ance to fatigue and
torsion. The material recommended for parts of this character is
either S. A. E. No. 6135 or No. 3135 steel, which have the chemical
composition given in Tables 9 and 7.

HEAT TREATMENT OF AXLES

Parts of this general type should be heat-treated to show the following
minimum physical properties: Elastic limit, 115,000 lb. per square
inch; elongation in 2 in., 16 per cent; reduction of area, 50 per
cent; Brinell hardness, 277 to 321.

The heat treatment used to secure these physical properties consists
in quenching from a temperature of from 1,520 to 1,540 deg.F. in water
and tempering at a temperature of from 975 to 1,025 deg.F. Where the
axle shaft is a forging, and in the case of steering knuckles and
arms, this heat treatment should be preceded by normalizing the
forgings at a temperature of from 1,550 to 1,600 deg.F. It will be
noted that these physical properties correspond to those worked
out for an ideal aviation engine crankshaft. If parts of this type
are designed with proper sections, so that this range of physical
properties can be used, the part in question will give maximum
service.

One of the most important developments during the Liberty engine
program was the fact that it is not necessary to use a high-analysis
alloy steel to secure a finished part which will give proper service.
This fact should save the automotive industry millions of dollars
on future production.

If the proper authority be given the metallurgical engineer to
govern the handling of the steel from the time it is purchased
until it is assembled into finished product, mild-analysis steels
can be used and the quality of the finished product guaranteed.
It was only through the careful adherence to these fundamental
principles that it was possible to produce 20,000 Liberty engines,
which are considered to be the most highly stressed mechanism ever
produced, without the failure of a single engine from defective
material or heat treatment.

MAKING STEEL BALLS

Steel balls are made from rods or coils according to size, stock
less than 9/16-in. comes in coils. Stock 5/8-in. and larger comes
in rods. Ball stock is designated in thousandths so that 5/8-in.
rods are known as 0.625-in. stock.

Steel for making balls of average size is made up of:

Carbon 0.95 to 1.05 per cent
Silicon 0.20 to 0.35 per cent
Manganese 0.30 to 0.45 per cent
C