Tracking supervision of service performance and life assessment of defective 15Cr1Mo1V steel pipeline

Abstract

A large number of inclusion defects were found during the metal supervision process of the Russian-made 15Cr1Mo1V steel main steam pipeline of a thermal power unit. In this paper, the tracking supervision research of the 15Cr1Mo1V steel pipeline (three elbows with the most serious inclusions at different service time) is carried out in order to find out the variation and distribution law of the material's strength, toughness, metallographic structure and creep rupture strength. And on this basis, a creep fatigue life assessment is performed. The results show that the tensile strength, impact absorption energy and creep rupture property of the material decreased with the increase of the service time. The ductile-brittle transition temperature increased as time prolonging, all above room temperature, which means that the material is brittle. The main inclusions are MnS and SiO2 which are usually plastic, and the most serious inclusions are grade 3. Longitudinal inclusions are mostly of long-chain features, but no crack was found at the inclusion tips; the transverse inclusions are granular. The creep fatigue crack evaluation for the defects of plastic inclusions such as MnS and SiO2 shows that the crack propagation length at the inclusion tip after over 105 h of service is tiny which is consistent with the microscopic result, which means that the influence of the plastic inclusion on the structural integrity can be neglected. With the increase of the service time, the endurance strength gradually and slowly decreased. The presence of the inclusions did not accelerate the decline rate of the endurance strength, but might have an impact on the initial endurance strength of the new material. Meanwhile, brittle inclusion defects tend to generate stress concentration and induce the formation of cracks, so it is also necessary to strengthen the supervision and inspection of the inclusions and judge the inclusion category in the subsequent operation.