Effect of Pre-Welding Tempering on Creep Properties of Grade 91 Steels Weldments

Thermal efficiency of fossil-fire power plants can be increased by higher steam temperature.  To meet this need, creep strength-enhanced ferritic (CSEF) steels such as the 9 Cr steel [ASTM A387 Grade 91] are widely used as tubing and piping in new generation of fossil fired power plants.  However, performance of welds made on CSEF steels, does not always meet expectations.  Microstructures in the fine-grained heat affected zone (FGHAZ) may significantly reduce creep strength leading Type IV failures.  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 M₂₃C₆ carbides in standard pre-welding temperedcondition [6 times higher] than that of the modified pre-welding tempered condition. In the same samples subjected to 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 M₂₃C₆ carbides was observed in FGHAZ.  As a result, during PWHT, new and fine carbides re-precipitated in the FGHAZ and thereby providing good creep strength.