Study On Fracture Toughness Of 617 Nickel-base Alloy Welded Joints At Different Temperature

617 alloy has been an important candidate material for rotors of advanced ultra-supercritical steam turbine?AUSC?because of its excellent high temperature performance.Recently,the welding rotor has become the mainstream owing to its low cost,high efficiency and good machinability.However,due to the uneven microstructures of welded joints,the high temperature properties of WM,HAZ and BM are quite different.Thus,it is necessary to compare and analyze the properties of different regions,especially the high temperature fracture property which is one of the most important indexes for the safe operation of welded components at high temperature.Therefore,it is necessary to study the high temperature fracture toughness of nickel-based alloy welded joints.In this paper,the fracture toughness at different temperatures of 617 nickel-base alloy welded joints was studied,the fracture toughness of different regions was tested,and the intrinsic correlation between fracture behavior and temperature as well as structure was studied.This study provides data support and theoretical basis for the safety design and evaluation for nickel-base alloy welded joints.Firstly,the microstructures of welded joints were characterized.All the BM are austenite equiaxed grains,and the precipitated phases are mainly M₂₃C₆ and Ti?C,N?.After stress relief annealing at 980?,M₂₃C₆ with uniform distribution and size ranging from 100 to 200 nm was precipitated.The microstructure of HAZ is similar to that of BM.However,during the welding heat input,Ti?C,N?in HAZ reacted with Co,Cr and Mo in austenite matrix and form mixed phases of?-austenite,M₂₃C₆ and M₆C,And there is no small size M₂₃C₆ precipitated after stress-relief annealing.The WM is mainly composed of coarse columnar grains,and a large amount of?-austenite,M₂₃C₆,M₆C and Ti?C,N?are precipitated in the grains and boundaries.The microhardness analysis shows that the lowest hardness in WM is similar to that in BM.Secondly,a series of basic mechanical test for welded joints were conducted.Tensile properties of BM and welded joints were tested at room temperature,650?,700?and 750?respectively.From room temperature to 750?,the yield strength of BM decreased from 526 MPa to 380 MPa,and tensile strength decreased from 947 MPa to 558 MPa.The yield strength of welded joint decreased from 592 MPa to 395 MPa,and the tensile strength decreased from 887 MPa to 528 MPa.WM is the weakest part of strength and plasticity in welded joint.Through Charpy impact test,it is further revealed that WM is the part that possesses the worst toughness.Subsequently,based on the data of tensile test and impact test,high temperature fracture toughness test was carried out.The J_0.2 of BM at room temperature,650?,700?and 750?are 270.2 kJ/m²,225.9 kJ/m²,204.0 kJ/m² and 173.2 kJ/m²,respectively,while the J_0.2 values of WM are151.1 kJ/m²,140.9 kJ/m²,111.8 kJ/m² and 102.1 kJ/m²,respectively.The fracture toughness of BM at high temperature is better than that of WM.The higher the temperature,the lower the fracture toughness of the welded joint.Finally,fracture was observed and characterized.The crack propagation in BM and WM is mainly transgranular,while the propagation path in BM is more tortuous.Ti?C,N?in the BM becomes an important source of voids when the temperature is above 700?,which leads to the deterioration of strength and toughness of BM.EBSD grain orientation analysis shows that there are about 17.1%large angle grain boundaries in the base metal,which would hinder the crack propagation.And the proportion of large angle grain boundaries in the weld is less than 2%.KAM diagram shows that the plastic deformation of BM is greater than that of WM.The total plastic deformation of BM decreases at 700?,and the plastic deformation concentrates on the grain boundary,which indicates that the grain boundary slip at high temperature affects the transfer of plastic deformation and leads to the decrease of fracture toughness of the BM.