Furthermore, the isothermal austenite formation data generated in this work was found to meet the effective site saturation criterion for additivity, implying that the austenitization process would be expected to be additive.
The effect of starting microstructure was evaluated by performing isothermal and continuous heating tests on two different pearlitic microstructures. It was found that, in agreement with published results, the transformation rate varied in inverse proportion with the pearlite spacing and colony size.Applied Science, Faculty of
Materials Engineering, Department of
Graduate
11
Sum, Maisie. "Variant selection during the austenite-to-martensite transformation." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1999. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape8/PQDD_0028/MQ50665.pdf.
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Sum, Maisie. "Variant selection during the austenite-to-martensite transformation." Thesis, McGill University, 1998. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=21328.
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Variant selection rules are described that predict the austenite-to-mariensite transformation textures of an Fe-30% Ni alloy. The occurrence of variant selection is explained in detail using the crystal plasticity model of Bishop and Hill and the Kurdjumov-Sachs transformation relationship. A correlation is established between the slip systems of the former and the rotation axes of the latter. The selection criteria are based on a combination of slip activity, that is, active slip systems defined in terms of positive shears, and permissible dislocation reactions. Thus some of the variants selected are associated with slip systems that are active, while the remainder are accounted for by the in-plane reaction of active dislocations to form inactive or unstressed dislocations. The variant selection criteria are tested against two strain paths; plane strain rolling and axisymmetric compression. Extensive studies were conducted on the former strain path and revealed excellent agreement between the measured and predicted transformation textures. A more detailed analysis is still required for the axisymmetric compression strain path, although, the results obtained indicate that the variant selection model does indeed generate a transformation texture that is consistent with the experimental one.
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Perevoshchikova, Nataliya. "Modeling of austenite to ferrite transformation in steels." Thesis, Université de Lorraine, 2012. http://www.theses.fr/2012LORR0342/document.
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La thèse porte sur la modélisation de la transformation de l'austénite en ferrite dans les aciers en mettant l'accent sur les conditions thermodynamiques et cinétiques aux interfaces alpha/gamma en cours de croissance de la ferrite. Dans une première partie, la thèse se concentre sur la description des équilibres thermodynamiques entre alpha et gamma à l'aide de la méthode CalPhad. Nous avons développé un nouvel algorithme hybride combinant la construction d'une enveloppe convexe avec la méthode classique de Newton-Raphson. Nous montrons ses possibilités pour des aciers ternaire Fe-C-Cr et quaternaire Fe-C-Cr-Mo dans des cas particulièrement difficiles. Dans un second chapitre, un modèle à interface épaisse a été développé. Il permet de prédire l'ensemble du spectre des conditions à l'interface alpha/gamma au cours de la croissance de la ferrite, de l'équilibre complet au paraéquilibre avec des cas intermédiaires des plus intéressants. Nous montrons que de nombreux régimes cinétiques particuliers dans les systèmes Fe-C-X peuvent être prévus avec un minimum de paramètres d'ajustement, principalement le rapport entre les diffusivités de l'élément substitutionnel dans l'interface épaisse et dans le volume d'austénite. Le troisième chapitre porte sur l'étude d'un modèle de champ de phase. Une analyse approfondie des conditions à l'interface données par le modèle est réalisée en utilisant la technique des développements asymptotiques. En utilisant les connaissances fournies par cette analyse, le rôle de la mobilité intrinsèque d'interface sur la cinétique et les régimes de croissance est étudié, à la fois dans le cas simple d'alliages binaires Fe-C et dans le cas plus complexe d'alliages Fe-C-MnTransformation in steels focusing on the thermodynamic and kinetics conditions at the alpha/gamma interfaces during the ferrite growth. The first chapter deals with the determination of thermodynamic equilibria between alpha and gamma with CalPhad thermodynamic description. We have developed a new hybrid algorithm combining the construction of a convex hull to the more classical Newton-Raphson method to compute two phase equilibria in multicomponent alloys with two sublattices. Its capabilities are demonstrated on ternary Fe-C-Cr and quaternary Fe-C-Cr-Mo steels. In the second chapter, we present a thick interface model aiming to predict the whole spectrum of conditions at an alpha/gamma interface during ferrite growth, from full equilibrium to paraequilibrium with intermediate cases as the most interesting feature. The model, despite its numerous simplifying assumptions to facilitate its numerical implementation, allows to predict some peculiar kinetics in Fe-C-X systems with a minimum of fitting parameters, mainly the ratio between the diffusivities of the substitutional element inside the thick interface and in bulk austenite. The third chapter deals with the phase field model of austenite to ferrite transformation in steels. A thorough analysis on the conditions at the interface has been performed using the technique of matched asymptotic expansions. Special attention is given to clarify the role of the interface mobility on the growth regimes both in simple Fe-C alloys and in more complex Fe-C-Mn alloys
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Hernando, Juan Carlos. "Morphological characterization of primary austenite in cast iron." Licentiate thesis, Tekniska Högskolan, Högskolan i Jönköping, JTH, Material och tillverkning, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:hj:diva-35585.
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Automotive industry products portfolio includes a wide variety of complex‐shaped cast iron products, such as truck engine components, that need to withstand a constant trend of higher demands, especially urged by stricter environmental regulations on emissions. Combined with this continued demand on properties improvement, cast iron industry faces a process problem related to the lack of understanding of solidification and mechanisms behind defect formation. Casting products are highly affected by the product design and the manufacturing method itself, which governs the final microstructure and hence the final mechanical properties. Wall thickness of the moulding material strongly influences the solidification time, varying the microstructural coarseness, resulting in a component with different properties depending on the local shape of the casting. The main objective of this work is the characterization of the primary austenite microstructure and its coarsening process, which has been poorly documented in cast iron literature, to allow the prediction and control of these microstructural features present in the casting. The microstructural evolution of the primary austenite in hypoeutectic lamellar graphite iron (LGI) is studied under isothermal coarsening conditions. The dendritic microstructure suffered major morphological changes that included dendrite fragmentation, globularization, and coalescence. Empirical relations based on morphological parameters are introduced to predict the microstructural evolution of primary austenite. A novel technique for colour‐etching and semi‐automatic image analysis for the characterization of quenched dendritic microstructures in cast iron is presented. A new experimental technique for production of graphitic iron with varying nodularity is presented as a solution to control the production of compacted (CGI) and spheroidal graphite iron (SGI) under laboratory conditions. The nodularity evolution is controlled as a function of the holding time and the residual Mg, allowing the study of the primary solidification and primary microstructures of hypoeutectic CGI and SGI in future investigations.
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Leguen, Claire. "Precipitation controlled prior austenite grain size in steels." Lyon, INSA, 2010. http://theses.insa-lyon.fr/publication/2010ISAL0018/these.pdf.
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Au cours de cette étude nous présentons la corrélation existant entre l’évolution de la croissance des grains austénitiques et celle de l’état de précipitation au cours de traitements thermiques d’aciers. Pour cela, nous avons, dans un premier temps, caractérisé l’état de précipitation expérimentalement. Les fractions volumiques précipitées sont mesurées par dissolution électrolytique. La Microscopie Electronique en Transmission (MET) permet d’obtenir les distributions de taille (imagerie HAADF) et la composition chimique des précipités (EDX, EELS pour le carbone et l’azote). Afin de traiter les spectres EELS obtenus sur des carbonitrures complexes du type (V,Nb,Ti)(C,N), nous avons mis en place une approche basée sur la méthode du ‘Least Mean Square Fitting’ (LMSF). Les résultats obtenus par cette routine sont en accord avec ceux obtenus par EDX sur les éléments lourds (Nb,V,Ti,…), ce qui crédibilise les résultats obtenus pour le carbone et l’azote. Dans un second temps, des distributions de taille de grains austénitiques ont été obtenues par observation métallographique après une attaque chimique spécifique (dite de "Bechet-Beaujard"). Dans le cadre de cette étude deux alliages ont été étudiés. (i) Un alliage modèle, le FeVNbCN, présentant des précipités NbCet VCN. La raison du choix de cet alliage est d’étudier l’influence de l’azote sur l’évolution de l’état de précipitation au cours de traitements d’austénitisation. Des modèles thermodynamiques combinés de prédiction des cinétiques de précipitation et de croissance des grains conduisent à des prédictions en bon accord avec les résultats expérimentaux de taille de grains, de tailles de précipités, de composition chimique et de fraction volumique. (ii) Un alliage industriel, le 16MnCr5+Nb a également été étudié. Cet alliage contient des précipités de NbC et de AlN. La corrélation obtenue entre la taille de grains austénitiques et l’évolution de l’état de précipitation montre que la composition chimique des précipités n’entre pas en jeu, mais plus leur taille est petite et plus leur fraction volumique est grande, plus le contrôle des grains par les précipités est efficace. Enfin, nous nous sommes intéressés au phénomène de croissance anormale (lors de traitements thermomécaniques comme ceux réalisés dans l’industrie notamment, la taille de grain austénitique peut croître de façon hétérogène). Ce type de croissance entraîne des instabilités mécaniques indésirables. Nous avons donc développé un modèle de prédiction du risque de croissance anormale pour un état de précipitation donné. Ce modèle présente un très bon accord avec les résultats expérimentaux des deux alliages étudiésDuring this study, the correlation between the evolution of the prior austenitic grain size and of the precipitation state during thermal treatment performed on steels is presented. To do this, the precipitation state has been finely characterized. Precipitate volume fractions were measured by plasma spectroscopy. Transmission Electron Microscopy (TEM) was used to determine the precipitate size distributions (HAADF images) and the precipitate chemical composition (EDX, EELS for carbon and nitrogen). In order to treat ELLS spectra obtained on complex carbonitrides (V,Nb,Ti)(C,N), a routine based on the Least Mean square Fitting have been developed. Results obtained with this method are in good agreement with those obtained by EDX analysis for metallic elements (Nb, V, Ti, …). Then, grain size distributions were determined using a special etching called "Bechet-Beaujard", which reveals the prior austenite grain boundaries. Two alloys have been characterized in this study. (i) A model alloy, the FeVNbCN, which presents two precipitate types, NbC and VCN. This alloy was chosen to study the role of nitrogen on the precipitation state during reversion treatments. A model predicting the precipitation kinetics, coupled with a model for grain growth, give a good agreement with experimental results on grain sizes, precipitate sizes and on precipitate volume fraction. (ii) An industrial steel, the 16MnCr5+Nb was also studied. This alloy exhibits the presence of AlN and NbC precipitates. The correlation obtained between the Prior Austenite Grain Size and the evolution of the precipitation state shows that a large volume fraction of small precipitates allows a great pinning of grain boundaries. Finally, during thermo-mechanical treatments performed in the industry, some large grains may grow faster than smaller grains, leading to the so-called abnormal grain growth. This kind of growth can lead to undesirable mechanical instabilities. We have developed a criterium for abnormal grain growth which predicts the risk of such growth for a given precipitation state. This model presents a good agreement with all experimental results for both studied alloys
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Frölund, Henrik. "Manual TIG welding of Fe10Cr4Al+RE: overlay on an austenitic substrate and butt weld using alumina forming austenite consumable." Thesis, KTH, Fysik, 2020. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-279448.
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Otubo, Jorge. "Desenvolvimento de ligas inoxidaveis com efeito de memoria de forma : elaboração e caracterização." [s.n.], 1996. http://repositorio.unicamp.br/jspui/handle/REPOSIP/263112.
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Orietadores: Paulo Roberto Mei, Sadamu KoshimizuTese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica
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Resumo: É apresentado neste trabalho, pioneiro no Brasil, o desenvolvimento de ligas inoxidáveis com Efeito de Memória de Forma (EMF) assistido por transformação martensítica não termoelástica. Mostrou-se que a fusão por indução à vácuo é uma técnica adequada e que tanto o trabalho a quente como o trabalbo a frio são perfeitamente possíveis nas ligas em questão à medida que os parâmetros operacionais como temperatura de aquecimento, redução por passe, etc., sejam controlados. A eficiênciada técnica de treinamento na melhora do Efeito de Memória de Forma foi comprovada com a recuperação de forma em torno de 95 % da liga CrNi e de 89 % da liga CrNiCo após 5 ciclos com pré-deformação de 4% e aquecimento a 600 °e. Estes resultados são bastante promissores e comparáveis aos melhores resultados encontrados na literatura. O caráter inoxidável e a recuperação de forma de praticamente 4% das ligas desenvolvidas promovem-nas para aplicações práticas como conexões de tubo sem solda e mecanismo para liberação de painéis solares de satélite. Considerações sobre processamento, propriedades mecânicas e microestruturais são também analisadas
Abstract: It is presented in this work, pioneer in Brazil, the development of stainless alloys with shape memory effect assisted by non thermoelastic martensitic transformation. It was shown that Vacuum Induction Melting (VIM) is a suitable technique for melting these alloys and that hot working or cold working are perfectly possible as long as the heating temperature, reduction per pass, etc. are controlled. The efficiency of training technique in improving the Shape Memory Effect was shown by the 95 % shape recovery of the CrNi alloy and by 89 % of shape recovery of the CrNiCo alloy after five training cyc1es of 4 % pre-strain and annealing at 600°C. These results are very promising and comparable to the best ones published in the literature. The stainless character and the shape recovery of almost 4% promote these alloys for practical applications such as for pipe fitting and for satellite solar panel release mechanism.The alloys' processing, mechanicaland microstructural properties are also analyzed.
Doutorado
Materiais e Processos de Fabricação
Doutor em Engenharia Mecânica
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Menezes, Henrique Thiago Freire de. "InfluÃncia do Resfriamento sub-zero apÃs o Envelhecimento na FormaÃÃo de Austenita Revertida em um AÃo Maraging 350." Universidade Federal do CearÃ, 2008. http://www.teses.ufc.br/tde_busca/arquivo.php?codArquivo=6601.
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FundaÃÃo de Amparo à Pesquisa do Estado do CearÃOs aÃos maraging sÃo materiais de ultra-alta resistÃncia sendo composto principalmente por uma base quaternÃria Ni-Co-Mo-Ti que possuem excelente tenacidade aliada com alta resistÃncia mecÃnica sendo utilizadas na indÃstria bÃlica, nuclear, componentes aeronÃuticos, vasos de pressÃo atà indÃstria esportiva. Uma de suas importantes aplicaÃÃes à na utilizaÃÃo nos motores de histerese os quais necessitam de materiais com caracterÃsticas de materiais magneticamente macios e elevada resistÃncia mecÃnica. Foram realizados ensaios de microdueza para determinar propriedades mecÃnicas nas diversas condiÃÃes de tratamento de envelhecimento e meios de resfriamento. Com o objetivo de descobrir as fases presentes no material nas diversas condiÃÃes de tratamento foram realizadas ensaios por difraÃÃo de raios-X bem como analisar a macrotextura atravÃs de FDOC`s. A microtextura foi analisada atravÃs da tÃcnica de EBSD (Electron Backscatterig Diffraction) com o intuito de estudar a influÃncia da textura da austenita na martensita nas determinadas condiÃÃes de envelhecimento e meios de resfriamento. Esta dissertaÃÃo à de grande relevÃncia para o desenvolvimento de novos produtos utilizando aÃos maraging, pois determina as condiÃÃes que oferecem as propriedades mecÃnicas, bem como as fases e textura presentes no material nas condiÃÃes de tratamento requeridas.
Maraging steels are materials of ultra high strength mainly composed of a base quaternary Ni-Co-Mo-Ti which have excellent toughness combined with high mechanical strength being used in war industry, nuclear, aircraft components, pressure vessels up to the sports industry . One of its important applications is in use in hysteresis motors which require materials with characteristics of magnetically soft materials and high mechanical strength. Microdueza tests were performed to determine mechanical properties in different treatment conditions of aging and cooling media. In order to discover the phases present in the material conditions in the various treatment trials were performed by X-ray diffraction as well as analyze the macrotexture through FDOC `s. The microtexture was examined using the technique of EBSD (Electron Diffraction Backscatterig) in order to study the influence of the texture of the austenite in the martensite aging in certain conditions and means of cooling. This thesis is of great importance for the development of new products using maraging steel, because it determines the conditions that provide the mechanical properties as well as the phases and texture present in the material conditions of treatment required.
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Silva, Anderson José Saretta Tomaz da. "Têmpera e partição em ferros fundidos nodulares." Universidade de São Paulo, 2013. http://www.teses.usp.br/teses/disponiveis/3/3133/tde-10072014-152701/.
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Um novo ciclo de tratamento térmico denominado como têmpera e partição vem sendo desenvolvido em aços com elevados teores de silício, como rota para obtenção de estruturas com frações consideráveis de austenia retida. Essa rota de tratamento m térmico consiste em realizar uma têmpera temperaturas intermediárias entre Ms e Mf, seguido de um reaquecimento com manutenção em patamares isotérmicos por certos intervalos de tempo, objetivando estabilizar a austenita remanescente através da partição do carbono a partir da martensita supersaturada. No presente trabalho, duas ligas de ferros fundidos nodulares convencionais, com diferentes teores de silício e manganês, foram submetidas a ciclos de têmpera e partição. As amostras foram austenitizadas a 900°C por duas horas. Uma das ligas foi temperada em óleo a 160°C e a outra a 170°C por 2 minutos. Imediatamente após a têmpera as amostras foram reaquecidas em temperaturas entre 300 e 450°C por intervalos de tempo que variaram entre 2 e 180 minutos. A caracterização microestrutural foi realizada através de microscopia eletrônica de varredura (MEV) e difração de raios x. A caracterização mecânica foi feita através de ensaios de energia absorvida ao impacto, dureza HRC e ensaios de tração. A caracterização microestrutural evidenciou que os ciclos de têmpera e partição são viáveis na obtenção de frações consideráveis de austenita retida nos ferros fundidos nodulares. A caracterização mecânica evidenciou que foi possível obter boas combinações de energia absorvida ao impacto, resistência à tração e alongamento. Em todas as condições testadas é possível perceber uma janela de processo bem definida caracterizada por valores crescentes das propriedades mecânicas nos primeiros minutos do ciclo de partição e que decrescem após certo intervalo de tempo. O conjunto de propriedades mecânicas obtidas através dessa rota de tratamentos térmicos indica que os ferros fundidos nodulares submetidos ao ciclo de têmpera e partição podem se constituir como alternativa tecnológica para aplicações comerciais nas quais os ferros fundidos nodulares austemperados já são materiais consolidados.A new heat treatment cycle known as quenching and partitioning has been developed in commercial steel alloys containing silicon as a way to obtain structures with controlled fractions of retained austenite. This heat treatment cycle consists in performing a quenching in temperatures between Ms and Mf, followed by a reheating with isothermal holding by different time intervals. The aim of this cycle is to achieve the austenite stabilization by diffusion of carbon from the supersaturated plates of martensite. In this work, two conventional ductile cast iron alloys, with two different contents of silicon and manganese were heat-treated in quenching and partitioning cycle. The samples were austenitized at 900°C for two hours, followed by quenching in oil at 160° C and 170° C for two minutes. Immediately after quenching, the samples were reheated at temperatures between 300 and 450°C for time intervals between 2 and 180 minutes. The microstructural characterization was performed using electronic microscopy (SEM) and x-ray diffraction. The mechanical characterization was performed using impact tests, hardness and tensile strength tests. The microstructural characterization showed that the cycles of quenching and partitioning are viable to obtain considerable fractions of retained austenite in nodular cast by this heat treatment route. The mechanical characterization showed that it was possible to obtain good combinations of energy absorbed in the impact, tensile strength and elongation. In all tested conditions was possible to perceive a well-defined process window characterized by increasing values of mechanical properties in the first minutes of the partitioning step, and decrease after certain time intervals. The set of mechanical properties obtained by this route of heat treatments indicates that nodular cast iron subjected to tempering and partitioning cycle can be constituted as an alternative technology for commercial applications in which austempered ductile irons are already consolidated materials.
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Azizi-Alizamini, Hamid. "Austenite formation and grain refinement in C-Mn steels." Thesis, University of British Columbia, 2010. http://hdl.handle.net/2429/30513.
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The present work deals with grain refinement and austenite formation in a plain C-Mn steel with 0.17C-0.74Mn (wt pct). To improve the limited work hardening capability of ultrafine grained ferritic steels, new approaches were adopted to develop bimodal ferrite grain size distributions and ultrafine grained dual phase microstructures. The first approach is based on deformation and annealing of a ferrite-martensite microstructure. Ultrafine grained dual phase steels were obtained through rapid heating of very fine ferrite-carbide aggregates into the intercritical annealing region where partial austenite formation takes place. Hence, austenite formation was systematically investigated using a combination of microstructure characterization and detailed dilatometry analysis. The effect of initial structure and heating rate on austenite formation was examined. The resulting microstructure characteristics and mechanical properties of dual phase steels were also investigated. A multi-phase field modelling approach was adopted to simulate austenite formation from a variety of initial structures including ferrite-spheroidized carbide aggregates, fully pearlitic and ferrite-pearlite structures.
The results show that a bimodal distribution of ferrite grains negates the Lüdering effect, yet the improvement of work hardening rate remains marginal compared to fine grained ferrite structures. Very fine grained initial structure and rapid heat treatment cycle are essential parameters to achieve ultrafine grained dual phase steels with improved mechanical properties in the steel employed in this study. For austenite formation, dilatation data can be used to distinguish different stages of microstructure evolution upon heating into the single austenite phase region including ferrite recrystallization, pearlite to austenite and ferrite to austenite transformation. Heating rate has a pronounced effect on the size and morphology of austenite grains in the intercritical annealing region. It is shown that phase field modelling is capable of predicting microstructural changes during austenite formation. It is well suited to capture complex interaction between microstructure processes such as spheroidization, carbide dissolution and coarsening during austenite formation especially in fine grained structures where the length scale is comparable with carbon diffusion distance.
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Yu, Wanhua. "Cellular automata modelling of austenite grain coarsening during reheating." Thesis, University of Sheffield, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.251524.
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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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Evteev, Alexander V., Elena V. Levchenko, Irina V. Belova, and Graeme E. Murch. "Carbon diffusion in austenite: computer simulation and theoretical analysis." Universitätsbibliothek Leipzig, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-193337.
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Dogan, Omer Nihat. "Interactions between austenite grain boundaries and aluminum nitride precipitates." Case Western Reserve University School of Graduate Studies / OhioLINK, 1990. http://rave.ohiolink.edu/etdc/view?acc_num=case1054927506.
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Takahashi, Manabu. "Reaustenitisation from bainite in steels." Thesis, University of Cambridge, 1993. https://www.repository.cam.ac.uk/handle/1810/221917.
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Kamat, Rajeev Gurunath. "The principle of additivity and the proeutectoid ferrite transformation." Thesis, University of British Columbia, 1990. http://hdl.handle.net/2429/30682.
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This study critically examines the additivity of the proeutectoid ferrite transformation. The study has been carried out in two parts with the first part involving experimental verification of the additivity of the proeutectoid ferrite transformation and the second part a theoretical assessment of the additive nature of the ferrite transformation with the aid of mathematical models.
Austenite-to-proeutectoid ferrite transformation kinetics were measured under a number of isothermal, stepped-isothermal and continuous-cooling conditions for three plain-carbon hypo-eutectoid steel grades (AISI 1010,1020 and 1040) using a dilatometer and a Gleeble 1500 thermomechanical simulator. Isothermal transformation kinetics were characterized using the Avrami equation. The stepped-isothermal transformation tests were designed to experimentally assess the additive nature of the austenite to proeutectoid ferrite transformation by measuring transformation kinetics partially at one temperature and after a rapid temperature change to another temperature. Results on the 1010 and 1040 steels showed that the proeutectoid ferrite transformation with polygonal morphology is additive under changing temperature in that the ferrite transformation kinetics at the second temperature are quite similar to the isothermal kinetics at that temperature.
Stepped-isothermal transformation measurements were made on the 1020 steel with the resulting ferrite morphology either remaining Widmanstatten at both temperatures or changing from allotriomorph to predominantly Widmanstatten at the two temperatures. In both cases the results showed additive behavior. However, the stepped-isothermal test in which the proeutectoid ferrite was transformed to an equilibrium amount and equilibrated at the first temperature and then rapidly changed to the second temperature was not additive.
Characterizing the isothermal formation of proeutectoid ferrite in the three steels using the Avrami equation resulted in a reasonably constant value of n and the b parameter increasing with increasing transformation temperature. Early site saturation was evident in a number of test specimens.
In the second part of the study, two mathematical models with planar and spherical interface geometries were developed to theoretically assess the additivity of the proeutectoid ferrite transformation. A finite-difference numerical technique was employed to describe the austenite to ferrite diffusion controlled moving-interface problem for a system having finite boundaries. A test of additivity of the proeutectoid ferrite transformation was made by predicting the ferrite growth kinetics and the associated carbon gradients under stepped-isothermal conditions. The predictions were consistent with the observed experimental additivity of the proeutectoid ferrite transformation in the 1010 steel. The spherical model predicted isothermal ferrite growth kinetics compared more favorably with the experimentally measured kinetics of the 1010 steel than the planar model.
An unusual phase was detected in the 1020 steel. A number of tests were performed to measure the isothermal transformation kinetics of the new phase. Scanning electron microscopy (SEM) and scanning-transmission electron microscopy (STEM) were used to further investigate the structural details in this new phase. The results indicated that the new phase is a type of bainite but having some of the characteristics attributable to massive transformation products.Applied Science, Faculty of
Materials Engineering, Department of
Graduate
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Soylu, B. "Phase tranformations in duplex stainless steels." Thesis, University of Cambridge, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.235281.
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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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Pariser, Gerhard Carolus. "Modeling the austenite to ferrite phase transformation for steel development /." Aachen : Shaker, 2006. http://bvbr.bib-bvb.de:8991/F?func=service&doc_library=BVB01&doc_number=014913109&line_number=0001&func_code=DB_RECORDS&service_type=MEDIA.
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Pandi, Rassoul. "Dynamic transformation of austenite to ferrite in low carbon steel." Thesis, McGill University, 1993. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=26064.
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The design of thermomechanical processing schedules to control microstructures requires the knowledge of the austenite-to-ferrite transformation start temperature (Ar$ sb3$). Further, in curved mold continuous casting, transverse cracking occurs during the unbending procedure to straighten the solidified curved strand, and this can also strongly depend on the austenite-to-ferrite transformation. In both of these industrial processes, during deformation, the temperature usually decreases continuously. Thus, two new methods to determine the Ar$ sb3$, based on continuous cooling compression (CCC) and continuous cooling torsion (CCT), have been developed. While the latter is applicable for low and high strains, the former can be only used for low strains.The aim of this investigation was to determine the effect of deformation in the single phase austenite and two phase austenite plus ferrite region on the transformation and dynamic transformation behaviour of austenite-to-ferrite. CCC tests were carried out on a low carbon steel and the influence of strain, strain rate, cooling rate and austenite grain size, was examined.
During the application of strain, the generated dislocations cause an increase in stored strain energy. This energy adds to the driving force for austenite-to-ferrite transformation, increasing the kinetics of this transformation, raising the Ar$ sb3$ in this way. The faster kinetics leads to a finer polygonal ferrite grain size after transformation. In contrast to the effect of increasing strain, accelerated cooling rates decrease the transformation start temperature, but can still lead to grain refinement via high nucleation rates. By increasing the cooling rate, fine acicular ferrite with a high aspect ratio could be obtained. Since, the ferrite grain size is directly related to austenite grain size, by varying austenite grain size, a wide range of ferrite grain sizes could be obtained. Finally, deforming close to the Ar$ sb3$ maximizes the strain effect on dynamically transformed ferrite.
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Wang, Fei. "Austenite grain growth behaviour of HSLA steel during reheating treatment." Thesis, University of Birmingham, 2017. http://etheses.bham.ac.uk//id/eprint/7415/.
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The grain growth behaviour during reheating between 950 ºC and 1300 ºC of as-cast Al-Nb steel (containing 0.019 wt% Nb and 0.057 wt% Al) and rolled Nb-containing steel (containing 0.028 wt% Nb and 0.031 wt% Al) have been investigated. In particular the role of microalloying element segregation during casting and, hence the spatial distribution of microalloying precipitates, on grain boundary pinning during reheating has been considered. The Al-Nb containing steel has been examined in separate initial conditions, including as-cast (segregated structure), homogenised and forged (reduced separation of segregated bands) samples. It was found that microalloy segregation occurred between the dendritic and interdendritic regions, where the secondary dendrite arm spacing (SDAS) was 150 ± 50 μm. Nb showed strong segregation into the interdendritic regions resulting in a higher number density of Nb(C,N) precipitates (2.64 × 104 /mm2) compared to the dendritic region (0.73 × 104 /mm2). However, Al did not show strong segregation resulting in relatively well-distributed AlN precipitates in the matrix (1.29× 104 /mm2 in the interdendritic region and 1.89× 104 /mm2 in the dendritic region). After forging, the separation between the segregated bands was reduced to 65 ± 10 μm from the previous 150 ± 50 μm in the as-cast sample. The increased Nb content in the rolled Nb-containing steel compared to the Al-Nb steel gave a greater extent of segregation in the solute-enriched regions resulting in a larger number density of Nb(C,N) present (5.9× 104 /mm2), whilst the separation between in the segregated bands in the as-rolled Nb-containing steel was 35 ± 10 μm.
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Nilsson, Robin. "Experimental Methodologies for Analyzing Austenite Recrystallization in Martensitic Tool Steels." Thesis, KTH, Materialvetenskap, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-176091.
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Revealing the prior austenite grain boundaries from a martensitic structure is well known to be very difficult and dependent on the chemical composition and the thermomechanical processing of the steel. In the present study, four different chemical etching reagents and additional thermal etching have been conducted for thermomechanical simulated tool steels Orvar Supreme and Stavax ESR. The etching results have been characterized using light optical microscopy and electron backscattered diffraction. The obtained results show that saturated aqueous picric acid, oxalic and sodium bisulfite based acid reveals prior austenite grain boundaries well for Orvar Supreme. For Stavax ESR, only aqueous CrO3-NaOH-picric acid provides good results in revealing the prior austenite grain boundaries. Thermal etching shows good potential and if conducted properly, thermal etching is a good alternative to the chemical reagents from a health- and environmental perspective.
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Black, Martin Peter. "Microstructural evolution of austenite in a microalloyed Fe30% Ni alloy." Thesis, University of Sheffield, 2002. http://etheses.whiterose.ac.uk/12774/.
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The study of the physical metallurgy of microalloyed steels has been an important field of research for nearly forty years. During this time the hot working characteristics have been comprehensively investigated, simulated and modelled. Unfortunately, the actual microstructural behaviour during hot working cannot be followed completely due to the unavoidable phase transformation of these steels upon cooling. This transformation prohibits direct study of the deformed austenite, by disordering the dislocation structures developed during hot working. In order to avoid the problem of transformation, a model alloy has been developed. This allows the retention of the austenitic structure to room temperature, while retaining similar thermodynamic and deformational properties to conventional microalloyed steels. The alloy was based on a matrix of iron with 30wt. % nickel, and niobium and carbon additions to the level of 0.1% and 0.09% respectively. The use of such an alloy to simulate the hot working behaviour of traditional microalloyed steels means that the study of softening and precipitation events in the austenite matrix is possible as the phase transformation is avoided. The Fe-30%Ni-Nb alloy has undergone thermomechanical processing. Hot plane strain compression tests have been carried out in order to study the precipitation kinetics. Before testing, the material was solution treated at 1250°C to allow supersaturation of the niobium at lower temperatures. Double-deformation plane strain compression testing has been carried out over a range of temperatures (900-1050°C) at a strain rate of 10s 1 and with delay times between deformations varying from is to 1000s. This testing has allowed the study of both the static precipitation and recrystallisation kinetics from the resulting flow stress behaviour. Precipitation has been evident from the stress-strain curves. Transmission electron microscopy of thin foils of the hot-worked material has been completed to investigate the dislocation structures produced. This shows the presence of a microbanded substructure. The particle populations have been studied using conventional transmission electron microscopy. Direct observation of particles precipitated in the austenite matrix has been achieved by electron spectroscopic imaging studies of the as-deformed material. This is important, as it shows preferential precipitation upon the dislocation structure, and not within the matrix. The overall study shows that the iron-nickel alloy is a good model austenite in many respects. It has similar hot-working characteristics: deformation behaviour, work hardening response, and recrystallisation behaviour. The present work also reports similar NbC precipitation behaviour to that found in conventional C-Mn based microalloyed steels. It appears that the alloy is an excellent model for microalloyed austenite under hot-working conditions, and should prove to be a valuable material for future investigations.
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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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Zhang, Feng. "Effects of austenite condition on bainite transformations in microalloyed bar steels." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp04/nq22504.pdf.
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Soliman, Mohamed A. "Phase transformations and mechanical properties of new austenite-stabilised bainite steels." Clausthal-Zellerfeld Pieper, 2007. http://d-nb.info/989405060/04.
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Liu, Weijie. "Precipitation of Ti(CN) in austenite : experimental results, analysis and modelling." Thesis, McGill University, 1987. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=75434.
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Stress relaxation measurements were carried out on a plain carbon and four Ti steels over the temperature range 850 to 1050$ sp circ$C. The results show that the stress relaxation of plain carbon austenite after a 5% prestrain can be described by the relation $ sigma$ = $ sigma sb0$-$ alpha$ln(1 + $ beta$t). By contrast, in the solution-treated Ti steels, relaxation is arrested at the start of precipitation and is resumed when precipitation is complete. As a result, this new mechanical method is suitable for following carbonitride precipitation in microalloyed austenite at hot working temperatures.PTT diagrams were determined by the present technique for the steels containing 0.05, 0.11, 0.18 and 0.25% Ti. The PTT curves obtained are C shaped for all the steels. The upper parts of these curves are shifted to significantly longer times as the Ti and C concentrations are reduced. By contrast, the positions of the lower arms of the curves are relatively independent of the current values of the solubility product (Ti) (C). This phenomenon is attributed to the catalytic effect of trace amounts of dissolved N on the nucleation rate of Ti(CN) in austenite.
Changes in the size distribution and morphology of the precipitates during relaxation of the 0.25% Ti steel were followed by means of electron microscopy. The cube shaped Ti(CN) precipitates were heterogeneously distributed in either a chain-like or a cell-like manner. Electron microanalysis was additionally carried out to determine the compositions of the Ti carbosulphide-Mn sulphide inclusions.
Finally, the experimental results are compared with the predictions of classical nucleation theory and of the diffusion controlled particle growth theory. Good agreement was obtained between the predictions of the theories and the experimental results. It is demonstrated by means of the thermodynamic analysis of nucleation that the Ti(CN) precipitate/austenite interface is of a semi-coherent nature. It is shown that the interface between a critical nucleus and the matrix can be characterized by the newly introduced coherency loss parameter C.
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Ghosh, Chiradeep. "The dynamic transformation of deformed Austenite at temperatures above the Ae3." Thesis, McGill University, 2013. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=117149.
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The dynamic transformation behavior of deformed austenite was studied in four steels of increasing carbon contents that had been deformed over the temperature range 743 – 917°C. These experiments were carried out in torsion under an atmosphere of argon and 5% H2 and the experimental temperatures were above the ortho and para-equilibrium Ae3 temperatures of the steels. Strains of 0.15 – 5 were applied at strain rates of 0.4 - 4.5 s-1. The experimental parameters were varied in order to determine the effects of strain and temperature on the formation of strain-induced ferrite and cementite. The structures observed are Widmanstätten in form and appear to have nucleated displacively. The onsets of dynamic transformation and dynamic recrystallization were detected in the four steels using the double-differentiation method. Two sets of second derivative minima were found to be associated with all the flow curves. It is shown that double minima can only be obtained when the polynomial order is at least 7. The first set of minima corresponds to the initiation of dynamic transformation (DT). The second set is associated with the nucleation of dynamic recrystallization (DRX). The critical strain for DT is always lower than for DRX in this range and increases slightly with temperature. Conversely, the critical strain for DRX decreases with temperature in the usual way.The mean flow stresses (MFS's) pertaining to each experimental condition were calculated from the flow curves by integration. These are plotted against the inverse absolute temperature in the form of Boratto diagrams. The stress drop temperatures, normally defined as the upper critical temperature Ar3*, were determined from these diagrams. These are shown to be about 40°C above the paraequilibrium and about 20 - 30°C above the orthoequilibrium upper critical transformation temperatures. This type of behavior is ascribed to the occurrence of the dynamic transformation of austenite to ferrite during deformation. The effect of deformation on the Gibbs energy of austenite in these steels was estimated by assuming that the austenite continues to work harden after initiation of the transformation and that its flow stress and dislocation density can be derived from the experimental flow curve by making suitable assumptions about two-phase flow. By further taking into account the inhomogeneity of the dislocation density, Gibbs energy contributions (driving forces) are derived that are sufficient to promote transformation as much as 100°C above the Ae3. The carbon diffusion times required for formation of the observed ferrite plates and cementite particles are calculated; these are consistent with the occurrence of interstitial diffusion during deformation. Similar calculations indicate that substitutional diffusion does not play a role during dynamic transformation. The Gibbs energy calculations suggest that growth of the Widmanstätten ferrite is followed by C diffusion at the lower carbon contents, while it is accompanied by C diffusion at the higher carbon levels.Le comportement de la transformation dynamique de l'austénite lors de sa déformation a été étudié dans quatre aciers, dont les teneurs en carbone sont croissantes, déformés à des températures allant de 743 à 917°C. Ces expériences ont été réalisées par torsion sous une atmosphère contrôlée (mélange argon et 5% H2) et les températures de déformation étaient au-dessus des températures d'ortho et para-équilibre Ae3 pour chaque acier. Des taux de déformation de 0,15 à 5 ont été utilisés avec des vitesses de déformation allant de 0,4 à 4,5 s-1. Les paramètres expérimentaux ont été modifiés afin de déterminer les effets du taux de déformation et de la température sur la formation de ferrite et de cémentite induite par déformation. Les structures observées sont de type Widmanstätten et semblent avoir nucléées de façon displacive. Les départs de la transformation dynamique et de la recrystallization dynamique ont été détectés pour les quatre aciers en utilisant la méthode de double différenciation. Deux ensembles de minima sur les dérivés secondes se trouvent être associés à toutes les courbes d'écoulement. Il est montré qu'un double minimum ne peut être obtenu que lorsque l'ordre du polynôme utilisé dans la procédure d'ajustement de l'intégralité de la courbe découlement est supérieur ou égale à 7. Le premier ensemble de minima correspond à l'initialisation de la transformation dynamique. Le second ensemble est associé à la nucléation de la recrystallization dynamique. Le taux de déformation critique de la transformation dynamique est toujours inférieur à celui de la recrystallization dynamique, dans la gamme de température étudiée, et augmente légèrement avec la température. A l'inverse, le taux de déformation critique de la recrystallization dynamique diminue, de manière connue, avec la température. Les contraintes d'écoulement moyennes relatives à chaque condition expérimentale sont calculées à partir de l'intégration des courbes d'écoulement. Celles-ci sont représentées en fonction de l'inverse de la température absolue sous la forme de diagrammes de Boratto. Les températures auxquelles la chute de contrainte se produit, normalement définie comme étant la température critique supérieure Ar3*, sont déterminées à partir de ces diagrammes. Celles-ci sont d'environ 40°C au-dessus de la température critique supérieure de para-équilibre et d'environ 20 à 30°C au-dessus de celle d'ortho-équilibre. L'effet de la déformation sur l'énergie de Gibbs de l'austénite dans ces aciers est estimé en supposant que l'austénite continue de s'écrouir après l'initiation de la transformation et que sa contrainte d'écoulement et sa densité de dislocations peuvent être dérivées à partir des courbes d'écoulement expérimentales en émettant des hypothèses appropriées sur l'écoulement conjoint de deux phases. En prenant en outre en compte l'hétérogénéité de la densité de dislocation, les contributions à l'énergie de Gibbs obtenues (forces motrices) sont suffisantes pour promouvoir une transformation jusqu'à 100°C au-dessus de la température Ae3. Le temps de diffusion du carbone requis pour la formation des plaques de ferrite observées est calculé; les résultats obtenus sont cohérents avec l'apparition de diffusion interstitielle au cours de la déformation. Des calculs similaires indiquent que la diffusion substitutionnelle ne joue aucun rôle lors de la transformation dynamique. Les calculs de l'énergie de Gibbs suggèrent que la croissance de la ferrite de Widmanstätten est suivie de la diffusion du carbone dans le cas des plus faibles teneurs en carbone tandis qu'elle est accompagnée par celle-ci pour les teneurs les plus fortes.
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MAMANI, JULIO DAMIAN SUNI. "QUANTIFICATION OF AUSTENITE-MARTENSITE IN LOW ALLOY STEEL BY IMAGE ANALYSIS." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 2013. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=22902@1.
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PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIROCONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO
O objetivo deste trabalho foi desenvolver uma metodologia para quantificação, por microscopia, do microconstituinte Austenita-Martensita (AM) em um aço de alta resistência e baixa liga (ABRL) da classe API5LX80. Foram obtidas imagens de Microscopia Ótica (MO) em campo claro e Eletrônica de Varredura (MEV), nos modos de elétrons secundários (SE) e retroespalhados (BSE). As imagens foram quantificadas por Processamento e Análise Digital de Imagens (PADI) e os resultados dos dois tipos de microscopia foram comparados. O principal desafio foi discriminar AM em meio a uma microestrutura multifásica complexa, com frações variadas de ferrita, bainita e do próprio AM. Para revelar a presença de AM foram testadas diferentes sequências de ataques químicos e eletrolíticos. Os resultados demostraram que o ataque químico misto, acrescentando um passo Lepera modificado gerou uma melhora acentuada do contraste em imagens de MO e de MEV no modo BSE. Imagens de MEV no modo SE apresentaram problemas de borda devido à posição do detector de elétrons, o que impediu a correta discriminação das regiões de AM. No modo BSE, a tensão de aceleração foi reduzida para 5 kV, para reduzir a penetração do feixe de elétrons e aumentar o contraste devido à fina camada de AM. Estas imagens foram filtradas para reduzir ruído e segmentadas por limiar simples para quantificar o AM. Nas imagens coloridas de MO, o AM foi segmentado por limiares nos espaços de cor RGB e HSB e em seguida foi quantificado. Utilizando Microscopia Co-Localizada (MCL) foram obtidas imagens de campos idênticos no MEV e no MO, permitindo comparar as técnicas. Mostrou-se que, no mesmo aumento, a microscopia ótica tende a subestimar a fração de AM quando comparada à microscopia eletrônica de varredura, no modo BSE.
This dissertation proposed the development of a quantification method, by microscopy, of the microconstituent Martensite-Austenite (MA) in a High Strength Low Alloy (HSLA) steel of the API5LX80 class. Images were obtained by Optical Microscopy (OM) and Scanning Electron Microscopy (SEM), in secondary electron (SE) and backscattered electron (BSE) modes. Digital Image Processing and Analysis (IA) was employed to process and quantify the acquired images and compare the results of the two types of microscopy. The main challenge was to discriminate the MA amidst a complex multiphase microstructure with varying fractions of ferrite, bainite and MA itself. To reveal the MA different chemical and electrolytic etching sequences were tested. The results showed that a mixed combination with an extra step of modified LePera etchant issued the best contrast for both OM and BSE mode SEM. SEM images in SE mode showed edge problems due to the location of the electron detector, what prevented the correct discrimination of MA regions. The accelerating voltage in the BSE mode was reduced to 5 kV which in turn decreased beam penetration and increase contrast due to the thin MA layer. These images were filtered to reduce noise and segmented by a simple threshold to quantify MA. In the color OM images MA was segmented by thresholds in the RGB or HSB color spaces and subsequently quantified. Employing Co-Site Microscopy images of identical fields acquired by OM and SEM, a direct comparison of the techniques was allowed. It was show that, for the same magnification, optical microscopy tends to underestimate the MA fraction when compared to electron microscopy in BSE mode.
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BERT, ALEXANDRE. "Modelisation micromecanique de la transformation de phase austenite-martensite par ecrouissage." Paris, ENSAM, 1998. http://www.theses.fr/1998ENAM0005.
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Les aciers inoxydables austenitiques sont dits instables lorsqu'au cours de la deformation une nouvelle phase se cree au detriment de la phase mere. Le but de cette etude est la modelisation micromecanique tridimensionnelle du comportement d'un tel materiau. Le comportement de la phase mere (austenite) est decrit en prenant en compte le glissement sur les plans denses 1,1,1 du systeme cubique faces centrees. Lorsque deux plans actifs s'interceptent, il se cree a l'intersection un germe de phase fille (martensite). Le modele ne prend en compte qu'un seul variant de martensite par grain. Le grain, avant deformation est donc monophase austenitique, puis il devient biphase, avec une fraction volumique de renfort qui evolue (la martensite est consideree comme le renfort). Le comportement du grain biphase est evalue par homogeneisation au sens de mori et tanaka. Chaque grain modelise par son comportement et son orientation participe au comportement macroscopique du materiau. L'homogeneisation sur un grand nombre de grains est effectuee grace a un schema autocoherent. Avec deux niveaux d'homogeneisation et une fraction volumique de renfort qui evolue, le materiau peut etre assimile a un composite de composite avec fraction evolutive de renfort. Les donnees experimentales necessaires (evolution de la fraction volumique de martensite, anisotropie du materiau, evaluation des contraintes dans les phases) sont obtenues par diffractions des rayons x.
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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, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=case1346250505.
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Guimarães, Junior Jamil Martins. "TRANSFORMAÇÃO MARTENSÍTICA INDUZIDA POR DEFORMAÇÃO EM AÇOS INOXIDÁVEIS AUSTENÍTICOS AISI 304 E AISI 316 DEFORMADOS POR DIFERENTES PROCESSOS." UNIVERSIDADE ESTADUAL DE PONTA GROSSA, 2016. http://tede2.uepg.br/jspui/handle/prefix/1476.
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Stainless steels with properties which allow high conformability and satisfactory mechanical resistance due to phase transformations are becoming prominently active for industries and metallurgical research centers. Therefore, the phenomena related to the work hardening must be understood and mastered, once the strain has an extremely important function on the phase transformation of these materials. For the investigations, are conducted different kinds of strains on the stainless steels by filing, compression and cold rolling at 12%, 31%, 50%, 70%, 80% and 90% reduction levels. In order to compare the materials, are conducted the following technics: optical microscopy, EDS, EBSD, X ray diffraction, hardness measurements and ferritoscope. The high hardness presented by these as received materials is attributed to the fact that they were not annealed. Low rolled steels showed high transformation into ε-martensite, and high rolled levels (like 90%) showed high grain elongation and very high α’- martensite fractions. Filling and compression also showed high α’- martensite fractions for the AISI 304. However, the AISI 316 didn’t show a significant fraction of α’- martensite after the compression test. Therefore, the high hardness showed by AISI 316 steel after the compression test is certainly due to classic work hardening mechanisms, not due to a α’- martensite transformation, which depends on the Stacking Fault Energy (SFE).
Aços inoxidáveis com propriedades que permitam alta conformabilidade aliada a resistência mecânica satisfatória devido a transformações de fase vêm sendo destaque nas indústrias e centros de pesquisa de metalurgia. Para tanto, os fenômenos relacionados ao encruamento devem ser compreendidos e dominados, pois os processos de deformação desempenham uma função extremamente importante na transformação de fases desses materiais. As investigações são conduzidas com deformações plásticas nos aços inoxidáveis por meio de limagem, compressão a 60% de redução da altura e laminação a níveis de 12%, 31%, 50%, 70%, 80% e 90% de redução. Para fins comparativos, são conduzidas análises microestruturais por meio de: microscopia óptica, EDS, EBSD, difração de raios X, medidas de dureza e ferritoscopia. Percebeu-se uma dureza acima do esperado nos materiais inicialmente caracterizados devido ao fato de os mesmos não estarem na condição de recozidos. Baixos níveis de redução na espessura promoveram elevados índices de transformação da austenita para martensita ε, enquanto que altos níveis de redução resultaram em elevado alongamento dos grãos e as maiores frações de martensita α’ entre todas as condições de deformação. Deformação por meio de limagem e por meio de compressão promoveram altos índices de transformação da austenita em martensita α’ para o aço inoxidável AISI 304, porém quando submetido à compressão, o aço AISI 316 não apresentou consideráveis frações de martensita na microestrutura. Sendo assim, conclui-se que o aumento de dureza do aço AISI 316 se deu por mecanismos clássicos de endurecimento, e não pela formação de martensita induzida por deformação. Isso porque a tendência em formar martensita induzida por deformação é dependente da EDE, e consequentemente, da composição química do material.
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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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Chang, Liou Chun. "Bainite transformation and novel bainitic rail steels." Thesis, University of Cambridge, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.360612.
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Sahay, Sunil Kishore. "The austenite → ferrite transformation in tungsten steels and some other ternary steels." Thesis, University of Cambridge, 1986. https://www.repository.cam.ac.uk/handle/1810/221895.
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Robinson, Isaac Dalton Gordon. "Grain growth and austenite decomposition in two niobium containing line pipe steels." Thesis, University of British Columbia, 2016. http://hdl.handle.net/2429/58975.
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Niobium is a common microalloying element for line pipe steels for the promotion of fine ferrite microstructures following hot rolling. During construction via welding, niobium carbonitrides formed during coiling may dissolve depending on peak temperatures reached. This has a strong influence on the microstructure and mechanical properties of the heat affected zone. The primary objective of the current work is to study the effect of niobium microalloying on heat affected zone microstructures. Two steel compositions were selected for this study, possessing a high (0.091 wt%) niobium content and differing (0.028 and 0.058 wt%) carbon contents. To study the effect of niobium on grain growth, thermal histories were designed in the context of continuous heating and isothermal holding for a range of heating rates and holding temperatures. Laser ultrasonics were used to observe in-situ grain growth during these histories and experimental results were confirmed by ex-situ metallography using appropriate etchant. Niobium is shown to be effective in restricting grain growth behavior at temperatures when precipitates are stable. An increase in heating rate was found to reduce overall grain growth (from 48 to 25 μm between 10 and 1000°C/s), eventually reaching a limiting behavior at the maximum heating rate (i.e. 1000°C/s). To study the effect of niobium on austenite decomposition, thermal histories were designed to produce two grain sizes (5 and 35 μm) and two states of niobium (in and out of solution). The latter was investigated through precipitation experiments. Cooling rates were varied between 3 and 30°C/s and mechanical dilatometry was used to measure the phase transformation. As expected, an increase in cooling rate, carbon content, and prior austenite grain size results in lower transformation temperatures. The effect of niobium in solution was strongly dependent on prior austenite grain size. Microstructures formed by austenite decomposition were characterized via metallography and microhardness. The microstructural constituents include ferrite, bainite, and M/A. Samples representative of each microstructure were selected to quantify M/A fraction through etching and point counting. Niobium was shown to have a strong effect on hardness when the prior austenite grain size was large (up to 70 HV).Applied Science, Faculty of
Materials Engineering, Department of
Graduate
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Djahazi, Mohammad. "Influence of boron distribution on precipitation and recrystallization in hot worked austenite." Thesis, McGill University, 1989. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=74307.
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The influence of boron distribution on the precipitation of Nb(C,N) and on austenite recrystallization was studied by means of a stress relaxation technique. A plain carbon steel containing 0.026% C, used as the reference material, and three other steels alloyed with 0.003% B, 0.055% Nb and 0.003% B + 0.055% Nb were employed. The microstructural evolution during relaxation was interpreted in terms of the pinning effect of precipitates on mobile dislocations.C-shaped precipitation-time-temperature diagrams were determined for both the Nb + B and Nb steels. The presence of boron accelerates precipitation so that it begins at higher temperatures and after shorter times. Also, the combined addition of Nb and boron delays the recrystallization start time at 1000$ sp circ$C and suppresses the partial recrystallization of austenite observed at 950$ sp circ$C.
Samples were quenched or control cooled at different stages of the tests and the evolution of precipitation and the location of boron and boron compounds were studied by various microanalytical techniques. The occurrence of the non-equilibrium segregation of boron after deformation and during recrystallization at austenite grain boundaries was revealed in this way. The observations are interpreted in terms of the interaction between vacancies and boron atoms.
The strong retardation of recrystallization and the acceleration of precipitation observed when both Nb and boron are present is explained in terms of the formation of Nb-B complexes and the increase in the effective concentration of precipitate forming interstitials. At deformation temperatures of 950$ sp circ$C and above, boron appears to increase the solute drag effect of Nb, leading to greater delays in recrystallization. At lower temperatures, boron plays its role in accelerating the nucleation stage of precipitation.
Finally, changes in the size distribution of the precipitates as a function of composition, temperature and time were measured. These data are employed in the framework of the diffusion controlled nucleation and particle growth theory. Expressions for the Nb(C,N) precipitation start times and for the diffusion of Nb in austenite were obtained which are in accord with the values reported in the literature.
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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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