The heat treatment of pinions produced from this material consists in carburizing at a temperature of from 1,600 to 1,650°F. for a sufficient length of time to secure a depth of case from 1/32 to 3/64 in. The pinions are then quenched in oil from a temperature of 1,500 to 1,525°F. to refine the grain of the core and quenched in oil from a temperature of from 1,340 to 1,360°F. To refine and harden the case. The use of this material however, is recommended only in an emergency, as high-nickel steel is very susceptible to seams, secondary pipe and laminations.
The main criterion on rear-axle and pinion shafts, steering knuckles and arms and parts of this general type is resistance 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°F. in water and tempering at a temperature of from 975 to 1,025°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°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.