The Working of Steel Part 7

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 1,600F.

in oil and annealing for good machineability at a temperature of from 1,300 to 1,350F. 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 subst.i.tuted by normalizing the forgings at a temperature of from 1,550 to 1,600F. 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 50F. per hour. For the two standard steels used, this consisted in cooling from the normalizing temperature down to a temperature of 1,100F., 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 1,440F. for the No.

X-3,340 steel, or from a temperature of from 1,500 to 1,540F.

for No. 6,140 steel, followed by tempering in saltpeter or in an electric furnace at a temperature of from 650 to 700F.

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 1,600F., followed by cooling in the furnace or in air. The forgings were then quenched in oil from a temperature of from 1,420 to 1,440F. for the No. X-3,335 steel, or from a temperature of from 1,500 to 1,525F.

for No. 6,135 steel, followed by tempering at a temperature of from 1,075 to 1,150F. At the option of the manufacturer, the normalizing treatment could be subst.i.tuted by quenching the forgings from a temperature of from 1,550 to 1,600F., in oil, and annealing for the best machineability at a temperature of from 1,300 to 1,350F.

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 500F. 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 quant.i.ty, 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 a.s.sembly. 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 1,600F., followed by quenching in water at a temperature of from 1,475 to 1,525F. and tempering at a temperature of from 1,000 to 1,100F.

It is absolutely necessary that the crankshafts be removed from the quenching tank before being allowed to cool below a temperature of 500F., 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 500F. were followed by retempering at a temperature of approximately 200F. below the original tempering temperature.

Another ill.u.s.tration 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 a.n.a.lysis 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 1,100F. 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 quant.i.ties 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 crus.h.i.+ng 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 1,675F., 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 1,575F. to refine the grain of core properly and then quenching in oil at a temperature of from 1,340 to 1,380F. 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 400F. 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 a.n.a.lysis 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 a.n.a.lysis 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 1,600F.

Cool from this temperature to a temperature of 1,100F. at the rate of 50 per hour. Cool from 1,100F., 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 1,540F., followed by tempering in oil at a temperature of from 375 to 425F. 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.