Eliminating the soft zone for grade 91 steel weldment via enhancing prior austenite grain size of the intercritical heat-affected zone

The soft zone is deemed unwanted for grade 91 steel weldments as it constitutes a cracking-sensitive region deleterious for long-term creep performance. In the present work, formation mechanisms of the soft zone in grade 91 steel weldments have been elucidated and an approach for its elimination has been proposed. It has been demonstrated that a soft zone forms within the intercritical heat-affected zone (ICHAZ) after post-weld tempering. Post-weld normalizing and tempering enables the elimination of the soft zone. Further analysis reveals that the ICHAZ of the as-welded specimen possesses a mixed structure, consisting of over-tempered martensite and fresh martensite with a fine prior austenite grain size. During post-weld tempering, the increase in grain size and the reduction in dislocation density are the main factors contributing to the decrease in hardness. The recrystallization rate of the ICHAZ is faster than that of other regions, resulting in a decrease in hardness from 350.8 HV0.3 to 204.6 HV0.3 and the formation of the soft zone. Increased prior austenite grain size in the ICHAZ, achieved through post-weld normalizing and subsequent tempering, facilitates the formation of tempered martensite and prevents the formation of the soft zone.