Intergranular susceptibility in failures of high pressure tubes

This work addresses the influence of metallurgical susceptibility to intergranular cracking on the repeated cracking and failure of thick wall curved steel tubes from a petrochemical reactor. These tubes are made of HP-4 steel, bent and heat treated, and then subjected to autofrettage. Internal pressure is around 250 MPa. All failures are characterized by strongly branched, mostly circumferential multiple intergranular cracks. Most cracks initiated in the outer surface, in contact with steam; these were related to stress corrosion cracking (SCC). Some cracks initiated in pre defects in the inner surface, in contact with a polymer, and in the mid thickness of the tube wall. This study includes the assessment of deformation and temperature induced embrittlement mechanisms, measurement of longitudinal residual stresses, and mechanical testing included tensile, Charpy and SCC tests. Susceptibility to intergranular cracking was experimentally assessed by recreating conditions of embrittlement by thermal treatments and tensile testing. Samples with 0, 3 and 5% plastic deformations were subjected to 24 h thermal treatments between 300 and 400 °C. Under the conditions of previous plastic deformation due to bending and autofrettage it was possible to recreate intergranular embrittlement at service temperatures, a phenomenon similar to temper embrittlement. The process of forming the bent created localized yielding and large longitudinal residual stresses. Recovery measures, mostly relying on thermal treatments, were defined.