Water wall tube burst occurred in a subcritical 300 MW unit boiler. To find out the reasons

the macro characteristics of the rupture area were examined and the material's metallographic structure was analyzed by optical microscope

the energy dispersive spectroscopy(EDS) and X-ray diffraction(XRD) were adopted to examine the scale(corrosion products) on the tubes. The results show that

the tube exhibited "window opening"brittle fracture

and thick scale formed on inner wall of the fire-facing side. The substrate under scale was obviously thinned by corrosion

and decarbonization and intergranular crack were evident at the base metal. The rupture failure was resulted from hydrogen corrosion under scale. To insure the unit operation safety

several suggestions were put forward

such as preventive measures including using nondestructive testing method to examine the water wall tube

changing tubes with defects

conducting examination test to effectively decrease the scaling rate of the inner wall of water wall tube

and enhancing the monitoring of steam water quality.