Oxy-Acetylene Welding and Cutting | Page 7

desired point, and allowed to cool in the air
until all traces of red have disappeared when held in a dark place. The
work should be held where it is reasonably free from cold air currents.

If, upon touching a pine stick to the piece being annealed, the wood
does not smoke, the work may then be cooled in water.
Better annealing is secured and harder metal may be annealed if the
cooling is extended over a number of hours by placing the work in a
bed of non-heat-conducting material, such as ashes, charred bone,
asbestos fibre, lime, sand or fire clay. It should be well covered with
the heat retaining material and allowed to remain until cool. Cooling
may be accomplished by allowing the fire in an oven or furnace to die
down and go out, leaving the work inside the oven with all openings
closed. The greater the time taken for gradual cooling from the red heat,
the more perfect will be the results of the annealing.
While steel is annealed by slow cooling, copper or brass is annealed by
bringing to a low red heat and quickly plunging into cold water.
_Hardening._--Steel is hardened by bringing to a proper temperature,
slowly and evenly as for annealing, and then cooling more or less
quickly, according to the grade of steel being handled. The degree of
hardening is determined by the kind of steel, the temperature from
which the metal is cooled and the temperature and nature of the bath
into which it is plunged for cooling.
Steel to be hardened is often heated in the fire until at some heat around
600 to 700 degrees is reached, then placed in a heating bath of molten
lead, heated mercury, fused cyanate of potassium, etc., the heating bath
itself being kept at the proper temperature by fires acting on it. While
these baths have the advantage of heating the metal evenly and to
exactly the temperature desired throughout without any part becoming
over or under heated, their disadvantages consist of the fact that their
materials and the fumes are poisonous in most all cases, and if not
poisonous, are extremely disagreeable.
The degree of heat that a piece of steel must be brought to in order that
it may be hardened depends on the percentage of carbon in the steel.
The greater the percentage of carbon, the lower the heat necessary to
harden.

[Illustration: Figure 5.--Cooling the Test Bar for Hardening]
To find the proper heat from which any steel must be cooled, a simple
test may be carried out provided a sample of the steel, about six inches
long can be secured. One end of this test bar should be heated almost to
its melting point, and held at this heat until the other end just turns red.
Now cool the piece in water by plunging it so that both ends enter at
the same time (Figure 5), that is, hold it parallel with the surface of the
water when plunged in. This serves the purpose of cooling each point
along the bar from a different heat. When it has cooled in the water
remove the piece and break it at short intervals, about 1/2 inch, along
its length. The point along the test bar which was cooled from the best
possible temperature will show a very fine smooth grain and the piece
cannot be cut by a file at this point. 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