A number of longitudinal cracks were detected in drum's water-side of a 300 MW boiler. The microstructures, residual stress, hardness of the drum near the cracks, and operating processes of the 300 MW thermal power generating unit were studied to explore the cause of crack formation. The results show that since carbides from the pearlite of the steel near the cracks were spheroidized, these spherical carbides gather in the alloy ferrite grain boundaries and parts of them connect into a chain organization. Crack initiates in the longitudinal grooves which caused by the barrel tile pressing processes, and there is a high stress concentration in these areas. Alternating thermal stress caused by water lever fluctuations and variable load operation leads to low cycle fatigue cracks initiation near the water line. The crack formation of the drum is mainly due to local stress concentration, larger alternating thermal stress, microstructure of the materials change, and stress corrosion caused by the boiler water.%某电厂300MW机组锅炉汽包中锅筒水侧出现多条纵向裂纹.通过对裂纹附近的金相组织、残余应力、硬度以及运行工况的研究,分析了裂纹形成的原因.研究结果表明:汽包裂纹附近材料珠光体中碳化物球化,球状的碳化物聚集在合金铁素体的晶界上部分连成链状组织;裂纹位于简体瓦片压制过程中形成的较深的纵向沟痕附近,在这些部位附近出现较高的应力集中;由于水位线的波动和变负荷运行产生的交变热应力导致水位线附近部位萌生低周疲劳裂纹;母材裂纹形成的主要原因是筒体中局部的应力集中、较大的交变热应力、材料组织变化以及锅水应力腐蚀的综合作用.
展开▼
