APPLICATION OF LIBERTY ENGINE MATERIALS TO THE AUTOMOTIVE INDUSTRY[1]
[Footnote 1: Paper presented at the summer meeting of the S. A.
E. at Ottawa Beach in June, 1919.]
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°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°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°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°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.
CARBON-STEEL FORGINGS
Low-stressed, carbon-steel forgings include such parts as carbureter
control levers, etc. The important criterion for parts of this type
is ease of fabrication and freedom from over-heated and burned
forgings. The material used for such parts was S. A. E. No. 1,030
steel, which is of the following chemical composition: Carbon, 0.250
to 0.350 per cent; manganese, 0.500 to 0.800 per cent; phosphorus,
0.045 maximum per cent; sulphur, 0.050 maximum per cent.
To obtain good machineability, all forgings produced from this
steel were heated to a temperature of from 1,575 to 1,625°F. to
refine the grain of the steel thoroughly and quenched in water
and then tempered to obtain proper machineability by heating to a
temperature of from 1,000 to 1,100°F. and cooled slowly or quenched.
Forgings subjected to this heat treatment are free from hard spots
and will show a Brinell hardness of 177 to 217, which is proper for
all ordinary machining operations. Great care should be taken not
to use steel for parts of this type containing less than 0.25 per
cent carbon, because the lower the carbon the greater the liability
of hard spots, and the more difficult it becomes to eliminate them.
The only satisfactory method so far in commercial use for the
elimination of hard spots is to give forgings a very severe quench
from a high temperature followed by a proper tempering heat to
secure good machine ability as outlined above.
The important carbon-steel forgings consisted of the cylinders,
the propeller-hubs, the propeller-hub flange, etc. The material
used for parts of this type was S. A. E. No. 1,045 steel, which
is of the following chemical composition: Carbon, 0.400 to 0.500
per cent; manganese, 0.500 to 0.800 per cent; phosphorus, 0.045
maximum per cent; sulphur, 0.050 maximum per cent.
All forgings made from this material must show, after heat treatment,
the following minimum physical properties: Elastic limit, 70,000;
lb. per square inch, elongation in 2 in., 18 per cent, reduction
of area, 45; per cent, Brinell hardness, 217 to 255.
To obtain these physical properties, the forgings were quenched in
water from a temperature of 1,500 to 1,550°F., followed by tempering
to meet proper Brinell requirements by heating to a temperature
of 1,150 to 1,200°F. and cooled slowly or quenched. No trouble
of any kind was ever experienced with parts of this type.
The principal carbon-steel pressed parts used on the Liberty engine
were the water jackets and the exhaust manifolds. The material
used for parts of this type was S. A. E. No. 1,010 steel, which
is of the following chemical composition: Carbon, 0.05 to 0.15 per
cent; manganese, 0.30 to 0.60 per cent; phosphorus, 0.045 maximum
per cent; sulphur, 0.045 maximum per cent.
No trouble was experienced in the production of any parts from
this material with the exception of the water jacket. Due to the
particular design of the Liberty cylinder assembly, many failures
occurred in the early days, due to the top of the jacket cracking
with a brittle fracture. It was found that these failures were
caused primarily from the use of jackets which showed small scratches
or die marks at this joint and secondarily by improper annealing of
the jackets themselves between the different forming operations.
By a careful inspection for die marks and by giving the jackets
1,400°F. annealing before the last forming operation, it was possible
to completely eliminate the trouble encountered.
HIGHLY STRESSED PARTS
The highly stressed parts on the Liberty engine consisted of the
connecting-rod bolt, the main-bearing bolt, the propeller-hub key,
etc. The material used for parts of this type was selected at the
option of the manufacturer from standard S. A. E. steels, the
composition of which are given in Table 11.
TABLE 11.--COMPOSITION OF S. A. E. STEELS Nos. 2,330, 3,135 AND 6,130
Steel No 2,330 3,135 6,130
Carbon, minimum 0.250 0.300 0.250
Carbon, maximum 0.350 0.400 0.450
Manganese, minimum 0.500 0.500 0.500
Manganese, maximum 0.800 0.800 0.800
Phosphorus, maximum 0.045 0.040 0.040
Sulphur, maximum 0.045 0.045 0.045
Nickel, minimum 3.250 1.000
Nickel, maximum 3.750 1.500
Chromium, minimum 0.450 0.800
Chromium, maximum 0.750 1.100
Vanadium, minimum 0.150
All highly stressed parts on the Liberty engine must show, after
heat treatment, the following minimum physical properties: Elastic
limit, 100,000 lb. per square inch; elongation in 2 in., 16 per
cent; reduction of area, 45 per cent; scleroscope hardness, 40
to 50.
The heat treatment employed to obtain these physical properties
consisted in quenching from a temperature of 1,525 to 1,575°F., in
oil, followed by tempering at a temperature of from 925 to 975°F.
Due to the extremely fine limits used on all threaded parts for
the Liberty engine, a large percentage of rejection was due to
warpage and scaling of parts. To eliminate this objection, many
of the Liberty engine builders adopted the use of heat-treated
and cold-drawn alloy steel for their highly stressed parts. On
all sizes up to and including 3/8 in. in diameter, the physical
properties were secured by merely normalizing the hot-rolled bars
by heating to a temperature of from 1,525 to 1,575°F., and cooling
in air, followed by the usual cold-drawing reductions. For parts
requiring stock over 3/8 in. in diameter, the physical properties
desired were obtained by quenching and tempering the hot-rolled bars
before cold-drawing. It is the opinion that the use of heat-treated
and cold-drawn bars is very good practice, provided proper inspection
is made to guarantee the uniformity of heat treatment and, therefore,
the uniformity of the physical properties of the finished parts.
The question has been asked many times by different manufacturers, as
to which alloy steel offers the best machineability when heat-treated
to a given Brinell hardness. The general consensus of opinion among
the screw-machine manufacturers is that S. A. E. No. 6,130 steel
gives the best machineability and that S. A. E. No. 2,330 steel
would receive second choice of the three specified.
In the finishing of highly stressed parts for aviation engines,
extreme care must be taken to see that all tool marks are eliminated,
unless they are parallel to the axis of strain, and that proper
radii are maintained at all changes of section. This is of the
utmost importance to give proper fatigue resistance to the part
in question.
GEARS
The material used for all gears on the Liberty engine was selected
at the option of the manufacturer from the following standard S.
A. E. steels, the composition of which are given in Table 12,
TABLE 12.--COMPOSITION OF STEELS NOS. X-3,340 AND 6,140
Steel No X-3,340 6,140
Carbon, minimum 0.350 0.350
Carbon, maximum 0.450 0.450
Manganese, minimum 0.450 0.500
Manganese, maximum 0.750 0.800
Phosphorus, maximum 0.040 0.040
Sulphur, maximum 0.045 0.045
Nickel, minimum 2.750
Nickel, maximum 3.250
Chromium, minimum 0.700 0.800
Chromium, maximum 0.950 1.100
Vanadium, minimum 0.150
All gears were heat-treated to a scleroscope hardness of from 55
to 55. The heat treatment used to secure this hardness consisted
in quenching the forgings from a temperature of 1,550 to 1,600°F.
in oil and annealing for good machineability at a temperature of
from 1,300 to 1,350°F. Forgings treated in this manner showed a
Brinell hardness of from 177 to 217.
RATE OF COOLING
At the option of the manufacturer, the above treatment of gear
forgings could be substituted by normalizing the forgings at a
temperature of from 1,550 to 1,600°F. The most important criterion
for proper normalizing, consisted in allowing the forgings to cool
through the critical temperature of the steel, at a rate not to exceed
50°F. per hour. For the two standard steels used, this consisted in
cooling from the normalizing temperature down to a temperature
of 1,100°F., at the rate indicated. Forgings normalized in this
manner will show a Brinell hardness of from 177 to 217. The question
has been repeatedly asked as to which treatment will produce the
higher quality finished part. In answer to this I will state that
on simple forgings of comparatively small section, the normalizing
treatment will produce a finished part which is of equal quality to
that of the quenched and annealed forgings. However, in the case of
complex forgings, or those of large section, more uniform physical
properties of the finished part will be obtained by quenching and
annealing the forgings in the place of normalizing.
The heat treatment of the finished gears consisted of quenching
in oil from a temperature of from 1,420 to 1,440°F. for the No.
X-3,340 steel, or from a temperature of from 1,500 to 1,540°F.
for No. 6,140 steel, followed by tempering in saltpeter or in an
electric furnace at a temperature of from 650 to 700°F.
The question has been asked by many engineers, why is the comparatively
low scleroscope hardness specified for gears? The reason for this is
that at best the life of an aviation engine is short, as compared with
that of an automobile, truck or tractor, and that shock resistance
is of vital importance. A sclerescope hardness of from 55 to 65
will give sufficient resistance to wear to prevent replacements
during the life of an aviation engine, while at the same time this
hardness produces approximately 50 per cent greater shock-resisting
properties to the gear. In the case of the automobile, truck or
tractor, resistance to wear is the main criterion and for that
reason the higher hardness is specified.
Great care should be taken in the design of an aviation engine
gear to eliminate sharp corners at the bottom of teeth as well
as in keyways. Any change of section in any stressed part of an
aviation engine must have a radius of at least 1/32 in. to give
proper shock and fatigue resistance. This fact has been demonstrated
many times during the Liberty engine program.
CONNECTING RODS
The material used for all connecting rods on the Liberty engine
was selected at the option of the manufacturer from one of two
standard S. A. E. steels, the composition of which are given in
Table 13.
TABLE 13.--COMPOSITION OF STEELS NOS. X-3,335 AND 6,135
Steel No. X-3,335 6,135
Carbon, minimum 0.300 0.300
Carbon, maximum 0.400 0.400
Manganese, minimum 0.450 0.500
Manganese, maximum 0.750 0.800
Phosphorus, maximum 0.040 0.040
Sulphur, maximum 0.045 0.045
Nickel, minimum 2.750
Nickel, maximum 3.250
Chromium, minimum 0.700 0.800
Chromium, maximum 0.950 1.100
Vanadium minimum 0.150
All connecting rods were heat-treated to show the following minimum
physical properties; Elastic limit, 105,000 lb. per square inch:
elongation in 2 in., 17.5; per cent, reduction of area 50.0; per
cent., Brinell hardness, 241 to 277.
The heat treatment used to secure these physical properties consisted
in normalizing the forgings at a temperature of from 1,550 to 1,600°F.,
followed by cooling in the furnace or in air. The forgings were then
quenched in oil from a temperature of from 1,420 to 1,440°F. for the
No. X-3,335 steel, or from a temperature of from 1,500 to 1,525°F.
for No. 6,135 steel, followed by tempering at a temperature of from
1,075 to 1,150°F. At the option of the manufacturer, the normalizing
treatment could be substituted by quenching the forgings from a
temperature of from 1,550 to 1,600°F., in oil, and annealing for
the best machineability at a temperature of from 1,300 to 1,350°F.
The double quench, however, did not prove satisfactory on No. X-3,335
steel, due to the fact that it was necessary to remove forgings
from the quenching bath while still at a temperature of from 300
to 500°F. to eliminate any possibility of cracking. In view of the
fact that this practice is difficult to carry out in the average
heat-treating plant, considerable trouble was experienced.
The most important criterion in the production of aviation engine
connecting rods is the elimination of burned or severely overheated
forgings. Due to the particular design of the forked rod, considerable
trouble was experienced in this respect because of the necessity
of reheating the forgings before they are completely forged. As
a means of elimination of burned forgings, test lugs were forged
on the channel section as well as on the top end of fork. After
the finish heat treatment, these test lugs were nicked and broken
and the fracture of the steel carefully examined. This precaution
made it possible to eliminate burned forgings as the test lugs were
placed on sections which would be most likely to become burned.
There is a great difference of opinion among engineers as to what
physical properties an aviation engine connecting rod should have.
Many of the most prominent engineers contend that a connecting rod
should be as stiff as possible. To produce rods in this manner in
any quantity, it is necessary for the final heat treatment to be made
on the semi-machined rod. This practice would make it necessary for a
larger percentage of the semi-machined rods to be cold-straightened
after the finish heat treatment. The cold-straightening operation
on a part having important functions to perform as a connecting
rod is extremely dangerous.
In view of the fact that a connecting rod functions as a strut,
it is considered that this part should be only stiff enough to
prevent any whipping action during the running of the engine. The
greater the fatigue-resisting property that one can put into the
rod after this stiffness is reached, the longer the life of the
rod will be. This is the reason for the Brinell limits mentioned
being specified.
In connection with the connecting rod, emphasis must be laid on the
importance of proper radii at all changes of section. The connecting
rods for the first few Liberty engines were machined with sharp
corners at the point where the connecting-rod bolt-head fits on
assembly. On the first long endurance test of a Liberty engine
equipped with rods of this type, failure resulted from fatigue
starting at this point. It is interesting to note that every rod on
the engine which did not completely fail at this point had started
to crack. The adoption of a 1/32-in. radius at this point completely
eliminated fatigue failures on Liberty rods.
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°F.,
followed by quenching in water at a temperature of from 1,475 to
1,525°F. and tempering at a temperature of from 1,000 to 1,100°F.
It is absolutely necessary that the crankshafts be removed from the
quenching tank before being allowed to cool below a temperature of
500°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°F. were followed by retempering
at a temperature of approximately 200°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°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.
PISTON PIN
The piston pin on an aviation engine must possess maximum resistance
to wear and to fatigue. For this reason, the piston pin is considered,
from a metallurgical standpoint, the most important part on the
engine to produce in quantities and still possess the above
characteristics. The material used for the Liberty engine piston
pin was S. A. E. No. 2315 steel, which is of the following chemical
composition: Carbon, 0.100 to 0.200 per cent; manganese, 0.500
to 0.800 per cent; phosphorus, 0.040 maximum per cent; sulphur,
0.045 maximum per cent; nickel, 3.250 to 3.750 per cent.
Each finished piston pin, after heat treatment, must show a minimum
scleroscope hardness of the case of 70, a scleroscope hardness of
the core of from 35 to 55 and a minimum crushing strength when
supported as a beam and the load applied at the center of 35,000
lb. The heat treatment used to obtain the above physical properties
consisted in carburizing at a temperature not to exceed 1,675°F.,
for a sufficient length of time to secure a case of from 0.02 to
0.04 in. deep. The pins are then allowed to cool slowly from the
carbonizing heat, after which the hole is finish-machined and the
pin cut to length. The finish heat treatment of the piston pin
consisted in quenching in oil from a temperature of from 1,525 to
1,575°F. to refine the grain of core properly and then quenching in
oil at a temperature of from 1,340 to 1,380°F. to refine and harden
the grain of the case properly, as well as to secure proper hardness
of core. After this quenching, all piston pins are tempered in oil
at a temperature of from 375 to 400°F. A 100 per cent inspection
for scleroscope hardness of the case and the core was made, and
no failures were ever recorded when the above material and heat
treatment was used.
APPLICATION TO THE AUTOMOTIVE INDUSTRY
The information given on the various parts of the Liberty engine
applies with equal force to the corresponding parts in the construction
of an automobile, truck or tractor. We recommend as first choice for
carbon-steel screw-machine parts material produced by the basic
open hearth process and having the following chemical composition;
Carbon, 0.150 to 0.250 per cent; manganese, 0.500 to 0.800 per
cent; phosphorus, 0.045 maximum per cent; sulphur, 0.075 to 0.150
per cent.
This material is very uniform and is nearly as free cutting as
bessemer screw stock. It is sufficiently uniform to be used for
unimportant carburized parts, as well as for non-heat-treated
screw-machine parts. A number of the large automobile manufacturers
are now specifying this material in preference to the regular bessemer
grades.
As second choice for carbon-steel screw-machine parts we recommend
ordinary bessemer screw stock, purchased in accordance with S. A.
E. specification No. 1114. The advantage of using No. 1114 steel
lies in the fact that the majority of warehouses carry standard
sizes of this material in stock at all times. The disadvantage
of using this material is due to its lack of uniformity.
The important criterion for transmission gears is resistance to
wear. To secure proper resistance to wear a Brinell hardness of
from 512 to 560 must be obtained. The material selected to obtain
this hardness should be one which can be made most nearly uniform,
will undergo forging operations the easiest, will be the hardest
to overheat or burn, will machine best and will respond to a good
commercial range of heat treatment.
It is a well-known fact that the element chromium, when in the form
of chromium carbide in alloy steel, offers the greatest resistance to
wear of any combination yet developed. It is also a well-known fact that
the element nickel in steel gives excellent shock-resisting properties
as well as resistance to wear but not nearly as great a resistance
to wear as chromium. It has been standard practice for a number of
years for many manufacturers to use a high nickel-chromium steel
for transmission gears. A typical nickel-chromium gear specification
is as follows: Carbon, 0.470 to 0.520 per cent; manganese, 0.500
to 0.800 per cent; phosphorus, 0.040 maximum per cent; sulphur,
0.045 maximum per cent; chromium, 0.700 to 0.950 per cent.
There is no question but that a gear made from material of such an
analysis will give excellent service. However, it is possible to
obtain the same quality of service and at the same time appreciably
reduce the cost of the finished part. The gear steel specified is
of the air-hardening type. It is extremely sensitive to secondary
pipe, as well as seams, and is extremely difficult to forge and
very easy to overheat. The heat-treatment range is very wide, but
the danger from quenching cracks is very great. In regard to the
machineability, this material is the hardest to machine of any
alloy steel known.
COMPOSITION OF TRANSMISSION-GEAR STEEL
If the nickel content of this steel is eliminated, and the percentage
of chromium raised slightly, an ideal transmission-gear material is
obtained. This would, therefore, be of the following composition:
Carbon, 0.470 to 0.520 per cent; manganese, 0.500 to 0.800 per
cent; phosphorus, 0.040 maximum per cent; sulphur, 0.045 maximum
per cent; chromium, 0.800 to 1.100 per cent.
The important criterion in connection with the use of this material
is that the steel be properly deoxidized, either through the use
of ferrovanadium or its equivalent. Approximately 2,500 sets of
transmission gears are being made daily from material of this analysis
and are giving entirely satisfactory results in service. The heat
treatment of the above material for transmission gears is as follows:
"Normalize forgings at a temperature of from 1,5.50 to 1,600°F.
Cool from this temperature to a temperature of 1,100°F. at the
rate of 50° per hour. Cool from 1,100°F., either in air or quench
in water."
Forgings so treated will show a Brinell hardness of from 177 to
217, which is the proper range for the best machineability. The
heat treatment of the finished gears consists of quenching in oil
from a temperature of 1,500 to 1,540°F., followed by tempering
in oil at a temperature of from 375 to 425°F. Gears so treated
will show a Brinell hardness of from 512 to 560, or a scleroscope
hardness of from 72 to 80. One tractor builder has placed in service
20,000 sets of gears of this type of material and has never had to
replace a gear. Taking into consideration the fact that a tractor
transmission is subjected to the worst possible service conditions,
and that it is under high stress 90 per cent of the time, it seems
inconceivable that any appreciable transmission trouble would be
experienced when material of this type is used on an automobile,
where the full load is applied not over 1 per cent of the time,
or on trucks where the full load is applied not over 50 per cent
of the time.
The gear hardness specified is necessary to reduce to a minimum
the pitting or surface fatigue of the teeth. If gears having a
Brinell hardness of over 560 are used, danger is encountered, due
to low shock-resisting properties. If the Brinell hardness is under
512, trouble is experienced due to wear and surface fatigue of
the teeth.
For ring gears and pinions material of the following chemical
composition is recommended: Carbon, 0.100 to 0.200 per cent; manganese,
0.350 to 0.650 per cent; phosphorus, 0.040 maximum per cent; sulphur,
0.045 maximum per cent; chromium, 0.550 to 0.750 per cent; nickel,
0.400 to 0.600 per cent.
Care should be taken to see that this material is properly deoxidized
either by the use of ferrovanadium or its equivalent. The advantage
of using a material of the above type lies in the fact that it will
produce a satisfactory finished part with a very simple treatment.
The heat treatment of ring gears and pinions is as follows: "Carburize
at a temperature of from 1,650 to 1,700°F. for a sufficient length
of time to secure a depth of case of from 1/32 to 3/64 in., and
quench directly from carburizing heat in oil. Reheat to a temperature
of from 1,430 to 1,460°F. and quench in oil. Temper in oil at a
temperature of from 375 to 425°F. The final quenching operation
on a ring gear should be made on a fixture similar to the Gleason
press to reduce distortion to a minimum."
One of the largest producers of ring gears and pinions in the automotive
industry has been using this material and treatment for the last 2
years, and is of the opinion that he is now producing the highest
quality product ever turned out by that plant.
On some designs of automobiles a large amount of trouble is experienced
with the driving pinion. If the material and heat treatment specified
will not give satisfaction, rather than to change the design it is
possible to use the following analysis material, which will raise
the cost of the finished part but will give excellent service:
Carbon, 0.100 to 0.200 per cent; manganese, 0.350 to 0.650 per
cent; phosphorus, 0.040 maximum per cent; sulphur, 0.045 maximum
per cent; nickel, 4.750 to 5.250 per cent.
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.
Steel for making balls of average size is made up of:
Carbon 0.95 to 1.05 per cent
Silicon 0.20 to 0.35 per cent
Manganese 0.30 to 0.45 per cent
Chromium 0.35 to 0.45 per cent
Sulphur and phosphorus not to exceed 0.025 per cent
For the larger sizes a typical analysis is:
Carbon 1.02 per cent
Silicon 0.21 per cent
Manganese 0.40 per cent
Chromium 0.65 per cent
Sulphur 0.026 per cent
Phosphorus 0.014 per cent
Balls 5/8 in. and below are formed cold on upsetting or heading
machines, the stock use is as follows:
TABLE 14.--SIZES OF STOCK FOR FORMING BALLS ON HEADER
-------------------------------------------------------
Diameter of | Diameter of | Diameter of | Diameter of
ball, inch | stock inch | ball, inch | stock, inch
-------------|-------------|-------------|-------------
1/8 | 0.100 | 5/16 | 0.235
5/32 | 0.120 | 3/8 | 0.275
3/16 | 0.145 | 7/16 | 0.320
7/32 | 0.170 | 1/2 | 0.365
1/4 | 0.190 | 9/16 | 0.395
9/32 | 0.220 | 5/8 | 0.440
-------------------------------------------------------
For larger balls the blanks are hot-forged from straight bars.
They are usually forged in multiples of four under a spring hammer
and then separated by a suitable punching or shearing die in a
press adjoining the hammer. The dimensions are:
-----------------------------------------------------------
Diameter of ball, | Diameter of die, | Diameter of stock,
inch | inch | inch
-------------------|------------------|--------------------
3/4 | 0.775 | 0.625
7/8 | 0.905 | 0.729
1 | 1.035 | 0.823
-----------------------------------------------------------
Before hardening, the balls are annealed to relieve the stresses
of forging and grinding, this being done by passing them through a
revolving retort made of nichrome or other heat-resisting substance.
The annealing temperature is 1,300°F.
The hardening temperature is from 1,425 to 1,475°F. according to
size and composition of steel. Small balls, 5/16 and under, are
quenched in oil, the larger sizes in water. In some special cases
brine is used. Quenching small balls in water is too great a shock
as the small volume is cooled clear through almost instantly. The
larger balls have metal enough to cool more slowly.
Balls which are cooled in either water or brine are boiled in water
for 2 hr. to relieve internal stresses, after which the balls are
finished by dry-grinding and oil-grinding.
The ball makers have an interesting method of testing stock for
seams which do not show in the rod or wire. The Hoover Steel Ball
Company cut off pieces of rod or wire 7/16 in. long and subject
them to an end pressure of from 20,000 to 50,000 lb. A pressure
of 20,000 lb. compresses the piece to 3/16 in. and the 50,000 lb.
pressure to 3/32 in. This opens any seam which may exist but a
solid bar shows no seam.
Another method which has proved very successful is to pass the
bar or rod to be tested through a solenoid electro-magnet. With
suitable instruments it is claimed that this is an almost infallible
test as the instruments show at once when a seam or flaw is present
in the bar.