Microstructural instability of a welded joint in P91 steel during creep at 600°C

Abstract

The martensitic 10CrMoVNb 91 (P91) steel has been developed for applications under high ultra supercritical pressure for steam power plants. The creep rupture strength of welded joints of this steel is limited by the fine grained region of its heat affected zone (HAZ). The microstructural changes occurring in this region during long term creep tests at 600°C and 70 MPa were examined using optical microscopy and scanning and transmission electron microscopy for comparison with the microstructure of this region after post-weld heat treatment. The factors which result in the reduction of creep rupture strength of the fine grained region of HAZ are discussed. It was found that the most effective factors reducing the creep rupture strength of the fine grained region of the HAZ of a welded joint in P91 steel, in comparison with the coarse grained region of the HAZ and the base metal of the welded joint, are the finer prior austenite grain size of the fine grained region, which accelerates the rate of growth of martensite lath subgrains and enhances creep cavitation, and also the lower peak temperature during welding, which results in a softer martensite matrix.