The effect of thermal history during fabrication on the mechanical properties of weldments in grade 91 creep resistant steel

Abstract

The creep resistant steel, grade 91, is in widespread use for steam plants. However, poor cross–weld creep strength of welded joints compared with that of the parent material has become a major concern for many operators. Creep cracking in the more refined regions of the heat affected zone, known as type IV cracking, is the main failure mechanism associated with poor cross–weld creep performance. The effect of unplanned or unusual heat treatments (HTs) or lack of HT during fabrication of boiler components on the mechanical performance of weldments in P91 has been investigated. Reduced pressure electron beam welding equipment was used to produce through thickness melt runs in 28-mm pipe, in order to produce a HAZ that is free of reheated regions. The HTs investigated were shown to have a negligible effect on the cross–weld creep rupture life at 630 °C. The implications of these findings for industry are discussed.