A comparison of creep rupture behaviour of 2.25Cr-1Mo and 9Cr-1Mo steels and their weld joints

Creep rupture behaviour of 2.25Cr-1Mo and 9Cr-1Mo base metals and weld join ts has been studied at 823 K over a stress range of 100-250 MPa. In the str ess range examined, 2.25Cr-1Mo steel showed better creep rupture strength c ompared to 9Cr-1Mo steel. Both the weld joints displayed a creep rupture st rength inferior to those of their respective base metals. Failure occurred in the respective intercritical region of the heat affected zone (HAZ) in b oth the steel weld joints. At a given applied stress, the difference in rup ture life of base and weld joint was more pronounced in 2.25Cr-1Mo steel th an in 9Cr-1Mo steel and this difference increased with decrease in applied stress. As the presence of Mo2C precipitate imparts creep resistance to 2.2 5Cr-1Mo steel, its absence in the intercritical HAZ region of the joint is held responsible for its poor creep life. On the other hand, 9Cr-1Mo steel which is strengthened by solid solution hardening and dislocation substruct ure, is less prone to degradation in the intercritical region. Also, the pr esence of F-ferrite in the coarse grain region of HAZ of 9Cr-1Mo steel redu ces its grain growth tendency and also its susceptibility to intergranular creep cavitation compared to 2.25Cr-1Mo steel.