Bundy tube, sometimes called Bundy pipe, is type of double-walled low-carbon steel

Steel

Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

 tube manufactured by rolling a copper-coated steel strip through 720 degrees and resistance brazing

Brazing

Brazing is a metal-joining process whereby a filler metal is heated above and distributed between two or more close-fitting parts by capillary action. The filler metal is brought slightly above its melting temperature while protected by a suitable atmosphere, usually a flux...

 the overlapped seam in a process called Bundywelding. It may be zinc- or terne- coated for corrosion protection. It is used in automotive hydraulic brake lines in cars manufactured in the USA since the 1930s.

A 1969 study by the SAE recommended the replacement of Bundy tube with 90-10 copper-nickel alloy UNS

Unified numbering system

The unified numbering system is an alloy designation system widely accepted in North America. It consists of a prefix letter and five digits designating a material composition. A prefix of S indicates stainless steel alloys, C for copper, brass, or bronze alloys, T for tool steels, etc...

 C70600 (Kunifer pipe) because of corrosion concerns. Kunifer pipe has since been adopted by European automakers Volvo

Volvo

AB Volvo is a Swedish builder of commercial vehicles, including trucks, buses and construction equipment. Volvo also supplies marine and industrial drive systems, aerospace components and financial services...

, Rolls-Royce

Rolls-Royce (car)

This a list of Rolls-Royce motor cars and includes vehicles produced by:*Rolls-Royce Limited *Rolls-Royce Motors , which was owned by Vickers between 1980 and 1998, and after that by Volkswagen...

, Lotus Cars

Lotus Cars

Lotus Cars is a British manufacturer of sports and racing cars based at the former site of RAF Hethel, a World War II airfield in Norfolk. The company designs and builds race and production automobiles of light weight and fine handling characteristics...

, Aston-Martin, Porsche

Porsche

Porsche Automobil Holding SE, usually shortened to Porsche SE a Societas Europaea or European Public Company, is a German based holding company with investments in the automotive industry....

, and Audi

Audi

Audi AG is a German automobile manufacturer, from supermini to crossover SUVs in various body styles and price ranges that are marketed under the Audi brand , positioned as the premium brand within the Volkswagen Group....

. Bundy pipe retains the advantage higher rigidity, which means less volume expansion under pressure.

The Bundy Tubing Company, started in the USA, was bought in the 1980s by what is now the British company TI Automotive.