Analysis and application of critical value of type IV cracking creep hole in P92 steel joint

Objective The factors influencing the critical value of the creep hole in the HAZ of P92 pipeline joint were analyzed, and the calculation method of the critical value of the creep hole was obtained, which provides a basis for evaluating the joint cracking risk. Methods The P92 joints with type IV cracking after service were collected, and the samples were taken from the uncracked parts for high temperature sustained creep test under different stress conditions at 610 ℃. Then, the distribution morphology of creep holes in the HAZ of the cracked joints and permanently fractured joints were observed by optical microscope and electron microscope, and the area fraction of the creep holes was measured by image analysis software. At the same time, hardness tester was used to test the softening of the HAZ. Finally, synthesizing the test results the influence of the softening degree of the fine crystal zone and the stress level of the joint on the critical value of the creep hole was analyzed, and the functional relationship between the stress, hardness value and the critical value of the hole were fitted. Results It was found that the fine-grained zone of P92 steel joint has a faster softening rate than other areas of the joint, but the softening was not the necessary condition for type IV cracking. When the stress level was high, the creep cracking could be triggered when the fine-grained zone did not significantly soften.The functional relations of the joint axial stress level, the critical value of creep hole area fraction and microhardness were obtained. Conclusion The critical value of creep hole area fraction causing type IV cracking was related to the relative stress level (the ratio of axial stress to hardness). The higher the relative stress level, the smaller the critical value of creep hole and the lower the safety of joint. The axial stress level of cracked joints could be estimated by measuring the creep hole area fraction and microhardness in the FGHAZ of the joints, and the type IV cracking risk of uncracked joints in service could also be assessed.