Failure Analysis on Fracture of 304 Stainless Steel Flange Connecting Bolts of a Liquid Ammonia Storage Tank

The 304 stainless steel flange connecting bolts of the pneumatic valve of the gas ammonia outlet above the liquid ammonia storage tank in a power plant fractured during service, the fracture failure reasons of the 304 stainless steel bolts were analyzed through the aspects of microstructure, fracture, hardness and tensile property by using metallographic microscope, scanning electron microscope, micro-hardness tester, tensile testing machine, etc. The results show that the failure mode of the bolts was stress corrosion cracking. The unqualified chemical compositions (high carbon content and low chromium content) of the bolts resulted in the decrease of corrosion resistance of the alloy and the increase of the tendency of intergranular stress corrosion. Because the bolts were exposed to the marine atmosphere near the coast which exhibited a higher concentration of Cl^-, the surface of the bolts were easy to be corroded during the long time exposure, and the cracks rapidly expanded along the grain boundaries under the combined effect of the pre-tightening force, the tensile stress produced during the service of the pneumatic valve of the gas ammonia outlet and the Cl^- until the occurrence of fracture. In addition, there were many casting defects in the bolts, which could significantly reduce the mechanical properties of the alloy and make the bolts fracture prematurely in the case of corrosion.