Correlation of precipitate stability to increased creep resistance of Cr–Mo steel welds

Abstract An order of magnitude decrease (from 16.0 × 10 −4 to 4.1 × 10 −4 % h −1 ) in steady-state creep rate was observed in the fine-grained heat-affected zone (HAZ) of a Cr–Mo steel weld by the reduction of the pre-weld tempering temperature from 760 °C (HTT) to 650 °C (LTT). The microstructure during each stage of the manufacturing path, including pre-weld temper, thermal cycling and post-weld heat treatment, was characterized using a suite of characterization techniques. The techniques included simulated thermal cycling, dilatometry and electron microscopy, as well as time-resolved X-ray diffraction using Synchrotron radiation. Both LTT and HTT steels before welding contain M 23 C 6 (M = Cr, Fe) and MX (M = Nb, V; X = C, N) precipitates in a tempered martensite matrix. During simulated HAZ thermal cycling with different peak temperatures, changes in M 23 C 6 carbide characteristics were observed between the HTT and LLT conditions, while MX precipitates remained stable in both conditions. Simulated post-weld heat treatment samples show larger M 23 C 6 in the HTT condition than in the LTT condition. The results provide a solution to extending the life of Cr–Mo steel welded structures used in power plants.