Bundy tube, sometimes called Bundy pipe, is type of double-walled low-carbon steel tube manufactured by rolling a copper-coated steel strip through 720 degrees and resistance brazing the overlapped seam in a process called Bundywelding.
Some key sources are Shelley Automation ltd in the UK and SVS Refcomp Pvt Ltd in India It may be zinc or terne coated for corrosion protection.
A 1969 study by the Society of Automotive Engineers (SAE) recommended the replacement of Bundy tube with 90-10 copper-nickel alloy UNS C70600 (Kunifer pipe) because of corrosion concerns.
[1] Kunifer pipe has since been adopted by European automakers Volvo, Rolls-Royce, Lotus Cars, Aston-Martin, Porsche, and Audi.
[2] Bundy tube still retains the advantage of higher rigidity, which means less volume expansion under pressure.