Improved creep strength of alumina-forming austenitic heat-resistant steels through W addition

We investigated the effect of W addition on the creep and precipitation behavior of alumina-forming austenitic (AFA) heat-resistant steels at 700 °C and 160 MPa. Our results showed that the creep rupture time of the W containing AFA (WAFA) increased more than that of the AFA due to enhanced precipitation hardening of Laves phase and suppression of sigma phase formation. W increased the volume fraction of Laves phase and enhanced precipitation hardening by higher driving force for the precipitation and affinity of the Laves phase. Therefore most W were consumed for the precipitation of the Laves phase, however Mo content contributed to the precipitation of M23C6 as well as precipitation of Laves phase due to the lower affinity of Laves phase. The M23C6 in AFA contained enough Mo to form the sigma phase, which served as an initiation site for cavitation and crack propagation. The sigma phase grew and coarsened at the expense of M23C6 and Mo-rich Laves phase to match the stoichiometric composition of the sigma phase, and finally creep strength of AFA was greatly reduced.