Stress Corrosion Cracking in Twin Ferrule Compression Fittings in SS31600

Abstract

Abstract Twin Ferrule Compression fittings are used worldwide in many industries for pressure containment including Oil and Gas. Stainless steel grade UNS S 31600 / 1.4401 is a common choice of material for such items as it offers corrosion resistance in its uncoated state and a high degree of internal cleanliness. As an austenitic stainless steel however the items are prone to chloride induced stress corrosion cracking (CSCC) in certain environments such as offshore marine and coastal areas where such failures were previously reported in SPE paper 157356-PP 2012. It has since been suggested that a contributory factor to these failures may be over tightening of the fittings during installation inducing excessive tensile stresses above design intent and shortening the time to failure. This paper will attempt to quantify these stresses and show how the act of over tightening the fitting (beyond the manufactures recommendation) may affect the susceptibility to CSCC and the time to failure. It will also detail the development of CSCC in the fittings over time and show how the degradation mechanism progresses to leak type failures. These items are numerous on oil and gas production platforms with several thousand typically employed per installation so the importance of knowledge in this area may be significant to some users. If the morphology or stages of cracking can be identified then targeted inspection of fittings may be carried out to plan change outs in a timely manner rather than allowing the cracks to develop to cause larger leaks or unplanned shutdowns. If the precise morphology of the cracking is identified a better understanding of the consequences can be made. Verifying tensile stress levels may also be used in the selection of alternative materials for the fittings. Fitting manufacturers offer a range of alternative materials which may be more resistant to stress corrosion cracking such as Duplex, SMo or Nickel alloys. The test took a sample of new ½″ tubing connector twin ferrule fittings of S31600 material and fixed them to tubing using various degrees of tightening. The fittings were then subjected to the standard accelerated corrosion test of ASTM G123 00.