| Reheat double valve group in nuclear power turbine unit can ensure the safe operation of steam turbine. Because its valve stem ran in the high temperature and pressure environment (500? , 16.7MPa) with no lubrication for a long time, which was easy to wear under steam shock and vibration load and seriously affected the performance and service life of the valve stem. In order to solve this problem, the high temperature self- lubrication and wear resistant coating was prepared by laser cladding on the stem surface to replace conventional wear-resistant coating with surfacing processes. On this basis, micro texture on coated surface was investigated preliminary to improve matching features of friction pair and significantly enhance high temperature wear resistance performance of valve stem. The main research contents are as follows:Firstly, the design and characteristics of high temperature self-lubricating and wear resistant coatings were studied. Based on the coupling law between the broadband beam and the coaxial powder flow,the coating with different powder composition was prepared by laser cladding. The macroscopic morphology, phase composition,microstructure and hardness distribution were analyzed. The results show that the powder composition had little influence on the width of the coating, and had significant effect on the height, the variation range was 0.1-1.16mm. The microstructure of Ni60 coating from the bottom to the top was plane crystal, cellular crystal, columnar crystal and equiaxed dendrite. Cr23C6, Fe2Si hard phase andy- (Ni, Fe) ductile phase were formed during solidification. The self-lubricating wear resistant coating of Ni60M+5%CaF2+5%MoS2 was more uniform and compact. FeMo2S4,CaMo04, CaF2 and other lubricating phases were detected, which played a positive role in the grinding of the coating. Ni60M+10%CaF2 coating and Ni60M+10%MoS2 coating were distributed in bulk, and partial lubrication phase was detected. Compared with the substrate, the micro-hardness of various coatings were increased greatly. The micro-hardness of the Ni60 coating was about 3.6 times as high as that of the 304 stainless steel substrate. However, the micro-hardness of the coating was reduced to some extent with the addition of CaF2 and MoS2.Secondly, the wear resistance of matrix, Ni60 coating and self-lubricating composite coating were experimentally compared. The effect of powder composition on the wear resistance of the coating was analyzed, and the feasibility of high temperature self-lubricating wear resistant coating was validated. The results show that the friction coefficient and wear rate of the coatings were significantly lower than that of the stainless steel substrate. Among them, the wear resistance of Ni60+5% CaF2+5% MoS2 self-lubricating wear-resistant coating is the best, with the wear amount 0.0077g, 6.5 times lower than the matrix.The wear of matrix was mainly abrasive wear, adhesive wear and oxidation wear, and the Ni60 coating was mainly abrasive wear,adhesive wear and fatigue wear. The wear surface of Ni60+5% CaF2+5%MoS2 coating was more smooth, and the fatigue wear was the main one.Finally, anti-friction process with micro texture on the surface of self-lubricating coating was carried out.The effect of processing parameters (scanning speed, scanning frequency, pulse frequency,average output power and focal distances) on the morphology and quality of micro texture was studied. Micro grooves on the surface of self-lubricating coatings were obtained by laser micro texturing technique, and wear resistance before and after texture was analyzed.The results show that the friction coefficient and wear loss of Ni60+5%CaF2+5% MoS2 self-lubricating coating after micro texturing were reduced by 18.87% and 28.57% respectively compared with the non textured coating. The coating wear surface has no obvious damage after the micro texture, only a slight furrow and fatigue spalling, which verified the feasibility of anti-friction process with micro texture. |
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