Study On Microstructure And Mechanical Properties Of 9%Cr Ferrite Heat-Resistant Steel Weld Metal After Aging

With the increasingly serious environmental pollution and the increasing shortage of fossil fuels, it has become a serious problem to improve the efficiency of coal- fired power plants and reduce emissions. Improving the parameters of thermal power units coal- fired power plant(both temperature and pressure of the steam) is an important way to solve the problem, but it is severely limited by the development of high temperature materials. 9% Cr heat-resistant steels are widely used in supercritical(SC) and ultra-supercritical(USC) unit boiler, such as the main steam pipes, superheater, reheater and so on.This investigation focus on 9% Cr ferritic heat-resistant steels which are used in 566 ℃ SC and 600 ℃ class USC boiler. The effects of long-term aging of P91 welded joint(625℃ aging) and P92 weld metal(650℃ aging) on microstructure as well as mechanical properties have been investigated. The stability of microstructure a nd mechanical properties of the two steels were compared. By advanced analysis methods of SEM-BSE, EBSD, TEM and XRD, the microstructure including precipitates, grain boundaries, subgrains and dislocation was discussed. Moreover, the correlation between microstructure and mechanical properties was established.In P91 base metal there is a certain orientation relationship not only between M23C6 type carbide and ferrite matrix but also between the Laves phase and M23C6 type carbide. In P92 weld metal there is a certain orientation relationship between Laves phase and ferrite matrix. A misorientation between the subgrain and the neighboring crystal that was lower than 2°. Subgrain constantly merges and grows after aging, eventually forming a 2-15° subgrain boundary. The formation and growth of subgrain results the increasing fraction of the small-angle grain boundaries after long-team aging process. The reasons which lead to reduction of hardness and tensile strength are different with aging time.This thesis has provided a reference for the choice of welding materials and welding process. Meanwhile, it has an important theoretical significance and engineering value for the development of ferritic heat-resistant steels applied in USC.