Failure analysis for type 321h superheater tubing bends

摘要

A metallurgical examination and root cause failure analysis was performed for two failed secondary superheater tube samples. The tubing material was specified as Type 321H stainless steel. The service life was approximately sixeen years. The failure location was on the trailing edge of the wrapper tube bends at the extrados of the 90 deg bend. The failure apperance is characterized by the follwing: window opening fracture appearance, axially oriented cracks initiating at the tube inside surface approximately 10 deg to 25 deg from the neutral axis towards the extrados, radial crack propagation direction and intergranular creep rupture cracks. One of the unfailed bends had inside surface connected cracking near the neutral axis position typical of the failed tubes. Based on the intergranular failure path and intergranular creep cracking found for both filed tubes, the failure mechanism was identified as creep rupture. The probable root cause of failure was the excessive ovality of the tube geometry due to either initial fabrication or system stresses, exacerbated by the higher than anticipated operating temperature. The magnitude of the stress intensification associated with the tube ovality was estiamted based on the Type 321H tubing rupture properties, the known failure time, and the estimated service temperature.