Microstructure Observation around Hardness Transition Area of Repeated Induction Heated S45C Shaft

It had been reported that repeated induction heating process decreased the prior austenite grain size and refined the martensitic structure of JIS SUJ2 samples. In the present work, we compared hardness values and structures of hard surface and soft core of three-times repeated induction heated and once tempered (IH Q3T1) JIS S45C steel bar specimen and once induction heated and once tempered (IH Q1T1) bar. Structures consist of three areas: Hard; Transition; and Soft Core Areas. Martensite, ferrite and martensite, and ferrite and pearlite were observed at the Hard, Transition, and Soft Core Areas. From these observations, it was found that ferrite ratio increased with depth from the surface, and repeated induction heating had little effect on the hardness distribution and internal structure.