Nitriding of ternary Fe–Cr–Mo alloys; role of the Cr/Mo-ratio

Ternary Fe–Cr–Mo alloys, all containing about 2at.% total alloying element content (Cr+Mo) but with atomic Cr/Mo-ratios ranging from 1.0 to 7.2, were nitrided at 580°C, 550°C, and 520°C. The nitrided zone was characterized by light microscopy, hardness measurements, electron probe microanalysis and X-ray diffraction. The alloying element nitrides forming in the nitrided zone were characterized by scanning and transmission electron microscopy. Initially, the continuous precipitation of finely distributed, “mixed”, cubic NaCl-type (Cr,Mo)Ny nitride platelets, exhibiting a Baker–Nutting orientation relationship with the ferrite matrix, occurs. The developing N-concentration-depth profiles indicate that alloys with a lower Cr/Mo-ratio show a slightly slower nitride-precipitation rate. The attainable surface hardness and the residual surface macrostress are of similar magnitudes for all Cr/Mo-ratios. Upon prolonged nitriding (=aging of the nitrided microstructure), a discontinuous transformation takes place leading to the formation of a coarsened lamellar microstructure. The nature of this discontinuous reaction changes from a discontinuous coarsening of the initial (Cr,Mo)Ny nitride platelets for alloys of high Cr/Mo-ratios to a discontinuous precipitation of hexagonal CrMoN2 for alloys of low Cr/Mo-ratios. The latter reaction, as compared to the first one, is driven by a larger driving force and thus the extent of the discontinuous transformation increases considerably with decreasing Cr/Mo-ratio. A summarizing discussion on precipitation of “mixed” vs. separate nitrides is presented.