Effect Of Control Mode On High Temperature Low Cycle Fatigue Behavior Of 9-12% Cr Steel And Cyclic Constitutive Modeling

The effect of high temperature and cyclic loading should be taken into account in the strength design and the life analysis of the critical components (such as rotor, blade, cylinder etc.) in high-end equipment including ultra-supercritical steam turbine and aero-engine. Especially in the local discontinuous area of components, fatigue damage under stress or strain control mode may arise. Therefore, clarifying the effect of control mode on high temperature low cycle fatigue behavior and describing accurately the stress-strain relationship under cyclic loading conditions, are very important in the improvement of structural strength design and life prediction in engineering and theory. In this work, the low cycle fatigue tests of modified ultra-supercritical steam turbine rotor steel X12CrMoWVNbN10-1-1 under stress and strain control modes were conducted at 600 ℃. Cyclic deformation behavior under different control modes was investigated. The difference and rule of microscopic deformation mechanisms under both control modes were analyzed. In addition, cyclic plastic constitutive equation under strain-controlled fatigue was built.The main research contents and conclusions are as follows:(1) The effect of stress and strain control mode on the low cycle fatigue behavior of 9-12%Cr steel under fully reversed condition. Symmetrical low cycle fatigue tests of modified ultra-supercritical steam turbine rotor steel X12CrMoWVNbN10-1-1 under strain-and stress-controlled conditions were conducted at 600 ℃. The effect of control mode on the cyclic deformation response and fatigue life evaluation was analyzed. The microscopic damage mechanism under different control modes was illustrated by TEM microscopic observation. Results revealed that the cyclic stress-strain curves under two control modes can be mainly divided into two domains, i.e., macro-yield domain and micro-yield domain. In micro-yield domain, the cyclic deformation response of X12CrMoWVNbN10-1-1 steel showed little difference under stress and strain control modes. In macro-yield domain, shorter fatigue life and more obvious fatigue damage under the stress-controlled condition were observed than the strain-controlled condition due to the significant cyclic softening and tension compression asymmetry behavior. A redistribution of dislocations due to sub-grain boundary disintegration was responsible for the more significant softening caused by stress cycling. The cyclic softening under strain cycling condition was ascribed to the subgrain recovery of martensitic laths. The additional ratcheting strain under the fully reversed stress-controlled condition has a detrimental effect on fatigue life. Based on the mechanism, a modification on Manson-Coffin life prediction equation was proposed. The predicted fatigue life agreed well with the corresponding experimental results under both control modes.(2) Mean stress relaxation behavior and damage mechanism of 9-12% Cr steel under strain-controlled fatigue. Asymmetrical strain cycling tests of 9-12%Cr steel were conducted at 600℃. Mean stress relaxation behavior under various strain amplitudes was investigated. Mean stress relaxation and its interaction with cyclic softening were analyzed based on the internal stress partition method and TEM microscopic observation. A new back stress component Xmiddle was put forward to clarify the microscopic physical mechanism of cyclic deformation response under various strain amplitudes. Results revealed that mean stress relaxation under strain-controlled fatigue was determined by combined effects of back stresses on different scales. In macro-yield domain (high strain amplitudes), the quick decrease of intra-granular back stress Xintra was responsible for the significant mean stress relaxation and cyclic softening. In micro-yield domain (low strain amplitudes), slight cyclic softening can be ascribed to the decrease of effective stress. Mean stress relaxation was retarded by the middle back stress Xmiddle resulted from the heterogeneous coarsening of martensitic laths during the cyclic process. A new internal stress analysis model was proposed on the basis of above physical mechanism. The proposed model can reasonably describe the damage mechanism of mean stress relaxation for 9-12%Cr steel under various strain amplitudes.(3) Static and cyclic stress relaxation behaviors of 9- 12%Cr steel. Static stress relaxation tests under different initial stresses and cyclic stress relaxation tests under various peak strains of 9-12%Cr steel were conducted at 600℃. The influence of cyclic loading on the stress relaxation behavior was explored by paying particular attention to the effect of peak strain and unloading amplitude. The physical mechanism of cyclic relaxation behavior was illustrated by TEM microscopic observation. Results revealed that cyclic stress relaxation rate increased with the increase of peak strain. The cyclic loading could either enhance or retard the relaxation behavior, depending on the unloading amplitude. For the unloading amplitudes in micro-yield domain, after a transient cyclic hardening at the beginning of fatigue test, the material displays slight cyclic softening involving localized recovery of martensitic laths, which enhanced cyclic relaxation compared with the relaxation rate for static application. For the unloading amplitudes in quasi-elastic domain, the material showed slight cyclic hardening accompanied with dislocation pinning caused by unidirectional plastic deformation, which retarded cyclic relaxation. The effect of cyclic loading on the relaxation behavior was determined by combined effects of cyclic plastic and unidirectional plastic accumulation.(4) Cyclic viscoplastic constitutive equation of 9-12%Cr steel under strain-controlled fatigue. Based on the symmetrical and asymmetrical strain cycling experimental results of 9-12% Cr steel at 600 ℃, the evolution of mean stress relaxation and cyclic softening was analyzed. The existing constitutive models modified on the A-F kinematic hardening rule did not reflect the strain-amplitude dependent competition mechanism between the cyclic softening and mean stress relaxation behavior. So the models cannot describe accurately the mean stress relaxation behavior. A new cyclic viscoplastic constitutive is proposed through introducing a mean stress relaxation factor into the Abdel-Karim-Ohno kinematic hardening model. The strain-amplitude dependent cyclic softening and mean stress relaxation behavior of 9-12%Cr steel were finely reproduced by the proposed model, which was achieved by taking into account the synthetic effect of the maximum plastic strain and the accumulated cyclic plastic strain.