Research On Co-free Ni-based Alloy And Laser Cladding Process Applied In Nuclear Valve Sealing Surface Hardening

Sealing surfaces of nuclear valves, the important nuclear equipment, are the most crucial working face, for their quality affecting the valves'service life directly. The valves'life can be obviously lengthened by using hardfacing materials with good properties. The Co-based alloy has long been thought as No.1 hardfacing materials which are suitable for high temperature and high pressure valves. But the element Co is a kind of rare metal and very expensive. The worst is, in radiation environment Co can be easily activated to be isotope Co60 which can cause nuclear pollution for its long radioactive half-life. As a result, the Co-based materials will be eliminated in the valves of third and fourth-generation nuclear plants.In this paper a kind of Ni-based alloy is designed to obtain the properties on wear resistance and corrosion resistance in high temperature close to Co-base alloy's. In this material Ni is selected as the main element and many elements such as Cr, W, Al, Mo, Si and etc. are added on the purpose of solid solution strengthening, second phase strengthening and grain boundary strengthening. The materials are cladded by laser. Then the crystal structure and phase compositions are analyzed. We turn back to adjust the material composition and finally this alloy is set down and named Ni-3.The smooth layers without any cracks or pores are made through improving the structure of laser head and optimizing the laser cladding parameters. The micro-structure of one layer and several layers with different processing parameters are analyzed and compared. The hardness of Ni-3 cladding is high, at about HV0.3500 in room temperature, close to Co-based alloy Stellite06 and lower than Fe-based alloy Norem02 which is a popular Co-substitute alloy. The high hardness of Ni-3 is mainly due to the hard phases likeM23(CB)6,M7C3,Ni3Aland limitedNi3B,Ni3Si. Moreover, the addition of B and Y2O3 which can refine the grains and harden the boundaries improves the hardness and toughness further.The results of friction and wear experiments at 360℃show that the wear volume loss of Ni-3 is nearly half as much as Norem02's and a little bigger than Stellite06's. And in the corrosion experiments at 350℃, the corrosion weight loss of Norem02 is the biggest. The corrosive resistance of Stellite06 is the first and Ni-3's comes second.The experiments and research demonstrate that Ni-3 is a good Co-free alloy. Its performances on wear resistance and corrosion resistance in high temperature are both better than Norem02. Compared with Stellite06, Ni-3 is near to it on high temperature wear resistance aspect, but a bit inferior on high temperature corrosion resistance aspect.