Failure Investigation of Boiler Tube: A Root Cause Analysis

In a thermal power plant, water wall boiler tube made of Cr–Mo low alloy steel, failed during operation. Post failure investigation revealed ‘fish mouth’ opening accompanied by thick scale formation, significant wall thinning and de-shaping of circular cross section of tube material. All the evidence indicated temperature rise over a period, for which martensitic transformation, matrix grain growth and globulerization of carbide precipitate occurred within microstructure of the alloy. The temperature rise was due to the over-firing of burner and the sudden change in flue gas constituents. Wall thinning due to oxide scale formation and plastic flow of material at elevated temperature tentatively below upper critical temperature, reduced the load bearing ability of component. The tube material ultimately failed due to overload during service.