Crankshaft





The crankshaft was the most highly stressed part of the entire

Liberty engine, and, therefore, every metallurgical precaution

was taken to guarantee the quality of this part. The material used

for the greater portion of the Liberty crankshafts produced was

nickel-chromium steel of the following chemical composition: Carbon,

0.350 to 0.450 per cent; manganese, 0.300 to 0.600 per cent; phosphorus,

0.040 maximum per cent; sulphur, 0.045 maximum per cent; nickel,

1.750 to 2.250 per cent; chromium, 0.700 to 0.900 per cent.



Each crankshaft was heat-treated to show the following minimum

physical properties: Elastic limit, 116,000 lb. per square inch;

elongation in 2 in., 16 per cent, reduction of area, 50 per cent,

Izod impact, 34 ft.-lb.; Brinell hardness, 266 to 321.



For every increase of 4,000 lb. per square inch in the elastic

limit above 116,000 lb. per square inch, the minimum Izod impact

required was reduced 1 ft.-lb.



The heat treatment used to produce these physical properties consisted

in normalizing the forgings at a temperature of from 1,550 to 1,600 deg.F.,

followed by quenching in water at a temperature of from 1,475 to

1,525 deg.F. and tempering at a temperature of from 1,000 to 1,100 deg.F.

It is absolutely necessary that the crankshafts be removed from the

quenching tank before being allowed to cool below a temperature of

500 deg.F., and immediately placed in the tempering furnace to eliminate

the possibility of quenching cracks.



A prolongation of not less than the diameter of the forging bearing

was forged on one end of each crankshaft. This was removed from

the shaft after the finish heat treatment, and physical tests were

made on test specimens which were cut from it at a point half way

between the center and the surface. One tensile test and one impact

test were made on each crankshaft, and the results obtained were

recorded against the serial number of the shaft in question. This

serial number was carried through all machining operations and

stamped on the cheek of the finished shaft. In addition to the

above tensile and impact tests, at least two Brinell hardness

determinations were made on each shaft.



All straightening operations on the Liberty crankshaft which were

performed below a temperature of 500 deg.F. were followed by retempering

at a temperature of approximately 200 deg.F. below the original tempering

temperature.



Another illustration of the importance of proper radii at all changes

of section is given in the case of the Liberty crankshaft. The presence

of tool marks or under cuts must be completely eliminated from an

aviation engine crankshaft to secure proper service. During the

duration of the Liberty program, four crankshafts failed from fatigue,

failures starting from sharp corners at bottom of propeller-hub

keyway. Two of the shafts that failed showed torsional spirals

running more than completely around the shaft. As soon as this

difficulty was removed no further trouble was experienced.



One of the most important difficulties encountered in connection

with the production of Liberty crankshafts was hair-line seams. The

question of hair-line seams has been discussed to greater length

by engineers and metallurgists during the war than any other single

question. Hair-line seams are caused by small non-metallic inclusions

in the steel. There is every reason to believe that these inclusions

are in the greater majority of cases manganese sulphide. There is

a great difference of opinion as to the exact effect of hair-line

seams on the service of an aviation engine crankshaft. It is the

opinion of many that hair-line seams do not in any way affect the

endurance of a crankshaft in service, provided they are parallel to

the grain of the steel and do not occur on a fillet. Of the 20,000

Liberty engines produced, fully 50 per cent of the crankshafts

used contain hair-line seams but not at the locations mentioned.

There has never been a failure of a Liberty crankshaft which could

in any way be traced to hair-line seams.



It was found that hair-line seams occur generally on high

nickel-chromium steels. One of the main reasons why the comparatively

mild analysis nickel-chromium steel was used was due to the very

few hair-line seams present in it. It was also determined that

the hair lines will in general be found near the surface of the

forgings. For that reason, as much finish as possible was allowed

for machining. A number of tests have been made on forging bars

to determine the depths at which hair-line seams are found, and

many cases came up in which hair-line seams were found 3/8 in.

from the surface of the bar. This means that in case a crankshaft

does not show hair-line seams on the ground surface this is no

indication that it is free from such a defect.



One important peculiarity of nickel-chromium steel was brought

out from the results obtained on impact tests. This peculiarity

is known as blue brittleness. Just what the effect of this is

on the service of a finished part depends entirely upon the design

of the particular part in question. There have been no failures of

any nickel-chromium steel parts in the automotive industry which

could in any way be traced to this phenomena.



Whether or not nickel-chromium-steel forgings will show blue

brittleness depends entirely upon the temperature at which they

are tempered and their rate of cooling from this temperature. The

danger range for tempering nickel-chromium steels is between a

temperature of from 400 to 1,100 deg.F. From the data so far gathered

on this phenomena, it is necessary that the nickel-chromium steel

to show blue brittleness be made by the acid process. There has

never come to my attention a single instance in which basic open

hearth steel has shown this phenomena. Just why the acid open hearth

steel should be sensitive to blue brittleness is not known.



All that is necessary to eliminate the presence of blue brittleness

is to quench all nickel-chromium-steel forgings in water from their

tempering temperature. The last 20,000 Liberty crankshafts that

were made were quenched in this manner.





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