Element Partitioning And Its Effects On Microstructure Evolution During Phase Transformations In Fe-Mn-C Alloys

Enrichment of alloying elements and carbon in austenite during ferritetransformations and intercritical annealing of martensite is crucial to the production oftransformation induced plasticity steels. But in alloy steels, alloying elements can noteasily diffuse accompanying with carbon in the above-mentioned transformations, sincethe diffusion coefficient of alloying elements is much less than that of carbon. It thuscauses considerable controversy on the partitioning behavior of alloying elements andcarbon between product and parent phases. Thus, in this study Fe-Mn-C ternary alloyswere employed to investigate the partitioning behavior of Mn and carbon in theabove-mentioned transformations, and to reveal the effects of Mn and carbonpartitioning on the microstructure evolution in the above-mentioned transformations. Inaddition, once pearlite formation occurs, it will consume large amounts of carbon inaustenite. To help suppress pearlite formation in the production of transformationinduced plasticity steels, pearlite formation conditions in hypoeutectoid Fe-Mn-C alloyswere studied in this study.The effects of transformation temperature, nominal carbon content, Mn content andaustenite grain size on the partitioning behavior of alloying elements and carbon inisothermal ferrite transformations were investigated by using Fe-Mn-C alloys. A steadystate occurs at later stages of ferrite transformation due to Mn partitioning between theproduct and parent phases. As for Fe-2Mn-C alloys, it was found that carbonenrichment of austenite in the steady state of ferrite transformation agrees relativelywell with the prediction of PLE/NPLE model (partitioning local equilibrium/negligiblepartitioning local equilibrium), but much lower than the prediction of PE model(paraequilibrium). Carbon enrichment of austenite in the steady state can be increasedby decreasing austenite grain size. It can be attributed to a transition of partitioningmode of alloying elements, namely from PE in the early stages of the transformation toNPLE in the later stages.Isothermal proeutectoid ferrite and pearlite transformations were performed inhypoeutectoid Fe-Mn-0.3C alloys. It was found that lamellar pearlite can form evenwhen carbon content in austenite is lower than the Acmcomposition. It is supposed to arise from a two-dimensional diffusion of carbon at austenite/pearlite interface. Inaddition, proeutectoid ferrite fraction with respect to pearlite increases with decreasingaustenite grain size, but the thickness of grain boundary ferrite is constant.The effects of annealing temperature, nominal carbon content and pre-temperingtreatment on microstructure evolution and transformation kinetics in isothermalintercritical annealing of martensite were investigated by using Fe-Mn-C alloys. Twotypes of austenite are formed: acicular and globular austenite, which have differentnucleation site and different orientation relationship with the parent martensite. In theearly stage of annealing, only carbon atoms are redistributed between the productaustenite and parent martensite. As isothermal annealing time increases, theredistribution of Mn atoms starts to occur. In addition, the carbon concentration ofaustenite in early stage of annealing is beyond orthoequilibrium Ae3line. It isinterpreted as that Gibbs free energy is dissipated at interface during austenite formation,leading interfacial concentrations to deviate from equilibrium concentrations.Pre-tempering at623K promote the formation of globular austenite in isothermalannealing. It can be attributed to the increase of number density and particle size ofcementite particles, which are formed during low-temperature tempering. Pre-temperingat923K retards the formation of globular austenite. It presumably arises from Mnenrichment in cementite, which leads the austenite formation accompanied withcementite dissolution being controlled by Mn diffusion.