Combined Effects of Different Overload Ratios and Nitriding Process on Fatigue Crack Growth of AISI 4140 Steel

 The combined effects of different overload ratios (OLRs) and nitriding process on fatigue crack growth (FCG) were analyzed. Steel compact tension specimens made of AISI 4140 steel were subjected to the plasma nitrification process using a gas mixture of 50% N2 and 50% H2 at 500 ℃ for 2 h. Structural properties of the samples were studied by scanning electron microscopy, optical microscopy, microhardness testing and digital imaging. Plasma nitriding slowed down FCG, but the crack growth retardation caused by the overload was reduced when the OLR was 2.0 and 2.5. Nitriding did not take effect when the OLR was 1.5 and 3.0.