Standard IN718 fasteners are widely used in aero-engine applications due to its excellent combined mechanical properties of tensile, fatigue and creep strength at temperatures up to 630°C. Additionally, IN718 is a cost-effective choice compared with other alloys [1]. However, the flexibility as allowed by the industrial manufacturing specification, in the choice of raw material, and in the selection of the solution treatment temperature range leads to variations in microstructure of bolt and consequently mechanical performance of the bolts [2].

Axial tension fatigue and dwell fatigue tests were conducted at 650C using both IN718 test bars with simulated microstructures and ½” bolt products to evaluate the influence of possible microstructural variation on the properties of the bolt and associated failure mechanism. In addition to the shank properties, the local properties in the threaded section were examined by conducting tests in bending configuration on splitted bolts. A profound influence of annealing temperature on dwell fatigue life has been confirmed with the observations of a change of failure mechanism. At the investigated temperature, improvement of fatigue strength by cold rolling in the threaded section is evident. Microstructural characterisation and fractographic examination were conducted throughout to aid understanding.

References

[1] T Ostra, U Alonso, F Veiga, M Ortiz, P Ramiro, A Alberdi; Analysis of the Machining Process of Inconel 718 Parts Manufactured by Laser Metal Deposition. Materials (Basel). 2019, 12(13), 2159.

[2] S Azadian, L Wei, R Warren; Delta phase precipitation in Inconel 718. Materials Characterization. 2004, 52, 7-16.