Constitutive Model of High-Temperature Fatigue Performance for N80Q Steel for Multiple Rounds Huff and Puff Thermal Recovery Casing Pipe

Uniaxial tensile tests, creep tests (stress of 350,450 MPa) and symmetrical load low cycle fatigue tests (average strain of 0, strain amplitude of 1.0%, 1.4%, 2.0%, 3.0%) were carried out on N80Q steel for thermal recovery casing pipe at 350 ℃. The material parameters were determined according to the test data. The Chaboche viscoplastic constitutive model was constructed and verified by asymmetric load low cycle fatigue tests with average strain of 0.5%. The results show that the elastoplastic energy parameters are determined according to the uniaxial tensile curve, fatigue initial curve and creep curve. The dynamic strengthening parameters, isotropic strengthening parameters and damage stage parameters were determined according to hysteretic curves under symmetrical loads. When the cycle number was half-cycle life and the strain amplitude was 1.0%, 1.4%, 2.0%, 3.0%, the relative error between the maximum stress of the test steel obatained by constructed constitutive model simulation and by the asymmetric load low cycle fatigue test was 4.10%, 3.15%, 3.10%, 0.87%, respectively, which were all below 5%, showing that the model was accurate.