Creep deformation and rupture behaviour of service exposed P91 weld and base steel measured by miniature tensile creep testing

Abstract Early cracking had occurred at a weld joint of a service-exposed P91 steel hot reheat pipe branch. In this study, a series of miniature tensile creep tests were conducted for both the base and weld materials extracted from the retired P91 pipe weld. Tests were conducted at various stress levels at 569 °C to measure the creep properties and investigate the failure mechanisms. Microstructural analyses were also carried out for post-test specimens. The measured creep strengths were represented in terms of the Larson-Miller parameter, and compared to literature data. Most of the data fell within the 95% error band of grade 91 steel. However, the weld metal exhibited higher creep rupture strengths. It showed creep brittle fracture, and base metal showed creep ductile behaviour. The weld metal sometimes had non-uniform microstructures, yielding a seriously tempered zones with minor tempered martensites, thereby causing early creep ductile failure of the weld.