Investigations On The Strength Of Stainless Steel Brazed Joint At High Temperature

Brazing technology has aroused increasing concerns in response to the need of manufacture of high-tech products. For instance, heat exchangers inside the aero-plane, turbine blades of the jet engine, micro chemo-mechanical system, recuperators at high temperature, etc. In order to save operating cost and to increase the efficiency, the brazed structures must suffer higher service temperature. Although the stress is generally smaller than the yield limit of the material, creep takes place at high temperature, which leads to stress redistribution and damage occurrence. Therefore the brazed joint becomes the weak part of structures serviced at high temperature.The high temperature strength data of the brazing material are essential to evaluate high-temperature performance of brazed joint. However for its high cost and time consuming, the data are so absent that the basic strength design appears rather embarrassing. Due to the difficulty in sampling testing specimens from the narrow brazed zone(usually with a width of 10-150 祄), the as-cast Ni-based filler is adopted in the creep test in the present paper. The measured creep properties are compared with those of OCrl8Ni9 stainless steel. It is shown that the brazing material has lower creep ductility and lower creep strain rate which is more sensitive to stress change. The Kachanov-Rabotnov constitutive equation, with material constants obtained from creep experiments of as-cast filler, can be used to describe the creep behavior effectively. At the same time, the gauge length variation of stainless steel overlapped brazed joints is measured and compared with the results of FEM simulation.Typical finite element models of stainless steel brazed joint are established. Based on the Norton creep constitutive equation the stress redistribution of the joints is analyzed, and effects of different conditions are discussed. From the simulated results, the features of stress redistribution are obtained. Because the joint failure depends on the stress redistribution, the influence of various factors on the strength is indicated. Optimized joint types are recommended accordingly.The major research work and conclusions of this thesis are given as following: (1) Creep and fracture performances of as-cast filler at high temperature are observed and the material constants are obtained. A Kachanov-Rabotnov equation is determined to describe the creep behavior of the brazing material. By comparison ofspecimen testing with the simulation, it is seen that the K-R equation is capable of representing the mechanical behavior of braze metal at high temperature. Because the creep strain rate of the braze metal is much lower than that of the OCrl8Ni9 stainless steel at the same conditions, the brazed joint is referred to as a creep-hard joint.(2) Typical finite element models of stainless steel brazed joints are constructed. Stress distribution and its time-dependant features are calculated. The features of stress redistribution are obtained.(3) Based on the results of the stress redistribution, factors affecting the strength of stainless steel brazed joints at high temperature are discussed. Optimized joint types for design are recommended.(4) FEM is also utilized to investigate the effect of repaired brazing on the strength of the structures. It shows that after brazing repair the local stress of the parent metal is reduced, which is beneficial to the life extension of the brazing structures.