Kinetics and Microstructure of Sulfonitriding of Die-Steels

Abstract

Sulfonitriding is one of the surface treatments of die made of steels for forging and casting. In this surface treatment, annealing under NH3 and H2S containing atmosphere provides the formation of top sulfide layer on the nitrided zone. The surface sulfide zone has anti-seizure and lubrication effects during forging and casting processes. In this study, microstructure development and its kinetics in sulfonitriding of die-steels. Commercial die-steels, JIS-SKT4 and JIS-SKD61, were used. Those steels were annealed under gas-mixture of N2 and H2S at 520°C for 0.5 h and NH3for 2.5 h. Microstructure of the samples was observed by using scanning electron microscopy (SEM) and electron probe micro-analyzer (EPMA). Phase identification of the samples was conducted with X-ray diffraction (XRD). Chemical profile from the surface was obtained by using glow discharge spectroscopy (GDS). A thin continuous layer with 2 -3 μm in thickness was developed on the surface of die-steels. XRD revealed the formation of FeS after sulfonitriding process. The FeS layer was so dense without significant pore network. Results of GDS and EPMA shows that the surface FeS layer include nitrogen and Cr. Inner nitrided zone consisted of mainly Fe3N. Repeating the sulfonitriding process, thickness of the top FeS layer and Fe3N zone increase parabolically. As a result of the marker experiments with Al2O3 particles, top FeS layer was developed by the outward diffusion of Fe and Fe3N zone was grown with inward diffusion of N. Because open pore network in the top FeS layer was not observed, inward diffuses of nitrogen passing through the top FeS layer occurs during sulfonitriding process.