Wrong heat treatment of martensitic steel welded tubes caused major cracking during assembly of resuperheaters in a fossil fuel power plant

摘要

During assembly of resuperheaters in a modernized power unit numerous cracks have occurred in 44.5 (OD) mm X 4 mm TIG welded tube joints made of dissimilar martensitic steels X20CrMoV 12.1 and T91. All cracks were located in the HAZ of the X20 steel. The failure investigation covered metallographic examination and Vickers hardness measurements of test pieces cut out from the damaged resuperheaters. In the cracked specimens very high Vickers hardness was measured (up to 549 HV5) in the X20 steel HAZ with the presence of martensite. The not cracked welded joints were of lower hardness and martensite was partly tempered. By comparison of the maximum hardness of the tested specimens with those ones measured on specially shop welded joints after heat treatment at different temperatures and also left without heat treatment, it was found that the X20-T91 tube joints at the failed resuperheaters have not reached the required soaking temperature of 760 deg C. Due to resistance heat treatment of the welded tube joints, grouped in bundles, their soaking temperatures were in the range between 550 deg C and 680 deg C and the cracked joints have not been tempered at all. After cutting out the defective joints new X20-T91 tubes have been welded and heat treated one by one. Vickers hardness measured on test samples of repair joints testifies the properly performed heat treatment.