Microstructure Evolution And Properties In Weld Heat Affected Zone Of GH3535 Nickel Based Alloy

Solid-solution strengthened GH3535 Nickle based alloy is the main candidate structural material for the molten salt reactor(MSR)– one of the Generation IV advanced nuclear power plants.It can be applied to endure the extreme service environment,such as high temperature high corrosion in MSR,meanwhile,very high performance of the welded joint is required to meet such service environment as well.Base metal adjacent to both sides of the weld becomes heating affected zone(HAZ)with high microstructure gradient by the rapid heating and cooling process during uneven welding thermal cycles,which will affect plastic deformation harmonization of welded joint.In addition,complex microstructure transformation usually occurs near the fusion zone of HAZ,the nonequilibrium microstructure generated in this zone may cause crack flaws.The high microstructure and performance inhomogeneity makes HAZ a possible weak link of welded joint in the long-term and high temperature service condition.Thus it is essential to study the evolution of microstructure,welding residual stress,and the mechanical behavior of HAZ and clarify the effect of HAZ as a whole part on the short-term and long-term performance of welded joint.In this work,cladding were firstly employed to obtain HAZ with a larger size,then the continuous changes of carbide morphology,grain size and dislocation density,twin boundary as the distance from the fusion line were characterized.It is shown that strong microstructure differentiation occurs in HAZ under the effect of welding thermal cycle.Eutectic transformation happens within 400?m from fusion line,the initial particle carbide are turned into lamellar and cribriform carbides.Within 100?m from fusion line,layers of lamellar eutectic carbide are further spherized under the action of repeated high temperature thermal cycles.As distance from the fusion line,the grain sizes decrease slowly and then go up due to coarse grain region and fine grain region were formed in HAZ.On contrary,? 3 twin boundary increased sharply and then slowly.The results of EBSD and numerical calculated by commercial code MSC.Marc show that the welding stress and strain are mainly concentrated in the interface between the weld metal and base metal.Moreover,high density dislocation and extremely serious dislocation entanglement were found near the fusion line,while as distance from the fusion line,the phenomenon mentioned above gradually reduced and disappearred.Hardness and tensile test of different parts of HAZ show that the area adjcent to the fusion zone pose the highest hardness and strength,but minimum elongation;As far away from the fusion line,the hardness and strength values gradually decline and keep stable in the base metal,while the elongation presents opposite trend to the hardness and strengh.It is noted that strong consistency and relevance between change of mechanical properties and microstructure in HAZ are found.The high strength and low plasticity near the fusion line are mainly caused by the welding residual strain with hardening effect and high density to impede dislocation motion.In combination with thermal simulation,the influence of welding thermal cycle parameters,such as peak temperature and cycling times on microstructure and mechanical properties were analyzed.Transformation mechanism of eutectic carbide near fusion zone were revealed,in addition,the model of eutectic transformation was also established.Thermal simulation test and differential scanning calorimeter(DSC)show that the transformation temperature of the eutectic reaction of carbide is about 1300 oC,eutectic carbides type are determined as(Ni+Cr)3Mo3C and(Ni+Cr)2Mo4C.Based on the research mentioned above,the transformation model of carbide eutectic reaction was established,and the transformation process and mechanism were illustrated as well.Tensile test of thermal simulation specimens shows that the alloy has a good high temperature tensile properties and stress relaxation resistance when the peak temperature below 1300 oC.The change of the thermal cycle times and peak temperature had no obvious effects on the mechanical properties of the thermal simulated alloy.However the tensile properties significantly deteriorate when the thermal cycle peak temperature over 1340 oC,it is mainly due to the loose microstructure formed under the high temperature of thermal cycles.The width of eutectic carbides region(HAZ(E))and HAZ were controlled by the welding process parameters to study the effect of HAZ on the mechanical properties of welded joint and determine the weak links in the welded joint after the short and long term service.The tensile test of short-term high temperature shows that no obvious damage were found on the properties of welded joint with the change of width of HAZ(E)and HAZ,moreover,almost all test specimens were fractured in the base metal.Welded joint shows a good high temperature mechanical stability within the scope of 650-725 oC.For long-term test,it was found the weak link of the joint is weld metal during endurance test.In this work,it is confirmed that no softening occurs in HAZ after welding,and the HAZ did not failed in the loading and service process of welded joint.Research in this work relieves the concern that the HAZ may be weak link of welded joint and component in the service,also has laid the foundation for the further application of GH3535 alloy.