It will be necessary to remember the exact color
of that point when taken from the fire, making another test if necessary,
and heat all pieces of this same steel to this heat. It will be necessary
to have the cooling bath always at the same temperature, or the results
cannot be alike.
While steel to be hardened is usually cooled in water, many other liquids
may be used. If cooled in strong brine, the heat will be extracted much
quicker, and the degree of hardness will be greater. A still greater degree
of hardness is secured by cooling in a bath of mercury. Care should be used
with the mercury bath, as the fumes that arise are poisonous.
Should toughness be desired, without extreme hardness, the steel may be
cooled in a bath of lard oil, neatsfoot oil or fish oil. To secure a result
between water and oil, it is customary to place a thick layer of oil on top
of water. In cooling, the piece will pass through the oil first, thus
avoiding the sudden shock of the cold water, yet producing a degree of
hardness almost as great as if the oil were not used.
It will, of course, be necessary to make a separate test for each cooling
medium used. If the fracture of the test piece shows a coarse grain, the
steel was too hot at that point; if the fracture can be cut with a file,
the metal was not hot enough at that point.
When hardening carbon tool steel its heat should be brought to a cherry
red, the exact degree of heat depending on the amount of carbon and the
test made, then plunged into water and held there until all hissing sound
and vibration ceases.
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