Studies On High-temperature Microstructures And Mechanical Properties Of The T92/HR3C And T92/TP347H Dissimilar Steel Weld Joints

Currently, some ferritic and austenitic heat resistant steels, such as T92, HR3Cand TP347H have been widely used in building the supercritical （SC） and/orultra-supercritical （USC） power units as the superheater and reheater. Dissimilarsteel welding among these new heat-resistant steels is known as a key factoraffecting the quality of construction and the running safety of the power units.Unfortunately, research on plastic deformation behavior and mechanical propertiesof T92/HR3C and T92/TP347H dissimilar steel welding joints at high-temperatureare limited till now. So, it should be paid more attentions. In this paper,high-temperature microstructures, mechanical properties and plastic deformationbehavior of these two dissimilar steel weld joints were studied, and the creeprupture strength of the joints was also investigated. All of them are much necessaryto evaluate the running safety of the USC units.The T92/HR3C and T92/TP347H dissimilar steel joints were formed through agas tungsten arc welding （GTAW） technique, respectively, using ERNiCrMo-3andERNiCr-3welding wires. After post weld heat treatment （PWHT） at1033K for30min, the high-temperature short-term tensile test and the creep rupture test werecarried out. After the high-temperature short-term tensile process, plasticdeformation behavior, mechanical properties and fractural modes of the joints wereresearched. Stress tri-axiality theory was employed to explain the tensiledeformation and facture mechanism of the tensile joints, and stress redistributiontheory was employed to learn the facture mechanism of the creep rupture joints.Finally, based on the Goldenberg model, two mathematical models for doping outthe creep rupture strengths of the T92/HR3C and T92/TP347H joints were derivedfrom the high-temperature short-term tensile testing results of the joints,respectively.For the T92/HR3C dissimilar steel joints, short-term tensile test at773-923Kand the creep rupture test at898K/190MPa were carried out, respectively. Thetensile fracture position of the joints is always located in the T92side fine-grainedheat-affected zone （FGHAZ）. The fracture mechanism of the joints changes from a normal mode to a mixed mode （normal plus shear） as the testing temperatureincreased. Due to the stress redistribution in the necking area, the plasticdeformation characteristics is different in the different mixed fracture regions. Inthe region adjacent to the normal fracture surface, the plastic deformationorientation is approximately parallel to the fracture surface. However, it is roughlyperpendicular to the fracture surface in the region far from the fraction surface. Inthe region adjacent to the shear fracture surface, the plastic deformation orientationis at a45°angle. So, shape of dimples in different mixed fracture regions ischanged. The dimples on the normal fracture surface are equiaxial and those on theshear fracture surface are parabolic. After the high-temperature creep rupture test at898K and190MPa, the fracture position of the joints is located in the T92BMwith a normal and dimple-aggregation type fracture mode. There are a lot of creepvoids on the fracture surface, which form and grow along the precipitates/matrixinterfaces, and the size of voids increases with that of the precipitates.For the T92/TP347H dissimilar steel joints, the short-term tensile test at848-923K was carried out. After the high-temperature short-term tensile test, fractureposition of the joints is located in the T92side FGHAZ. The fracture mechanism ofthe joints changes from a mixed mode （normal plus shear） to shear mode as thetesting temperature increased. Similarly, in the high-temperature short-term tensileprocess, the plastic deformation orientation in the region adjacent to the fracturesurface is different from that in the region far from the fraction surface. And, shapeof dimples is also different for the different fracture surface.Moreover, an equation relating to the short-term tensile strength of theT92/HR3C and T92/TP347H dissimilar steel joints to the creep rupture strength atdifferent temperature is derived. Creep rupture strength of these two joints weredetermined, i.e.σ₁0⁵⁸⁷³105=64.7MPa for the T92/HR3C joints andσ₁0⁵⁸⁷³105=49.0MPa forthe T92/TP347H joints, respectively. So, two kinds of the joints have a highreliability under a long-term service condition of the USC power units.