Structural Development of Expanded Austenite on Duplex Stainless Steel by Low Temperature Thermochemical Nitriding Process

This paper describes the results of gaseous thermochemical treatment of nitriding duplex stainless steel using tube furnace. The nitriding was performed in temperature range between 400°C and 500°C for 6 hours, forming a dual layer structure with hard nitrogen layer. The nitriding gas composition used for this process is 50% NH4 + 50% N2 and 25% NH4 + 75% N2. The structural development was characterised using hardness tester, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Based on the experimental results, it was found that nitrided layer produced maximum thickness nitrided layer about 13.96µm and hardness 666.3 HV0.025 at 450°C with 50% NH4. The formation of expanded austenite was observed in temperature range between 400°C and 500°C. However, the cromium nitride has been developed at temperature 500°C which decreased the corrosion properties of duplex stainless steel.