The effect of elevated temperature and tensile force loading on the relaxation of residual stresses in surface-hardened elements of the rod structure under creep conditions

A mathematical model for the relaxation of residual stresses in surface-hardened cylindrical elements of statically indefinable rod systems under creep conditions with elevated temperature and tensile force loading was developed. The following problems were solved during the modeling: reconstruction of the stress-strain state in a cylindrical rod after the surface treatment by microspheres; consideration of the influence of temperature loading on the magnitude and the fields of residual stresses due to the temperature dependence of Young's modulus; calculation of relaxation of residual stresses in hardened elements of the system under the influence of elevated temperature and tensile force loading under creep conditions; analysis of the final residual stresses after creep and reduced temperature and tensile force unloading.
The problems were solved within the first two stages of creep of the system of material elements. For a detailed analysis a three-element statically indefinable system with hardened elements at the temperature of 20$\,^\circ$C and an operating temperature of 675$\,^\circ$C made of ZhS6U alloy was used.
To implement the solutions of the problems mentioned, numerical algorithms were developed using discretization by spatial and temporal coordinates and using the method of time steps. For a posteriori estimation of the convergence and stability of the numerical method the numerical results were compared for large values of the calculation time with the asymptotic values of the stress-strain state characteristics corresponding to the steady-state creep stage obtained by the analytical method. The results obtained by both approaches are consistent.
The results of calculations were illustrated the kinetics of residual stresses in all three rods of the system during creep under the influence of elevated temperature and tensile force loading, starting from the moment of their formation after hardening. It was shown that a stepwise change in the magnitude and the distribution of residual stresses occurs only due to the “instantaneous” temperature heating of the elements of the rod structure due to the temperature dependence of the Young's modulus. It was also established by calculations that the relaxation of residual stresses in the most loaded rods system is much slower than in less loaded ones. To illustrate the main results obtained in this paper, we plotted the distribution of residual stresses along the depth of the hardened layer.