e. In that case the filling would keep the water
away from the surface and no hardening would take place. Where
filling is to be done, various materials are used by different
hardeners. Fireclay and common putty seem to be favored by many.

Every mechanic who has had anything to do with the hardening of
tools knows how necessary it is to take a cut from the surface of
the bar that is to be hardened. The reason is that in the process
of making the steel its outer surface has become decarbonized.
This change makes it low-carbon steel, which will of course not
harden. It is necessary to remove from 1/16 to 1/4 in. of diameter
on bars ranging from 1/2 to 4 in.

This same decarbonization occurs if the steel is placed in the
forge in such a way that unburned oxygen from the blast can get at
it. The carbon is oxidized, or burned out, converting the outside
of the steel into low-carbon steel. The way to avoid this is to use
a deep fire. Lack of this precaution is the cause of much spoiled
work, not only because of decarbonization of the outer surface
of the metal, but because the cold blast striking the hot steel
acts like boiling hot water poured into an ice-cold glass tumbler.
The contraction sets up stresses that result in cracks when the
piece is quenched.

PREVENTING DECARBONIZATION OF TOOL STEEL

It is especially important to prevent decarbonization in such tools
as taps and form cutters, which must keep their shape after hardening
and which cannot be ground away on the profile. For this reason
it is well to put taps, reamers and the like into pieces of pipe
in heating them. The pipe need be closed on one end only, as the
air will not circulate readily unless there is an opening at both
ends.

Even if used in connection with a blacksmith's forge the lead bath
has an advantage for heating tools of complicated shapes, since
it is easier to heat them uniformly and they are submerged and
away from the air. The lead must be stirred frequently or the heat
is not uniform in all parts of the lead bath. Covering the lead
with powdered charcoal will largely prevent oxidization and waste
of lead.

Such a bath is good for temperatures between 620 and 1,150 deg.F. At
higher temperatures there is much waste of lead.

ANNEALING TO RELIEVE INTERNAL STRESSES

Work quenched from a high temperature and not afterward tempered
will, if complex in shape, contain many internal stresses which may
later cause it to break. They may be