Role of residual stress in the failure of the bend of superheater Cr-Mo steel tube working in high-temperature conditions

This study deals with the material analysis of a Cr-Mo steel superheater at the bending point. The crack occurred on the inside of the bend after 4,300 h of boiler operation, which resulted in a leak of the working medium and a forced device shutdown. The planned service life of the superheater tubes was 34,000 h. The operating parameters of the boiler were as follows: steam output pressure 3.2–3.6 MPa, steam output temperature 440–450 °C. Cracks were found on the lateral surfaces of the bends of the superheater lower tubes. Visual, microscopic, and EDS SEM analyses were performed to determine the cause of the damage. Microscopic analysis showed that crack initiation occurred near the maximum bend on the inner surface of the tubes, where the present pits acted as stress concentrators. EDS SEM confirmed that the predominant iron content in the corrosion layer was related to the formation of tubercles above the corrosion pits, which participated in the corrosion-mechanical nature of the failure. The calculated residual stress values were significant and concentrated in both the intrados and extrados bending regions of the tube. Finite element model (FEM) was used to locate the post-bending stress distribution of the tube and confirmed that a combination of post-bending residual stresses, steam pressure and temperature, and steel tube pitting caused cracks to form and propagate. The distribution of local mechanical stresses through a cross-section of the wall thickness of the superheated tube was experimentally carried out using an electron backscattered diffraction (EBSD) analysis. The obtained results were correlated with the Mises residual stress.