Effects of nitriding temperature on gas nitriding property of SACM645 steels were investigated in order to develop a new nitriding process. In this investigation, TEM observation was performed to study surface hardening mechanism in gas nitriding.
Effective case depth increased with nitriding temperature was raised. On the other hand, surface hardness decreased as nitriding temperature increased. Precipitation of complex nitride (Al, Cr) N which has f.c.c structure was observed in the nitrided layer. It is proved that, surface hardness increased at lower nitriding temperature because precipitates became fine and were inducing large coherent strain on matrix. In examining a few patterns of temperature profiles in nitriding, it is concluded that the nitriding process with continuous heating showed better nitriding property than other ordinary nitriding processes. The reasons for this are as follows, i) high surface hardness obtained at lower temperature in the early nitriding stage is maintained in higher temperature, ii) enough amount of nitrogen diffuse into the steel during continuous heating.
