PLASMA NITRIDING TIME ON THE HARDNESS AND CRYSTAL STRUCTURE/PHASE OF SUS403 AND SCS6 MARTENSITIC STAINLESS STEELS: AN ANALYTICAL STUDY

The purpose of this study is to analyze the effect of plasma nitriding time on the physicochemical properties and mechanical hardness of commercially available martensitic stainless steels, SUS403 (JIS G4304) and SCS6 (JIS G5121). Both steels were chosen as the model materials because of their general applications, such as for pressure vessel, valve, turbine, pump, and conveyor belt in petroleum industry, in which high strength and good corrosion resistant materials are demanded. The nitriding processes were carried out by plasma process at fixed temperature of 450 degrees C with varied processing times of 20, 30, and 40 hours. The results show that the addition of nitrogen has a significant role in modifying the microstructure, phase balance, and hardness of both steels. Varying the processing time has a direct impact to the penetration depth of nitrogen into the material. Interestingly, despite similar martensitic properties, we found that the nitrided layer on SUS403 and SCS6 specimens exhibits different iron nitride layer characteristics, i.e., crystal structure/phase and hardness profile. It was revealed that the processing time of 20 hours is suitable to form an Fe4N compound layer. Longer than this period is just increasing the iron-nitride layer thickness. In addition, the present study also discussed the formation process of iron nitride compound-layer during plasma nitriding process.