Failure analysis of high-pressure fire extinguisher

The failure analysis of the high-pressure cylinder of a fire extinguisher fabricated by quench and tempered AISI/SAE 4130 steel is presented. The metallographic and mechanical examination indicated that the cylinder's fabrication material was not properly heat-treated since the microstructure contained an excessive amount of alpha-ferrite and low hardness. The failure mechanism was identified as Stress Corrosion Cracking induced by the combination of pressure induced and residual stresses, a corrosive environment due to the presence of water and other contaminants in the extinguishing agent, and an incorrectly heat-treated material. A fracture mechanics assessment demonstrated that a critical crack, of 3.35 mm (0.250 in.) deep and 254 mm (10 in.) long, caused the failure at the operation pressure. To prevent similar failures, it is recommended to periodically inspect the high-pressure cylinder by nondestructive testing to prevent internal cracks. Furthermore, the cylinders must be perfectly dry and free of corrosive species.