Transition from Type IV to Type I cracking in heat-treated grade 91 steel weldments

Increasing the post-weld heat-treating temperature from 600 °C to 840 °C causes a transition of creep rupture locations from the Type IV cracking in the intercritical heat-affected zone to the Type I cracking in the fusion zone in the Grade 91 steel weldments. The specimens following a heat-treatment at temperatures below the A1 temperature of the base metal failed in the Type IV cracking mode after high-temperature creep tests at 650 °C. The creep-damaged region associated with the Type IV cracking mode is consistently located in a softened zone, which is identified as the intercritical heat-affected zone. In the specimens heat-treated at temperatures close to or above the A1 temperature of the base metal, the fusion zone enters the intercritical temperature between its A1 and A3 temperatures. The fusion zone experienced a partial austenitization to become a mixture of retained ferrite and new martensite, which is believed the most creep-susceptible. The same microstructure, a mixture of retained soft ferrite and new hard martensite, caused nucleation and growth of cavities in the both Type IV and Type I damaged regions. Therefore, it is suggested that the same creep mechanism may have contributed to both cracking modes in Grade 91 steel.