PSI - Issue 66

Johannes L. Otto et al. / Procedia Structural Integrity 66 (2024) 256–264 Johannes L. Otto et al. / Structural Integrity Procedia 00 (2025) 000–000

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1. Introduction Vacuum brazing is an established manufacturing process in the field of joining technology for a wide range of applications in various industries. It enables high-quality joints at a high degree of design freedom to be created between similar and even dissimilar materials, such as metals and ceramics, Boretius (2006) and Fedotov et al. (2024). When steel joints with good corrosion properties and high mechanical strength at elevated temperatures are required, nickel-based filler metals are often used, where the brazing process itself is usually characterized by diffusion mechanisms. This so-called diffusion brazing can also be referred to as transient liquid phase (TLP) bonding, Cook III and Sorensen (2011) and Binesh (2020). For brazing austenitic stainless steels, Ni-based filler metals offer several advantages over other base alloys. These include comparatively low material costs, moderate brazing temperatures, good flow and wetting properties during brazing, but above all, the mechanical and corrosive properties are comparable to those of the base material. To enable diffusion brazing, the melting point of the nickel-based filler must be significantly lower than that of the base material. For this purpose, the metalloids B, P and Si have proven to be effective, Way et al. (2020). Although Cr can lower the melting point, its main function is to increase strength, heat and corrosion resistance, Penyaz et al. (2021). However, intermetallic brittle phase formations such as chromium borides and nickel silicides are a limiting factor for the service life, especially when it comes to corrosion fatigue, Otto et al. (2023). Considering that industrial applications for nickel-based filler metals include applications like exhaust gas heat exchangers, honeycomb seals on turbine components, steam-carrying pipes or injection moulding tools with integrated cooling channels, it is of particular interest to determine the fatigue life of these brazed joints not only in air but also simultaneously in a corrosive environment and at elevated temperatures. For this purpose, a newly developed application-oriented corrosion fatigue test setup will be presented. The mechanisms that are leading to the corrosion fatigue failure of the joints at different environments are of particular interest in this study. To understand these mechanisms, besides the influence of the brazing seam, the behaviour of the metastable austenitic steel must also be considered. Due to the brazing process, the steel develops a high ductility as the dislocation density get reduced and grain size growth occurs, which leads to high plastic deformation under creep fatigue. The resulting slip bands and deformation-induced martensite formation can lead to the formation of micro galvanic elements, as shown by Chen et al. (2021). These factors can increase susceptibility to stress corrosion cracking, Shin (2023). The experimental filler metal used in this study, was produced by melt-spinning in form of an amorphous foil of a thickness of about 20 µm. A description of the process and its challenges is given by Bobzin et al. (2024). The used filler metal composition and the melting range, determined by DSC is shown in Table 1. It was developed after numerous corrosive and mechanical preliminary tests with various alloys. The B content is relatively low at 1.5% to reduce the formation of chromium borides, while the Si content is relatively high at 7.5%, like the industrial filler metal BNi-5a. The Cr content of 15% provides enough Cr to form a sufficient passive layer in the area of the brazing seam, even with inhomogeneous distribution. Contents of 7% Cr, as in the widely used industrial BNi-2, caused severe corrosion attack. The addition of 2% Mo further improved the corrosion resistance against acids without significantly changing the melting range, while higher values of Mo strongly promoted the formation of brittle phases and thus negatively influenced mechanical (and in some cases also corrosive) properties. The addition of 10% Fe provided a more homogeneous microstructure to the base material and good mechanical properties, even though this further increased the melting range. The metastable austenite AISI 304L (X2CrNi18-9, 1.4307) was used as base material, which was available in the form of cold-drawn rod material with a diameter of 20 mm. Table 1. Compositions (in wt.-%) and melting ranges (in °C) of the base material and the filler metal. Alloy / name C Cr Ni Mn Si P S Fe Mo B T S T L AISI 304L 0.023 18.14 8.03 1.54 0.3 0.045 0.03 bal. 0.34 – ~ 1400 ~ 1450 Filler metal – 15 bal. – 7.5 – – 10 2 1.5 1058 1134 2. Materials and experimental procedures 2.1. Materials and diffusion brazing process