Nitriding of AISI 316L Austenitic Stainless Steel at Low Temperature for the Enhancement of Surface Properties and Corrosion Properties

Austenitic stainless steel (ASS) makes up over 70% of total stainless steel production and has been used widely in many industrial fields such as automotive, oil & gas, chemical, medical and food industries due to their excellent corrosion resistance. However, the applications of this material are severely limited by very poor surface hardness as well as wear resistance. Many investigations have been conducted to improve surface hardness of ASS and thus enlarging their possibility of wider application, but led significant loss of its corrosion resistance. This tendency occurs due to the sensitivity effect. The objectives of this investigation are to improve the surface hardness as well as corrosion properties of AISI 316L austenitic stainless steel. Theoretically, the low temperature treatment is to avoid the formation of nitride and carbide precipitation which can reduce the corrosion resistance of stainless steel. AISI 316L stainless steel has been treated by surface hardening technique, which is low temperature gas nitriding treatment (LTGN) in quartz tube furnace. The variable of nitriding temperature of this treatment is at 400°C, 450°C and 500 °C with constant 8 hours of nitriding time. The morphological changes, structural phase and elemental profile of the treated samples were observed by optical microscope (OM), field emission scanning electron microscopy (FESEM) and energy dispersive X-ray (EDX). From the results, the microstructure and phase composition depend on the nitriding temperatures. X-ray diffraction (XRD) has been used to confirm the formation of nitrides on the surface layer of the samples. The surface microhardness was investigated using Vickers hardness tester. The results showed that the surface microhardness values increased as the treatment temperature increased which has close relationship with elemental depth profile. The corrosion properties of unnitrided 316L austenitic stainless steel and nitrided 316L austenitic stainless steels at low temperature, e.g: 400oC and high temperature e.g: 500oC were evaluated by measuring polarization curves in 3.5% NaCl solution in CO2 environment. From the result, gas nitriding at a low temperature significantly improved the corrosion properties of sample. However, when gas nitriding is carried out at a high temperature of 500oC , chromium nitrides form in the nitrided case so that the bulk of the nitrided case had very poor corrosion resistance.