| In the industry of thermal power plants, boiler tubes, especially water wall tubes in enginery of over 300MW, water wall tubes are easily corroded in high temperature and wore by dust when the coal with high content sulphur is used, which made water wall tubes become thin and burst. In traditional, the burst tubes are replaced by new tubes. However, this method is not economical in time and economy. With thermal spray technology, the surface of water wall tubes is easily covered by varied high performance coatings .In this paper, several new alloy and cermet coatings applied to water wall tubes were designed and their performance was studied according to the research item of Hanchuan power station of Hubei province "the study and development on the corrosion-resistant and wear-resistant coatings of water wall tubes in boiler".In the aspect of thermal spraying processes the main techniques applied to water wall tubes include flame spray, arc spraying, plasma spray and high velocity oxy-fuel spray(HVOF).Flame spray possesses the characteristics of simple equipment, easy of use and low operating cost, but the coating adhesive strength is low, and porosity and oxidation is high. Although arc spraying has the advantages of high deposition efficiency and low cost, only electrically conductive wire materials can be adopted for arc spraying .The quality of coatings produced with plasma spraying and HVOF is high, but large volume of plasma spraying equipment makes it difficult to be used on site, and the high investment of equipment, the large consumption in fuel and high cost prevent from spreading of HVOF. The coatings deposited with a new subsonic flame spraying possess high bonding strength, low porosity and is low costly, at the same time, it is applied to the worksite of boiler tubes specially .The coatings applied to water wall tubes were deposited by subsonic flame spraying in this study.In the aspect of spraying materials, NiCr alloy and cermet coating have become main materials applied to boiler tubes. In the thesis, we developed newalloy and cermet coating materials by adjusting the content of Cr in Ni and add some amount of Si and Cr3C2. The test of high temperature oxidation and corrosion, erosion, bonding strength, hardness, thermal cycling and thermal shock of coatings tests were carried out in this study. The mechanism of corrosion and erosion resistance of NiCr alloy and cermet coatings were explored and their performance were compared with 20G, 20G covered by dope and the coatings of FeCrAl, NiCrTi used widely at present. Increscent weight of oxidation, graphic and micrographic were explored ,meanwhile, the mechanism of anti-erosion and wear-resistant of new coating material are also studied.At last, a program applied to calculate the temperature distribution and heat transmission between water wall tubes with sprayed coatings and metal anti-erosion tile was developed by use of Visual Basic. |
