Long term creep behaviour of welded joints of P91 steel at 650 °C

The martensitic P91 steel (9 Cr-1Mo-V-Nb) has been used for high temperature components in the highly ultra super critical pressure steam power plants. The creep behaviour in terms of creep rupture strength and minimum creep rate of the welded joints of this steel is controlled by that of the fine-grained heat affected zone (FGHAZ). The factors which results in the reduction of the creep rupture strength and the increase in the minimum creep rate of FGHAZ at 650 °C were determined and discussed. It was found that the main difference between the welded joint zones is the prior austenite grain size. The finer the prior austenite grain size as in FGHAZ, the higher is the rate of recovery of dislocations, the rate of coarsening of the subgrain structure and coarsening of the intergranular Cr-rich carbide particles, consequently, the lower the creep resistance.