in
the shop sense, signifies the making of a piece of steel about as hard
as it can be made--"glass hard"--while tempering indicates some stage in
an infinite range between the fully hardened and the annealed or
softened condition. As a matter of convenience only, hardening is
usually a stage in the work of tempering. It is easier to harden first,
and "let down" to the temper required, than to secure the exact heat for
tempering by raising the material to it. This is partly due to the long
established practice of estimating temperature by colour tints; but this
is being rapidly invaded by new methods in which the temper heat is
obtained in furnaces provided with pyrometers, by means of which exact
heat regulation is readily secured, and in which the heating up is done
gradually. Such furnaces are used for hardening balls for bearings,
cams, small toothed wheels and similar work, as well as for tempering
springs, milling cutters and other kinds of cutting tools. But for the
cutting tools having single edges, as used in engineers' shops, the
colour test is still generally retained.

In the practice of hardening and tempering tools by colour, experience
is the only safe guide. Colour tints vary with degrees of light; steels
of different brands require different treatment in regard to temperature
and quenching; and steels even of identical chemical composition do not
always behave alike when tempered. Every fresh brand of steel has,
therefore, to be treated at first in a tentative and experimental
fashion in order to secure the best possible results. The larger the
masses of steel, and the greater the disparity in dimensions of adjacent
parts, the greater is the risk of cracking and distortion. Excessive
length and the presence of keen angles increase the difficulties of
hardening. The following points have to be observed in the work of
hardening and tempering.

A grade of steel must be selected of suitable quality for the purpose
for which it has to be used. There are a number of such grades, ranging
from about 1-1/2 to 1/2% content of carbon, and each having its special
utility. Overheating must be avoided, as that burns the steel and
injures or ruins it. A safe rule is never to heat any grade of steel to
a temperature higher than that at which experience proves it will take
the temper required. Heating must be regular and thorough throughout,
and must therefore be slowly done when dealing with thick masses.
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