Analysis of creep behavior of welded joints of P91 steel at 600 °C

The martensitic P91 steel (9Cr-1Mo-V-Nb) has been developed for ultra-supercritical pressure application in steam power plants. The creep rupture strength of the welded joint of this steel is limited by the fine-grained region of its heat affected zone (HAZ). The factors which result in the reduction of the creep rupture strength of the fine-grained region of HAZ were discussed. It was found that the most effective factor reducing the creep rupture strength of the fine-grained region of the HAZ of the welded joint of P91 steel, in comparison with other regions of the welded joint, is the finer prior austenite grain size. These fine prior austenite grains of the fine-grained region of the HAZ accelerate the rate of growth of martensite lath subgrains which results in a softer martensite matrix. The minimum creep rate dependence on applied stress for base metal, welded joints and the samples simulating the fine-grained region of the welded joints was described by means of the conventional power-law equation. The experimental results, both in the form of strain versus time creep curves and of minimum creep rate versus time creep curves were used to study the creep behavior of the welded joints of P91 steel at 600 °C.