Fatigue-crack propagation and fractographic analysis of 18Ni(250) maraging steel tested in argon and hydrogen environments

Abstract

Fatigue-crack propagation studies on an 18Ni(250) maraging steel were performed at room temperature in dry and humid argon and hydrogen environments (atmospheric pressure) to examine the effects of moisture and hydrogen on the rates of fatigue-crack propagation and on the fracture path through the microstructure. The results showed that the rate of fatigue-crack propagation in the 18Ni (250) maraging steel was insensitive to moisture in the test environment. Nearly the same rates of crack propagation were observed in dry argon, humid argon, and humid hydrogen. The influence of dry hydrogen was, however, pronounced, the rate of fatigue-crack propagation in dry hydrogen was nearly three times that observed in the other environments. Analyses of the fracture surfaces by electron fractography showed that similar surface characteristics existed for the specimens tested in dry argon, humid argon, and humid hydrogen. The fracture paths appeared to be transgranular, at least with respect to the prior austenite grain boundaries, and the fracture surfaces consisted largely of ductile fatigue striations. Specimens tested in dry hydrogen, on the other hand, exhibited numerous regions with a cleavage type of surface structure. Relationships between the observed fracture-surface topography and measured crack-propagation rates are considered. The role of hydrogen and water vapor in promoting crack growth are also discussed.