Overview of nickel-based filler metals for brazing of austenitic stainless steels

Abstract

The austenitic stainless steels, which are given the most attention in this overview, are widely used in many industries. The exceptional complex of properties of these steels allows them to be used as structural materials for components of nuclear reactors, diesel, aircraft and rocket engines, stationary gas turbines. The most complicated technological operations in the manufacture of structural elements are intermediate and final assemblies. In most cases, these operations are performed using various types of cutting and welding. Brazing is used for the manufacture of thin-walled precision structures with complex geometries, requiring a low quality of accuracy [1–2]. Brazed joints are able to withstand high temperatures, static and dynamic mechanical loads, and the influence of aggressive media for a long time without noticeable deterioration of their properties. Depending on the tasks and the shape of the finished product, capillary brazing or “sandwich” brazing is used. In the case of capillary brazing, the filler metal is placed on the surface of the product outside the gap so that the molten braze alloy flows into the gap. This method can be used for products with complex geometries, but it requires good wettability and high fluidity of the filler metal. An example is the brazing of stepped outer air seal segment [3] made of stainless steel. This segment is a plate with honeycombs brazed by Ni – 7Cr – 4.5Si – 3Fe – 3B, wt.% filler metal. In the work [4], this technology successfully solves the problem of brazing honeycombs from a thin nickel superalloy foil using a nickel-based filler metal Ni – 18Cr – 10Si, wt.%. Thus, the processes of wetting and flowing of braze alloy into the gap are important parameters that must be taken into account in order to obtain a high-quality connection [5]. The ability of the brazed alloy to flow into the gap determines the height of its rise in the capillary, which is critical when joining parts of honeycomb structures, such as heat exchangers. For example, in the work [6], vacuum brazing of a steel intermediate heat exchanger (IHX) using a nickel-based filler metal Ni – 7Cr – 4.5Si – 3Fe – 3B, wt.% is considered. IHX is an important component of a high-temperature gas-cooled reactor (HTGR) [7], designed for a power and hydrogen production system. In addition to the growing reactor Overview of nickel-based filler metals for brazing of austenitic stainless steels