A Very Brief Introduction to Brazing

What is Brazing?

Brazing, simply put, is joining metal to metal by filling the joint with a different, melted metal at temperatures over 840F. The melted metal filling the gap must be able to wet the pieces being joined so that it is drawn into the gap by capillary action. Below 840F, the equivalent process is called soldering. If the process is hot enough to melt the metals being joined, it becomes welding or braze-welding.

The pieces being joined together are referred to as the base metal, while the melted metal that fills the gap between them is called the filler metal. Filler metal is also called brazing rod, spelter, or brazing alloy.

The following discussion of brazing focuses on the techniques and materials used in brazing bicycle frames, but most of the information applies to any similar brazing of light-weight steel components.

Safety

All types of brazing generate toxic fumes, and most brazing torches burn unstable fuels. Read the instructions and Material Safety Data Sheets that come with all tools and supplies and take the warnings seriously! Ask for the MSDS when you buy an unfamiliar filler or flux.

Brass Brazing

As the name implies, brazing is usually done with a filler made of some form of brass or bronze, alloys of copper with zinc, tin, and other metals. Brass and bronze brazing alloys typically melt at temperatures over 1500F, sometimes as high as 2000F. This is hot enough to anneal steel, destroying any strength added through heat treatment. For this reason, heat-treated bicycle tubing such as Reynolds 731 should not be brass brazed.

For tubing that has not been heat treated, such as Reynolds 531 or plain 4130 chromoly steel, brass brazing is entirely appropriate. It produces joints that are more than strong enough for a bicycle frame, it costs much less than silver, and it is a more forgiving process, allowing wider tolerances in tube mitering and temperature control. The vast majority of brazed production bicycle frames, and a large share of custom frames, are all brass or bronze brazed. Especially for a beginning frame builder, brass is often a good choice.

Silver Brazing

Lower temperatures and stronger joints can be achieved with more expensive alloys composed largely of silver. Silver fillers melt and flow at lower temperatures than most brass or bronze fillers, and are better at wicking into very fine joints. Both of these characteristics improve final joint strength.

Note: While modern U.S. usage calls the use of these alloys silver brazing, obsolete U.S. works and many current British sources often use the terms silver soldering or hard soldering to describe the same process. This is a source of continuing confusion, since silver solder is a term also used to describe much weaker alloys used at low temperatures for plumbing work. To avoid confusion, try to refer to an alloy's composition, standardized classification, or melting and flowing temperatures. When in doubt, seek clarification.

Common silver brazing alloys contain as much as 60% silver or as little as 20%, with the remainder made up of metals including copper, zinc, nickel, and tin. Silver brazing alloys melt at temperatures as low as 1145F, greatly reducing the impact on heat treatment of bicycle tubing. There is still a heat affected zone near the joint, but the butting of the tubing provides more material here to make up for its reduced strength. Away from the joint, the lower joint temperature means less heat travels along the tubing, and the heat treatment remains effective. The skill of the brazer also plays an important role in reducing heat damage, since the longer a joint is heated, the more heat is available to damage surrounding material.

Often, silver brazing alloys also include cadmium, a poisonous metal with deadly fumes. Cadmium lowers the brazing temperature and improves handling characteristics slightly, but should only be used by expert workers with commercial grade ventilation equipment. Using it at home in your garage is asking for trouble, so if you want to do silver brazing, look for cadmium-free alloys. All the major manufacturers of silver brazing alloys produce cadmium free fillers.

Silver is also the filler of choice for brazing stainless steels. Most brass fillers will not wet the surface of stainless, and if they do they will produce low-quality joints. Brazing stainless steel takes more practice than brazing ordinary steels, and in some cases special filler alloys are needed to prevent corrosion between the filler metal and the base metal. A small amount of nickel is added to the filler alloy to prevent this sort of corrosion, which can otherwise cause rapid failure of joints continuously or repeatedly exposed to water.

While each manufacturer has its own trade names for its alloys, most alloys sold in the U.S. conform to one of a number of specifications established by the American Welding Society (AWS). A common cadmium-free alloy used in bicycle work is AWS No. BAg-7, which has the same characteristics whether sold as Safety-Silv 56, All State 155, or Silvaloy 355. The alloy is 55 to 57 percent silver, 21 to 23 percent copper, 15 to 19 percent zinc, and 4.5 to 5.5 percent tin.

Nickel Silver Brazing

One final class of brazing filler that should be mentioned is nickel silver alloy. Despite the name, this is not a silver alloy at all, but mainly nickel and copper. It has a much higher melting point than genuine silver alloys, and should not be used on heat treated tubing.

On the other hand, nickel silver is stronger than brass filler, often much stronger, and some nickel silvers will work well on some alloys of stainless steel. All State specifically recommends one of its nickel silver alloys, No. 11, for brazing bicycle frames, and this alloy does make strong, reliable frame joints. According to his Manual, this is Tim Paterek's preferred fillet brazing alloy, too. It has a very wide working range, from 1200 F to 1750 F, and a tensile strength of up to 85,000 psi.

Fluxes

At the temperatures involved in brazing, most metals oxidize very rapidly. This would interfere with the melted silver or brass wetting the steel and flowing smoothly into the joint. To prevent this, chemicals called fluxes that dissolve oxidized metals and protect the surface from further oxidation are added to the process. Most commonly, these fluxes are placed in and around the joint as pastes, though they can also be caked around the filler rod or added to the torch flame itself.

Brazing filler will usually not flow onto un-fluxed base metal. While this can be a problem inside your joints, it can be a useful property outside them: if you are making a small joint relative to the size of your heat source, filler will tend to flow all over the heated base metal. Limiting your fluxed area will also limit this flow, making cleanup easier. Don't overdo this -- you do want flux in the joint, and you don't want to burn the metal around the joint. You can still file off the excess filler when the joint has cooled, and with practice you will learn to keep the filler where you want it by controlling the heat rather than by limiting the flux.

Different fluxes provide protection at different temperature ranges, so it is important to get a flux designed for use with the type of filler you are using. Commonly fluxes are designated for either silver or brass brazing, being active at one temperature range or the other but not both. There are also special fluxes that have greater protection for use on stainless steels or when the work must be heated for a prolonged period.

Each flux manufacturer will provide information on the temperature ranges, base metals, and filler metals a flux is designed for.

Brazing Methods

Typically, custom bicycle frames are brazed by hand. The frame pieces are held in place with a jig, and the builder heats the joints one at a time with a hand held oxy-acetylene torch. This is the same sort of torch used for welding, but it is used somewhat differently in brazing, since the base metal is not melted.

In the past, many bicycles were brazed over an open hearth, rather than with a torch. This limited the use of jigs, so that the frame joints had to be pinned or tack-brazed in place before the whole frame could be moved over to the hearth, where one joint after another would be exposed to the heat of the flames. This ancient method remained in widespread use as recently as the 1970s among conservative custom builders, but has largely been displaced by the easier and more precise torch brazing method.

How to Learn to Braze

Obviously, this is a very brief introduction to brazing. To really learn to braze your best option is to find a course in brazing at a local community college or trade school, or to find a bicycle frame builder who will teach you for a fee. You can teach yourself to braze without these aids, but brazing is art as well as science, and many techniques and tips will not be found in a book.