Creep Micro-damage Mechanism And Constitutive Model Of A Novel Martensitic Heat-Resistant G115 Steel

The development and construction of advanced ultra-supercritical（USC）coal-fired power plants with high efficiency are important measures to achieve the strategic goal of energy conservation and emission reduction in China.G115 steel,independently developed by China Iron and Steel Research Institute（CISRI）and Bao Steel,is a candidate material for the next generation of USC thermal power plants in China.In this thesis,the creep micro-damage mechanism and constitutive model of a new martensitic heat-resistant steel G115 are systematically studied.The involved scientific problems are clarified by means of experimental research,theoretical development,and computational modelling,which provides basic design data for the safe operation of the next generation of USC thermal power plants.The innovative research work of this thesis is as follows:（1）The high-temperature tensile deformation behavior and micro-damage mechanism of G115 steel were studied.After high-temperature tensile deformation,the interactions between dislocations with dislocations and precipitates in G115 steel are significantly enhanced.Laves phase is not observed.The average sizes of M₂₃C₆ and Cu-rich precipitate（CRP）are respectively increased by～1.6 and～2.4 times with increasing temperatures from 625°C to 675°C.M₂₃C₆ shows higher thermal stability than CRP.The preferential recovery of martensite laths during high-temperature tensile deformation at 675°C is attributed to the low dense CRPs.The strain-compensated constitutive model of G115 steel is established,and the exponential equation can accurately describe the high-temperature tensile deformation behavior of G115 steel.（2）Creep deformation behavior and micro-damage mechanism of G115 steel were studied.After creep at high stress,there are dense and fine CRPs.Laves phase is not observed,and martensite lath cracking and fracture occur.After creep at low stress,there are coarse CRPs and Laves phases,and martensite laths obviously coarsen and creep cavities are formed.Creep deformation leads to the obvious interactions between dislocations with dislocations and precipitates.Ductile fracture is the creep fracture mechanism of G115 steel.The relationship of the minimum creep rate and stress obeys the Bird-Mukherjee-Dorn（BMD）equation.The threshold stresses at 625°C,650°C,and 675°C are calculated to be 177.8 MPa,91.4 MPa,and 87.6 MPa,respectively.（3）The microstructure evolution of G115 steel during creep process was studied by interrupted creep tests.There is no obvious texture after long-term creep.The dislocation density decreases rapidly in the early stage of creep,then keeps stable,and finally decreases rapidly.Martensite lath coarsens after creep for 1045 h due to low density and coarse CRPs.Creep strain induces the movement of dislocations with solute atoms,therefore,CRPs rapidly coarsen.Two paths of Laves phase formation are observed,and M₂₃C₆-assisted Laves phase growth mechanism is proposed.（4）The constitutive model considers creep deformation mechanisms and can accurately predict the creep deformation behavior of G115 steel under a wide range of service conditions.The dislocation-based creep constitutive model describes the evolution of microstructure during creep process and represents creep deformation behavior of G115 steel.The high-temperature strength prediction model similarly considers the microstructure evolution and can accurately predict the high-temperature strength of G115 steel with creep damage.