Investigation Of Anisotropic Creep Performance Of Modified CrMoV Steel Weld Metal

Creep failure is the major security threat of welding parts at high temperature and high pressure. A key part of the steam turbine in power plant-steam turbine rotor is often manufactured by welding several forging together since its structure is too large to forge. But the microstructure of weld metal is extremely heterogeneous because of the welding heating and cooling cycles in the welding process, which generates anisotropic creep performance at high temperature, it affects the structural integrity of welded joint as like a life reduction factor. However, the systematic research of the mechanism, the influence and research methods of anisotropic creep performance are limited. In this paper, the systematic research of anisotropic creep properties of weld metal is conducted by experimental methods and numerical simulation analysis. The main research tasks and conclusions are summarized as follows:(1) The metallographic observation and micro Vickers hardness test of the weld metal was conducted. It can be seen that the weld metal is mainly composed of the columnar grain zone, coarse grain zone, equiaxed grain zone and welding center equiaxed grain zone. The columnar grain and coarse grain have the similar shape, and the columnar grain is a little larger than coarse grain; the grain boundaries of columnar grain are misty, but the coarse grain has clear outline; the equiaxed grain has clear granule, while the welding center equiaxed grain has no grain boundaries. There are a lot of strip bainites and granular bainites across the weld metal, whose equivalent size of subgrains in coarse grain, equiaxed grain, columnar grain and welding center equiaxed grain decreases prospectively. And the welding center is the weakest part of the weld metal by the results of micro Vickers hardness tests.(2) The tensile tests were performed on the circumferential, radial and axial micro plate specimens of weld metal. It can be found that the tensile strength of the circumferential, radial and axial specimens decreases progressively; the radial and axial specimens fracture at columnar grain zone and welding center equiaxed grain zone respectively, which is affected by the arrangement of the columnar grain zone, coarse grain zone, equiaxed grain zone and welding center equiaxed grain zone on the specimen.(3) The creep tests were carried out on the circumferential and radial micro bar specimens extracted on the weld bead, and radial micro bar specimens extracted in the welding center of weld metal. It can be obtained that the creep strength of circumferential specimens on the weld bead, radial specimen on the weld bead and radial specimen in the welding center decreases progressively. Then fracture analysis, metallographic observation and TEM analysis of the fracture specimens combined with creep analysis and damage creep analysis; which reveals that the creep properties of coarse grain, equiaxed grain, columnar grain and welding center equiaxed grain decreases progressively; and the anisotropic creep behavior of welded joints is affected by the stress redistribution generated by the distribution of columnar grain zone, coarse grain zone, equiaxed grain zone and welding center equiaxed grain zone on the specimens.(4) The creep tests were implemented on the circumferential and radial micro flat specimens based on the modified in-situ creep testing system. It can be gained that the creep life of circumferential specimens is larger than that of radial specimens and the steady creep rate of circumferential specimens is less than that of radial specimens, which agrees with the result of micro bar specimens. The micro flat specimen can be used to characterize the creep anisotropy of weld metal well.