Research On Laser Surface Modification Of Alloy 690 Tube For Nuclear Power Steam Generator

As the main material of PWR steam generator and heat pipe, Inconel 690 alloy had a history of more than 30 years, reflecting high temperatures and the advantages of resistance to intergranular corrosion, stress corrosion. However, corrosion resistance deteriorated when the temperature of the solution and the concentration is high. There was a small amount of this kind of material of the pipe stress corrosion cracking existed abroad. Due to the fretting wear between the the pipe and support plate caused by the flow, the wear resistance of Inconel 690 alloy was not ideal.To modify the surface of Inconel 690 alloy pipe, using 3500 W CO2 slab(DC035)laser and 3000 W(DISTA-3000) semiconductor laser, the main process included the aging heat treatment after laser melting of the Inconel 690 alloy tube surface, laser melting and auxiliary nitrogen nitriding processing simultaneously, as well as laser melting and Cr powder simultaneously. In addition, the scanning electron microscope(SEM), energy spectrometer(EDS) and X-ray diffraction(XRD) were used to characterize the microstructure morphology, composition and phase of modified Inconel 690 alloy. The polarization curve was performed using electrochemical workstation, representing the corrosion resistance of alloy Inconel 690. The hardness distribution was measured by micro-hardness meter. The results can be summarized as follows:The melting layer of samples modified by three kinds of processing technique all represented high metallurgical quality, lustrous surface, low surface roughness,relatively flat and no defects such as cracks or porosity. In addition, melting thickness improved with the increase of laser power. The melted grain became more refined and the element distributed more homogeneously.The perfect passive film can be attributed to the increasing content of Cr between grain boundaries.The corrosion resistance of samples in solution containing Cl-and OH-increased. At the same time,the grain refinement and the increased hardness of fusing layer were responsible for its improved corrosion resistance.The samples processed by CO2 laser melting nitriding, fusing nitriding layer composition mainly composed of Ti N, Cr N and gamma phase. The penetration of N slightly lowered the corrosion resistance of Inconel 690 alloy tube in alkaline solution.In addition, the corrosion resistance decreased with the power increasing.The melting nitriding layer obtaining higher hardness was attributed to the precipitation of the Cr N and Ti N in grain boundaries.With 3000 W semiconductor laser(laser of 915 nm and 808 nm double optical coupling output) combined with aging composite processing of Inconel 690 alloy pipe,owing to the higher laser output frequency of the coupling of semiconductor, in the process of alloying a larger amount of heat input matrix, and the formated temperature gradient is bigger,molten pool convection is more intense. which eventually result in forming Cr2O3. At the same time,the convection in molten pool also made the solute of Ni、 Cr in matrix form Ni0.64 Cr0.36 phase.