Studies On Plasma Transferred Arc Welding Coatings Of Ni-Cr-B-Si Hardfacings

Water lubricated bearing guide is a Engineered Safety Feature for nuclear power plants, which is commonly used in nuclear containment spray pump or low-pressure safety injection pump in secondary loop.In this paper, its plasma arc welding coating is studied. The substrate is Z2CN18-10austenitic stainless steel and the welding material is Ni-Cr-B-Si alloy. The austenitic stainless steel is widely used in nuclear power plants for its good corrosion resistance, but its wear resistance is not high, so the surface treatment is used to enhance its wear resistance.The Ni-Cr-B-Si alloy power was deposited on austenitic stainless steel substrate by plasma arc welding. The surfacing layer structure and phase composition are measured with the use of optical microscopy (OM), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The phase is measured by X-ray diffraction (XRD) and the microhardness is measured by microhardness tester. The corrosion resistance is measured by CS350potentio dynamic scanning. The results are as follows.Welding speed influences geometrical dimensions of the weld bead, the bead height, bead width, penetration depth and The ratio of width to height w/h of the single bead increase as welding speed is increased, the weld bead is going to be flat. The extent of dilution increases with increasing welding speed, when increasing the welding speed, the performance of the alloy is changing,the microstructure refine and The hardness of Ni-Cr-B-Si coating is decreased.Arc oscillation can affect microstructure. It is observed that the dendrite arm spacing is reduced significantly when transverse arc oscillation speed is comparable with welding speed. The strength and hardness of the weld metal increase, when the arc oscillation speed is far greater than welding speed, the dendrite arm spacing is decided by welding speed.The Ni60coatings by PTAW can be divided into three layers, the hardfacings of Ni60consist of γ-Ni, carbides, borides and the eutectic of y-Ni and borides. The amount of daisy-like borides and y-Ni eutectic in middle layer is large, so its hardness is high; but the amount of daisy-like borides and γ-Ni eutectic in bottom layer and top layer is small, so its hardness is low. In the borate, Ni60alloy has higher corrosion potential and wider passivation width, and the middle layer has the best corrosion resistance. In seawater the passivation is weakening due to more Cl-, the middle layer has the best corrosion resistance. The corrosion resistance of M50in the three-layer is basically same in the borate.Strengthening methods of Ni-Cr-B-Si surfacing are solid solution strengthening and hard phase strengthening, Strengthening methods of Ni40is solid solution strengthening, Strengthening methods of Ni60is hard phase strengthening, Strengthening methods of Ni50are both solid solution strengthening and phase strengthening. Adding Nb properly can improve the hardness of Ni-Cr-B-Si. The laser welding and PTA welding are similar, but the organization of the laser welding coating is small and the hardness is high.Post weld heat treatment can reduce welding stresses and cracking susceptibility. The hardness can be increase by appropriate heat treatment.