Effect of W addition on the low cycle fatigue behavior of high Cr ferritic steels

A study has been made to investigate the effect of tungsten (W) addition on the microstructures, tensile properties and low cycle fatigue (LCF) properties of 9Cr–1Mo steels at 298 and 873 K. Four steels containing different amount of W (0, 1.2, 1.8 and 2.7W) were normalized at 1050°C for 1 h and tempered at 750°C for 1 h. Microstructural analysis showed that no significant difference was observed in their tempered martensite microstructures of 0, 1.2 and 1.8W, but soft δ-ferrite was observed to form at the prior austenite grain boundaries of 2.7W alloy. With the increase in W content, the yield and tensile strength were increased at both temperatures. LCF life was also increased with the W content up to 1.8%, but decreased in the 2.7W alloy, which was primarily due to the presence of soft δ-ferrite acting as a crack initiation site. The fatigue life of 873 K was reduced as compared with that of 298 K, due not only to the decrease in the strength at high temperature but also to the formation of oxide layers along slip bands, increasing oxide film ruptured slip irreversibility during cyclic deformation.