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.

Orientadores: Sergio Tonini Button, Mauro Moraes de Souza / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-08-11T13:19:22Z (GMT). No. of bitstreams: 1 SouzaFilho_Valterde_M.pdf: 4133313 bytes, checksum: a0db5181ce553fa4a0c17b20db4a876b (MD5) Previous issue date: 2008 / Resumo: A conformação a quente, especificamente em prensa horizontal, é um tema de pouco estudo no meio acadêmico, mas interessante pelo emprego da alta taxa de deformação que alcança 90s-1. A microestrutura resultante desse processo é influenciada por algumas variáveis como temperatura, taxa de deformação, tamanho de grão austenítico inicial e taxa de resfriamento. A proposta deste trabalho é prever o comportamento da microestrutura dos aços perante essa alta taxa de deformação com a utilização da simulação numérica. Para tanto, os materiais DIN 20NiCrMo8 MOD e DIN 16MnCr5 MOD foram ensaiados nessa taxa de deformação em um processo de conformação industrial. A comparação do tamanho de grão austenítico obtido da conformação industrial com o tamanho de grão austenítico obtido através da simulação numérica é demonstrada. A influência da taxa de resfriamento sobre a microestrutura para cada material também foi demonstrada. Concluiu-se que a previsão do tamanho de grão austenítico é adequada utilizando-se o software comercial MSC.Superform acrescido do cálculo para crescimento de grãos. A previsão do comportamento mecânico após o processo de conformação utilizando-se de equações da literatura foi insatisfatória, porém pode-se demonstrar a influência da variação da taxa de resfriamento na microestrutura das peças conformada a quente / Abstract: A conformação a quente, especificamente em prensa horizontal, é um tema de pouco estudo no meio acadêmico, mas interessante pelo emprego da alta taxa de deformação que alcança 90s-1. A microestrutura resultante desse processo é influenciada por algumas variáveis como temperatura, taxa de deformação, tamanho de grão austenítico inicial e taxa de resfriamento. A proposta deste trabalho é prever o comportamento da microestrutura dos aços perante essa alta taxa de deformação com a utilização da simulação numérica. Para tanto, os materiais DIN 20NiCrMo8 MOD e DIN 16MnCr5 MOD foram ensaiados nessa taxa de deformação em um processo de conformação industrial. A comparação do tamanho de grão austenítico obtido da conformação industrial com o tamanho de grão austenítico obtido através da simulação numérica é demonstrada. A influência da taxa de resfriamento sobre a microestrutura para cada material também foi demonstrada. Concluiu-se que a previsão do tamanho de grão austenítico é adequada utilizando-se o software comercial MSC.Superform acrescido do cálculo para crescimento de grãos. A previsão do comportamento mecânico após o processo de conformação utilizando-se de equações da literatura foi insatisfatória, porém pode-se demonstrar a influência da variação da taxa de resfriamento na microestrutura das peças conformada a quente / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica

Ce travail est une contribution à l'étude des conséquences mécaniques des transformations de phase à l'état solide dans les aciers, en particulier la plasticité de transformation (TRIP), en prenant en compte l'effet de la taille de grain austénitique (AGS). L'évolution de l'AGS a été étudiée sous différentes conditions d'austénitisation. Des essais de plasticité de transformation ont été conduits avec les mêmes conditions d'austénitisation afin d'observer l'évolution du TRIP avec l'AGS. Trois types de chargement mécanique sont considérés : la traction uniaxiale, la torsion uniaxiale et le cas biaxial de traction + torsion. Les résultats montrent que : i) le TRIP augmente avec l'AGS dans le cas de la traction ; ii) il est indépendant de l'AGS pour la torsion; iii) pour le cas du changement biaxial, le TRIP diminue légèrement. Ceci montre que les modèles décrivant le TRIP doivent être revus afin de prendre en compte l'AGS. / This work is a contribution to the study of mechanical consequences of solid-solid state phase transformations in steels, particularly the Transformation Induced Plasticity (TRIP), and the effect of the Austenite Grain Size (AGS). The evolution of AGS was studied taking into account different austenitization conditions. Then, TRIP tests were carried out with the same conditions to observe the evolution of TRIP with AGS. Three types of loading are considered: the uniaxial tension case, torsion case and biaxial tension and torsion case. The resuslts show that: i)the TRIP increase with AGS in the tensile case. ii) It is independant for the torsion case. iii) For the biaxial loading case, the TRIP decreases slightly. This shows that the micromechanical models describing the TRIP should be reviewed to take account of the AGS.

Firstly, an existing thermodynamic model for the predicting of martensite start temperature of commercial steels has been improved to include more elements such as N, Si, V, Mo, Nb, W, Ti, Al, Cu, Co, B, P and S and their corresponding composition ranges for Martensitic transformation. The predicting ability of the existing model is improved considerably by critical assessment of different binary and ternary systems i.e. CALPHAD approach which is by wise selection of experimental data for optimization of the interaction parameters. Understanding the degree of variation in multi-component commercial alloys, various ternary systems such as Fe-Ni-X and Fe-Cr-X are optimized using both binary and ternary interaction parameters. The large variations between calculated and the experimental values are determined and reported for improvements in thermodynamics descriptions.Secondly, model for the prediction of Epsilon martensite start temperature of some commercial steels and shape memory alloys is newly introduced by optimizing Fe-Mn, Fe-Mn-Si and other Fe-Mn-X systems considering the commercial aspects in the recent development of light weight steels alloyed with Al and Si.Thirdly, the effect of prior Austenite grain size (pAGS) on martensite start temperature is introduced into the model in the form of non-chemical contribution which will greatly influence the Gibbs energy barrier for transformation. A serious attempt has been made to describe the dependency of transition between lenticular and thin-plate martensite morphologies on the refinement of prior Austenite grain size.Finally, the model is validated using a data-set of 1500 commercial and novel alloys. Including the newly modified thermodynamic descriptions for the Fe-based TCFE9 database by Thermo-Calc software AB, the model has the efficiency to predict the martensite start temperature of Multi-component alloys with an accuracy of (±) 35 K. The model predictability can be further improved by critical assessment of thermodynamic factors such as stacking faults and magnetism in Fe-Mn-Si-Ni-Cr systems.

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.

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.

The use of pre-hardened mold steels has increased appreciably over the years; more than 80% of the plastic mold steels are used in pre-hardened condition. These steels are delivered to the customer in finished state i.e. there is no need of any post treatment. With hardness around ~40HRC, they have properties such as good polishability, good weldability, corrosion resistance and thermal conductivity. Machinability is a very important parameter in pre-hardened mold steels as it has a direct impact on the cost of the mold. In normal machining operations involving intricate or near net shapes, machining constitutes around 60% of the total mold cost. Efforts are underway to explore every possible way to reduce costs associated with machining and to make production more economical. All the possible parameters which are considered to affect the machinability are being investigated by the researchers. This thesis work focuses on the effect of prior austenite grain size on the machinability of pre-hardened mold steel (Uddeholm Nimax).  Austenitizing temperatures and holding times were varied to obtain varying grain sized microstructures in different samples of the same material. As it was difficult to delineate prior-austenite grain boundaries, experimental and empirical methods were employed to obtain reference values. These different grain sized samples were thereafter subjected to machining tests, using two sets of cutting parameters. Maximum flank wear depth=0.2mm was defined for one series of test which were more akin to rough machining, and machining length of 43200mm or maximum wear depth=0.2mm were defined for second series of tests which were similar to finishing machining. The results were obtained after careful quantative and qualitative analysis of cutting tools. The results obtained for Uddeholm Nimax seemed to indicate that larger grain sized material was easier to machine. However, factors such as retained austenite content and work hardening on machined surface, which lead to degradation of machining operations were also taken into consideration. Uddeholm Nimax showed better machinability in large grained samples as retained austenite(less than 2%) content was minimal in the large grained sample. Small grained sample in Uddeholm Nimax had a higher retained austenite (7+2%) which resulted in degradation of machining operation and a lesser cutting tool life.