The P91 steel (9Cr-1Mo-V-Nb) was developed for superheating applications in steam plant facilities. The creep resistance of the welded joint of this steel is limited by the fine-grain area of the heat-affected zone (HAZ). Factors leading to the reduction of creep strength have been examined. It was found that the most effective factor decreasing the rupture creep strength of the fine-grain region of the heat-affected zone in the welded joint of P91 steel, compared to other regions of the weld, is the smaller size of the prior unstable grains. These fine prior unstable grains in the fine-grain area of the HAZ accelerate the growth rate of martensite subgrains, resulting in a softer martensite matrix.

The dependence of the minimum creep rate on the applied stress for the base metal, welded joints, and samples simulating the fine-grain region of welded joints is described using the traditional power law equation. Experimental results, both in the form of creep strain versus time curves and minimum creep rate versus time curves, were used to study the creep behavior of P91 steel welds at 600°C.