Investigation on the welding-induced multiple failures in boiler water wall tube

Abstract In this study, by performing visual inspection, chemical composition analysis, mechanical performance testing, micro-structure and micro-morphology analyses, a failure investigation is presented on the surfacing weld induced bursting of 20G water wall tube in a thermal power plant. Besides, to identify the environmental medium and deposits, the Energy Dispersive Spectroscopy (EDS) and X-Ray Diffraction (XRD) have been conducted. The tube has undergone the localized wall thinning by fly-ash erosion burst after being repaired for 4000 h. As revealed from the macroscopic examination on the tube, significant corrosive products have been produced on the water-side of the surfacing layer position of the tube. According to the metallographic analysis of the rupture region, the massive widmanstatten overheated structure has been found in the heat-affected zone (HAZ). Furthermore, in the failure parent metal, extensive intergranular cracks, severe decarburization and mechanical performance degradation (∼20 HV10) have been identified. The micro-morphology analysis has described the partial-melting process, spherical particles and the dendrite on the fracture surface. As revealed from the EDS and XRD results, the accumulation of Cu on the inner surface of the rupture region of the tube was demonstrated. The comprehensive study has suggested that the excessive welding heat-input leads to the generation of local over-heating and over-burning. Lastly, the electrochemical corrosion and the hydrogen corrosion cracking are generated in the inner wall of the welding region.