One way to control the formation of oxides during brazing and reduce oxides after precleaning is to surround the braze area with an appropriate controlled atmosphere in a furnace. This article presents an overview of the right use of controlled atmospheres in brazing that are used at Kepston Ltd. of Wednesbury, U.K. The atmospheres used include pure dry hydrogen (H2), exothermic or endothermic gas in continuous furnaces, and wet or dry H2 in batch furnaces. This flexibility enables the company to braze components as small as a few grams to more than 100 kg, in a variety of metals from mild steel to stainless steel, in batches from prototypes to very large production runs. Controlled atmospheres are most commonly used in furnace brazing, but may also be appropriate in induction or resistance brazing. Furnace brazing requires an atmosphere to protect the assemblies from oxidation and, in the case of steels from decarburization during brazing and also during cooling, which takes place in chambers close to, or contiguous with, the brazing furnace. Benefits of controlled atmospheres are described as well as effects of the gases on brazed parts. It is concluded that the only components of the usual brazing atmosphere that act as fluxes or reducing agents are H2 and CO. All other constituents are either neutral or may even promote reactions potentially harmful to the service life of the brazement. Even more significant is the fact that the absolute levels of H2 and CO are not critical. The two ratios of H2 to H,O and the ratio of CO to CO2 determine the reducing capacity of the atmosphere. Aspects of exothermically and endothermically generated atmospheres as well as high-H2 and nitrogen-based atmospheres are discussed with view on the material that has to be brazed.
Controlled atmospheres for bright brazing
Welding Journal, New York ; 83 , 10 ; 25-29
2004
5 Seiten, 9 Bilder, 6 Quellen
Aufsatz (Zeitschrift)
Englisch
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