The influence of the parent metal condition on the cross-weld creep performance in Grade 91 steel

There are thousands of technical papers, reports and other documents which seek to identify the key factors affecting the performance of Grade 91 steel. Yet, as evidenced by the recent reduction in stress allowable values for Grade 91 steel and the introduction of a Type II specification in the ASME B&PV Code, considerable uncertainty still exists regarding the performance, composition and heat treatment. Knowledge regarding the creep performance of weldments is even more limited and opinions on damage development even more diverse. This point is pertinent whether consideration is given to laboratory cross-weld creep behavior or whether the assessment considers thick section components operating under high pressure and temperature. In part, this additional complexity is because the welding thermal cycle introduces a heat affected zone (HAZ). Although it is widely recognized that the HAZ is linked to creep failures of welds which is likely to be life-limiting in structures, there is no agreement over basic approaches such as the correct methods to characterize the microstructures formed, how to classify the structures and the specifics of the region where creep failure occurs. For example, published work states that the locations for failure should be described as the soft-zone, the subcritical heat affected zone (SCHAZ), the over tempered zone (OTZ), the intercritical heat affected zone (ICHAZ) or the fine-grained heat affected zone (FGHAZ). It is thus apparent that for cross-weld behavior in Grade 91 steel these descriptions cannot be simultaneously and technically accurate.