Category: APPLICATION OF LIBERTY ENGINE MATERIALS TO THE AUTOMOTIVE INDUSTRY
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
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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