In Situ Study of the Stress Relaxation During Aging of Nickel-Base Superalloy Forgings

Nickel-base superalloys are conventionally aged in order to develop a favorable microstructure as well as to relieve residual stresses. Here neutron diffraction was used to follow the evolution of residual stress during aging in Inconel 718 and Udimet 720LI for the first time by a combination of in situ and ex situ measurements. First, the quench-induced stress profiles were determined for different geometries of Inconel 718 forgings confirming that the amplitude of stress is not significantly affected by the thickness of the component. It was followed by in situ residual stress analysis using neutron diffraction during aging/annealing treatments at 650 degrees C, 720 degrees C, and 750 degrees C. Almost 90pct of stress relaxation was found to occur primarily during heating to the aging temperature as a result of a combination of plasticity and early-stage creep relaxation. Creep-like stress relaxation was observed to evolve at a diminishing rate during the isothermal treatment of Inconel 718 at 720 degrees C and 750 degrees C, while no further stress relaxation occurred at 650 degrees C. This means that a change in hold temperature might have a more immediate impact on stress relaxation than a change in duration of heat treatment. The post-aging ex situ measurements showed that a heat treatment at 750 degrees C for 8hours reduced the stresses by approximately 70pct in Inconel 718. By comparison, when heat treating Udimet 720LI in the same way only a 20pct stress reduction was observed, which is explained by the higher creep resistance of this alloy.