Mathematical modeling of relaxation of residual stresses in thin-walled pipelines in the delivery state and after bilateral surface hardening at creep

A mathematical model of reconstruction of the fields of residual stresses and plastic deformations in thin-walled cylindrical tubes in the delivery state and after bilateral surface plastic hardening was developed. It included a method for identification of the model parameters using the example of thin-walled mm tubes made of X18N10T steel based on experimental data for the axial and circumferential components of the residual stress tensor for samples in the delivery state and after bilateral mechanical ultrasonic hardening. The adequacy of the mathematical model for the reconstruction of residual stresses in the thin-walled tubes made of X18N10T steel was verified by the experimental data in the state of delivery and after bilateral surface plastic hardening, taking into account the anisotropy of the distribution of plastic deformation after hardening in the axial and circumferential directions. A method for calculating the two-way relaxation of residual stresses on the outer and inner surfaces of thin-walled tubes under creep conditions was developed based on the generalization of the corresponding method for unilateral hardening. The relaxation process in the thin-walled tubes made of 08X18N9 steel (an early analogue of X18N10T steel) under conditions of thermal exposure, axial tension, internal pressure and the combined action of axial tension and internal pressure was investigated on the basis of the constructed phenomenological creep theory for this steel. A detailed analysis of the kinetics of the fields of residual stresses during creep in the thin-walled samples in the delivery state and after bilateral hardening at different times was performed. It is shown that under these conditions, almost a complete relaxation of residual technological stresses occurs both in the samples in the delivery state and after bilateral surface plastic deformation within 50 hours.