Phase transformations and numerical modelling in simulated HAZ of nanostructured P91B steel for high temperature applications

This paper critically assesses phase transformations occurring after welding and subsequent post weld heat treatments in simulated sub-heat affected zones (HAZ) of P91B steel. Samples for weld-HAZ simulation were produced corresponding to grain-coarsened HAZ, grain-refined HAZ and inter-critical HAZ. Analyses revealed diverse phase transformation mechanisms (for GCHAZ = pipe-diffusion and for GR/ICHAZ = GB-diffusion) owing to manipulation in grain size and boron-enriched nanosized particles as regards virgin steel after welding. However, after PWHT, same phase transformation mechanism (interface diffusion) in all simulated sub-HAZs is observed. Hardness evaluations and prior austenite grain boundaries dissolution confirm GB embrittlement after welding. Boron segregation, the presence of borides and boron-enriched particles heads to ~ 50% drop in hardness deviations enhancing GB hardening after PWHT. Particle refinement is observed after PWHT which is further validated by numerical modelling. In addition, particle evolution during cooling from peak temperature of weld thermal cycle and isothermal holding of PWHT is analysed. Apparent activation energy of nucleation/growth follows descending order, i.e. GC/GR/ICHAZ for nanosized particles during welding.