Modelling and measurements of thermally induced residual stress in aerospace materials during quenching

Residual stress induced during and as a result of manufacturing processes can have a significant impact on the later stages of manufacturing (e.g., machining), and in-service performance (e.g., resistance to fatigue) of a component. In this work, a novel approach is presented by combining FE based residual stress predictions and experimental validation at scales comparable to industrial components. Instrumented samples of IN718 nickel-based superalloy have been water-quenched and air-cooled from solution annealing temperature (980 °C) and the associated cooling curves were measured at specified locations. The cooling curves were used as boundary conditions for inverse calculation of zone-specific heat transfer coefficient (HTC), which is the main parameter to estimate the heat exchange rate between different regions of a heated part and its surrounding environment. The HTCs have then been implemented in an elastic-plastic finite element model, which included temperature dependant thermo-mechanical properties to predict thermally induced residual stress fields during heterogeneous water/air quenching from 980 °C. For the verification of the model, identical plates were heterogeneously quenched (i.e., half in water and half in air) from 980 °C, both vertically and horizontally, and residual stress was then measured using the contour method and incremental central hole drilling. Significant level of residual stress was generated in both samples due to the heterogeneous plastic deformation occurring around the outer surfaces, the extent of which depends strongly upon the severity of the cooling rate. Due to the non-uniform cooling and the quenching configurations (i.e., vertical vs. horizontal), different residual stress fields, developed during quenching, were simulated and measured for both sample conditions. A very good agreement was achieved between the results of model predictions and those of the residual stress measurement methods.