Repair of Cracks in High Thickness Quench and Tempered Steel

During the planned shutdown in one of the Indian Oil, Petrochemical complex, Linear low density polyethylene plant fine longitudinal cracks were observed in the fillet weld of the heavy thickness sieve plate to bracing plate weld joint of a low pressure separator vessel. The crack was observed to be propagated (80-90% of total weld length) on the both side of fillet weld (toe region) of bracing and sieve plate weld joint in bracing. The ischolar_main cause of the cracks was suspected to be improper heat treatment in shop resulting in cracking while in service for six months. Since the sieve plate and bracing are made of heavy thickness chrome-molybdenum quench- tempered steel also known as high strength steel, in-situ repair and in-situ post weld heat treatment is difficult. Repair of eight no. of joints with total length 12 meters, took much time. This paper discusses the various problems encountered during the repair of cracks. A thorough understanding of the technicalities involved in in-situ repair and heat treatment involved of low alloy high strength heavy thickness steel will help in proper planning and in reducing the repair time.