Effect of pre-welding tempering on creep properties of grade 91 steels weldments

Current research suggest that modification of the ASME A387 Code-specified tempering of Grade 91 [i.e. reducing pre-weld tempering temperature from 760°C to 650°C] has the potential to increase creep life in the welded plate. To understand this improvement, weld samples were made with standard condition [normalizing (1080°C), tempering (760°C), welding and post-weld heat treatment (760°C)], as well as, samples with pre-weld tempering temperature at 650°C. Creep life of welds made with modified tempering was almost 5 times longer than that of standard welds. Optical and electron microscopy characterization exhibited higher volume fraction of M ₂₃C₆ carbides in standard pre-welding tempered condition [5 times higher] than that of the modified pre-welding tempered condition. In the samples subjected to standard pre-weld tempering and welding, carbides at martensite lath and prior austenite grain boundaries did not dissolve completely in the FGHAZ region. Therefore, reduced creep strength in these samples is correlated to coarsening of these carbides during PWHT. In contrast, the samples subjected to modified tempering; nearly 100% dissolution of small M ₂₃C₆ carbides was observed in FGHAZ. As a result, during PWHT, new and fine carbides reprecipitated in the FGHAZ and thereby providing good creep strength.