Evolution of Microstructure during Welding Simulation of Boron Modified P91 Steel

Formation of heat affected zone (HAZ) when welding is considered as weakest link across transition joint. Characterization of this zone is a key issue to reduce the effect of embrittlement; hence appropriate welding parameters were adopted. In the present study, HAZ simulation was done for boron modified P91 steel using Gleeble. It has been observed that matrix was martensitic in nature for coarse-grain HAZ (CGHAZ)/fine-grain HAZ (FGHAZ) whereas for inter-critical HAZ (ICHAZ), matrix was ferrite + martensite containing primary carbides. The size and fraction of carbides varied with peak temperature. Presence of boron influenced the microstructural characteristics of different zones by influencing precipitation nature and fractional characteristic of phases. Substantial variation in hardness was obtained before and after post-weld heat treatment (PWHT). In this respect, simulated samples exhibited maximum hardness at CGHAZ and after PWHT the same region showed minimum hardness. Short-term impression creep testing of simulated sub-HAZs was done, which revealed embrittling effect in ICHAZ, whereas embrittlement shifted to CGHAZ after PWHT. Based on conventional nomenclature for weldment failures, it is confirmed that PWHT shifted type-IV failure (ICHAZ) to type-III failure (CGHAZ) in boron modified P91 steel.