Limitations of calculating stress relaxation limit by function-fitting of Inconel718 superalloy

•Stress relaxation limit of Inconel718 is calculated by function-fitting.•Function-fitting is precise at service temperatures and low initial stress.•Function-fitting deviates from the reality at over-temperature or high initial stress.•Stress relaxation mechanism differs with the variation of testing conditions.•Over-temperatures and high initial stress make plastic deformation aggravating. Stress relaxation tests were carried out to identify stress relaxation limit of Inconel718 under different testing conditions. Experimental results indicated that stress relaxation limits calculated by function-fitting were consistent with testing data at service temperatures and low initial stress because of the rapid reaching of steady state. However, there exist large deviations using the same method under testing conditions of over-temperatures or high initial stress (near yield stress). Reasons for these are related to stress relaxation mechanism which can be illustrated by microstructure evolution: diffusion motion accelerates remarkably at high temperature, while multiple forms of plastic deformation occurs at high initial stress.