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

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