Microstructures And Mechanical Properties Of The T92/Super304H Dissimilar Steel Weld Joints

Energy shortage and environmental pollution around the world urges people todevelop new techniques of clean and high-efficiency thermal power generation.Among them, ultra super-critical （USC） power generation technique is regarded asthe most proven green-energy technology. In order to meet the tough conditions forrunning the USC units, several new-style heat-resistant steels, such as T92martensite steel and Super304H austenite steel have been developed. Therefore, akey issue of welding of these new-style heat-resistant steels arises. Study on theT92/Super304H dissimilar steel welding is limited so far, compared with that on theT92/Super304H similar steel welding. It seriously restricts the development andprogress on construction and servicing management of the USC units in ourcountry.The T92/Super304H dissimilar steel joints were prepared through gas tungstenarc welding （GTAW）, using two Ni-based welding wires of ERNiCr-3andERNiCrMo-3, respectively. Microstructures and mechanical properties of the jointswere studied. The joints were then aged at650°C for times up to3000h toinvestigate the effect of artificial annealing on the microstructures, fractographiesand mechanical properties of them. In terms of the short-term tensile tests at550-650℃of the joints, the high-temperature plastic deformation behaviors andfracture mechanisms of the joints were also studied. Finally, an equation of thecreep rupture strength related to the short-term tensile strength of the joints wasderived, based on the Goldenberg model.In the T92side heat affect zone （HAZ）, lots of second-phase particles precipitatealong the grain boundaries and inside the grains of the prior austenites. The finegrained zone of the T92side HAZ is of sorbite. In the Super304H side HAZ, theaustenitic gains grow up with a large amount of precipitates generated along thegrain boundaries. The ERNiCr-3welding seam is of a coarse cellular structure.Comparatively, the ERNiCrMo-3welding seam is of a dense columnar structure.Between these two welded joints, the ERNiCrMo-3weld joints have higher strength,ductility and hardness values, however, the ERNiCr-3weld joints have a higherimpact toughness value. Moreover, the tensile fracture of the former joints takesplace in the Super304H base metal, and that of the latter occurs in the welding seam. Because the ERNiCrMo-3welded joints have more excellent comprehensivemechanical properties, the ERNiCrMo-3Ni-based welding wire was recommendedfor welding the T92/Super304H dissimilar steel joints.As the aging time increased, second-phase grains in the austenitic grains of theT92side HAZ aggregate and coarsen, accompanying with the growth of theδ-ferrite grains. The carbides at the prior austenitic grain boundaries of the T92base metal graduaaly grow. The Super304H side HAZ maintains unchangeable inthe early aging stage. After aging at1000h, the MX phase particles in it precipitateand dispersively distribute. The carbides in the Super304H base metal do notobviously increase in amount, but aggregate and grow. The tensile fracture of theaged joints takes place in the T92base metal. The tensile strength, impacttoughness and hardness values o f the joints all decrease.In the high-temperature tensile process, the fracture positions of all joints are inthe T92base metals; however, the fracture mechanisms of these joints are changedfrom a normal mode to a shear mode as the test temperature risen. Moreover,according to the equation of the creep rupture strength of the joints firstly derivedby the authors’ group, the creep rupture strength of the joints was then doped out,（?）105873=42.7MPa。...