Fastener Failure - Creep Rupture

This case study details the failure of five threaded rods from a support system within a high-temperature furnace. The furnace operated at approximately 1250F, and the rod material was type 310 stainless steel. The objective of the evaluation was to determine the failure mechanism for the threaded rods.

The work scope for this analysis consisted of light microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and microstructure analysis (metallography). For specific details on these methods, please see the Handbook of Analytical Methods of Materials on our website or contact us for more information.

The fracture for each of the five threaded rods was macroscopically transverse and followed the roots of the threads. The microscopic fracture features consisted of small, irregularly-shaped dimples and facets. These fracture features were consistent with a preferential fracture along secondary phase microstructure constituents. The texture of the fracture surfaces suggested preferential fracture propagation at or near the material grain boundaries.

The metallographic examination showed a high density of voids throughout the load-bearing cross section of the rod. The voids occurred along the austenite grain boundaries and at secondary-phase islands in the microstructure. The microstructure of the rod material also contained massive carbides within the load bearing section of the rod. Massive carbides are consistent with high-temperature degradation of the rod material.

The grain boundary voids and fracture morphology were characteristic of damage due to creep. Creep is a progressive failure mechanism that occurs due to a combination of an elevated temperature and a static tensile stress. The transverse orientation of the fractures indicated that the stresses causing these failures were due to tensile forces on the rods. Deformation in the threads of the mating nuts indicated that the rods may have been subjected to relatively high tensile forces.

In summary, the failure mechanism for the threaded rods was creep rupture. The creep fractures occurred due to a combination of an elevated-temperature environment and a sustained tensile force. The rod material had sustained high-temperature degradation resulting in the formation of massive carbides, which may also have contributed to the failures. No manufacturing anomalies were observed that would have contributed to the failures.