Surface nitriding influence on the fatigue life behavior of ASTM A743 steel type CA6NM

Surface treatment such as plasma nitriding has been largely used in several industrial applications because it improves the mechanical, tribological, and chemical properties of materials. This work evaluates the influence of the treatment time on the nitrided layer formation of the martensitic stainless steel ASTM A 743 CA 6NM and its resulting fatigue life behavior. Samples were nitrided at 500 °C and a pressure of 4 mbar (400 Pa) in a gaseous mixture containing hydrogen (H2: 80 vol %) and nitrogen (N2: 20 vol %). Treatment times were 1, 3 and 5 h. The properties of the nitrided layers obtained were assessed through microhardness tests, scanning electron microscopy analysis and X-ray diffractometry to identify the phases formed in the layers, construct the S–N curve and determine the fatigue life for each treatment time. For all treatment times, there was a substantial increase in surface hardness, as well as in fatigue life with respect to the non-nitrided material. These results are associated with the generation of the compound layer along with the diffusion layer and the formation of iron (ε-Fe2–3N and γ′-Fe4N) and chromium nitrides. During nitriding at 500 °C for 1 h, the surface hardness increased by 385% and the fatigue limit increased by 23.76% at 2 million cycles. Higher microhardness gradient between the specimenʹs surfaces and nuclei was observed for lower nitriding times, resulting in higher compressive residual stress values of the surface for 1 h nitriding time and higher fatigue limit.