Failure analysis on a fractured 34CrMo4 steel high pressure cylinder filled with a mixture of inert gases

After 10 years of service, one high-pressure cylinder made by 34CrMo4 low alloy steel fractured catastrophically, causing extensive damages. The cylinder was filled with a commercially available mixture of nitrogen, argon and carbon dioxide. A failure analysis has been carried out in order to identify the cause of rupture. The examination of cross sections specimens has revealed several branched cracks originating at cylinder inner wall side and propagating across the bulk. The observation of fracture surfaces supported the hypothesis of an environmental attack that affected the cylinder inner surface. No evidences of other damage form have been found: the chemical analysis, the microstructure examination and the mechanical tests performed provided results in full compliance with the technical specifications of the alloy. On the basis of such considerations, it can be assumed that the cylinder fractured by stress corrosion cracking. The exposition to a CO/CO2 containing atmosphere in presence of undesired traces of moisture promoted the local damage, as long as the applied tensile stress triggered cracks formation and propagation. Then, a progressive reduction of the effective toughness occurred, leading to a sudden, catastrophic, overload rupture.