Effect of hydrogen on fatigue crack growth in austenitic Fe-Mn-Al steel

The decrease in fatigue resistance of an austenitic Fe-Mn-Al steel, due to presence of cathodically precharged hydrogen has been evaluated in the present study. Compact tension (CT) specimens precharged with hydrogen and a control without hydrogen charging were subjected to sinusoidal, tension-tension, cyclic loading at stress ratios (R) of 0.1,0.3, and 0.5. The tests were conducted in the applied stress intensity range (Delta K) of 10-60 MPa root m at 10 Hz frequency and plots of fatigue crack growth rate (FCGR) versus Delta K were obtained. It has been observed that presence of internal hydrogen leads to enhancement of FCGR by a factor of 2 to 15 depending on the value of Delta K acid R. The enhancement increased with increase in R and Delta K, and was maximum for ail R values around 40 MPa root m. The fractographic features observed using scanning electron microscope (SEM) support the trends in FCGR data. The crack growth enhancement has been found to increase with increase in Delta K upto a value of 40 MPa root m. Around Delta K = 40 MPa root m, where maximum crack growth enhancement due to hydrogen has been observed, the fracture face had prominent cleavage facets compared to a few small regions of cleavage facets at lesser stress intensities. At stress intensities greater than 40 MPa root m inclusion interface separation and formation of microvoids appear to play a significant role on the fatigue fracture process and correspondingly, the role of hydrogen seems to be insignificant. The origin of the cleavage facets and the consequent enhancement in FCGR due to hydrogen is explained on the basis of hydrogen accumulation and transport at the edge crack tip.