Handbook-Braze Welding

2 Filler Metal and Flux for Braze Welding The filler metal used for most braze welding is a copper alloy containing roughly 60% copper, 40% zinc, and small amounts of tin, iron, manganese, and silicon. OXWELD No. 25M bronze welding rod is an example of a material formulated specifically for braze welding. It tins readily, flows freely, forms weld metal with excellent strength and high ductility. For bronze-surfacing – a process used for building up wearing surfaces rather than making up joints – a filler metal of slightly different composition, designed to achieve greater hardness at some sacrifice in ductility, is frequently specified. OXWELD No. 31T rod is an example of such a material. Silicon bronze, which contains only copper and silicon (no zinc), and phosphor bronze (a copper-tin alloy) are also sometimes used for braze- welding steel. Whenever a copper-zinc filler metal is used for braze welding, a flux must be used. Without flux, proper tinning action, even on the cleanest steel or cast iron surfaces, is virtually impossible to attain. The flux also serves other purposes, such as reducing the amount of fuming which occurs because of the rather low boiling point of zinc. The flux can be precoated on the welding rod, or transferred to the rod by dipping the heated end of the rod in a can of powdered flux. A vapor flux, which is picked up by the acetylene on its way to the torch, is sometimes used in production braze welding applications. Precoated rod (such as OXWELD Flux-Coated 25M) is the choice of many welders, since it eliminates the need to interrupt the actual welding operation in order to pick up flux on the rod.