Creep Deformation and Rupture of 9Cr-1Mo and 2.25 Cr-1 Mo Weld Joints

Creep rupture behaviour of 2.25Cr-1Mo and 9Cr-1Mo base metals and weld joints has been studied at 823 K over a stress range of 100 - 250 MPa. In the stress range examined, 2.25Cr-1Mo showed better creep rupture strength than that of 9Cr-1Mo steel. Both the weld joints displayed inferior creep rupture strength than their respective base metals. Failures occurred in ths intercritical region of heat affected zobe (HAZ) for both the weld joints. At a given applied stress, the difference in rupture life of base and weld joint was more pronounced for 2.25Cr-1Mo than for 9Cr-1Mo steel and this difference increased with decrease in applied stress. As presence of MOj,C precipitates imparts creep resistance to 2.25 Cr-1 Mo steel, its absence in intercritical region of joints is responsible for poor creep life. On the other hand, 9Cr-1Mo, which is strengthened by solid solution hardening and dislocation substructure, is less prone to degradation due to heating in intercritical region. Also presence of 5-ferrite in the coarse-grained region of HAZ of 9Cr-1Mo reduces its grain growth tendency and also its susceptibility to intergranular creep cavitation compared to 2.25Cr-1Mo steel.