Study on element diffusion behaviour of vacuum-furnace brazing 316 L/BNi-2 joints based on Boltzmann-Matano model

Brazed joints have been widely used in the assembling process of compact structures due to its series of advantages. It is the weakest part of the whole system and directly determines the safety and reliability of long-term operation. In order to improve the performance of brazed joints, it is necessary to study the diffusion behaviour of major elements. This paper investigates the wedge-shaped specimen composed by 316 L stainless steel with BNi-2 filler alloy in vacuum brazing furnace. The microscopic morphology is studied by scanning electron microscopy (SEM), energy disperse spectroscopy (EDS) and electron probe microanalysis (EPMA). The elements concentration distribution is depicted by using Boltzmann function according to the EDS results. Then a calculation method based on Boltzmann-Matano model is proposed. The results show that the athermal solidification zone, the isothermal solidification zone and the diffusion-affected zone consist of eutectic compound, γ-Ni solid solution and boride precipitates, respectively. The brazing temperature have a great influence on the diffusion behaviour of the elements Si, Fe and Ni. The diffusion coefficient values at different positions in brazed joints can be accurately calculated by the proposed model. The thermal vibration energy are the main reasons for the diffusion behaviours at brazing temperature.