Study on the structure and wear resistance of two-step salt bath nitrocarburized steel

Abstract

This paper deals with the investigation of two-step salt bath nitrocarburizing technologies. For comparison, ferritic nitrocarburizing was also conducted. By means of metallographic examination, microhardness test, X-ray diffraction analysis, the microstructure, phase construction, surface hardness and hardness profiles of case hardened layers on steel treated by various nitrocarburizing techniques were investigated. The friction coefficient and ring-on-block apparatuses were employed to investigate the friction coefficient and wear-resistance of case hardened layers. Meanwhile, the surface roughness of various nitrocarburized specimens was also tested by a stylus profilometer. The experimental results indicate that thicker compound layers are achieved on the specimen surfaces treated by two-step nitrocarburizing than by ferritic nitrocarburizing; the phase constructions of two-step salt bath nitrocarburized specimen surfaces are different from that observed on ferritic nitrocarburized specimens and two distinct layers are clearly visible on the cross-sections of two-step nitrocarburized specimens which consists predominately of ε-compound (Fe3N) with a tiny minority of Fe3O4 (outer layer) and carbon/nitrogen martensite with a high percentage of austenite (inner later), while the phase construction of specimen surfaces treated by ferritic nitrocarburizing is composed mainly of ε-compound (Fe3N) and a small amount of Fe4N and Fe2–3N.