merely martensite wholly transformed into
a mixture of ultramicroscopic crystals of ferrite (alpha iron)
and cementite (Fe3C).

"Tempering" or "drawing" should be restricted to mean moderate
reheating, up to about 350 deg. C., forming troostitic steel. "Toughening"
represents the practice of reheating hardened carbon steels from
350 deg. C. up to just below the lower critical, and forms sorbitic
steel; while "annealing" refers to a heating for grain size at
or above the transformation ranges, followed by a slow cooling.
Any of these operations not only allows the transformations from
austenite to pearlite to proceed, but also relieves internal stresses
in the steel.

Normalizing is a heating like annealing, followed by a moderately
rapid quench.

JUDGING THE HEAT OF STEEL

While the use of a pyrometer is of course the only way to have
accurate knowledge as to the heat being used in either forging or
hardening steels, a color chart will be of considerable assistance
if carefully studied. These have been prepared by several of the
steel companies as a guide, but it must be remembered that the colors
and temperatures given are only approximate, and can be nothing
else.

[Illustration: FIG. 53.--Finding hardening heats with a magnet.]

_The Magnet Test_.--The critical point can also be determined by
an ordinary horse-shoe magnet. Touch the steel with a magnet during
the heating and when it reaches the temperature at which steel fails
to attract the magnet, or in other words, loses its magnetism,
the critical point has been reached.

Figures 53 and 54 show how these are used in practice.

The first (Fig. 53) shows the use of a permanent horse-shoe magnet
and the second (Fig. 54) an electro-magnet consisting of an iron
rod with a coil or spool magnet at the outer end. In either case
the magnet should not be allowed to become heated but should be
applied quickly.

[Illustration: FIG. 54.--Using electro-magnet to determine heat.]

The work is heated up slowly in the furnace and the magnet applied
from time to time. The steel being heated will attract the magnet
until the heat reaches the critical point. The magnet is applied
frequently and when the magnet is no longer attracted, the piece
is at the lowest temperature at which it can be hardened properly.
Quenching slightly above this point will give a tool of satisfactory
hardness. The method applies only to carbon steels and will not
work for modern high-speed