CFB锅炉用MMC防磨涂层的失效原因与机理

FAILURE ANALYSIS OF MMC COATING USED IN CFB BOILERS

在465 t/h循环流化床锅炉(CFB)水冷壁管上用高速电弧喷涂(HVAS)方法制备Cr3C2颗粒增强金属基复合材料(MMC)防磨涂层,运行180天后,在失效涂层部位取样,研究涂层的失效原因与失效机理.结果表明,涂层上端与管壁过渡处最容易发生早期失效,原因是涂层厚度形成的凸台造成的局部涡旋流;在低攻击角颗粒冲蚀下,MMC涂层的失效机理主要是微观切削和腐蚀;在高攻击角颗粒冲蚀下,涂层的失效机理主要是涂层被大攻击角颗粒撞击后碎片的剥落.两种情况下均伴随有涂层表面的高温氧化和低温热腐蚀.MMC涂层在低攻击角颗粒冲蚀下的耐磨性优于高攻击角颗粒撞击的耐磨性.最后,给出了采用涂层厚度过渡处理的解决方法.

A Cr3 C2 particles reinforced metal matrix composite （ MMC ） coating was prepared on the surface of the water-wall tubes in a 465 t/h circulating fluidized bed （CFB） boiler by suing high velocity arc spraying （HVAS） method. Samples were made from the failure coating after being in service for 180 days and then examined by SEM. The failure cause and mechanism of the coating were studied in detail through analysing the operation conditions. The results show that the premature failure easily occur at the upper edge of the coating, which may be induced by a local gaseous vortex in the CFB boiler, the later may be initiated on the convex periphery of coatings on the upper part of the surface of the coated tubes. In the condition of abrasion by particles impacted at small angles, the failure mechanism of the MMC coating is mainly micro-plough and corrosion. Nevertheless, if the abrasion particles impact the coating by large angles, the coating appears some characteristics as such as brittle materials. Now, the hard particles become the sources of micro-cracks, and the failure mechanism is the fragment-spallation by abrasive wear. The above two cases were also accompanied by oxidation and hot corrosion proceses on the surface of the coating. The abrasive-wear resistance of the MMC coating under the impact condition of particles by small angles is higher than that by large angles. In the paper, a measure to overcome the troubles was proposed that the thickness of the coating on its upper edge should be gradully decreased upstream.