Failure analysis on tube weldment cracking of a high-temperature carbon black air preheater

In this study, the failure analysis on weldment joining the medium-to-high temperature steel tube (the jointed materials are S30815 and HK40Nb, respectively) sections of a high-temperature carbon black air preheater was conducted. The root cause of the cracking is identified as metal dusting appearing in the high-temperature carbon black flue gas environment, as indicated by the result of the macroscopic analysis, the chemical composition analysis, the metallographic examination, and the corrosion product analysis. As revealed by the result, the metal was directly exposed to the flue gas due to the damage of the surface protective film (i.e., Cr2O3) of the stainless-steel tube weldment. The result suggested that elemental sulfur within the flue gas and overheating in the operation destroyed the oxide film. Under the effect of the high carbon activity in the flue gas, carbon atoms were driven to attack metals. The accumulated carbon particles arising from weld reinforcement promoted the failure of the weldment. To prevent metal dusting, it is recommended to increase the amount of water vapor in the flue gas for the reduction of the carbon activity, choose high-temperature resistant materials for the prevention of direct contact between the flue gas and the material, and improve the welding quality to prevent the flue gas from being disturbed.