Internal nitriding of Fe-Cr-Mo alloys; precipitation of mixed nitrides and role of the CrIMo-ratio

Abstract

Controlled gaseous nitriding of ternary Fe-Cr-Mo alloys leads to the development of ternary, mixed nitrides in the ferrite matrix, which show complex chemical, structural, and morphological transformations as a function of nitriding time: initially continuous precipitation of fine, coherent, cubic, NaC1- type (Cr,Mo)N nitride platelets develop, which later transform to a novel, hexagonal CrMoN2 nitride by a discontinuous precipitation reaction. Some of relatively coarse cubic nitrides also occur in the ferrite lamellae. The Fe-Cr-Mo alloys with varying Cr/Mo ratio, but all containing a total alloying element (Cr+Mo) content of 2 at.%, showed similar kinetics of continuous precipitation of cubic (Cr,Mo)N nitride. The kinetics of the discontinuous precipitation reaction is faster for the alloys with lower Cr/Mo ratio. © 2015 ASM International®.