Surface modification of duplex stainless steel using TIG torch and nitriding processes for tribological applications

Abstract

Duplex stainless steel (DSS) is widely used in petrochemical, waste incineration and gas turbine industries. However, DSS suffers from poor hardness and wear resistance that limits the applicability of the DSS for wider applications. Surface modification is required to improve the surface characteristics of DSS surface. The main objective of this study is to develop hard surface on DSS using TIG torch and nitriding processes. In TIG torch, the process was performed using different parameters such as silicon carbide (SiC) particle size of 20 μm - 100 μm, current of 80 - 100 A, voltage of 20 - 40 V, transverse speed of 1 -2 mm/s and argon flow rate of 15 -25 L/min. For nitriding, the experiments was conducted at different temperature of 400 ºC – 550 ºC, holding time of 4 - 16 hours and gas mixture ratio of 0.2 - 0.5 NH3/N2. The surface modified DSS was characterized using optical microscopy, scanning electron microscopy and x-ray diffraction. The hardness measurement was carried out using Vickers hardness tester based on ASTM E384, while wear behavior was assessed using ball-on-disc tribometer according to ASTM D6079. A design of experiment based on Taguchi method has been adopted to optimize the TIG torch and nitriding process parameters. The results show that hard surface DSS was successfully developed using TIG torch and nitriding processes. Characterization of the TIG melted layer DSS reveals the existence of new phases containing a dendritic structure, while nitrided layer DSS demonstrated the presence of expanded austenite. The surface hardness was achieved approximately 85 % higher than the DSS substrate with hardness value of 1700 Hv for TIG torch melting and 1598 Hv for nitriding. The wear rate of TIG melted and nitrided surface DSS was reduced by 70 % and 85 % compared to DSS substrate with value of 2.1 x 10-4 mm3/Nm and 1.06 x 10-4 mm3/Nm, respectively. Furthermore, the friction coefficient obtained 0.27 for TIG melted layer and 0.36 for nitrided layer DSS representing 50 % and 63 % better than DSS substrate, respectively. It was revealed that the worn surface showed very mild abrasive wear compared to substrate which demonstrated very severe wear with ploughing marks. The optimum TIG torch and nitriding process parameters have been identified using the Taguchi method. The optimum process parameters for TIG torch on surface hardness and friction coefficient were; current of 80 A, voltage of 20 V, transverse speed of 1.5 mm/s and argon flow rate of 25 L/min. Meanwhile, for wear rate, the optimum parameters were; 80 A, 30 V, 1.5 mm/s and 25 L/min. For nitriding, the optimum process parameters for hardness and friction coefficient were; temperature of 550ºC, holding time of 16 hours and gas mixture ratio of 0.3 NH3/N2, while for wear rate; 550ºC, 16 hours and 0.4 NH3/N2. Current was the most influencing parameter for the improvement of TIG torch melting, whereas, temperature and holding time for nitriding process. It was revealed that both processes are comparable and capable to produce hard surface layer on DSS with higher hardness and wear resistance properties which can be used for tribological and high temperature applications.