Using OM,SEM,TEM,hardness,impact and other test methods,the microstructure and mechanical properties of the P91 heat-resistant steel 90° reheat section elbow that failed after 51 381 hours in service in an ultra-supercritical power plant were studied.The results show that the second precipitation in the microstructure is ripening,its ability to pin grain boundaries is reduced,and a small part of the martensite laths are broadened to form sub-grains;the roughened M23C6 precipitates and Laves phases keep the hardness at a high level,but the impact toughness is significantly reduced.The stress analysis of the outer arc of the elbow shows that the stress of the outer arc is significantly higher than that of the inner arc.The high stress causes the outer arc to produce creep damage and accumulate prematurely,forming creep holes and gradually converge and merge,and finally generating creep cracks at the maximum surface stress of the outer arc.Creep cracks caused premature failure of the P91 reheated hot section elbow.
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