Effects of Alloying Element on Bainitic Transformation in Fe-0.3N Alloy

Abstract

The effects of alloying elements on the bainite transformation behaviors were investigated for Fe-0.3N and Fe-0.3N-1M (M: Si, Cr, Mn, Mo) (mass%) alloys. Nitrogen (N) was introduced into austenite by gas nitriding and subsequent isothermal holding were employed at 500°C. Bainitic ferrite starts to form from the austenite grain boundaries in all alloys. Bainite structure changed from nitride-free bainite (B-I type) to bainite accompanying film-like nitride precipitation between bainitic ferrite (B-II type) with increasing holding time in Fe-0.3N alloy. The transformation kinetics was retarded by addition of all the alloying elements investigated, resulting in larger retained austenite fraction after subsequent cooling to room temperature. The retardation was significant with Cr and Mn addition.

Quantitative evaluation of energy dissipation calculated from the interfacial N concentration showed that the effect of additive elements was small at short holding times. In this region, the dissipation due to the interfacial friction and the transformation strain were dominant. Whereas the effect of alloying elements appeared as the interface velocity decreased with increasing holding time.