Failure Analysis of Expansion Bellows Serving in Steam Pipe Network

The failure of Type 316L stainless steel expansion bellows serving in steam pipe network has been analyzed. The expansion bellows cracked after approximately 4-year service in steam network at 230°C. The caustic stress corrosion cracking (caustic-SCC) resulting from a combined function of caustic concentrating, metal temperature and residual stress was responsible for the failure. Cracking initiated on the inside surface of the bellow at the peak where served as stress raiser by observation of sample taken from failed expansion bellows. The pattern of cracking was shown as a spider web of several small cracks in photomicrographs. The metallographic examination also indicated that cracks were predominantly transgranular. The oxides on the surface of cracks can be confirmed to be caustic NaOH through X-ray diffraction (XRD) examination.