Chemical elements diffusion in the stainless steel components brazed with Cu-Ag alloy

Abstract

Abstract. The paper presents the study of diffusion of chemical elements through a brazing joint, between two thin components (0.5mm) made of stainless steel 304. An experimental brazing filler material has been used for brazing stainless steel component and then the diffusion phenomenon has been studied, in terms of chemical element displacement from the brazed separation interface. The filler material is in the form of a metal rod coated with ceramic slurry mixture of minerals, containing precursors and metallic powders, which can contribute to the formation of deposit brazed. In determining the distance of diffusion of chemical elements, on both sides of the fusion line, were performed measurements of the chemical composition using electron microscopy SEM and EDX spectrometry. Metallographic analysis of cross sections was performed with the aim of highlight the microstructural characteristics of brazed joints, for estimate the wetting capacity, adherence of filler metal and highlight any imperfections. Analyzes performed showed the penetration of alloying elements from the solder (Ag, Cu, Zn and Sn) towards the base material (stainless steel), over distances up to 60 microns. IntroductionNowadays, stainless steels like 304 and 316 are used extensively for steam pipes and exhaust systems, due their resistance to elevated temperature, oxidation, and corrosion, being the choice for food preparation equipment, including steam-heated boilers and storage tanks. The addition of chromium and nickel to the iron matrix creates a significant percentage of chromium and nickel atoms at the surface [1].The corrosion resistance of stainless steel depends on the chromium content. Long maintenance at higher temperatures, in the range of 425-870 °C, causes the atoms diffusion and changes the metal's properties. Such high temperatures allow the chromium to diffuse away from the grain boundaries to form chromium carbides, its preferred crystalline structure at this temperature range [2]. During brazing, the assembly is heated from room temperature to the filler material melting temperature, and significant diffusion phenomena of chemical elements occur between the parent material and the filler metal. This process is particularly important when the filler metal layer is very thin [3].

Brazing Behaviour of Ag-Cu Filler Materials

“…To analyse the joint between very thin parts made of stainless steel, the spreading mode in the brazing gap and the diffusion of chemical elements in transition zones from the heataffected zone (HAZ), two types of joints were analysed: corner joint (Figure 2a [3].…”

“…The samples were washed with hot water and dried in hot air. The representative areas from each brazed sample were cut using a cutting machine with a cooling agent to avoid the alteration of the microstructure [3]. The 'overlap' joint (Figure 2a) has the average size width (j) of 412 μm.…”

“…As it can be seen, the transition from the filler material to the Table 1. Chemical composition measured in 5 points in the cross-section of the parent material (Figure 3a) [3]. Table 2.…”

“…At higher temperatures, between 700 and 800°C, the second ceramic coating, containing a mixture of silver particles, starts to melt, completing the final chemical composition of the filler material. In this way, the filler metal can spread rapidly and it can completely fill the gap between the joined components [3][4][5][6]. The most used filler material brands are 244 Ag, 134 Ag and 145 Ag, which are designed to perform connections for drinkable water pipes a) b) The operating conditions involve corrosive effects and oxidation at high temperatures (300°C), tensions and contractions in the joint.…”

“…Due to the fact that additional surfaces of the brazing zone are affected by the dissolution effects favoured by the brazing fluxes, it is important to rapidly clean the surrounding surfaces after brazing [9]. The brazing flux preferred for this type of filler material is FB 10 [2,3]. To ensure a good mechanical resistance of the joint, the brazing alloy must be sufficiently fluid, in order to penetrate the gaps between the components, on large distances.…”