Contribution of coarsening of MX carbonitrides to creep strength degradation in high chromium ferritic steel

Abstract

The coarsening process of MX carbonitrides during creep and its effect on creep behaviour have been investigated for P92 steel (9Cr-0.5Mo-1.8W-VNb). At 1023 K after long term aging, creep rupture strength and tertiary stage creep rate rapidly decrease and increase, respectively. The stress exponent n of minimum creep rate decreases with increasing temperature, and the value of n at 1023 K was evaluated to be 5.7. MX carbonitrides coarsen during creep and the coarsening process corresponds to the early stage of Ostwald ripening. Creep deformation accelerates the coarsening of the MX carbonitrides. At 1023 K, the coherent strain between the MX carbonitrides and matrix reduces to a negligibly small value at times longer than 550 h, leading to a decrease in the resistance to dislocation motion. This contributes to the rapid increase of creep rate in the tertiary stage and the abrupt decrease of creep rupture strength in the long term.