Effect of pre-existing carbides prepared by different heat treatments on the nitriding behaviour during a carburizing and nitriding duplex treatment of an M50NiL steel

This article reports the effect of pre-existing carbides prepared by different heat treatments on the nitriding behaviour during a carburizing and nitriding duplex treatment of an M50NiL steel. The goal was to achieve an improved understanding of the relationship between the pre-existing carbides and nitriding behaviour. The M50NiL steel was carburized at 950 degrees C, heat treated at different quenching temperatures (ranging from 1000 to 1150 degrees C) and tempering temperatures (ranging from 500 to 580 degrees C), and finally nitrided at 500 degrees C. The surface layers were characterized by means of X-ray diffraction (XRD), laser scanning confocal microscopy (LSCM), scanning electron microscopy (SEM), Vickers microhardness measurement and transmission electron microscopy (TEM). The results show that the formation of intergranular precipitates was greatly related to the presence of pre-existing carbides, which led to a change in the carbon content in the matrix and showed different dissolving capacities. The compound layers consisted of gamma' (Fe4N) and e (Fe2-3N) phases as well as a minor amount of Fe3O4. The pre-existing carbides had no effect on the composition of the compound layer. For the diffusion of nitrogen, a density of pre-existing carbides that was too high or too low was unfavorable, as it greatly decreased the thickness of the nitrided layer. At the quenching temperature of 1100 degrees C and the tempering temperature of 540 degrees C, the thickest nitrided layer herein and a good hardness distribution were obtained, which was attributed to the optimum density of pre-existing carbides. This study provides information regarding how to obtain a thick nitrided layer by controlling pre-existing carbides during the duplex treatment of M50NiL steels.