Failure analysis of 12Cr1MoVG outlet header for the reheater of the boiler under high temperature and high-pressure environment

After operating for 11,640 h, a boiler failed due to multiple cracks generated in the inner wall of the outlet header for the reheater. A thorough analysis was conducted in this work, including evidence and specimen collection, chemical composition analysis, tensile and impact test analysis, metallographic analysis, energy dispersive spectroscopy (EDS) analysis, and electron backscattering diffraction (EBSD) characterisation, to reveal the causes of cracks in the outlet header of the reheater. The tensile strength-Rm and the average micro-Vickers hardness of the outlet header is 496 MPa and 168 HV, respectively. Besides, the microstructure was found to meet the requirements of the relevant standards. The grains near the crack exhibited different degrees of deformation, and the deformation degree increased with the decreasing distance from the crack. In addition, the grains with the Schmid factor value greater than 0.3 accounted for 99.8% near the crack, indicating that the grains surrounding the crack were predominantly soft orientation grains. Thus, many deformation bands mainly composed of low-angle boundaries were induced inside the grains near the crack. Eventually, it was concluded that alkali stress corrosion and thermal fatigue resulted in the failure of the outlet header.