Experimental investigations and modelling of relaxation behaviour of shot peening residual stresses at high temperature for nickel base superalloys

Abstract

Residual stresses induced by two shot peening conditions in nickel base superalloy Inconel 718 and their relaxation after different exposure times (10 and 100 h) at 600 and 650°C were evaluated by an X-ray diffraction technique. The behaviour of stress relaxation (effects of exposure time and applied temperature) is particularly analysed and discussed. A significant decrease of the initial stress values was observed in the first period of exposure time, followed by slowing down and then stabilisation. Experimental results showed that the maximum relaxation always occurred at the surface. It was also observed that a higher applied temperature produced greater relaxation, but had no significant effect on the pattern of the depth profile of stress. The relaxation mechanism is essentially interpreted by thermal recovery characterised by a rapid annihilation of the unstable crystalline defects in highly deformed materials. Complex changes of various microscopic magnitudes under high temperature are described by macroscopic magnitudes related to the plastic strains and associated residual stresses. An existing model is developed and actualised by integrating the limiting state of recovery and the integrated width of X -ray diffraction peaks in order to predict relaxation of the residual stresses. The relaxation predicted by the model agrees satisfactorily with the experimental results in the studied cases.