Abstract: For an imported 350 MW subcritical boiler of a power plant in China, multiple leakage failure phenomenon occured to the third stage reheater T91 steel tubes at the slider welding positions during hydraulic test after operation for 1.2×10⁵ h. Two typical leakage tubes were cut and the leakage reasons were analyzed by means of macroscopic analysis, chemical composition analysis, mechanical property test, metallographic examination, scanning electron microscope and energy spectrum analysis and so on. The results show the improper welding process resulted in local tube wall burn-through and pore defect when welding sliders, which was the main reason for the leakage of the T91 steel tubes during hydraulic test.