Internal nitriding of ternary Fe-Cr-Mo alloys; nitride development

Abstract

The precipitation of alloying element nitrides upon gaseous nitriding of iron-based ternary Fe-Cr-Mo alloys was studied employing XRD, LM, SEM and TEM. Hardness- And EPMA N-content-depth profiles were determined. The Cr/Mo-ratio of the alloys influences the nitriding response. Upon decreasing the Cr/Mo-ratio a more gradual transition of hardness and N-content from nitrided zone to unnitrided core and a slower increase of surface hardness and N-content indicate a "weakening" of the nitriding kinetics. Initially nano-sized, cubic, NaCl-type crystal structure nitride platelets, exhibiting a Baker-Nutting OR with the ferrite matrix, precipitate in the nitrided zone. Upon prolonged nitriding, the NaCl-type nitrides coarsen discontinuously in high Cr/Mo-ratio alloys whereas discontinuous precipitation of MoN occurs in low Cr/Mo-ratio alloys.