Role of microstructures on heterogeneous creep behaviour across P91 steel weld joint assessed by impression creep testing

Abstract

Microstructure across the single-pass activated-tungsten inert gas (A-TIG) weld joint of P91 steel consisting of weld metal, heat affected zone (HAZ) and base metal were characterized using scanning electron microscopy and electron back scattered diffraction (EBSD-SEM). Prior austenite grain size, boundary misorientations angle, coincidence site lattice (CSL) boundaries, substructure size, combination of recovered and deformed structure, M23C6 precipitates size, decoration of boundaries by precipitates were found to vary significantly within different regions of the joint. Creep deformation across the P91 steel weld joint assessed by impression creep at 923 K in the base metal, weld metal, coarse grain (CGHAZ), fine grain (FGHAZ) and inter-critical (ICHAZ) region revealed heterogeneous creep deformation behaviour; ICHAZ region deformed at higher rate as compared to other regions in the joint. Synergistic effect of the lower fraction of low misorientation and coincidence site lattice (CSL) boundaries, finer grain size, heterogeneous combination of more recovered and deformed structure within the ICHAZ, coarser M23C6 precipitates, and boundaries relatively devoid of precipitates as compared to other regions facilitates preferential localized deformation and cavitation in the soft ICHAZ eventually resulting in premature failure of the actual cross-weld joint. Heterogeneous microstructure and variations in high temperature strength across the joint have led to reduction in creep rupture strength of the single-pass A-TIG weld joint by about 30% as compared to its base metal at 923 K for 104 h.