Investigation Of Creep Crack Growth And Constraint Effect Of HastelloyC276/BNi2 Brazed Joint

The plate fin heat exchanger is a key equipment to improve energy efficiency.However,the creep deformation of the plate fin heat exchanger is inevitably generated at high temperature with long time operation.The creep crack growth in the brazed joint are the main reasons for the failure of the plate fin structure for heat exchanger.Different constraint levels have great influence on creep crack growth.The study of creep crack growth and constraint effect in brazed joints are of great significance to ensure the structural integrity of platefin structure at high temperature.Up to now,the research on the creep crack growth of brazed joints started late,and the related creep theory is not completed.The fracture mechanism of brazed joints is not clear.It is still unknown whether the current creep damage constitutive model can be applied to predict the creep crack growth of brazed joints or not.The theoretical and practical research on creep crack tip constraint effect of brazed joints are very limited.In this paper,the uniaxial,multiaxial creep and crack growth properties of brazed joints are studied by experimental method,and the creep fracture mechanism is analyzed.Based on the existing creep damage constitutive model,the creep damage constitutive model of brazed joint is derived.On the other hand,the effects of different constraint levels on creep crack growth of brazed joints are analyzed by numerical simulation based on the established creep damage constitutive model,and a creep constraint parameter of brazed joints is defined.The creep crack growth rate equation depending on constraint of brazed joints is established.The main work and conclusions are as follows:(1)The brazed joint consists of four regions,i.e.filler zone,isothermally solidified zone,interface reaction zone and parent zone.The microhardness of the diffusion zone is the largest and the microhardness of the brazing zone is the smallest.The conventional mechanical properties and creep properties of brazed joints were tested by means of experimental method.The mechanical properties and creep properties of the joints at 600?were obtained.The creep fracture behavior of brazed joint is stress dependent.The creep deformation behavior of brazed joint should be described by a creep constitutive model related to stress range.The creep fracture morphology of brazed joint was analyzed by SEM.It was found that the fracture morphology of brazed joint was a step ductile under lower stress and“rock candy”shape under higher stress.Creep fracture mechanisms of brazed joint at higher and lower stress levels were identified as intergranular brittle and transgranular ductile fracture,respectively.(2)The creep fracture behavior of brazed joint under multiaxial stress was investigated.It is found that the notch weakening effect exists in the brazed joint specimen.Intergranular brittle fracture is the main fracture mode of brazed joints under multiaxial stress.A new creep damage constitutive model for brazed joints is established.The established creep damage constitutive model can accurately predict the creep life of brazed joints under multiaxial stress.The error of predicted and experimental results is less than 17%.The proposed damage evolution equation can accurately predict the relationship between creep fracture ductility and stress multiaxiality of brazed joints.The stress and damage evolution behavior of brazed joints under multiaxial stress state are discussed.It is found that the brazed joints with shorter distance between the skeletal point and the notch root have lower creep life.(3)The creep crack growth behavior of brazed joints was studied by the experimental and finite element method.The creep crack of brazed joint propagates along the brazed seam.The growth of creep voids,formation,propagation and consolidation of micro-cracks are the internal reasons of the fracture of brazed joints.With the decreases of specimen thickness,the fracture morphology of creep crack growth shifed from ductile fracture to brittle fracture with grain boundary microcrack and creep cavity,leading to the decreases of creep life.The simulation results of creep crack growth obtained by using the established creep damage constitutive model are in good agreement with the experimental results,and the error between the predicted creep crack growth life and the experimental value is less than 13%.The predicted crack growth morphology is basically consistent with the experimental fracture morphology.(4)The effects of different constraint levels on the creep crack growth of brazed joints were analyzed by finite element method.The creep crack growth rate of SENT,MT and DENT specimen are much higher than that of CT,CST,DCT and SENB specimen under the same C*value.The creep life increases with the increases of specimen thickness and the decreases of pre-crack depth.The brazing defect can accelerate the creep crack growth and reduce the creep life of the joint.The defect has no effect on the creep crack growth rate after the defect merged.With the increase of the defect length and defect distance,the creep life decreases gradually.With the increase of creep coefficient,stress exponent and the decreases of creep ductility,the creep crack growth rate increases gradually under the same C* value,which leads to the decrease of creep life.(5)Based on the creep parameter R* and A_c,a new creep constraint parameter A_s is defined,which is related to the area of the tensile stress isoline.When the steady state is reached,the constraint parameter A_s is almost unchanged with the increases of creep time.Compared with the constraint parameters R*and A_c,the constraint parameter A_s has better load independence,and the relationship between A_s and crack depth is linear.The fitting results of constraint parameter A_s and creep crack growth rate are better.Based on the constraint parameter A_s,a constraint dependent creep crack growth rate equation for brazed joints is established.The crack growth data predicted by this proposed equation is in good agreement with the experimental data,and it can be used to predict the creep crack growth rate at other constraint levels.