Role of δ-ferrite in fatigue crack growth of AISI 316 austenitic stainless steel

Abstract

FF sample (nearly free of δ-ferrite) and CF sample (containing ∼4% δ-ferrite) were prepared from the AISI 316 austenitic stainless steel plate to elucidate the role of δ-ferrite in the fatigue crack growth under the solution treated and accelerated aged conditions. It is found that the fatigue crack growth resistance of the CF sample is higher than the FF sample under the solution treated condition. However, a significant deterioration of the fatigue crack growth resistance is observed in the CF sample while little variation is found in the FF sample after accelerated aging treatment at 750 °C for 10 h. In the solution treated condition, deflected crack growth path is present when the main crack encounters the δ-ferrite in the CF sample due to the differences in the fatigue responses between austenite and δ-ferrite. The measured growth rate of the deflected crack is significantly slower than that of the flat crack of the same length. After the accelerated aging treatment, microcracks are produced at the M23C6/δ interface due to the strain incompatibility between M23C6 and retained δ-ferrite when the decomposed δ-ferrite is subject to plastic deformation in the crack tip plastic zone. The preexisting microcracks in the front of crack tip provide a viable path for crack propagation, resulting in the relatively flat crack path.