Creep Behaviours And Mechanisms Of Combined Strengthened Novel Austenitic Heat-resistant Steels And Welded Joints

Combined strengthening austenitic heat-resistant steels have broad application prospect in construction of advanced ultra supercritical units(A-USC).Welding is an indispensable processing for austenitic heat-resistant steels application.This work investigates microstructure and mechanical properties,creep deformation and strengthening,creep fracture and damage,and creep microstructure evolution of Sanicro 25 steel and its welded joint and SP2215 welded joint.The creep extrapolation strengths of above materials are predicted.The study provides theoretical support for candidate selection of super/reheater of A-USC,which promotes the industrialization and localization of austenite heat-resistant steel.The work has important scientific and engineering application value.The main conclusions of this paper are as follows:(1)Based on microstructure and mechanical properties of Sanicro 25,the domestic homogeneous welding wire of Sanicro 25 was developed,and the equal strength matched argon arc welded joint was realized.The tensile strength of the joint at room temperature,923 K,and 973 K is 99.8%,98.4%and 93.9%of base metal's,respectively.The impact toughness of the weld metal at room temperature is 51.7%higher than that of base metal.The influence of second phase on strength and toughness of base metal and weld metal is discussed.For SP2215 welded joints,the tensile behavior,impact toughness and microhardness distribution of welded joint are explained in terms of difference of microstructures in each welding region.(2)The creep deformation and fracture mechanisms of Sanicro 25 steel were studied.The creep threshold stresses of Sanicro 25 at 973 K,998 K,and 1023 K are129.5 MPa,111.5 MPa and 82.0 MPa,respectively.Interactions between dislocation and second phase in creep of Sanicro 25 steel is clarified,that is,shear mechanism for nanosized Cu phase and climbing mechanism for nanosized MX phase.The creep damage tolerance of Sanicro 25 is between 4-6,and creep acceleration stage accounts for 20%of Sanicro 25's creep life.The creep cavities of Sanicro 25 nucleate around the coarsened intergranular carbide Cr₂₃C₆,and aggregate into cracks when them grow up,leading to final creep fracture.At the same time,low angle grain boundaries in Sanicro25 increase with the generation of dislocation cells during creep.(3)Creep tests at 973 K under 180-240 MPa were conducted for Sanicro 25 welded joint specimens.By analyzing the characteristics of creep ductility,crack initiation position,grain boundary deformation behavior,and fracture frontier microstructure,the differences of creep behavior and mechanism of Sanicro 25 welded joint under different test stress levels are explained.Creep tests at 923 K under 180-240 MPa were conducted for SP2215 welded joint specimens.The difference of creep behavior of SP2215 welded joint under different test stress levels is explained by the characteristics of creep ductility theory,effective stress theory,fracture morphology,and grain deformation behavior.(4)The distributions of nanosized phase in Sanicro 25 welded joint after creep are compared.The size of Cu phase and MX phase is 5.0 nm and 7.0 nm,respectively,after creep at 973 K and 180 MPa.The distribution of nanosized phase in SP2215 welded joint after creep is analyzed.The size of Cu phase and MX phase is stable at 3.8 nm and16.0 nm,respectively,and both the two precipitates have high uniform distribution and strong anti-coarsening ability.Nucleation mechanism of intergranular M₂₃C₆ in SP2215alloy crept at high temperature is observed.Evolution mechanism of Nb Cr N phase to sigma phase in SP2215 alloy crept at high temperature is presumed.(5)The time temperature parameter method and the method based on the activation energy of creep fracture are used to predict the 100,000 hours creep extrapolation strength of SP2215 and Sanicro 25.The most accurate extrapolation 100,000 hours creep strength is 83.71 MPa for SP2215 steel at 923 K,154.54 MPa for Sanicro 25 at923 K,and 91.63 MPa for Sanicro at 973 K.The main factors controlling the creep fracture of welded joints in long-term creep test are predicted by using the creep strength weakening coefficient of welded joints.