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 ma