Analysis On Microstructare Of T92Steel After High Temperature Multiaxial Creep

Now T92steel widely is used in high temperature steam pipes of boiler because of its excellent high temperature strength and creep properties. Under high temperature and complex stress, the multiaxial creep failure would be the main failure mode. In order to understand the multiaxial creep behavior, it’s necessary to carry out multiaxial creep tests and research the microscopic mechanism。The high temperature creep tests of standard specimen and double U-type notch specimen of T92steel were carried out at stress range from120MPa to250MPa at600℃and650℃. Then scanning electron microscopy was used to observe the fracture morphology and microstructure of the unfractured notch section and straight part section. The elements were analyzed by EDS. The results show that,(1)Compared with uniaxial stress state, the dimples in multiaxial stress state are smaller in size and shallower in depth, and there are a lot of carbide and second phase particles in dimple.(2)The notch makes plastic deformation marked with necking weaken, and the sharper the notch is, the smaller the plastic deformation is.(3)Under multiaxial stress state, the failure mode of T92steel gradually transfers from ductile dimple fracture to brittle quasi-cleavage fracture.(4)The fracture mechanism of notch specimens is different from standard specimens, and the cracking of notch specimen originates at the root of notch while the cracking of standard specimen initiates in the interior of specimen.(5)The notch makes the stress state of the unfractured notch section different from straight part section, which leads to the difference in the precipitates morphology of the two sections, and the precipitates of the unfractured notch section are larger in size and quantity, and are much coarser.(6)In the process of long-term creep, the main precipitates are Laves phases, M23C6type carbides and MX type carbonitrides. Coarsening of M23C6carbide is one reason of the creep life reduction, and Laves phase not only reduces the solid solution strengthening effect of T92steel and is related with cavities’ produce, and the anti-coarsening of MX type carbonitrides is strong, it can keep stable and is not obvious, so it plays a key role to the strength of the material.(7)In the process of long-term creep, the production of precipitates lead to migration of elements, the reason of transformation from ductile to brittle is related with oxidation and element compositions change and transform under multiaxial stress state.