Grain boundary carbide structure in tempered martensitic steel with serrated prior austenite grain boundaries

An Fe-2%Mn-0.36%C martensite with serrated prior-austenite grain boundaries was derived from work hardened austenite deformed at 1023 K. The martensite was tempered and microstructure of grain boundary carbide was observed by scanning electron microscopy and transmission electron microscopy, and compared with a conventional quench-tempered martensite. Coarse carbide films formed along flat prior-austenite grain boundaries in the conventional quench-tempered martensite. These carbide films disappeared, and the fine and uniform carbide structure was obtained in the tempered martensite derived from the work hardened austenite. On a serrated prior-austenite grain boundary, carbide particles had various sizes and shapes, and some parts of the boundary were free from carbide precipitation. Although carbide particles with various orientations were observed on one serrated prior-grain boundary, almost identically oriented carbide particles existed at a locally flat region of the grain boundary. These changes of carbide structure can be accounted for by the fluctuation of orientation on the serrated prior-austenite grain boundary.