Effect of Boron Content and Brazing Temperature on Braze Ability of Foil-type Nickel-based Brazing Filler Metal

Abstract

The exhaust gas recirculation (EGR) cooler uses stainless-steel brazing. The EGR cooler is a heat exchanger that cools exhaust gases, rendering these cooled gases useful for optimizing combustion. This application considerably benefits from stainless steel due to its excellent heat and corrosion resistance. In this context, stainless-steel brazing is achieved by applying various forms of brazing filler metal tailored to different brazing conditions and joint designs. Nickel-based brazing filler metals are classified into foil and powder forms. Foil-type brazing filler metal is known for its propensity to induce interfacial reactions, even below the conventional brazing temperature. Previous studies have shown a pronounced correlation between the boron content, a primary constituent of brazing filler metal, and brazing and liquidus temperatures. Heightened boron content has been empirically proven to decrease brazing and liquidus temperatures. Therefore, the effect of boron addition on brazing is investigated using coessential observations and assessments of joint strength. Ferritic stainless steel (SUS444) was used as the base metal, whereas brazing metal is a nickel-based foil-type augmented with varying boron concentrations. The brazing procedure entails assembling components using a stainless-steel jig, followed by brazing under vacuum conditions at 1050 °C for 10 min with a heating rate of 20 K/min. Post-brazing protocols encompass vaporizing binders at 600 °C for 15 min, succeeded by air cooling. Subsequent evaluations encompass visual inspections of the fillet, cross-sectional microstructural analyses conducted via optical microscopy, and tensile testing. This multifaceted investigation provides insights on the impact of boron content on the brazing process and delves into the structural and mechanical integrity of the brazed joints.