Influence of thermal aging on the microstructure and fatigue properties of modified 9Cr-1Mo steel.

摘要

Results of elevated-temperature low cycle fatigue and creep-fatigue tests are for one heat of modified 9Cr-1Mo steel in the normalized and tempered condition, after pre-aging 50,000 h at 538 and 593C, and after pre-aging for 75,000 h at 538C. These data show that pre-aging reduces the low cycle fatigue and creepfatigue lives in comparison to unaged material. The magnitude of these reductions are discussed along with the impact of pre-aging on the creep-fatigue damage diagrams. The effect of environment on creep-fatigue life of unaged modified 9Cr-1Mo steels is also addressed. Transmission electron microscopy explains changes in mechanical properties due to thermal aging. In the unaged alloy, TEM shows that dynamic recovery/recrystallization is occurring after significant strain-induced dislocation hardening around a stationary and stable array of as-tempered carbides during creep-fatigue. In contrast creep-fatigue testing of the pre-aged alloy produced a much coarser cellular subgrain structure and dislocation recovery without recrystallization. Aging causes as-tempered carbide dissolution and/or reprecipitation together with additional precipitation of Laves (Fe(sub 2)Mo) phase, which removes some of the precipitate-strengthening effects, and depletes solid-solution hardening effects on the dislocation networks and subgrain boundary structures.