Improving the fatigue performance of vanadium and silicon alloyed medium carbon steels after nitriding through increased core fatigue strength and compressive residual stress

The fatigue performance of nitrided medium carbon steels, alloyed with V and Si, and heat treated to form martensite and bainite microstructures, was evaluated using cantilever bending fatigue testing. Hardness testing and X-ray diffraction residual stress measurements were used in conjunction with an analysis of the applied stress distributions to determine vulnerable regions of crack initiation. The analysis was correlated with experimental scanning electron microscopy observations of crack initiation location. Increases in V and Si content lead to higher core hardness and higher core fatigue strength which increase the applied stress needed to initiate fatigue failure originating subsurface below the case. Increases in V and Si content also increase the magnitude of compressive residual stress, which increases the applied stress needed for surface initiated failure. The combination of increased core fatigue strength and magnitude of compressive residual stress improve the fatigue performance after nitriding compared to other nitrided medium carbon steels reported in literature.