The potential for carbon contamination of high purity and ultra-pure
stainless steel piping/tubing systems for the use of High Speed Steel
(HSS) tool bit 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 cutting material for achieving a burr-free tube
end that does not contain carbon. A stainless steel tool bit 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 with HSS. Cermets use HSS for a metallic bond matrix. Diamond is
nothing but carbon. Stelite contains carbon. HSS tool bits have been
used for squaring stainless steel tube in the microelectronics,
pharmaceuticals 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 the 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 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 metal on the surface of the tube being
machined.
