ining results.
By careful control of the heat-treating operation and with the aid
of the Brinell hardness tester and the microscope it is possible
to continually give forgings that will machine uniformly and be
soft enough to give desired production. The following gives a few
of the hardness numerals on steel used in gear manufacture that
produce good machining qualities:

0.20 per cent carbon, 3 per cent nickel, 1-1/4; per cent
chromium--Brinell 156 to 170.

0.50 per cent carbon, 3 per cent nickel, 1 per cent chromium--Brinell
179 to 187.

0.50 per cent carbon chrome-vanadium--Brinell 170 to 179.

THE INFLUENCE OF SIZE

The size of the piece influences the physical properties obtained in
steel by heat treatment. This has been worked out by E. J. Janitzky,
metallurgical engineer of the Illinois Steel Company, as follows:

[Illustration: FIG. 55.--Effect of size on heating.]

"With an increase in the mass of steel there is a corresponding
decrease in both the minimum surface hardness and depth hardness,
when quenched from the same temperature, under identical conditions
of the quenching medium. In other words, the physical properties
obtained are a function of the surface of the metal quenched for
a given mass of steel. Keeping this primary assumption in mind, it
is possible to predict what physical properties may be developed in
heat treating by calculating the surface per unit mass for different
shapes and sizes. It may be pointed out that the figures and chart
that follow are not results of actual tests, but are derived by
calculation. They indicate the mathematical relation, which, based
on the fact that the physical properties of steel are determined
not alone by the rate which heat is lost per unit of surface, but
by the rate which heat is lost per unit of weight in relation to
the surface exposed for that unit. The unit of weight has for the
different shaped bodies and their sizes a certain surface which
determines their physical properties.

"For example, the surface corresponding to 1 lb. of steel has been
computed for spheres, rounds and flats. For the sphere with a unit
weight of 1 lb. the portion is a cone with the apex at the center
of the sphere and the base the curved surface of the sphere (surface
exposed to quenching). For rounds, a unit weight of 1 lb. may be
taken as a disk or cylinder, the base and top surfaces naturally do
not enter into calculation. For a flat, a prismatic or cylindrical