Effects of microstructure on fatigue crack initiation and propagation of 16Mn steel

In the present study, the effect of microstructure of 16Mn steel on fatigue crack initiation (FCI) life and fatigue crack propagation (FCP) rates was experimentally investigated under two different conditions,i.e., as-received condition and high-temperature normalized (H.T.N.) condition. The microstructure of 16Mn steel under the as-received condition is ferrite and pearlite, which corresponds to that of the base metal of welded elements, and the microstructure under the H.T.N. condition is mainly coarse Widmanstätten structure, which can be thought of as the simulated microstructure at the weld toe. The fatigue test results show that the high-temperature normalization results in the increase of FCP rates in near-threshold region and the decrease of both FCI and FCP thresholds, and FCI life of 16Mn steel. Little effect of the microstructure is observed on the FCP mechanism in the intermediate range (da/dN=10^?8 to 10^?6 m/cycle). Based on the test results and analysis, the general expressions are given for both FCI life and FCP rates under the two conditions. It is pointed out that which of the test results should be applied to prediction of FCI life and FCP life depends upon the FCI location and FCP path in the welded elements.