Surface hardening of IF steel by plasma nitriding: Effect of a shot peening pre-treatment

Shot peening and plasma processes are widely used to improve surface properties of several alloys. In this work, triode plasma nitriding (TPN) was applied to Ti-stabilized interstitial free (IF) steels in an attempt to increase their hardness without compromising their excellent conformability. Shot peening was also trialed before triode plasma nitriding in an attempt to enhance nitriding kinetics and achieve deeper case depths. Triode plasma nitriding was performed at 450 °C, 475 °C and 500 °C for 4 h on Ti-stabilized IF steel. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to characterize the steel structure. Instrumented indentation hardness–depth profiles indicated that a significant hardening effect was achieved after plasma nitriding at 500 °C for 4 h. These nitriding conditions doubled the near-surface hardness of the parent IF steel and led to a case depth of 500 μm. Instrumented indentation hardness-profile data also indicated that the shot peening pre-treatment did not have any beneficial effect on nitriding kinetics as a reduction in case depth occurred after this mechanical pre-treatment. Increased surface roughening promoted by shot peening reduced the nitrogen uptake during nitriding. Dry sliding wear tests also corroborated the benefits of plasma nitriding on Ti-stabilized IF steels, as significantly lower wear volumes resulted after this surface hardening treatment. Although oxidative wear was found to occur in all IF steel samples, wear performance was found to be influenced by load support provided by underlying steel substrates and thickness of compound layers. The best wear performance of solely plasma nitrided samples could be attributed to thicker compound layers and deeper hardened cases compared to shot-peened + plasma nitrided samples, which exhibited shallower case depths and thinner compound layers.