Fatigue crack propagation in martensitic and austenitic steels

Abstract

Fatigue crack growth rates were measured in an annealed and in an aged maraging steel and in three different austenitic steels. Microhardness measurements were used to determine the plane strain plastic zone sizes as a function of ΔK and to evaluate the cyclic flow stress of the material near the crack tip. The presence of a reversed cyclic plastic zone within the monotonic plastic zone was confirmed. The two maraging steels work soften near the tip of the crack while the three austenitic steels work harden. The fatigue crack growth rates of the maraging steels are independent of the monotonic yield stress and are typical of the growth rates of steels with a bcc crystal structure. The crack growth rates in the stainless steels are an order of magnitude lower than for maraging steels for ΔK< 30 ksi √in. The excellent fatigue crack growth resistance of austenitic stainless steels is related to the de-formation induced phase transformations taking place in the plastic zone and to the low stacking fault energy of the alloys.