Microstructure Changing of P91 Heat Resistant Steel during Short-Term Creep at 873K

The working temperature of main steam pipe in (ultra-)supercritical power generating units is 873K, and creep behavior of P91 was tested under this temperature with the stress of 140MPa for 5534h, and the minimum creep rate was 0.2μm/h. Then creep test for short period were conducted for 100h and 1800h, and samples considered as primary creep stage and steady-state creep stage respectively. These samples as well as initial state and creep ruptured samples were observed by transmission electronic microscope(TEM) mainly under scanning mode(STEM). The precipitates in P91 heat resistant steel at different states were studied in detail with two methods: carbon extraction replica and twin-jet thin foil. It was shown that microstructure of P91 heat resistant steel after creep was degraded, especially for accelerated stage. That means the density of dislocation decrease, and the width of martensitic lath as well as the size of sub-grain were coarsen. It can be seen that the mean size of precipitates was coarsen slightly too. There were Laves phases in sample of steady-state creep stage with the size of 150nm. While it was coarse easily to large size more than 500nm in creep-ruptured samples. It would exert strengthening effect by precipitating along lath boundaries and sub-grain boundaries in early stage of creep. But Laves phases were prone to coarsen, which would weaken their strengthening effect and become detrimental to long term creep.