PHASE TRANFORMATIONS UPON CARBURIZING Fe-2wt.%Mn Alloy

Abstract

Steels based on Fe-Mn-C system are at the heart of materials research owing to their tunability of properties and both Mn and C being relatively less expensive. On the other hand carburization of steels is a well-known thermochemical surface treatment by which the surface properties of steel components can be drastically enhanced. Considering the general importance of understanding the role of Mn and C in steels, and also to understand the role of Mn on carburizability of steel, this thesis has focused at understanding the phase transformations in Fe-2wt.%Mn model alloy upon pack carburization treatment. Temperatures of carburization were chosen to understand the role of the phases present in the microstructure of base Fe-2wt.%Mn alloy; fully ferritic, ferritic + austenitic and fully austenitic state samples were carburized. For exposing the influence of Mn on carburization, pure Fe specimens were also carburized at identical conditions in selected experiments. Resulting microstructures were characterized by X-ray diffraction, Light and scanning electron microscopies and hardness measurements. In case of Fe-Mn specimens it appears that preexisting austenite phase either as a second phase together with ferrite or entirely to have carburization reaction. Experiments carried at temperatures above the eutectoid temperature and below the fully austenitizing temperature of Fe-2wt.% Mn alloys resulted in the occurrence of dual phase microstructure during carburizing which upon quenching to room temperature resulted in the duplex microstructure. In this case, the carbon seems to be absorbed only by the austenite along the ferrite matrix grain boundaries and it got diffused into the deeper regions along the grain boundary austenite network with simultaneous thickening of the austenite layers on grain boundaries near the surface. Experiments carried at fully austenitized condition resulted in the usual layer type growth of austenite at the surface. In comparison to pure Fe, the carburized layer on Fe-Mn alloy is thicker which has been attributed to the reduced C solubility of Mn-alloyed austenite. Although literature indicates that carburizing cannot be carried in ferritic state, in this study we have observed the growth of austenite layer on ferritic pure Iron substrate carburized at 850oC. This suggests the possibility of applying carburizing to ferritic steels by tuning the carburizing temperature and composition of the steel.