Study On Mechanical Properties Of SS304/BNi-2 Brazed Joint Based On Small Punch Tests

In view of the developing requirements of cost reduction,efficiency enhancement,energy saving and emission reduction,nuclear energy has attracted wide attention because of high cleanliness and low pollution.At present,high temperature gas-cooled reactor technology has become one of the core technologies in nuclear energy system.The compact multi-channel heat exchanger is an important component of high temperature gas-cooled reactors.Its reliability directly affects the energy conversion efficiency of high temperature gas-cooled reactors.The multi-channel heat exchangers are mainly encapsulated by brazing process,and the position of brazing joints is often the dangerous area of fracture failure.Therefore,mastering the fracture damage law of 304/BNi-2 stainless steel brazed joint has important engineering application value.However,the research on crack propagation failure of brazed joints mainly focuses on brazing process,residual stress and so on.There is few report has been found on the mechanical properties of brazed joints by means of miniature sample testing.In this paper,304/BNi-2 brazed joints taken as the research object,by combining finite element analysis with experimental research,the simulation of small punch testing at room temperature is completed on the basis of GTN micro-damage model.In addition,a user subroutine is developed based on the concept of ductility exhaustion model.The crack propagation process of brazed joints under the condition of small punch creep tests is studied,providing a reference for the design of plate-fin brazing structure.The main research work and conclusions are as follows:(1)Load-displacement curves of brazed joints are obtained through small punch tests at room temperature.Based on inverse method,non-destructive elastic-plastic parameters and GTN model parameters of brazing zone are obtained by simulation.(2)Based on the plastic damage model of GTN,the finite element analysis of brazed joints is carried out.The crack propagation law under uniform loading is obtained.It is proved that the GTN model can accurately simulate the crack propagation of brazed joints by experiments and the accuracy of the mechanical properties parameters of brazing filler is also verified.Finally,the influence of friction coefficient,indenter diameter,low die aperture,base metal thickness and joint gap on the crack propagation is studied and the reasons are discussed.(3)Based on fracture mechanics and damage mechanics,a ductility exhaustion creep damage subroutine for brazed joints is developed,and a small punch creep test model is established.The creep damage crack initiation and propagation of brazed joints are simulated by finite element method,and the creep law is obtained and life prediction equations are proposed.It is found that under the same stress,the minimum creep rate of brazed joint is larger than that of base metal and the creep property of brazed joint is weaker than that of base metal.(4)The effects of mesh,friction coefficient,solder layer thickness and material restraints on creep crack initiation and propagation of brazed joints are studied in depth and the reasons are analyzed.The results show that the creep strength and life of 304/BNi-2 brazed joints can be improved by optimizing brazing filler metal with lower creep coefficient,stress index and higher creep ductility,increasing the thickness of brazing filler metal properly.