A high-strain, biaxial (tension/torsion) fatigue study has been conducted on Type 316 stainless steel at 550°C. The tests were under strain control, some with dwell periods up to 10 min at the positive strain peaks. The deformation and fracture behavior under these conditions revealed a strengthening effect due to the fast carbide precipitation which extended life in some cases. Also it is shown that the form of the multiaxial fatigue failure criterion, modes, and directions of cracking were functions of hold time. The multiaxial cyclic stress-strain curve is best described in terms of shear stress and strain components on the maximum shear plane for a given hold time.

A metallographic examination of elevated temperature multiaxial fatigue specimens of 316 stainless steel was conducted to identify the type of damage and other microstructural changes that occurred during testing. The work hardening which takes place at the beginning of the test is associated with the development of either a cell or a maze dislocation structure. Two main factors contribute to the flow stress, that is, dislocation and carbide precipitation hardening. Dislocation hardening prevails, but precipitation hardening, which increases with testing time, becomes significant for long hold-time and low-strain range tests. Fatigue cracks are predominantly transgranular. However, intergranular cracks have been observed in hold-time specimens and are related to grain-boundary sliding and grain-boundary precipitation. The fatigue fracture is a three stage process, the initial stage (Stage I) is crystallographic, extending for one to three grains. The intermediate stage (Stage II) is dominated by striations, with the exception of some featureless facets in biaxial specimens. The final stage (Stage III) is a dimple type fracture (tension fracture), beginning at shorter crack lengths in the hold-time and high-strain range specimens. Striation spacing calculations give approximate life estimates in low-cycle fatigue.