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

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 5μ m 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.