2018 年 104 巻 11 号 p. 664-672
The effect of prior cold deformation on the dissolution speed of carbide in SUJ2 bearing steel subjected to induction heating austenitization at 920°C was examined. Microscopic observations using scanning electron microscopy and transmission electron microscopy, in addition to crystallinity measurements using powder X-ray diffraction, found no trace of the cold deformation effect in the carbide and few crystal defects in the spheroidal carbide particles of an as-received specimen. However, it is important to note that the cold deformation resulted in a refinement of austenite grains during austenitization, which accelerated the dissolution of chromium from the carbide compared to an unprocessed specimen. This led to enhancement of the resultant carbon diffusion in a matrix.
The volume fraction of carbide present at grain boundaries was estimated assuming a simple geometrical model. The relationship between the holding time and the volume fraction of undissolved carbide was derived based on a bulk diffusion model of chromium into an austenite matrix. Experimental results on the dissolution speed of carbide were qualitatively explained by considering the combination of grain boundary diffusion and bulk diffusion. The austenitization behavior of SUJ2 steel is remarkably influenced by the microstructure, in particular for short austenitization times.
