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.