FAILURE ANALYSIS OF DUPLEX STAINLESS STEEL IN AN ATMOSPHERIC TOWER

An atmospheric tower head, manufactured using a clad plate of carbon steel + 2205 duplex stainless steel (DSS), was reported to show clear cracks in the DSS layer after serving for four years at an atmospheric distillation unit of a refinery. The cracks propagated in a dendritic manner within the heat-affected zone (HAZ) and the weld metal, accompanying locations with a higher hardness than that of locations without cracks. Some nondestructive methods were used to analyze the chemical composition, microstructure, hardness of the base metal, HAZ and weld metal. An analysis based on these results and the statistical data of an HC1-H2S-H(2)o corrosion environment was proposed to explain the formation of cracks from the viewpoint of physical metallurgy. The analysis showed that there were conditions favorable for the precipitation of the sigma (o) phase in the DSS layer during the manufacturing process of the head, resulting in the occurrence of stress corrosion cracking (SCC) in the DSS layer under the harsh HC1-H2S-H2O environment. Moreover, some solutions, i.e., the enhancement of anticorrosion measures, the optimized microstructure of the DSS, and modified welding parameters were recommended to avoid a similar failure.