Application Of Liberty Engine Materials To The Automotive Industry





The success of the Liberty engine program was an engineering achievement

in which the science of metallurgy played an important part. The

reasons for the use of certain materials and certain treatments

for each part are given with recommendations for their application

to the problems of automotive industry.



The most important items to be taken into consideration in the

selection of material for parts of this type are uniformity and

machineability. It has been demonstrated many times that the ordinary

grades of bessemer screw stock are unsatisfactory for aviation

purposes, due to the presence of excessive amounts of unevenly

distributed phosphorus and sulphide segregations. For this reason,

material finished by the basic open hearth process was selected,

in accordance with the following specifications: Carbon, 0.150 to

0.250 per cent; manganese, 0.500 to 0.800 per cent; phosphorus,

0.045 maximum per cent; sulphur, 0.060 to 0.090 per cent.



This material in the cold-drawn condition will show: Elastic limit,

50,000 lb. per square inch, elongation in 2 in., 10 per cent, reduction

of area, 35 per cent.



This material gave as uniform physical properties as S. A. E. No.

1020 steel and at the same time was sufficiently free cutting to

produce a smooth thread and enable the screw-machine manufacturers

to produce, to the same thread limits, approximately 75 per cent

as many parts as from bessemer screw stock.



There are but seven carbon-steel carbonized parts on the Liberty

engine. The most important are the camshaft, the camshaft rocker

lever roller and the tappet. The material used for parts of this

type was S. A. E. No. 1,020 steel, which is of the following chemical

analysis: Carbon 0.150 to 0.250 per cent; manganese, 0.300 to 0.600

per cent; phosphorus, 0.045 maximum per cent; sulphur, 0.050 maximum

per cent.



The heat treatment consisted in carbonizing at a temperature of

from 1,650 to 1,700 deg.F. for a sufficient length of time to secure

the proper depth of case, cool slowly or quench; then reheat to a

temperature of 1,380 to 1,430 deg.F. to refine the grain of the case,

and quench in water. The only thing that should limit the rate of

cooling from the carbonizing heat is distortion. Camshaft rocker

lever rollers and tappets, as well as gear pins, were quenched

directly from the carbonizing heat in water and then case-refined

and rehardened by quenching in water from a temperature of from

1,380 to 1,430 deg.F.



The advantage of direct quenching from the carbonizing heat is

doubtless one of economy, and in many cases will save the cost

of a reheating. Specifications for case hardening, issued by the

Society of Automotive Engineers, have lately been revised; whereas

they formerly called for a slow cooling, they now permit a quenching

from the pot. Doubtless this is a step in advance. Warpage caused

by quenching can be reduced to a minimum by thoroughly annealing

the stock before any machine work is done on it.



Another advantage obtained from rapid cooling from the carbonizing

heat is the retaining of the majority of the excess cementite in

solution which produces a less brittle case and by so doing reduces

the liability of grinding checks and chipping of the case in actual

service.



In the case of the camshaft, it is not possible to quench directly

from the carbonizing heat because of distortion and therefore excessive

breakage during straightening operations. All Liberty camshafts

were cooled slowly from carbonizing heat and hardened by a single

reheating to a temperature of from 1,380 to 1,430 deg.F. and quenching

in water.



Considerable trouble has always been experienced in obtaining uniform

hardness on finished camshafts. This is caused by insufficient

water circulation in the quenching tank, which allows the formation

of steam pockets to take place, or by decarbonization of the case

during heating by the use of an overoxidizing flame. Another cause,

which is very often overlooked, is due to the case being ground off

one side of cam more than the other and is caused by the roughing

master cam being slightly different from the finishing master cam.

Great care should be taken to see that this condition does not occur,

especially when the depth of case is between 1/32 and 3/64 in.





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