Rate Of Cooling

: The Working Of Steel

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 th
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 normalizing

treatment will produce a finished part which is of equal quality to

that of the quenched and annealed forgings. However, in the case of

complex forgings, or those of large section, more uniform physical

properties of the finished part will be obtained by quenching and

annealing the forgings in the place of normalizing.

The heat treatment of the finished gears consisted of quenching

in oil from a temperature of from 1,420 to 1,440 deg.F. for the No.

X-3,340 steel, or from a temperature of from 1,500 to 1,540 deg.F.

for No. 6,140 steel, followed by tempering in saltpeter or in an

electric furnace at a temperature of from 650 to 700 deg.F.

The question has been asked by many engineers, why is the comparatively

low scleroscope hardness specified for gears? The reason for this is

that at best the life of an aviation engine is short, as compared with

that of an automobile, truck or tractor, and that shock resistance

is of vital importance. A sclerescope hardness of from 55 to 65

will give sufficient resistance to wear to prevent replacements

during the life of an aviation engine, while at the same time this

hardness produces approximately 50 per cent greater shock-resisting

properties to the gear. In the case of the automobile, truck or

tractor, resistance to wear is the main criterion and for that

reason the higher hardness is specified.

Great care should be taken in the design of an aviation engine

gear to eliminate sharp corners at the bottom of teeth as well

as in keyways. Any change of section in any stressed part of an

aviation engine must have a radius of at least 1/32 in. to give

proper shock and fatigue resistance. This fact has been demonstrated

many times during the Liberty engine program.