Our team in Australia recently conducted an extensive failure analysis on bolts used in a hand valve at an offshore oil and gas facility.
The bolts were manufactured from a chromium nickel stainless steel, broadly consistent with a 304 type grade. Upon examination, it was determined that the mechanical properties of the bolts aligned with those of A2-80 / Grade 8.8 high-strength structural bolts, which inherently possess high hardness and residual stress.
Our analysis revealed that these bolts' failure was primarily attributed to Chloride induced Stress Corrosion Cracking (SCC). This form of corrosion occurs when a combination of tensile stress, specific environmental conditions, and the presence of chloride ions act upon susceptible materials. In this case, all the necessary conditions for SCC to take place were found to be present.
Through the application of metallurgical failure analysis on the stainless steel bolts, our team successfully pinpointed the underlying cause of the fractures as stress corrosion cracks induced by chloride. As a result of our findings, we recommended a review of the bolt application, to assess the suitability of these bolts in their specific operating conditions. In addition, we advised conducting inspections on any other bolts operating under similar conditions to prevent any further failures caused by SCC. This proactive approach aimed to mitigate potential risks and ensure the integrity and reliability of the facility's equipment.