THE EFFECT OF COLD ROLLING ON THE CREEP BEHAVIOR OF INCONEL ALLOY 718

摘要

The creep behavior of INCONEL alloy 718 (IN 718) was investigated to identify processing-creep 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 (σ{sub}0). Creep-rupture time (T{sub}r) and elongation-to-failure (ε{sub}f) were also evaluated at 649°C and 758MPa, The lowest σ{sub}0 values (300MPa<σ{sub}0<310MPa) were exhibited for the most severely CR microstructures (60%, 80%, and IN 718SPF), while the baseline 0%CR microstructure exhibited a significantly greater σ{sub}0 value (540MPa). The greatest σ{sub}0 values, 645 and 630MPa, were exhibited by the 20% and 30%CR conditions, respectively. The σ{sub}0 values were related to the overall creep resistance as the 20%CR condition exhibited the lowest secondary creep rates for a given applied stress (σ{sub}a), 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 (σ{sub}e=σ{sub}a-σ{sub}0) and the transition occurred at σ{sub}e=135MPa for a temperature of 638°C. Increased CR tended to increase T{sub}r and ε{sub}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.