A Method for Calculating the Relaxation of Residual Creep Stresses in a Surface-Hardened Cylinder with a Series of Periodically Arranged Semicircular Incisions under Thermal Exposure Conditions

A numerical method for calculating the relaxation of residual stresses in a solid cylinder with a series of periodically located semicircular notches after advanced surface plastic hardening under creep conditions during thermal exposure has been developed and implemented based on the finite element method. The capabilities of the method and the results of its application are illustrated on surface-hardened cylinders made of EI698 alloy with a radius of 3.76 mm and a single notch of radius 0.1; 0.3; 0.5} mm and a series of 3, 5, and 7 periodically repeating notches of the same type with a radius of 0.1 and 0.3 mm. Relaxation of residual stresses for all calculation variants using the law of steady-state creep at T = 700°C on a time base of 1000 h is investigated. A comparative analysis of calculation data for a smooth cylindrical sample and cylinders with notches is performed. It is shown that when a single notch of 0.1 mm radius is applied after hardening of the cylinder, residual circumferential and axial compressive stresses along the depth from the bottom of the notch are comparable, and sometimes even higher, than for a smooth sample, and when notches are applied, the radius of which is greater than the thickness of the hardened layer, the values of these components are significantly lower. For a series of notches, the circumferential and axial residual compressive stresses are significantly reduced, decreasing nonlinearly from the extreme notches of the system to the central one. During creep during thermal exposure, relaxation of residual stresses occurs, but their value, although decreasing (in absolute value), retains a negative sign over time. This result is a positive factor in assessing the performance of surface-hardened cylinders with defects such as semicircular notches under high-temperature creep conditions. The calculation results are illustrated by numerous graphs and tabular data regarding the kinetics of residual stresses.