Failure analysis of bolts in deluge valve bonnet in cooling tower system in petrochemical plant

Machine bolts installed on the bonnet of OS&Y (outside screw & yoke) deluge gate valve 6″, were found cracked or broken in service. The gate valves are part of the petrochemical plant fire protection system. The result of chemical analysis of the received bolts samples, indicates that their composition corresponds to a TWIP steel, having high manganese (16 wt.%) and high chromium (9 wt.%). The microstructural examination of cross sections of bolts, revealed an austenite microstructure with high number density of deformation twinning and slip lines specially at the failure zones, which is typical of the cold deformed TWIP steels. The hardness distribution within the bolt's head and the head to shank zone was not homogeneous, ranging between 225 HV_0.1 in the shank central zone and 470 HV_0.1 at the curved highly stressed zone of the bolt neck. Both the cracked and broken bolts appeared corroded specially at the failure zone of the head to shank neck. The EDS analysis of corrosion products inside cracks indicated high concentration of Cr, Mn, Cu, O and traces of mineral elements of Si, S, Cl, K and Ca. The XRD analysis identified the Chromium Iron Manganese intermetallic Cr₅Fe₆Mn₈ and Iron Manganese Carbide Fe_2.7Mn₃C in the bolt matrix and formation of "Spinel" Copper Chromite CuCr₂O₄ and Iron Manganese Oxide MnFe₂O₄. The cracks appeared to originate at corrosion pits formed at the crevice between the bolt head and the underneath washer and nut at the head to shank fillet radius and on bolt neck zone, which have high concentration of corrosive species from the humid salty marine atmosphere. The cracks then propagate in perpendicular direction to the tensile stress direction of the bolt torque. The fractography examination of the cracked bolts cross sections and the broken bolts fracture surface depicted mixed ductile/dimpled mode with areas of quasi-cleavage transgranular and intergranular crack propagation. These observations suggest the failure mechanism to be hydrogen embrittlement-assisted chloride-stress corrosion cracking HE-assisted Cl-SCC.