Combined Gleeble physical welding simulation and low-cycle thermo-mechanical fatigue for heat-affected zone material for 9Cr steel: Experimental testing and through-process model

There is an urgent need to operate thermal power plant at significantly higher temperatures, pressures and flexibility, in order to reduce emissions, increase efficiency and facilitate uptake of renewable energy. This demands significantly improved design of welded connections for thermo-mechanical fatigue (TMF). A common mode of high temperature failure for welded 9Cr steels in such plant is Type IV failure, due to reduced hardness in the inter-critical heat affected zone (IC-HAZ). Little or no work has been previously conducted on TMF characterisation of HAZ of 9Cr steels. This work presents development of a combined Gleeble physically-simulated welding process for P91 heat affected zone, based on measured thermal histories from bead-on-plate welding trials, with in-situ low cycle thermo-mechanical fatigue up to 650°C. The simulated welding process, including post-weld heat treatment (PWHT), is shown to have significant effect on both microstructure and TMF behaviour, including life. The as-welded condition is shown to have the cyclically hardest stable response and the longest life, whereas the PWHT and parent material (PM) cases have similar cyclically soft responses and lives. A recently-developed through-process, physically-based, thermal-metallurgical-mechanical model is adapted and applied to the simulated welding thermal cycle and TMF testing for PM and HAZ specimens. The model is calibrated and validated against high temperature low-cycle fatigue and low-cycle TMF data for PM in the range 400 to 600°C, for different strain-ranges and strain-rates. It is also shown to capture some observed general trends for the simulated HAZ-TMF testing, especially the significant softening effect of PWHT and the significant increase in cyclic strength for the as-welded condition.