瞬时过热对高铬镍奥氏体钢断裂和析出相形变的影响机理

Influence mechanisms of instantaneous overheating on fracture and precipitate distortion of high chromium nickel austenitic steel

某火电厂锅炉在进行试验管屏风压试验测试时,发现再热器管(22Cr-15Ni钢)与套管(18Cr-8Ni-Nb钢)的密封焊接处存在裂纹,通过样管宏观分析、裂纹金相分析和断口及析出相的SEM/EDS分析了其裂纹产生原因。结果表明,失效处附近存在机械应力输入和焊瘤应力集中的明显痕迹,焊瘤热输入导致局部组织过热,同时基体M_(23)C_(6)球状析出相异常长大造成微观拉应力。上述原因的综合作用最终导致机械应力从应力集中处输入产生微裂纹,并沿着过热弱化的晶界扩展,与基体析出相产生的微裂纹汇合,最终扩展成为环向断口。风压泄漏吹击断口,造成断面上因热输入异常长大的M_(23)C_(6)球状析出相上产生韧性变形凹坑和脆性开裂,并显露内部枝状晶的微观断口形貌。

Cracks were found in the sealing weld between the reheater tube(22Cr-15 Ni steel)and the sleeve(18Cr-8Ni-Nb steel)during the test tube screen pressure test in a power plant.The cracking cause was studied through macro and micro methods,including macroanalysis of the sample tube,metallographic analysis of the crack,and SEM/EDS analysis of the fracture and precipitates.The results indicate that near the failure site there are obvious traces of mechanical stress input and weld stress concentration,meanwhile the welding heat input causes local microstructure overheating,and the abnormal growth of M_(23)C_(6) globular precipitate causes microscopic tensile stress.Finally,the combination of the above reasons causes that the mechanical stress is input from the stress concentration to produce microcracks that propagate along the overheating-weakened grain boundary.Such cracks coalesce with the microcracks generated by the matrix precipitates and eventually propagate into a circumferential fracture.The wind with pressure erupts and leads to the distortion and fragmentation of the globular precipitates,finally pits on surface and internal dendritic crystal of M_(23)C_(6) globular precipitates come to appear.