Microstructural Assessment of IN718: A Closed-Loop Fatigue Approach

Abstract

The goal of relating a local fatigue life approach with different microstructures requires the consideration of the main forging process dependent influence factors and their effect on grain size, grain shape, grain contiguity and others. The presented methodology shows the generation and use of a microstructural based evaluation method to link the grain-shape based texture and morphology to the low-cycle-fatigue behavior of superalloy 718. The developed microstructural based energy approach supports an alternative description of the microstructure and grain shape texture. Both, the morphology and the statistical distribution, although covering as-large-as grains, are assessed by two independent numerical parameters. A parametric link to local fatigue parameters was established using this alternative microstructural characterization technique. A prediction of the microstructural evolution during the forging process is already available at design stage for hot-forging of this nickel-base superalloy. In this regard the value of the first parameter e correlates with the mean grain size; the value of the second parameter b is affected by the local forging process history. This enables the lifetime assessment of local forging process at design stage using advanced forging simulation tools. This holistic approach of establishment an experimental based methodology from specimen tests, extensive companying metallographic evaluations, linking them with local forging parameters, implementing the supported microstructural parameters to forging simulation codes and calculating the local component lifetime closes the simulation chain for superalloy 718.