Effect Of Post-weld Heat Treatment On The Mechanical Properties And Corrosion Behavior Of ENiCrFe-7 Weld Overlay Cladding Materials

Nowadays,the nuclear power plays an increasingly important role in China.The pressure vessel,pressurizer and steam generator are the main equipments in nuclear power plants.They are all connected to each other by the main pipe using welding,where the dissimilar metal weld joints exist.After the joints is completed by welding,the post-weld heat treatment is often required to eliminate residual stress and optimize the property of each material in the joint.In the harsh service environment,the properties of ENiCrFe-7 weld cladding materials were critical to the long-term operation of nuclear power plants.Mechanical and corrosion properties are the main factors to evaluate the performance of ENiCrFe-7 weld cladding materials.Combined with the actual performance of each material in the joint,the mechanical properties and corrosive behavior including intergranular corrosion and corrosion resistance of as-weld ENiCrFe-7 and aged ones after different heat treatments(615℃/16h and 615℃/48h)at room-high temperature were comparatively investigated in this paper.The results were as follows:(1)Post-weld heat treatment could affect the precipitates,residual stress and Cr content around(Cr,C)-rich precipitates in the dendrites of ENiCrFe-7 weld cladding materials.The heat treatment could promote not only the formation of intergranular precipitates(Nb5Si3 and(Cr,C)-rich),but also the nucleation and growth of NbC precipitates in the interdendrites.The Cr content around(Cr,C)-rich precipitates in the dendrites and the residual stress decreased,which resulted from the reduced lattice distortion when the heat treatment time increased to 16h.When the heat treatment time increased to 48h,the Cr content around(Cr,C)-rich precipitates in the dendrites obviously increased,while the residual stress slightly increased due to the increase of the preciptates.The residual stress was compressive in ENiCrFe-7 samples with different heat treatments.(2)Post-weld heat treatment could affect the strength and elongation of ENiCrFe-7 weld cladding materials.The strength depended on the residual stress and precipitates,while the elongation was related to the lattice distortion in the interdendrites.When the heat treatment time increased to 16h,the residual stress decreased,however,the precipitates in the interdendrites and grain boundary increased,resulting in the decrease of the strength at room temperature;The precipitation of NbC in the interdendrites reduced the lattice distortion,which were coordinated with the dendrites,resulting in the increase of the elongation.When the heat treatment time increased to 48h,the residual stress had no obviously changed,while the precipitates in the interdendrites and grain boundary continuously increased,resultin in the increase of the strength at room temperature.The lattice distortion in the interdendrites further decreased and the elongation further increased.The fracture feature gradually transformed from the brittleness to ductility with increasing the heat treatment time.When compared with the condition at room temperature,the strength of all the samples including as-weld,615℃/16h and 615℃/48h decreased and the elongation increased at high temperature.The results of small punch test at room temperature were consistent with that in tensile test,and the relation between themt was linear.(3)The intergranular corrosion of Alloy ENiCrFe-7 with different heat treatments were related to the oxidizing capacity of medium.All of them had good resistance to intergranular corrosion with as-weld alloy and those after heat treatments of 615℃/16h and 615℃/48h in a solution with weakly oxidizing.However,in a solution with strong oxidizing,the intergranular corrosion properties of ENiCrFe-7 weld cladding materials was gradually decreased due to the continuous precipitation of precipitates with increasing the heat treatment time.The corrosion in the grain boundary and interdendrites increased,which resulted from the precipitation of(Cr,C)-rich precipitate along the grain boundary and NbC in the interdendrites when the heat treatment time increased to 16h.When the heat treatment time increased to 48h,the corrosion in the grain boundary and interdendrites was further aggravated which resulted from the continuous precipitation.(4)Post-weld heat treatment could change the corrosion potential and pits distribution of ENiCrFe-7 weld cladding materials.The corrosion potential in the dendrites of as-weld sample were higher than that of the interdendrites,and the dendrites were easy to be corroded.On the other hand,the as-weld sample had a lot of homogeneously distributed microstructure defects(e.g.composition segregation),which leaded to a uniform distribution of pits on the surface of the sample after polarization.After the heat treatment of 16h,the dendrites of sample had the most serious Cr-depletion area,resulting in the concentration of a large number of pits.After the heat treatment of 48h,the corrosion potential between dendrites and interdendrites were similar,and the rate of corrosion was similar.But the corrosion potential of NbC precipitates in the interdendrites was much higher than that of matrix,resulting in the preferred corrosion of the matrix near the precipitates.(5)Effect of post-weld heat treatment on the corrosion rate and mechanism of ENiCrFe-7 weld cladding materials were not obvious in the high-temperature and high-pressure water.When the heat treatment time increased to 16h,the corrosion rate did not obviously change,but the number of dissolved NbC increased.When the heat treatment time increased to 48h,the dissolved NbC precipitates further increased.But the effect of dissolved NbC on the corrosion rate of ENiCrFe-7 was not also obvious.On the other hand,the rate of weight gain in ENiCrFe-7 weld cladding materials including as-weld and heat treatment samples first increased,then decreased with the increase of expose time to 2250h in high temperature water,which resulted from the formation of denser oxide film.The oxide film of ENiCrFe-7 was composed of double-layer oxide,the outer layer was mainly composed of Fe3O4 and FeCr2O4 while the inner layer was Cr2O3.Ni was not oxidized during corrosion.