Low Potential For Carbon Contamination from High Speed Steel Tool Bits

The potential for carbon contamination of high purity and ultra-pure stainless steel piping/tubing systems from the use of High Speed Steel (HSS) tool bits has been discussed and debated for many years. In most cases the question comes up on systems using electropolished stainless steel tubing which has to be squared for autogenous welding.

There is no acceptable alternative tool bit material for achieving a burr-free tube end that does not contain carbon. A stainless steel that is carbon free does not exist.

Tungsten carbide has been recommended and used, but two problems exist, it is difficult to make a tool bit for burr-free cut with Tungsten Carbide and theoretically Tungsten Carbide which is a molecule of one Tungsten atom and one carbon atom can decompose under high heat to release a carbon atom and form an alternate carbide compound. The possibility of depositing carbon from Tungsten carbide is just as great as when using HSS. Cermets use HSS for a metallic bond matrix. Stellite™ type steel contains some carbon and Diamond is nothing but carbon!

HSS tool bits have been used for squaring stainless steel tube in the microelectronics, pharmaceutical and experimental physics programs for many years. Weld joints are continuously monitored in some of these programs and there has never been any evidence reported that a problem exists from carbon contamination as a result of using HSS tool bits.

From a practical point of view, the amount of material that could be deposited from a hardened and ground tool steel with only about .2% carbon is very small. In practice, in order to achieve a burr-free end prep, the tool bits must be run slow and not create high temperature levels in either the tube or the tool bit.

These conditions automatically reduce the probabilities of depositing carbon on the surface of the tube being machined.

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