Variation of carbon concentration in proeutectoid ferrite during austenitization in hypoeutectoid steel

摘要

The variation of the C concentration in proeutectoid ferrite (alpha(PF)) during austenitization in hypoeutectoid steels was quantitatively investigated using the massive transformation start temperature (T-m) of alpha(PF) to austenite (gamma) measured by high-temperature confocal laser scanning microscopy and hardness of alpha(PF). The C concentration in alpha(PF) at T-m in hypoeutectoid steels increased with increasing total C concentration up to approximately 0.2 wt.% during heating. The hardness of alpha(PF) with isothermal holding time at 775 degrees C in S20C steel revealed C enrichment in alpha(PF) at the early stage of isothermal holding and its reduction with further holding. These results explain the redistribution of the C in alpha(PF) during austenitization as follows: free C atoms released from cementite during pearlite decomposition diffuse excessively into neighboring alpha(PF) as well as pearlitic ferrite. The supersaturated C concentration in alpha(PF) is reduced during the long-range diffusive transformation of alpha(PF) to gamma. However, some of the excess C atoms still remain in alpha(PF) until alpha(PF) starts to massively transform to gamma. (C) 2014 Elsevier Inc. All rights reserved.