Stress relaxation behaviour in IN718 nickel based superalloy during ageing heat treatments

Abstract

Designing microstructure of components made from Inconel 718 nickel based superalloy (IN718) with tailored mechanical properties for high temperature applications, require sequential thermo-mechanical processing. This often includes straining and annealing at solution annealing temperature (i.e. ≈980℃) followed by water quenching and subsequent aging heat treatments at lower temperatures. In addition to the microstructure development (i.e. precipitation) the aging heat treatment partially relieve the residual stresses generated at previous stages of forging and water quenching, however the stress field will not be completely relaxed. In this study, a series of experiments were conducted on round tensile specimens made from IN718 bar to investigate tensile stress relaxation behaviours at elevated temperatures used for aging heat treatments. The stress relaxation curves obtained can be described by a hyperbolic function with a non-zero asymptotic stress (σ∞), which seems to be proportional to the initially applied stress (σ0) for a fixed temperature. This behaviour is investigated at temperatures between 620℃ and 770℃ that is a temperature range used in industry to perform the aging heat treatments to obtain microstructures with tailored mechanical properties. It has been shown that the σ∞/ σ0 ratio has decreased rapidly with increasing temperature at this range. The relaxation behaviour has been assessed numerically and an empirical relationship has been defined for each temperature that can be used for modelling purposes

Comparison Between Surface and Near-Surface Residual Stress Measurement Techniques Using a Standard Four-Point-Bend Specimen

“…Residual stress, a tensor quantity, are locked-in stresses within a component without external loading, generated as a result of complex non-linear thermal-mechanical processing during manufacturing. Most manufacturing processes introduce residual stress that has a direct bearing on manufacturing (e.g., undesirable distortion) and on the resilience of products in service and their design life [1][2][3][4][5][6]. Historically, residual stresses have primarily been incorporated into structure critical component design through a significant safety factor because they are challenging to characterise and control, and there is little design guidance in codes and standards.…”

Effect of Cold Pressing and Aging on Reduction and Evolution of Quenched Residual Stress for Al-Zn-Mg-Cu T-Type Rib

“…Bai [25] proposed a novel residual stress relaxation model considering the dislocation evolution mechanism and the plasticity theory to predict the change of residual stress during the annealing process. The aging heat treatment partially relieved the residual stresses generated at previous stages of forging and water quenching; however, the stress field would not be completely relaxed [26]. Zhang [27] determined the through-thickness residual stress distribution of urbine discs during the isothermal forging, solution treatment and aging treatment using neutron diffraction and 2D-detector X-ray diffraction.…”

Effect of stress relaxation on the evolution of residual stress during heat treatment of Ti-6Al-4V

“…These elevated levels of residual stress are associated with distortion during machining and a reduction in fatigue life which is undesirable [2]. Aging treatments are therefore often conducted at elevated temperatures to decompose any α′ into α-and β-phases and promote stress relaxation [1,3]. Whilst higher aging temperatures promote stress decay, this may lead to partial dissolution of the bimodal microstructure formed during solution treatment; adversely affecting strength [1,4].…”