L’objet de cette thèse est la prise en compte des effets du fraisage de finition dans un modèle d’endommagement en fatigue. Les procédés d’usinage tels que le fraisage sont connus pour imposer de fortes sollicitations thermomécaniques, pouvant altérer les propriétés géométriques (rugosité, arrachements) et mécaniques (contraintes résiduelles, écrouissages) en surface et sous-surface des pièces produites. Ces propriétés, regroupées sous le terme d’intégrité de surface, sont susceptibles d’affecter significativement la tenue en fatigue des pièces fabriquées. Cette problématique a été traitée pour le cas particulier du fraisage de finition à l’outil hémisphérique d’aciers à microstructure bainitique. Plusieurs axes d’étude ont été suivis, à commencer par la mesure et la caractérisation de l’intégrité de surface, et en particulier de son caractère anisotrope. Dans un deuxième temps, le rôle joué par l’intégrité de surface lors de sollicitations en fatigue a été mis en évidence au travers d’une campagne d’essais de flexion portant sur différents types de surface. Ces travaux à dominante empirique ont été complétés par l’étude et l’amélioration d’une approche hybride visant à mêler données expérimentales et modélisation pour prévoir rapidement et efficacement les profils de contraintes résiduelles induits par le procédé. Enfin, la dernière partie de l’étude a porté sur la prise en compte des résultats ainsi obtenus dans un modèle d’endommagement dit à deux échelles, dans le but de représenter, et dans la mesure du possible de prévoir, l’effet de l’intégrité de surface sur le comportement en fatigue du matériau. / This thesis focuses on the effects of finishing milling on fatigue damage. Machining processes such as milling are known to incur high thermomechanical loadings, which alter the geometrical (roughness) and mechanical (residual stresses, strain hardening) properties of the surface and sub-surface of produced parts. These properties, designated as surface integrity, are likely to affect significantly the fatigue strength of machined parts. The problem has been studied here in the case of the ball-end tool finishing milling of bainitic steels. Several approaches were followed, starting with the measurement and characterization of surface integrity, and especially of its anisotropic nature. Secondly, the role played by surface integrity during fatigue behaviour was highlighted through a bending test campaign including different surface types. These mainly empirical works were completed with the study and improvement of a hybrid approach aiming at combining experimental data and modelling in order to predict quickly and efficiently the residual stress profiles induced by the process. Finally, the last part of the study has dealt with taking account of the results thus obtained in a so-called two-scale damage model, in order to describe, and insofar as possible, to predict the fatigue behaviour of the machined material.

Made available in DSpace on 2014-06-11T19:28:35Z (GMT). No. of bitstreams: 0 Previous issue date: 2005-06Bitstream added on 2014-06-13T18:34:54Z : No. of bitstreams: 1 ranieri_a_me_guara.pdf: 1386692 bytes, checksum: c47cd01ee98e83332bd255dbd49cadad (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Universidade Estadual Paulista (UNESP) / Os principais objetivos deste trabalho foram desenvolver estruturas bainíticas e multifásicas através de diversas rotas de tratamentos térmicos, visando as melhores combinações de propriedades mecânicas, fornecendo subsídios científicos/tecnológicos para as indústrias brasileiras. Em certos componentes de veículos aeroespaciais tem sido usado aço de baixa liga e ultra-alta resistência temperados e revenidos com elevada resistência devida a estrutura martensítica mas com baixa tenacidade. Uma melhoria na tenacidade é conseguida com redução controlada de resistência através do revenimento. O novo conceito, para aços avançados que combinam alta resistência com boa tenacidade, está simbolizado pelas microestruturas bainíticas e multifásicas. Neste projeto foi feito um estudo do efeito das microestruturas nas propriedades mecânicas de um aço AISI 4340. Foram analisadas diversas microestruturas, desde aquelas inteiramente bainíticas até microestruturas multifásicas com teores variados de ferrita, bainita, martensita e austenita retida. Os resultados foram comparados com aqueles obtidos por têmpera por resfriamento continuo e com as diversas rotas de transformação isotérmica. As combinações de propriedades mecânicas estão relacionadas com as frações volumétricas das fases e a bainita melhora significativamente a ductilidade do aço, mantendo a resistência elevada e melhorando a combinação resistência/ductilidade. O aço possui baixo coeficiente de encruamento e é possível conseguir resistências entre 1000 MPa e 1400 MPa com alongamento entre 13% e 25%, combinação esta superior aquelas encontradas para o mesmo aço quando temperado e revenido em óleo. / The main goals of this study were to develop bainitic and multiphasic structures through several routes of heat treatment, in order to reach the better combination of mechanical properties, providing scientific/technological subsidies to Brazilian industries. In some of aerospatial vehicles components have been used quenched and tempered ultra-high-strength low-alloy steel where the martensitic structure is responsible for the high-strength and low toughness levels. Toughness improvements can be achieved by strength reduction control during tempering. The new concept for advanced steels, that combine high-strength and good toughness, is correlated with the bainitic and multiphasic microstructures. In this work the effect of microstructures on the mechanical properties of AISI 4340 steel. Has been analysed several microstructures, from those totally bainitic until multiphasics microstructures with various ferrite, bainite, martensite and retained austenite content. The results were compared with those obtained by quenching through continuous cooling transformation and several routes of isothermal transformation. The combinations of mechanical properties are related with volume fraction of present phases and the bainite improved significantly the toughness steel., keeping the high strength and improving the strength/toughness combination. This steel has low coefficient of hardness and is possible to achieve strengths between 1000 MPa e 1400 MPa with percentual elongation between 13% e 25%, this combination is better than that found to the same steel when quenched and tempered in oil.

Resumo: Os principais objetivos deste trabalho foram desenvolver estruturas bainíticas e multifásicas através de diversas rotas de tratamentos térmicos, visando as melhores combinações de propriedades mecânicas, fornecendo subsídios científicos/tecnológicos para as indústrias brasileiras. Em certos componentes de veículos aeroespaciais tem sido usado aço de baixa liga e ultra-alta resistência temperados e revenidos com elevada resistência devida a estrutura martensítica mas com baixa tenacidade. Uma melhoria na tenacidade é conseguida com redução controlada de resistência através do revenimento. O novo conceito, para aços avançados que combinam alta resistência com boa tenacidade, está simbolizado pelas microestruturas bainíticas e multifásicas. Neste projeto foi feito um estudo do efeito das microestruturas nas propriedades mecânicas de um aço AISI 4340. Foram analisadas diversas microestruturas, desde aquelas inteiramente bainíticas até microestruturas multifásicas com teores variados de ferrita, bainita, martensita e austenita retida. Os resultados foram comparados com aqueles obtidos por têmpera por resfriamento continuo e com as diversas rotas de transformação isotérmica. As combinações de propriedades mecânicas estão relacionadas com as frações volumétricas das fases e a bainita melhora significativamente a ductilidade do aço, mantendo a resistência elevada e melhorando a combinação resistência/ductilidade. O aço possui baixo coeficiente de encruamento e é possível conseguir resistências entre 1000 MPa e 1400 MPa com alongamento entre 13% e 25%, combinação esta superior aquelas encontradas para o mesmo aço quando temperado e revenido em óleo. / Abstract: The main goals of this study were to develop bainitic and multiphasic structures through several routes of heat treatment, in order to reach the better combination of mechanical properties, providing scientific/technological subsidies to Brazilian industries. In some of aerospatial vehicles components have been used quenched and tempered ultra-high-strength low-alloy steel where the martensitic structure is responsible for the high-strength and low toughness levels. Toughness improvements can be achieved by strength reduction control during tempering. The new concept for advanced steels, that combine high-strength and good toughness, is correlated with the bainitic and multiphasic microstructures. In this work the effect of microstructures on the mechanical properties of AISI 4340 steel. Has been analysed several microstructures, from those totally bainitic until multiphasics microstructures with various ferrite, bainite, martensite and retained austenite content. The results were compared with those obtained by quenching through continuous cooling transformation and several routes of isothermal transformation. The combinations of mechanical properties are related with volume fraction of present phases and the bainite improved significantly the toughness steel., keeping the high strength and improving the strength/toughness combination. This steel has low coefficient of hardness and is possible to achieve strengths between 1000 MPa e 1400 MPa with percentual elongation between 13% e 25%, this combination is better than that found to the same steel when quenched and tempered in oil. / Orientador: Tomaz Manabu Hashimoto / Coorientador: Antonio Jorge Abdalla / Banca: Marcelo dos Santos Pereira / Banca: Jorge Otubo / Mestre

Ce travail traite de l'optimisation des propriétés en traction et en résilience d'un acier 2.25 Cr – 1 Mo par le contrôle de sa microstructure via des traitements thermiques appropriés. Ainsi, les transformations de phases ayant lieu au cours de l'austénitisation, de la trempe et du revenu doivent être correctement appréhendées. Des observations en microscopies électroniques à balayage et en transmission, ainsi que des analyses par diffraction des rayons X, ont été effectuées afin de caractériser et de modéliser la microstructure de l'acier à chaque étape du traitement thermique. L'évolution de la phase austénitique lors de l'étape d'austénitisation, ainsi que son influence sur la microstructure après trempe, ont été étudiées. La croissance du grain austénitique a été modélisée afin de comprendre ses mécanismes sous-jacents, en particulier le phénomène de croissance limitée observé aux plus basses températures. L'effet des conditions d'austénitisation sur la décomposition de l'austénite ainsi que sur les propriétés mécaniques du matériau après trempe et revenu a été étudié expérimentalement. Une condition d'austénitisation optimale a été déterminée et utilisée pour étudier la précipitation au revenu. La précipitation des carbures a été étudiée pour différents temps et températures de revenu. La séquence de précipitation suivante a ainsi été mise en évidence : la cémentite M3C précipite en premier, suivie des carbures M2C et M7C3 ; les carbures à l'équilibre étant les M23C6. Enfin, l'influence de la précipitation des carbures sur les propriétés mécaniques de l'acier a été étudiée. Les propriétés en traction sont particulièrement sensibles aux conditions de revenu dans le domaine d'étude, alors que les propriétés en résilience restent stables. / This work aims at optimizing tensile and toughness properties of a 2.25Cr – 1Mo steel by controlling its microstructure through heat treatments. To this aim, phase transformations during austenitization, quenching and tempering have to be understood. Quantitative microstructural analyses were performed by means of SEM, TEM and XRD to characterize and model metallurgical evolution of the steel at each step of the heat treatment. The evolution of austenite during the austenitization stage, and its influence on the resulting as-quenched microstructure were thoroughly investigated. Austenite grain growth was modelled in order to understand its mechanisms, including the limited growth phenomenon observed at lower temperatures. The effect of austenitization conditions on further decomposition of austenite and on mechanical properties after quenching + tempering was experimentally determined. An optimal austenitization condition was selected and applied to study the tempering stage. Carbide precipitation was studied for various tempering temperatures and amounts of time. M3C carbides precipitate first, followed by M2C and M7C3; M23C6 are the equilibrium carbides. The influence of carbide precipitation on mechanical properties was studied. Tensile properties are closely linked to the tempering conditions in the range investigated, while impact toughness remains stable.

The average size of hot-work tools has gradually increased over the past years.This affects the effective temperature cycle tools experience during hardening,as large dimensions prevent uniform and rapid cooling, and thereby the resulting microstructures and properties. In order to avoid the formation of coarse structures or cracking during heat treatment it has become common practise to lower the austenitising temperature below that recommended by the steel manufacturer.In this work, therefore, the effects of austenitising at temperatures lower thancommonly recommended are investigated. Three 5% Cr hot-work tool steelsalloyed with Mo and V were heat treated, resulting microstructures andtempering carbides were studied and transformation characteristics determined for different austenitising temperatures and different cooling rates. The temperatures and cooling rates have been chosen to be representative for heat treatments of different sizes of tools. Bainite rather than martensite formed during slow cooling regardless of austenitising temperature. A lowered austenitising temperature produced largeramounts of both bainite and retained austenite while a higher caused graingrowth. Carbon partitioning during the bainitic transformation resulted in anincrease of the carbon content in the retained austenite of at least 0.3 wt.%. The austenitising temperature influences also the type and amount of tempering carbides that precipitate, which affects the properties of the steel. Higher austenitising temperatures favour the precipitation of MC carbides during tempering. The Mo rich M2C type carbides were proven to be more prone to coarsening during service at 560°C-600°C, while V rich MC carbides preserve their fine distribution. A best practice heat treatment needs to balance the increase of grain size with increasing austenitising temperatures, with the possibility to form more tempering carbides. Higher austenitising temperatures also give less retained austenite, which can affect dimensional stability and toughness negatively after tempering

A aplicação do novo conceito de tratamento térmico, chamado de têmpera e partição (Q&P), desenvolvido para a obtenção de aços da terceira geração da classe AHSS (Advanced high strengh steel ou aços avançados de alta resistência), mostra-se uma alternativa para o processamento de ferros fundidos nodulares com alta resistência mecânica. No processo Q&P, o carbono presente na martensita supersaturada, formada na etapa de têmpera, é utilizado para estabilizar a austenita não transformada durante a etapa de partição, mantendo-a estável na temperatura ambiente. Essa rota de tratamento térmico consiste em realizar uma têmpera no material (após uma etapa prévia de austenitização) numa faixa de temperatura entre o Ms e Mf (temperatura de início e fim da transformação martensítica, respectivamente), seguido de um reaquecimento e manutenção a uma temperatura acima do Ms (etapa isotérmica de partição) com o objetivo de que o carbono migre da martensita supersaturada para a austenita remanescente promovendo sua estabilização. Essa partição do carbono só é possível caso a precipitação da cementita seja suprimida, e isso é conseguido com a presença de elementos de liga, como o Si e/ou Al. Neste trabalho foi feito um estudo sobre as características microestruturais e mecânicas de um ferro fundido nodular (3,47%C; 2,47%Si; 0,2%Mn) submetido ao tratamento térmico de têmpera e partição, o qual foi denominado neste trabalho como, Q&PDI (Quenched and Partitioned Ductile Iron ou Ferro Fundido Nodular Temperado e Particionado). Para isso foi realizada uma austenitização plena nas amostras, a 880°C, por duas horas, seguida de uma têmpera em óleo pré-aquecido nas temperaturas de 140 e 170°C. A etapa de partição foi feita nas temperaturas de 300, 375 e 450°C, com intervalos de tempo variando de 5 a 120 minutos. A caracterização microestrutural foi realizada através de microscopia ótica, microscopia eletrônica de varredura e EBSD. A técnica de difração de raios-X foi empregada para quantificar a fração volumétrica e o teor de carbono na austenita retida. Ensaios de dilatometria, difração de raios-X \"in situ\" e nanoindentação foram empregados para auxiliar na análise das transformações de fases que ocorreram na etapa de partição, como a transformação bainítica e a precipitação de carbonetos de transição nas placas de martensita. A caracterização mecânica foi feita através de ensaios de tração, impacto, dureza, nanoindentação, tenacidade à fratura e resistência à fadiga. Ensaios de compressão auxiliaram na análise da transformação martensítica induzida por deformação. Os resultados obtidos mostraram que é possível obter ferros fundidos nodulares com alta resistência mecânica (limite de resistência >1450 MPa), com consideráveis ductilidade (de até 9%) e energia absorvida sob impacto (de até 81 J), bem como tenacidades à fratura de 55 MPa.m1/2 e limites de fadiga de 550 MPa. Este comportamento é proporcionado por uma microestrutura singular, constituída por uma dispersão homogênea de placas de martensita numa matriz de ausferrita bastante refinada, com consideráveis frações volumétricas de austenita retida (max. 23%). / Quenching and partitioning (Q & P), a new heat treatment concept developed to obtain third generation AHSS (Advanced High Strength Steel), is an alternative for processing of nodular cast irons in order to obtain high mechanical strength. In the Q & P process, the carbon present in the supersaturated martensite formed in the quenching step diffuses towards the untransformed austenite during the partition step, keeping it stable at room temperature. This heat treatment route consists of quenching the material (after a previous step of austenitization) in a temperature range between Ms and Mf (beginning and end temperature of the martensitic transformation, respectively), followed by reheating and maintenance at a temperature above the Ms (isothermal stage of partition) allowing the carbon to migrate from the supersaturated martensite to the remaining austenite, promoting its stabilization. This partition of carbon is only possible if precipitation of cementite is suppressed; this is achieved adding alloying elements such as Si and/or Al. In this work a study was made on the microstructural and mechanical characteristics of a ductile iron (3.47%C; 2.47%Si; 0.2%Mn), submitted to a Q&P heat treatment, in this work named Q & PDI (Quenched and Partitioned Ductile Iron). A full sample austenitization was carried out at 880 ° C for two hours, followed by a pre-heated oil quanching at temperatures of 140 and 170 ° C. The partitioning step was at temperatures of 300, 375 and 450Â ° C, with time intervals ranging from 5 to 120 minutes. Microstructural characterization was performed through optical microscopy, scanning electron microscopy and EBSD. The X-ray diffraction technique was used to quantify the volumetric fraction and the carbon content in the retained austenite. Dilatometry, X-ray diffraction \"in situ\" and nanoindentation were also used to aid in the analysis of the phase transformations that occurred in the partitioning stage, such as the bainitic transformation and the precipitation of transition carbides in the martensite plates. Mechanical characterization was performed through tensile, impact, hardness, nanoindentation, fracture toughness and fatigue strength tests. Compression tests aided in the analysis of the deformation induced martensitic transformation. The results showed that it is possible to obtain nodular cast irons with high mechanical strength (resistance limit> 1450 MPa), with considerable ductility (up to 9%) and energy absorbed under impact (up to 81 J), as well as fracture toughness of 55 MPa.m -1 / 2 and fatigue limits of 550 Mpa. This behavior is provided by a unique microstructure, consisting of a homogeneous dispersion of martensite plates in a very refined ausferrite matrix, with considerable volumetric fractions of retained austenite (max. 23%).

Investigou-se o efeito do tamanho de grão austenítico na cinética e na morfologia do produto da reação bainítica de um ferro fundido nodular austemperado (FFNA). Foram estudados 3 tamanhos de grão austeníticos, a saber: GG (grão grosseiro), GM (grao mediano); e GR (grao refinado). A condição GR foi obtida pela austenitização rápida de microestruturas martensíticas. A condição GG foi obtida por tratamento de austenitização em duas etapas, de modo a, respectivamente, provocar o crescimento de grão e manter o teor de carbono igual aos das demais condições. Na segunda etapa do tratamento da condição GG ocorreu precipitação de grafita secundária, que acelerou significativamente a taxa de reação bainítica, possibilitando estudar o efeito da.variação na quantidade de interfaces austenita/grafita e austenita/ austenita sobre a cinética e a morfologia da reação bainítica. O refino do grão austenítico acelerou a cinética de reação, aumentou a proporção de ferrita alotriomorfa de contorno de grão, refinou a microestrutura bainítica (ferrita + austenita retida) e melhorou em 14% o limite de escoamento dos FFNA. Finalmente, a predominância de ferrira alotriomorfa na condição mais fina indica que a formação de interfaces incoerentes (mecanismo difusional ao inves de reação displaciva) durante a austenitização rápida da martensita (aquecimento de 100°C/ s). / The effect of austenite grain size on the kinetics and the morphology of the bainitic reaction in an austempered ductile iron (ADI) has been investigated, Three austenite grain sizes were produced: GG (coarse grain), GH (medium grain) and GR (fine grain), The GR condition was obtained by the rapid austenitization of martensitic microstructures The secondary graphite precipitation observed in GG condition strongly accelerated the rate of bainite formation and made possible the study of the effect of austenite/graphite interface on the kinetics of this reaction, The austenite grain refinement also accelerated the bainite precipitation, increased the proportion of grain boundary alotriomorphs ferrite, refined the bainitic microstructure and improved by 14% the yield stress of ADI, The predominance of grain boundary alotriomorphs in GR was associated to the structure of austenite/austenite interface formed during the rapid austenitization of the studied ductile iron at heating rates of 100oC/s. It is suggested that the dominant mechanism of austenitization in this condition is a thermally activated one (not a displacive transformation). The different morphologies of the austenite decomposition used the Duhê\'s morphological system.

Estudou-se a cinética das transformações de fase em resfriamento contínuo e em tratamentos isotérmicos de cinco ligas de aços TRIP microligados com Nb, contendo teores variáveis de Mn e Si, através de ensaios dilatométricos, de caracterização morfológica dos produtos de transformação e de cálculos termodinâmicos e simulações numéricas usando os programas Thermocalc ® e Dictra®. Foram determinados os diagramas RC para a transformação da austenita, e foi estudada a influência da precipitação de ferrita pró-eutetóide e de bainita na fração volumétrica de austenita retida. Através dos diagramas de resfriamento contínuo foi possível delimitar a extensão do campo intercrítico dos cinco aços analisados, com determinação da janela de resfriamento e seus intervalos de temperaturas. Isso permitiu projetar os ciclos de resfriamento controlado a serem aplicados durante o processamento termomecânico dos Aços TRIP-D, TRIP-E e TRIP-H. Os cálculos pelo modelo numérico de redistribuição de carbono e de elementos substitucionais na interface ferrita/austenita, bem como as medidas de microanálise química por WDS e EDS permitiram verificar que a taxa de crescimento da ferrita pró-eutetóide é controlada pela difusão do carbono na austenita. Para tempos curtos de tratamento, o modelo de crescimento que melhor se ajusta é o do equilíbrio local com partição negligível de soluto. Verificou-se através de tratamentos isotérmicos no campo bainítico, que o silício atrasa a precipitação de carbonetos durante a reação bainítica, o que justifica o aumento da estabilidade da austenita retida no aço de maior Si (TRIP-H), quando comparado com o aço de menor Si (TRIP-E). Baseado nos resultados dos estudos das transformações de fase por resfriamento contínuo foram selecionadas as ligas TRIP-D, TRIP-E e TRIP-H, para simular dois esquemas de laminação controlada por meio de ensaios de torção a quente. Nesses ensaios foram variados o grau de deformação e a temperatura de acabamento, de modo a estudar os efeitos dos parâmetros de deformação mecânica na fração transformada dos diferentes constituintes microestruturais, e em particular na fração volumétrica de austenita retida. O primeiro ensaio refere-se à laminação controlada por recristalização estática (LCRE) e o segundo à laminação convencional (LCC), com temperatura de acabamento de 1030°C e 850°C, respectivamente. O resfriamento consistiu em dois tratamentos isotérmicos consecutivos: o primeiro no campo intercrítico (austenita + ferrita), e o segundo no campo bainítico. O aumento do grau de deformação na simulação por torção a quente da laminação controlada por recristalização estática, levou a um aumento da porcentagem de austenita retida obtida durante o resfriamento controlado (de 9 a 14,0 %). O acúmulo de energia de deformação abaixo da TNR na simulação do processo de laminação controlada convencional provocou uma diminuição da fração volumétrica de austenita retida bem como da concentração de carbono contido nela. Os perfis de Mn e C obtidos a partir de análises químicas com EDS e WDS em amostras do aço TRIP-E, deformadas com deformação total de 2,1 e deformação total de 2,8, mostram a contribuição do refinamento de grão para a difusão destes elementos na frente da interface ferrita/austenita, durante a precipitação de ferrita pró-eutetóide. / The phase transformation kinetics of five Nb microalloyed Si-Mn TRIP steels was studied under continuous cooling and isothermal treatments, using dilatometric techniques, morphologic characterization, Thermocalc computational thermodynamics and Dictra numerical simulation. WDS and EDS X-ray microanalysis and Dictra numerical modeling of C, Mn and Si distribution during transformation showed that the reaction is carbon diffusion controlled and growth occurs under local equilibrium with negligible partition. CCT diagrams for austenite transformation were determined and the effect of the amount of proeutectoid ferrite and bainite precipitation on the volume fraction of retained austenite was also estimated. The CCT diagrams allowed determining the boundaries of the critical zone and the processing window to obtain bainite plus austenite microstructures. Based on this information cooling cycles were selected to perform thermomechanical treatments. Three TRIP steels were selected to simulate, in a hot torsion testing machine, two different controlled rolling sequences: Recrystallization Controlled Rolling and Conventional Controlled Rolling. The influence of the degree of deformation and the finishing temperature on the amount of retained austenite was studied. After rolling the cooling cycle comprised two isothermal treatments, one in the austenite + ferrite field and the other in the bainitic field. Increasing the strain during simulation of Recrystallization Controlled Rolling led to an increase in the volume fraction of retained austenite to the range 9 to 14 %. The energy stored during simulation bellow TNR of the Conventional Controlled Rolling led to a decrease in the volume fraction and in the carbon content of retained austenite. The Mn and C contents measured by EDS and WDS analysis of TRIP-E steel, showed that grain refinement due to recrystallization contributes to diffusion of these elements in front of the ferrite/austenite interface during precipitation.

Ou study focuses on the effects of phase transformations on crack propagation. We want to understand the changes of fracture toughness during welding. In this work, fracture toughness is expressed by J-integral. There are many experimental methods to obtain the critical toughness JIC but they are impractical for our investigation during phase transformation. That is the reason why we have proposed a method coupling mechanical tests, digital image correlation and finite element simulation. The fracture tests are implemented on pre-cracked single edge notched plate sample which is easy for machining and heat conduct during phase transformation. The tests are conducted at different temperatures until rupture. Digital image correlation gives us the displacement information on every sample. Each test is then simulated by finite element where the fracture toughness is evaluated by the method G-Theta at the crack propagation starting moment found by potential drop method and digital image correlation technical. Two materials have been studied, 15Cr-5Ni martensitic precipitation hardening stainless steel and 16MND5 ferritic low carbon steel. For these two materials, different test temperatures were chosen before, during and after phase transformation for testing and failure characterization of the mechanical behavior. Investigation result shows that metallurgical phase transformation has an influence on fracture toughness and further crack propagation. For 15-5PH, the result of J1C shows that the as received 15-5PH has higher fracture toughness than the one at 200°C. The toughness is also higher than the original material after one cycle heat treatment probably due to some residual austenite. Meanwhile, pure austenite 15-5PH at 200°C has higher fracture toughness than pure martensitic 15-5PH at 200°C. For 16MND5, the result also proves that the phase transformation affects fracture toughness. The as received material has bigger J1C than the situation where it was heated to 600°C. On the other hand, the material at 600°C just before isothermal bainite transformation after the austenitization during cooling process also has higher fracture toughness than the one at 600°C before austenitization. These two conclusions are consistent well with the result of 15-5PH. But the final situation of 16MND5 after one cycle heat treatment has a slightly smaller J1C than the receiving situation. It means that one cycle heat treatment hasn't an significant influence on 16MND5fracture toughness. Conclusions show that one should pay attention to the heating period before austenitization of the substrate material when people do the welding as the higher temperature will bring the lower fracture toughness during this process. While during cooling period, the fracture toughness doesn't change a lot during, before or after the cooling induced phase transformation. Even for 15-5PH, it has a better fracture toughness after the martensite transformation than before.

The need to develop light-weight and high strength materials for car frames which improve fuel efficiency and provide increased passenger safety during dynamic events such as automobile crashes has been the focus of the steel and automobile industries for the past 30 years. In recent years, the development of high strength steels such as multi-phase TRIP (Transformation-Induced Plasticity)-aided steels have shown great promise due to their excellent combination of high strength and ductility. The savings in automobile weight is provided by the inherent strength of TRIP steels which allows for the use of thinner sections. The TRIP effect is characterized by the phenomenon known as strain-induced martensitic transformation (SIMT) which enhances the work hardenability of such steels as the austenite phase transforms to the much harder martensite phase during plastic straining. This results in a resistance to local necking which subsequently enhances the strength, ductility, and formability of such steels. However, various factors exist which affect the mechanical behaviour of TRIP steels. This study will aim, through the use of finite element models, to investigate the role and influence of each of these factors on the TRIP effect in type 304 austenitic and multi-phase TRIP steels. These factors include the rate at which the martensitic transformation proceeds, the state of stress to which the material is subjected to, the interaction between the surrounding matrix and embedded retained austenite islands in multi-phase TRIP steels, and the volume fraction and morphology of the retained austenite islands. Investigation of these factors will provide further insight on each of their contributions to the TRIP effect in order to exploit the potential benefits offered by these steels.