Effect of Prior Austenite Grain Size on the Morphology and Mechanical Properties of Martensite in Medium Carbon Steel

In industrial application, unintentional manufacturing line troubles often consequence in heating raw materials excessively, in terms of either time or temperature. One of the effects of such occurrence is a product with a variation of prior austenite grain size, even if after the heat treatment the end result is the same, martensite. The variation of the prior austenite grain size is believed to vary the end results of the martensite. This undesirable variation includes the variation of fatigue resistance, impact strength, yield strength, hardness, etc. This research studies the effect of the prior austenite grain size on the morphology of the martensite microstructure. The results show that within the typical industrial application of temperature and holding time set up, as holding time or the temperature increases, the prior austenite average diameter increases. The block and packet sizes in the martensite also increase. The variation of mechanical property dependence on the grain size is indeed due to the different characteristics reflected in the martensite morphology. With respect to the same area, smaller grain has more blocks and packets, which agrees with higher dislocation density verified with transmission electron microscopic evaluation.