nsus of opinion among
the screw-machine manufacturers is that S. A. E. No. 6,130 steel
gives the best machineability and that S. A. E. No. 2,330 steel
would receive second choice of the three specified.

In the finishing of highly stressed parts for aviation engines,
extreme care must be taken to see that all tool marks are eliminated,
unless they are parallel to the axis of strain, and that proper
radii are maintained at all changes of section. This is of the
utmost importance to give proper fatigue resistance to the part
in question.

GEARS

The material used for all gears on the Liberty engine was selected
at the option of the manufacturer from the following standard S.
A. E. steels, the composition of which are given in Table 12,

TABLE 12.--COMPOSITION OF STEELS NOS. X-3,340 AND 6,140

Steel No X-3,340 6,140
Carbon, minimum 0.350 0.350
Carbon, maximum 0.450 0.450
Manganese, minimum 0.450 0.500
Manganese, maximum 0.750 0.800
Phosphorus, maximum 0.040 0.040
Sulphur, maximum 0.045 0.045
Nickel, minimum 2.750
Nickel, maximum 3.250
Chromium, minimum 0.700 0.800
Chromium, maximum 0.950 1.100
Vanadium, minimum 0.150

All gears were heat-treated to a scleroscope hardness of from 55
to 55. The heat treatment used to secure this hardness consisted
in quenching the forgings from a temperature of 1,550 to 1,600 deg.F.
in oil and annealing for good machineability at a temperature of
from 1,300 to 1,350 deg.F. Forgings treated in this manner showed a
Brinell hardness of from 177 to 217.

RATE OF COOLING

At the option of the manufacturer, the above treatment of gear
forgings could be substituted by normalizing the forgings at a
temperature of from 1,550 to 1,600 deg.F. The most important criterion
for proper normalizing, consisted in allowing the forgings to cool
through the critical temperature of the steel, at a rate not to exceed
50 deg.F. per hour. For the two standard steels used, this consisted in
cooling from the normalizing temperature down to a temperature
of 1,100 deg.F., at the rate indicated. Forgings normalized in this
manner will show a Brinell hardness of from 177 to 217. The question
has been repeatedly asked as to which treatment will produce the
higher quality finished part. In answer to this I will state that
on simple forgings of comparatively small section, the n