超临界锅炉末级过热器爆管原因的分析

Burst Cause Analysis of Final-stage Superheater Tubes for a Supercritical Boiler

为研究超临界锅炉末级过热器爆管的原因,对1台600 MW超临界锅炉末级过热器爆管上游管和相关管样的内壁氧化物的宏观形态、微观结构,脱落氧化皮的微观结构、物相、各区域的微观形态和成分等进行了分析.结果表明:堵塞末级过热器下弯头造成过热器爆管的脱落氧化皮来自于T23/T91钢管的T23管段内壁,而非T91管段内壁;T23钢管内壁氧化皮为二层结构,外层为粗大柱状晶的纯磁铁矿(Fe3O4),内层为等轴细晶的含W和Cr的尖晶石;原生氧化皮内层存在一条或多条沿圆周方向排列的孔洞链,氧化皮容易沿孔洞链分离,从而造成氧化皮脱落.

In order to find the causes of tube burst failures encountered by the final-stage superheater of a 600 MW supercritical boiler,an analysis was carried out to both macrostructure and microstructure of the oxide scales taken from inner surface of relevant tube samples,and to the microstructure,phase,micromorphology and composition of fall-off scales.Results show that the failure has been caused by oxide scales split off from inner surface at T23 side of the T23/T91 joint,which blocked the lower elbow of the superheater and finally led to the tube burst.The scale on inner surface of T23 tube has a two-layer structure,of which the outer layer is the coarse columnar grain Fe_3O_4 while the inner layer fine equiaxed spinel,containing W and Cr element.One or more hole-chains on inner surface of primitive oxide skin formed along the circumference direction are easy to break and cause the oxide skin to fall off.