Creep and microstructural development in P91 weldments at elevated temperature

Abstract

This research concerns weldments in P91 steel and their creep behaviour. Its scope covers three main topics: the microstructure and creep response of the (i) weld metal, (ii) parent metal, and (iii) the effect of extended thermal exposure and creep on the weldments. Microstructural examination of the weld metal revealed an inhomogeneous structure, with each bead consisting of a columnar region, a coarse-grained region and a fine-grained region (the latter two regions resulting from heat-treatment of the weld bead by deposition of subsequent beads). The columnar regions exhibited high hardness whereas the coarse and fine grained regions exhibited lower hardnesses. SEM imaging revealed that the precipitate distribution throughout the weld was somewhat inhomogeneous, due to inadequate mixing in the weld pool during welding, leading to segregation and liquation effects. Examination by TEM revealed a fine martensitic structure with a distribution of chromium carbides, in addition to Mn-rich inclusions. Anisotropy of microstructure was assessed by metallographic examination on planes with normals parallel to and perpendicular to the welding direction. Creep tests on this material were performed, with the stress axis both parallel and perpendicular to the welding direction. Anisotropic creep behaviour was observed and correlated with the microstructural anisotropy. Failure life is significantly longer when uniaxial creep stress is parallel to the welding direction. The columnar regions of the weld were observed to be creep-strong with a low strain to failure whereas the coarse and fine grained regions were observed to be creep-weak with a higher strain to failure. Microstructural variations within weldments as a function of time and temperature have also been investigated. Specimens were aged at five temperatures between 760°C and 650°C for up to 12000 hours. At all exposure temperatures, the parent metal showed little change in terms of fine (subgrain) microstructure and hardness. Significant degradation of the weld metal microstructure was observed. This consisted of recrystallisation, emanating from the weld bead boundaries; in some cases, the recrystallised areas made up approximately 40 % of the metallographic section. The hardness of the recrystallised regions was typically 170 kgf mm-2, whereas that of the non-recrystallised areas was 240 kgf mm-2. TEM examination of the weld metal showed significant change, in the form of transformation of fine martensitic lath structure to larger, more equi-axed subgrains. Creep tests of aged crossweld samples showed accelerated minimum strain rates and reduced failure lives. It was also observed in crossweld specimens creep-tested at three stress levels between 70 MPa and 93 MPa that the failure location moved from the fine-grained HAZ to the parent at the highest test stress. The HAZ failures exhibited extensive cavitation restricted to the HAZ, and low failure ductility. The high stress parent metal failure, on the other hand, showed high ductility and extensive voiding and grain deformation within the parent metal microstructure. An assessment of the effect of strain on microstructural evolution has been made. This is deemed significant, and strain is believed to accelerate precipitate coarsening and martensite recovery processes