Plastic collapse of a stainless steel pressurized tube

The failure of a stainless steel tube which conducted oil at 300 °C has been analysed. The fracture surface of the broken tube was studied in the scanning electron microscope and the fracture mechanism found was the nucleation, growth and coalescence of voids. This mechanism is characteristic in materials plastically deformed before failure. Specimens for metallographic examination were cut from the damaged tube and from an intact tube to analyse both microstructures. No significant changes which could justify a microstructure’s embrittlement were detected. Hardness measurements were performed on the damaged and intact tubes. The broken tube was harder than the intact tube due to plastic deformation accumulated during failure. The pressure which is necessary to reach this hardening was analysed by the deformation theory of plasticity and it was found this pressure is close to that corresponding to the plastic instability. Consequently, the most plausible hypothesis of failure was due to an over-pressure which leads to the tube’s plastic collapse.