低温再热器12Cr2MoWVTiB钢接头断裂失效机理

Fracture mechanism of 12Cr2WMoVTiB steel joint in low-temperature reheater tubes

电厂低温再热器12Cr2MoWVTiB（R102）钢小径管接头焊接时取消预热和焊后热处理，工作温度在500～530℃，运行过程中发生脆性断裂失效。研究导致失效的裂纹启裂部位及扩展方向、裂纹附近的显微组织、硬度分布及沉淀相特征，判断裂纹性质，揭示裂纹的形成机理。结果表明，裂纹产生于粗晶区，终止于细晶区，性质为再热裂纹。粗晶区仍然为板条马氏体组织，板条内有高密度的位错，硬度值高达450HV；晶内的析出相很少，但原始奥氏体晶界上析出富Ti的复合沉淀相，尺寸达到5μm，它们的析出使晶界附近形成C和合金元素的贫化区，降低晶界强度，同时削弱晶间结合力，这是R102钢接头产生再热裂纹的主要原因。粗晶区的高淬硬状态也促进再热裂纹的形成。

The brittle failed 12Cr2WMoVTiB(R102)tube joint that operated at temperature of approximately 500-530 ℃ in lower-temperature reheater of power plant was invesgated.The joint was welded under no preheat and post weld heat treatment.The crack growgth,microstructures,hardness,precipitation phase near the crack were characterized,thus the failure mechanism could be understood based on these.The results showed that the cracking initiated at the coarse-grained heat-affected zone(CGHAZ)and terminated at the fine-grained heat-affected zone,which was typically the reheating crack.The CGHAZ remained the as-quenched martensite with hardness above 450 HV after expereienced long-time service,and there were high density dislocations in laths.There was little precipates in the grains.In CGHAZ,precipitated phases with rich Ti and larger than 5 μm were found to be distributed along the prior austenite grain boundaries which depleted the adjacent matrix carbon and alloying elements,leaving a soft zone.Therefore,the creep strain would be concentrated in these relatively soft zones leading to intergranular failure.This was the main reason for reheat crack.Moreever,the high hardness of CGHAZ also contributed to the crack.