Leakage failure analysis on water wall pipes of an ultra-supercritical boiler

•Unqualified manufacturing processes are the root cause of the malfunction.•Internal cracks occurred during the manufacturing of the parts, which were passivated at an excessively fast quenching rate.•The complex thermal stress environment and coarse grains promote crack propagation.•To prevent similar blade failures, some preventive strategies have been proposed. Water wall pipe is one of the key components of thermal power plant, and their premature failure is a common appearance. The occurrence of a leakage accident involving water wall pipes in a thermal power plant boiler prompted an analysis of the causes of cracking and leakage. This analysis was conducted using various methods including optical light microscope (OLM) observation, chemical composition testing, hardness testing, metallographic organization observation, and energy dispersive spectroscopy (EDS) analysis. Based on the findings, relevant suggestions were provided to prevent the recurrence of similar accidents. The test results indicate that the proximity of the leaking pipes to the soot blower and manhole door has created a complex stress environment. Also, the inner wall of the pipes displays passivated quenching cracks, while the outer wall exhibits black dense materials in the high temperature oxidation cracks. The fracture surface clearly exhibits a fatigue fracture pattern, with all cracks being transgranular and oriented perpendicular to the pipe wall. Additionally, coarse grains are observed near the outer surface of the cracks and in some areas in the middle of the pipe wall. The root cause of the failure is inferred to be that the passivation quenching cracks were generated due to improper process control during the manufacturing of the pipe, and the quenching cracks were further expanded by the thermal alternating stress, finally penetrated and leaked under the promotion of large stress environment, uneven grain structure and other factors.