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A superheater creep-fatigue interaction failure and its stress assessment
Engineering Failure Analysis ( IF 4 ) Pub Date : 2020-10-15 , DOI: 10.1016/j.engfailanal.2020.105004
William Liu

Creep-fatigue interaction (CFI) is a common failure mode in turbine blades. However, this failure mode has rarely been reported in boiler superheaters. This article presents a failure analysis of CFI on a superheater in a black liquor-fired boiler. Two cracks in the HAZ of stitch weld were identified. The “beach marks” on fracture surfaces indicated the cyclic stress. Fractographic analysis distinguished the different fracture modes. Crack 1 initiated and propagated in the transgranular mode. Crack 2 initiated in intergranular mode and propagated in the mixture of intergranular and transgranular modes. On the base of the hoop stress and axial stress, the dynamic stress was the main root cause of CFI fracture. The dynamic stress can be calculated from the fracture orientation angle. The greater the fracture orientation angle, the higher the dynamic stress. The fracture-mechanism map was applied in this case study. In combination of fractographic analysis and stress calculation with the fracture-mechanism map, the crack fracture sequence was revealed. The two cracks did not initiate simultaneously, and their stress levels were significantly different. Through this case study the fracture-mechanism map was recommended as a guide to superheater fracture analysis.



中文翻译:

过热器蠕变疲劳相互作用失效及其应力评估

蠕变疲劳相互作用(CFI)是涡轮叶片中常见的故障模式。但是,这种故障模式在锅炉过热器中鲜有报道。本文介绍了黑液锅炉中过热器上CFI的故障分析。在缝焊的热影响区中发现了两个裂纹。断裂表面上的“海滩痕迹”表示循环应力。分形分析分析了不同的断裂模式。裂纹1以跨晶模式萌生并扩展。裂纹2以晶间模式萌生,并以晶间和跨晶模式混合传播。基于环向应力和轴向应力,动应力是CFI断裂的主要原因。动应力可以从断裂取向角算出。断裂取向角越大,动应力越高。在本案例研究中应用了断裂机制图。将断裂分析和应力计算与断裂机理图相结合,揭示了裂纹的断裂顺序。这两个裂纹不是同时发生的,它们的应力水平也有很大差异。通过本案例研究,建议将断裂机理图作为过热器断裂分析的指南。

更新日期:2020-10-30
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