Low-temperature nitrocarburized 316L stainless steel: Residual stress and mechanical properties

Low-temperature nitrocarburizing combines the benefits of nitriding and carburizing, offering significant potential for enhancing the surface properties of austenitic stainless steel. However, studies on the residual stress distribution and cracking resistance of low-temperature nitrocarburized steel are lacking. This study investigates the effects of treatment temperatures and durations on the residual stress distribution, nano-hardness distribution, and macroscopic tensile properties of low-temperature nitrocarburized 316 L stainless steel. The results indicate that within the range of 410-450 degrees C, the surface hardness is consistently the highest. As the treatment time increases, both the surface nano-hardness and the depth of hardness distribution increase. For samples treated at 470 degrees C for 20 and 25 h, large cluster-like precipitates within the nitrocarburized layer led to the peak nano-hardness appearing near the middle of the layer. Residual compressive stress exists within the nitrocarburized layer, peaking at the interface between the nitrogen-expanded austenite layer and the carbonexpanded austenite layer. At treatment temperatures of 410 degrees C and 430 degrees C, the peak residual compressive stress increases with longer treatment times. Conversely, at treatment temperatures of 450 degrees C and 470 degrees C, the subsurface residual compressive stress peaks remain constant at approximately -1.9 GPa, regardless of treatment duration. Post-nitrocarburizing at 430 degrees C, the yield strength of 316 L increases, but its elongation decreases due to the brittleness of the nitrocarburized layer, which causes cracking at strains of 0.9-1.5 %. This study provides new insights for regulating the residual stress and mechanical properties of low-temperature nitrocarburized austenitic stainless steel.