Creep Behavior Of Dissimilar Welded Joints Of Martensitic Heat-resistant Steel In Ultra-supercritical Unit

12Cr10Co3W2Mo Ni VNb NB(Co3W2 for short)steel is a new type of martensitic heat resistant steel independently developed in China.F92 belongs to the 92 series of martensitic heat resistant steel developed in Japan,which has been domestically produced.The two steels have excellent high temperature properties,and the F92/Co3W2 dissimilar welds are used by Dongfang to manufacture ultra-supercritical units used/reheater components.There has been no research report on the welded joints of F92 and Co3W2 with different materials at home and abroad.In order to ensure the service safety of the welded joints of F92/Co3W2 with dissimilar steel at high temperature,it is necessary to carry out systematic and in-depth research on their weldability and creep properties at high temperature.In this paper,F92/Co3W2 welded joint was used as the main test object,and high temperature tensile creep experiments were carried out at 115-200 MPa and 873-893 K to study the creep fracture behavior and microstructure changes.Combined with the results of scanning and transmission electron microscopy,the degradation behavior of the creep microstructure was studied from the aspects of dislocation,nucleation coarsening of precipitated phase,creep pore,secondary crack and section morphology.Norton power law formula,Monkman-Grant relation,Larson-Miller parameter and creep damage tolerance coefficient were used to study the creep data.The creep fracture stress corresponding to100,000 hours of service life at different temperatures was extrapolated to analyze and compare the influence of temperature and stress on the difference of creep life.Nano-indentation technology was used to obtain the local nano-hardness,reduced modulus,plastic index,strain rate sensitivity and activation volume of the welded micro-zone with different creep damage degrees,and to study the influence of high temperature creep on the mechanical properties of the welded micro-zone.The experimental results show that the fracture location and fracture mode of welded joint change from transgranular ductile fracture of F92 base metal to intergranular brittle fracture of F92 fine grain heat affected zone(type IV fracture)with the decrease of applied stress in high temperature creep,and the critical stress of type IV failure is σ∈(140,120)MPa.The creep fracture stress corresponding to 873 K and 893 K is 112.3 MPa and 84.0 MPa at100,000 hours based on LMP extrapolation.The calculated normalized creep lifetime growth rate indicates that stress changes have a greater effect on creep lives than temperature changes over the specific temperature and stress ranges studied in this study.The evolution of creep damage tolerance coefficient indicates the existence of different major damage microscopic mechanisms,which are related to material degradation,such as coarsening of precipitates near the fracture site,subgrain coarsening,dislocation density reduction,nucleation and growth of creep cavity,etc.After 2735 hours of creep,Co3W2 still has a positive effect on its creep strength.The local physical and mechanical properties after long-term creep are better than those after short-term creep,which is related to the slow evolution of Laves particles in Co3W2 and other microstructural changes,while F92 side is the opposite.The change of activation volume during creep is consistent with the change of strain rate sensitivity,both of which are related to the temporal and spatial changes of structural creep damage.