Failure Causes of Cracking of 12Cr1MoVG Steel Superheater Tube in a Boiler

A 12Cr1MoVG steel superheater tube of a power plant boiler burst and cracked after half a year of operation. In order to identify the cause of the cracking, the failure analysis of superheater tube was carried out by means of morphology observation, chemical composition detection, mechanical property test, hardness test, nonmetallic inclusion analysis and microstructure analysis. The results show that the superheater tube did not undergo heat-treatment for stress relief after cold bending, and there were machining defects such as micro-cracks on the inner wall of the bent tube. During service, the oxidation layer on the inner wall of the tube greatly reduced the heat transfer efficiency, resulting in overheating operation of the superheater tube, thus decarbonizing the metal and reducing the corrosion resistance of the metal. The Na₃PO₄ content and alkalinity in the furnace water had exceeded the standard. The alkali solution formed by the deposition and concentration of free alkali ions in the oxide layer of the inner wall of the bending tube, which caused alkali stress corrosion to the superheater tube at the processing defect. And the superheater tube cracked along the grain boundary under the action of bending stress.