CREEP-BEHAVIOR OF CR-MO STEEL WELDMENTS

The common modes of creep failure occurring in different Cr-Mo steel w eldments, such as circumferential cracking in the coarse-grain region of heat affected zone (HAZ), transverse cracking in columnar weld meta l and circumferential cracking in the intercritical and overtempered r egions at outer edge of the HAZ (Type IV cracking) are reviewed and re lated to the microstructures in different regions of the weldment Cr-M o steels with high chromium are found to be inherently more resistant to these modes of failure than the steels with low-chromium The micros tructure varies from wrought base metal through transformed HAZ to cas t weld metal. HAZ consists of coarse-grain, fine-grain, intercritical and overtempered regions. Presence of delta-ferrite in the coarse-grai n region of HAZ of high-chromium Cr-Mo steels greatly reduces its grai n growth during welding compared to that in low-chromium Cr-Mo steels. These microstructures and associated mechanical properties in weldmen ts result in creep failure occurring by cracking in the HAZ or weld me tal.