Reseach On Cobalt-based Alloy Laser Welding Of Nuclear Valve Sealing Surface And Cladding Ion Implantation

It is imperative to develop more advanced sealing surface overlaying welding materials, constantly optimize the process plan of sealing surface overlaying welding, and seek effective surface modification techniques for overlay cladding, so as to enhance functionality, processability and economy of nuclear valve, thereby to meet the needs of the development of nuclear power construction and the improvement of working parameters of nuclear power unit.As one representative material of Co Cr-A class of cobalt-based alloy, Stellite 6 greatly meets the performance requirements of the sealing surface of nuclear valve in corrosion resistance, erosion corrosion resistance, cavitation resistance, anti-scratch, anti-pad injury, red-hardness and oxidation resistance at high temperature, etc., therefore enhancing the tightness and reliability of metal sealing nuclear valve. However, due to the high percentage of Co(more than 50%), it can absorb neutrons in the long-term nuclear irradiation conditions which results in long-term radioactive contamination beyond the requirements of nuclear system. With the development of new materials for surfacing to significantly reduce the content of Co, and the comparative advantages in laser cladding process such as the fine cladding organization, composition and dilution rate under control, small thermal distortion and high hardness, making a breakthrough of the restrict of Co substituting material, we can get the surfacing layer which meets the technical and testing requirements and the acceptance criteria of the nuclear valve. It is no doubt that it has significant scientific and application value.By virtue of the in-depth study of the use of nuclear valve operating conditions, the damage of sealing surface mechanism and failure pattern, through lucubrating the strengthening function of the various elements of cobalt-based alloys in laser cladding layer, the angle of welding processes and dilution rate, the range of main components and the elements content of the alloy powder in forming cladding layer can be ascertained, and thus preparing FCo-5 alloy powder of with no tungsten low carbon moderate cobalt. With the application of pre-coating method, high quality laser cladding with 2.04 mm thickness and 2.69 mm fitting radius of cross-section by a single arc can be prepared on the surface of 0Cr18Ni12Mo3 Ti nuclear valve specimen. SEM, XRD analysis and hardness test results show:Crystalline form of the cladding layer from the interface to the surface followed by a transition from a flat crystal to cellular dendrites, multi-directional growth of dendrites and fine dendrites; Middle and upper tissue mainly consist of γ-Co austenite dendrites, inter-dendritic lamellar eutectic and Cr23C6 hard particles dispersed; the normal temperature micro-hardness threshold of the area above binding layer is 390 ~ 470HV; The minimum thickness of the cladding layer should be retained with excellent creep resistance and high temperature hardness properties, so that it has strong secondary hardening effect after high tempering. FCo-5 as a nuclear valve sealing surface material has better applicability.Under the clarification of FCo-5 cladding cross-section geometry parameters, the microscopic hardness curve and the relationship between the laser processing parameters, with the full consideration of the energy loss of a variety of factors, a good calculation model was established matching the cladding of cross-sectional shape of the radius of curvature. The cladding cross-sectional area was deemed as objective function, while the laser output power and scanning speed was adopted as the design variables under identical thickness cladding conditions, in order to reduce nuclear valve dosage of cobalt base alloy, thus providing scientific fundamentals and design method.With the application of laser synchronous delivering powder method, Co-based alloy Stellite 6 powder were processed with large area and big thickness laser cladding on austenitic stainless steel 316 L, resulting in the range of processing parameters which didn’t appear cracks and gas holes defects. Combining solidification theory, dentritic growth theory and fusion welding principle, phase distribution, micro-hardness law and the corresponding relationship were analyzed. Under the analysis, thickening of the oxide film was the root cause of the pores in the bead lap; The oxide inclusions, loose organization and surface tensile stress concentration were the root cause of crack initiation and growth on the interface between the coating and substrate; The brittle phase migration and accumulation of high hardness WC were the root cause of the cladding layer cracking in scanning remelting process.High temperature and large dose C+ implantation to the laser cladding layer and corresponding friction and wear experiments and electrochemical corossion were conducted. Results show that the concentration of C+ follows a Gaussian distribution on the surface of coating layer and was above 8% within the area 18 μm away from the surface. High hardness carbides such as M23C6, Mn7C3, Si C etc. was obtained on the near surface area. The effect of C+ bombardment and sputtering improve the support conditions and the surface roughness of cladding layer. The root cause to improve the wear state of cladding layer surface is the crystal distortion produced by the ion implantation, dislocation network and the pinning effect of crack produced by a large number of interstitial atoms, the form of high hardness carbide and the improvement of surface quality; The corrosion resistance of cladding layer was improved by high density of defects, disordered state, diffuse distribution of carbide hardening phase and alloy phase, Rsm reducing greatly and etc which Formed in the cladding layer surface by C+ implantation of high temperature, high energy and high doses.