Influence of long-term aging at 520 °C and 560 °C and the superimposed creep stress on the microstructure of 1.25Cr-0.5Mo steel

Abstract

In order to understand the influence of high-temperature aging effects and those of the superimposed creep stress on the microstructural variations in a 1.25Cr-0.5Mo steel, the shoulder as well as gage portions of specimens subjected to stress-rupture tests at 520 °C and 560 °C have been studied by transmission electron microscopy. In the normalized and tempered condition, the microstructure of the steel consists of 90 pct ferrite and 10 pct bainite, and M₃C is the only carbide present in bainite and at a few ferrite grain boundaries. On aging at 520 °C for 5442 hours, Cr₂N precipitates in a fibrous form at ferrite-bainite interfaces, and the creep stress has enhanced this mode of precipitation. On holding for 13,928 hours at 520 °C, fibrous carbide is still present but its composition has changed to Mo₂C, while the superimposed creep stress has promoted the precipitation of Mo₂C needles with fine globular precipitates of M₂₃C₆. Aging at 560 °C for 1854 or 10,338 hours has resulted in the precipitation of longer Mo₂C needles and ellipsoidal M₂₃C₆ carbide precipitation; the superimposed creep stress has resulted in a more dense precipitation of shorter needles in both cases. There is some recovery of bainitic ferrite at 560 °C, though the cementite coarsening is negligible.