The Surface Modification Research Of Laser Cladding And N~+ Injection After Cladding On Nuclear Valve Sealing Surface Of Stellite 6 Alloy

As one of the key equipments of nuclear power plant, valve plays a more and more prominent role in the position in the plant. For nuclear valve workers, to develop more advanced sealing surface surfacing materials, make a more ideal process plan and find a more effective surface modification method is one of the highlights of the present study.Represented by Stellite 6, Co Cr-A cobalt base alloy well meets the performance requirements of the resistance of corrosion, erosion, abrasion and scratch, high temperature red-hardness which are required in nuclear valve sealing surface. Compared with conventional welding methods, cobalt-based hard alloy coating with denser structure, higher hardness and well metallurgical bonding coating between cladding and substrate was abtained on the nuclear valve sealing surface with laser cladding technology. However, how to solve the problem of poor hardness between the nuclear valve sealing surfaces, how to effectively carry out laser cladding with large area and thick coating and how to further improve the hardness,wear resistance and corrosion resistance of Stellite 6 alloy laser cladding coating has been troubling nuclear valve workers.In this paper, experiments of single-layer single-channel, lti-layer multi-channel laser cladding, N+ injection under the condition of high temperature, high beam density on Stellite 6 laser cladding coating were carried out and analyzed on nuclear valve sealing surface.Experiments of single-layer single-channel,ulti-layer multi-channel laser cladding were carried out on the 316 L austenitic stainless steel flange, and the continuous, uniform, dense structure was abtained. The cladding coating and the substrate was well metallurgically bonded.Based on the experiments, calculation model of cladding sectional shape curvature radius which fits well with fitting model was abtained by analyzing the relationship among geometric parameters of single-layer single-channel Stellite 6 alloy laser cladding road cross section, the microhardness curve and laser processing parameters. Optimization design model was constructed under the condition of the same thickness, in which the cross-sectional area is the objective function, and the laser output power and scanning speed is the design variables. And this provides scientific basis and design method to reduce nuclear valve dosage of cobalt base alloy.By using Synchronous feeding method, the feasibility of large area big thickness laser cladding on the nuclear valve sealing surface was researched. Under the condition of the laser power P=2500 W, the scanning speed v=5 mm/s, spot diameter D=5.05 mm, macro porosity and crack did not appear in laser cladding coating of double layers 4 channel and double layers 4 channel. With the increase of the number of channels, the roughness of cladding surface decreases and the bonding strength between coating and substrate becomes weak. From bonding zone to the surface, the microstructure of the cladding coating changes from planar to cellular, llular crystal, dendrites. The microhardness of cladding coating decreases slightly with the increase of the channels, but the whole cladding layer microhardness curve shape was not affected.The experiments of N+ injection under the condition of high temperature, high beam density and large dose on Stellite 6 laser cladding coating were carried out, and the result showed that when the N+ injection in cladding depth reaches 15μm, the injection depth increased two orders of magnitude compared with that under the normal temperature, the density of the secondary beam, and the moderate dose injection depth, and the concentration distribution is quasi Gaussian. Compared with results of the surface roughness of the injected and not injected samples, the bombardment and sputtering effect of the N+ improve the coating surface roughness Ra 、Rz 、Rsm, and the support conditions, improves the corrosion resistance of sealing surface and crack resistance; XRD phase detection analysis results show that a lot of diffusing distributed nitrogen compounds, such as micro crystalline Fe3 N, tiny Fe2 N and Fe Nm(m<1) and a certain amount of Cr3N0.4C1.6 roduced on the injected sample. Precipitation strengthening, solid solution strengthening and reinforcement of microcrystalline caused by the injection of N+ increase the microhardness of cladding coating; the results of friction and wear experimental show that the change of the chemical composition and metallographic, the increase of microhardness, the improvement of the surface roughness and dislocation network to the inside of the welding layer are the main reasons to the improvement of Stellite 6 laser overlaying welding coating wear resistance after the injection of N+; electrochemical corrosion experiment results show that the generation of high density defects, disordered state, micro crystalline nitrides produced under high temperature, high beam density, large dose of N+ injection on the surface cladding and the increase of Rsm improve the corrosion resistance of cladding coating. The improvement of hardness, wear resistance and corrosion resistance is of great value to high parameter value manufacturers.