Fabrication Induced Stress Corrosion Cracking of Waste Water Volatile Organic Compounds Stripper Domes

摘要

Codes used during the design and fabrication of plant equipment are sometimes not enough to prevent premature failures. Codes should be supplemented by expert knowledge. It is for this reason that the involvement of subject matter experts during the early stages of a project is important. This article presents the failure of a volatile organic compound (VOC) stripper after about six years of operation. Several leaks occurred due to short cracks on the domes of the stripper, close and transverse to the dome-to-shell welds. Visual, metallography, and scanning electron microscope (SEM) evaluation showed that the cracking mechanism is intergranular stress corrosion cracking (IG-SCC) that initiated at the internal surface of the dome. Metallography and hardness testing indicated that the dome side of the weldment has substantial forming residual stresses. XRF/CS analysis showed that the material of construction is UNS S30400 stainless steel as per design specification. The waste water contains a very low content of chlorides (about 10ppm). The combination of chlorides from the waste water, residual dome forming stresses and a sensitized microstructure next to the weld metal is the root cause of the cracking. No solution annealing was done on the formed dome because the design code does not require solution annealing for formed components with calculated strains that are less than 20%. The failed domes were replaced with low carbon equivalent grade UNS S30403 domes that were solution annealed after forming to avoid sensitization during welding and to reduce the level of residual stresses respectively.