12Cr1MoV锅炉过热器管长期过热开裂原因分析

Long-term Overheating Cracking Analysis of 12Cr1MoV Boiler Superheater Tube

某厂的12Cr1MoV锅炉过热器管运行39 840 h后发生开裂。宏观检测发现有堵管现象,断口呈厚唇状,表面覆盖着氧化物。采用光学显微镜、X射线衍射、扫描电镜、能谱仪、透射电子显微镜等测试方法对失效管进行检测。研究表明,堵管物的成分是Na3PO4、Fe2O3和NaFeO2;内壁氧化物在未堵管位置是Fe3O4,在堵管位置是Fe2O3。断口呈沿晶形貌,有沿晶界的二次裂纹和晶界孔洞。靠近裂纹和远离裂纹的金相组织相同,原始态的片状珠光体消失,碳化物在晶界聚集长大,组织发生了严重球化,晶界碳化物是M23C6型碳化物;根据电力行业标准评定材料球化级别为5级。由球化级别和运行时间估算壁温,得知过热器管超温运行。靠近裂纹和远离裂纹的试样的硬度值相近,数值较低,约为135 HV。12Cr1MoV锅炉过热器管的失效模式是高温蠕变开裂,失效主要是由内壁氧化层和堵管物造成的长期过热引起。

After running 39 840 h, a boiler superheater tube made by 12Cr1MoV steel in a plant cracked. The tube is blocked and the thick-lip-shape fracture is covered with oxides. By utilizing optical microscope（OM）, X-ray diffraction（XRD）, scanning electron microscopy（SEM）, energy dispersive spectroscopy（EDS）, transmission electron microscopy（TEM） and other methods to detect the tube, it is found that the compositions of blocking substances are Na3PO4, Fe203 and NaFeO2. In the unblocking position of the inner wall oxide is Fe304, however, in the blocking position, it is Fe203. In terms of the shape of the fracture morphology, it is inter granular, along with secondary cracks and pores in the grain boundary. The research results illustrate that the metallographic microstructures in samples of near crack and away from the crack are the same. With the lamellar pearlite of the initial state dissolves, the carbides gather and grow at the grain boundaries. The organization spherifies seriously, and the carbide is M23C6. According to the electric power industry standard, the spherical degree is identified as level 5. By utilizing spheroidization level and run-time to estimate the wall temperature, it can be know the tube run in over-temperature environment. To be specific, the hardness in all the samples is about 135 HV, which is relatively low. In all, the failure mode of superheater tube is high temperature creep cracking which is caused by long-term overheating resulted from the oxide layer and the blocking substances.