The effect of cold rolling on the creep behavior of inconel® alloy 718

Abstract

The creep behavior of INCONEL® alloy 718 (IN 718) was investigated to identify processingcreep property relationships. The alloy was cold rolled (CR) to 0, 10, 20, 30, 40, 60, and 80% followed by annealing and aging. In addition, this alloy can be superplastically formed (IN 718SPF) to a significantly finer grain size and the corresponding microstructure and creep behavior were evaluated. The creep behavior was evaluated in the applied stress range of 300-758MPa and the temperature range of 638-670°C. Constant-load tensile-creep experiments were used to measure the values of the steady-state creep rate and the consecutive load reduction method was used to determine the values of backstress σo). Creep-rupture time (Tr) and elongation-to-failure (εf) were also evaluated at 649°C and 758MPa. The lowest σo values (300MPa <310MPa) were exhibited for the most severely CR microstructures (60%, 80%, and IN 718SPF), while the baseline 0%CR microstructure exhibited a significantly greater σo value (540MPa). The greatest σo values, 645 and 630MPa, were exhibited by the 20% and 30%CR conditions, respectively.σo The values were related to the overall creep resistance as the 20%CR condition exhibited the lowest secondary creep rates for a given applied stress (σa), while the samples CR to more than 40% exhibited the greatest creep rates. The values for the effective stress exponent suggested that the transition between the rate-controlling creep mechanisms was dependent on effective stresses (σe=σa-σ0) and the transition occurred at σe=135MPa for a temperature of 638°C. Increased CR tended to increase Tr and ε f, and the 30%CR condition exhibited the greatest creep rupture properties. Overall, the 20%CR and 30%CR microstructures exhibited the greatest creep strength, while the most severely CR materials exhibited the poorest creep strength.