Steel has the overall best qualities for building and riding bicycles. It can easily be butted, shaped, cut, welded, or brazed. There are many choices in tube shape and size. Combine this with a variety of geometries to create countless types of ride qualities to suit each rider's needs. Read on to learn more about steel joining methods.

Metallurgy
I like metallurgy. I studied it in college and have a passion for the processes involved in frame building. One of the great things about building steel bikes is that they can be joined by welding or brazing.

Metal Joining Methods

Welding

TIG welding is a high-tech version of arc welding. TIG stand for Tungsten Inert Gas. The tip of the torch is made of tungsten. It's called the electrode and that's where the electric arc emanates. The inert gas used is Argon. Argon gas is fed out all around the tungsten tip and it pours all over the welding area. The weld is unaffected by the argon but it's role is crucial. It keeps harmful gasses away while the arc melts the two tubes together. Gases like oxygen, nitrogen, or helium can contaminate the metal in the weld making it brittle. Once contaminated there's no telling when the joint will crack but it is flawed and will never last. A good weld with no contamination is as strong as the parent tube.

Brazing

Brazing often gets confused with welding. There's a fundamental difference. The main tubes that are being joined do not melt. They are being "glued" together by a second metal filler, in our case brass or silver, that has a lower melting point than the steel tubes. The joint is heated to the melting temperature of the filler metal. Brass has a melting point around 1600°F. Silver melts lower around 1000°F. Once the main tubes are hot enough the secondary metal will "wet" the surface. Wetting means is will liquefy and attach or run into the joint. Atoms of the metal, whether brass or silver, are diffusing into the steel tubes creating a metallurgical bond. If you've ever made stained glass or fixed circuit boards you've done brazing. It's called soldering at such low temperatures and tin or lead are used as the filler material.

If that weren't enough, there's 2 ways to braze steel bike frames; fillet brazing and lugged construction. To fillet braze, the builder heats the joint and adds brass directly at the joint building a radius, or fillet. Later the joint is filed and sanded so there's a super smooth transition from one tube to the other. The beauty of fillet brazing is the way the tubes seemingly run from one tube to the other similar to carbon bikes. Lugged construction is achieved by using a lug which is a socket or sleeve the main tubes slide into. This is the oldest construction method used and, I think, the most durable. Once the tubes are fitted inside the lug, it is heated and another metal is fed in between the lug and tube to "glue" them together. I prefer silver for lugs over brass for 2 reasons. First, silver is really slick when it's liquid and it flows beautifully inside the lug to create the joint. Brass tends to be thick and slow moving and therefore, more difficult to penetrate the thin opening between the lug and tube. Secondly, the silver is a lower temperature metal. This is crucial to maintaining the strength of the main steel tubes. Heating the tubes to the brass melting point anneals the steel tubes. That is, it softens them is that area. This is by no means unsafe if done properly, and fillet brazing is a prime example. However, silver's melting point is so low that the steel will never experience any annealing. Consequently the steel tubes will be as strong before heating as after. The tubing manufacturers know this problem of overheating and have developed alloys which resist annealing. Alloys specifically designed for TIG welding have arrived which are actually strengthened by the heat (above 1600°F) during welding. These so-called "air-hardened" alloys are found in all major tubing brands, Columbus FOCO, True Temper OX Platinum Gold, and Dedachai UNO.

I generally use Columbus tubing. Let's face it, they're the best. It comes in all different buttings, wall thicknesses, strengths and shapes to suit my needs. I can order stock tubesets, swap out individual tubes, or pick one by one. The material has superior strength, great workability, excellent tolerances and an incredible legacy and reputation. I can get Reynolds, Dedachai, or True Temper, and for the most part they work great. But Columbus continues to improve their alloys while maintaining their top-notch quality. Need I say more?