The original production system as outlined for the manufacturers had called for a heat treatment in the rough-forged state for the connecting rods, and then semi-machining the rod forgings before giving them the final treatment. The Lincoln Motor Company insisted from the first that the proper method would be a complete heat treatment of the forging in the rough state, and machining the rod after the heat treatment. After a number of trial lots, the Signal Corps acceded to the request and production was immediately increased and quality benefited by the change. This method was later included in a revised specification issued to all producers.

The original system was one that required a great deal of labor per unit output. The Lincoln organization developed a method of handling connecting rods whereby five workmen accomplished the same result that would have required about 30 or 32 by the original method. Even after revising the specification so as to allow complete heat treatments in the rough-forged state, the ordinary methods employed in heat-treating would have required 12 to 15 men. With the fixtures employed, five men could handle 1,300 connecting rods, half of which are plain and half, forked, in a working period of little over 7 hr.



The increase in production was gained by devising fixtures which enabled fewer men to handle a greater quantity of parts with less effort and in less time.

In heat-treating the forgings were laid on a rack or loop A, Fig. 14, made of 1-1/4-in. double extra-heavy pipe, bent up with parallel sides about 9 in. apart, one end being bent straight across and the other end being bent upward so as to afford an easy grasp for the hook. Fifteen rods were laid on each loop, there being four loops of rods charged into a furnace with a hearth area of 36 by 66 in. The rods were charged at a temperature of approximately 900 deg.F. They were heated for refining over a period of 3 hr. to 1,625 deg.F., soaked 15 min, at this degree of heat and quenched in soluble quenching oil.

In pulling the heat to quench the rods, the furnace door was raised and the operator pulls one of the loops A, Fig. 15 forward to the shelf of the furnace, supporting the straight end of the loop by means of the porter bar B. They swung the loop of rods around from the furnace shelf and set the straight end of the loop on the edge of the quenching tank, then raise the curved end C, by means of their hook D so that all the rods on the loop slide into the oil bath.

Before the rods cooled entirely, the baskets in the quenching tank were raised and the oil allowed to partly drain off the forgings, and they were stacked on curved-end loops or racks and charged into the furnace for the second or hardening heat. The temperature of the furnace was raised in 1-1/2 hr. to 1,550 deg.F., the rods soaked for 15 min. at this degree of heat and quenched in the same manner as above.

They were again drained while yet warm, placed on loops and charged into the furnace for the third or tempering heat. The temperature of the furnace was brought to 1,100 deg.F. in 1 hr., and the rods soaked at this degree of heat for 1 hr. They were then removed from the furnace the same as for quenching, but were dumped onto steel platforms instead of into the quenching oil, and allowed to cool on these steel platforms down to the room temperature.

PICKLING THE FORGINGS

The forgings were then pickled in a hot solution of either niter cake or sulphuric acid and water at a temperature of 170 deg.F., and using a solution of about 25 per cent. The solution was maintained at a constant point by taking hydrometer readings two or three times a day, maintaining a reading of about 1.175. Sixty forked or one hundred single rods were placed in wooden racks and immersed in a lead-lined vat 30 by 30 by 5 ft. long. The rack was lowered or lifted by means of an air hoist and the rods were allowed to stay in solution from 1/2 to 1 hr., depending on the amount of scale. The rods were then swung and lowered in the rack into running hot water until all trace of the acid was removed.

The rod was finally subjected to Brinell test. This shows whether or not the rod has been heat-treated to the proper hardness. If the rods did not read between 241 and 277, they were re-treated until the proper hardness is obtained.



CHAPTER IV

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 deg.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 deg.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 deg.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 deg.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 deg.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 deg.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 deg.F., followed by tempering to meet proper Brinell requirements by heating to a temperature of 1,150 to 1,200 deg.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 deg.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 deg.F., in oil, followed by tempering at a temperature of from 925 to 975 deg.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 deg.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 deg.F. in oil and annealing for good machineability at a temperature of from 1,300 to 1,350 deg.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 deg.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 deg.F. per hour. For the two standard steels used, this consisted in cooling from the normalizing temperature down to a temperature of 1,100 deg.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 deg.F. for the No. X-3,340 steel, or from a temperature of from 1,500 to 1,540 deg.F. for No. 6,140 steel, followed by tempering in saltpeter or in an electric furnace at a temperature of from 650 to 700 deg.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 deg.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 deg.F. for the No. X-3,335 steel, or from a temperature of from 1,500 to 1,525 deg.F. for No. 6,135 steel, followed by tempering at a temperature of from 1,075 to 1,150 deg.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 deg.F., in oil, and annealing for the best machineability at a temperature of from 1,300 to 1,350 deg.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 deg.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 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.

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 deg.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 deg.F. to refine the grain of core properly and then quenching in oil at a temperature of from 1,340 to 1,380 deg.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 deg.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 deg.F. Cool from this temperature to a temperature of 1,100 deg.F. at the rate of 50 deg. per hour. Cool from 1,100 deg.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 deg.F., followed by tempering in oil at a temperature of from 375 to 425 deg.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 deg.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 deg.F. and quench in oil. Temper in oil at a temperature of from 375 to 425 deg.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 deg.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 deg.F. to refine the grain of the core and quenched in oil from a temperature of from 1,340 to 1,360 deg.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 deg.F. in water and tempering at a temperature of from 975 to 1,025 deg.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 deg.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 deg.F.

The hardening temperature is from 1,425 to 1,475 deg.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.



CHAPTER V

THE FORGING OF STEEL

So much depends upon the forging of steel that this operation must be carefully supervised. This is especially true because of the tendency to place unskilled and ignorant men as furnace-tenders and hammer men. The main points to be supervised are the slow and careful heating to the proper temperature; forging must be continued at a proper rate to the correct temperature. The bar of stock from which a forging was made may have had a fairly good structure, but if the details of the working are not carefully watched, a seamy, split article of no value may easily result.

HEATING.—Although it is possible to work steels cold, to an extent depending upon their ductility, and although such operations are commonly performed, "forging" usually means working heated steel. Heating is therefore a vital part of the process.

Heating should be done slowly in a soaking heat. A soft "lazy" flame with excess carbon is necessary to avoid burning the corners of the bar or billet, and heavily scaling the surface. If the temperature is not raised slowly, the outer part of the metal may be at welding heat while the inner part is several hundred degrees colder and comparatively hard and brittle.

The above refers to muffle furnaces. If the heating is done in a small blacksmith's forge, the fire should be kept clean, and remade at intervals of about two hours. Ashes and cinders should be cleaned from the center down to the tuyere and oily waste and wood used to start a new fire. As this kindles a layer of coke from the old fire is put on top, and another layer of green coal (screened and dampened blacksmiths' coal) as a cover. When the green coal on top has been coked the fire is ready for use. As the fuel burns out in the center, the coke forming around the edge is pushed inward, and its place taken by more green coal. Thus the fire is made up of three parts; the center where coke is burning and the iron heating; a zone where coke is forming, and the outside bank of green coal.

STEEL WORKED IN AUSTENITIC STATE.—As a general rule steel should be worked when it is in the austenitic state. (See page 108.) It is then soft and ductile.

As the steel is heated above the critical temperature the size of the austenite crystals tends to grow rapidly. When forging starts, however, these grains are broken up. The growth is continually destroyed by the hammering, which should consequently be continued down to the upper critical temperature when the austenite crystals break up into ferrite and cementite. The size of the final grains will be much smaller and hence a more uniform structure will result if the "mother" austenite was also fine grained. A final steel will be composed of pearlite; ferrite and pearlite; or cementite and pearlite, according to the carbon content.

The ultimate object is to secure a fine, uniform grain throughout the piece and this can be secured by uniform heating and by thoroughly rolling it or working it at a temperature just down to its critical point. If this is correctly done the fracture will be fine and silky. Steel which has been overheated slightly and the forging stopped at too high a temperature will show a "granular" fracture. A badly overheated or "burned" steel will have iridescent colors on a fresh fracture, it will be brittle both hot and cold, and absolutely ruined.

STEEL CAN BE WORKED COLD.—As noted above, steel can be worked cold, as in the case of cold-rolled steel. Heat treatment of cold-worked steel is a very delicate operation. Cold working hardens and strengthens steel. It also introduces internal stresses. Heat-treatments are designed to eliminate the stresses without losing the hardness and strength. This is done by tempering at a low heat. Avoid the "blue" range (350 to 750 deg.C.). Tempering for a considerable time just under the critical is liable to cause great brittleness. Annealing (reheating through the critical) destroys the effect of cold work.

FORGING

HIGH-SPEED STEEL.—Heat very slowly and carefully to from 1,800 to 2,000 deg.F. and forge thoroughly and uniformly. If the forging operation is prolonged do not continue forging the tool when the steel begins to stiffen under the hammer. Do not forge below 1,700 deg.F. (a dark lemon or orange color). Reheat frequently rather than prolong the hammering at the low heats.

After finishing the forging allow the tool to cool as slowly as possible in lime or dry ashes; avoid placing the tool on the damp ground or in a draught of air. Use a good clean fire for heating. Do not allow the tool to soak at the forging heat. Do not heat any more of the tool than is necessary in order to forge it to the desired shape.

CARBON TOOL STEEL.—Heat to a bright red, about 1,500 to 1,550 deg.F. Do not hammer steel when it cools down to a dark cherry red, or just below its hardening point, as this creates surface cracks.

OIL-HARDENING STEEL.—Heat slowly and uniformly to 1,450 deg.F. and forge thoroughly. Do not under any circumstances attempt to harden at the forging heat. After cooling from forging reheat to about 1,450 deg.F. and cool slowly so as to remove forging strains.

CHROME-NICKEL STEEL.—Forging heat of chrome-nickel steel depends very largely on the percentage of each element contained in the steel. Steel containing from 1/2 to 1 per cent chromium and from 1-1/2 to 3-1/2 per cent nickel, with a carbon content equal to the chromium, should be heated very slowly and uniformly to approximately 1,600 deg. F., or salmon color. After forging, reheat the steel to about 1,450 deg. and cool slowly so as to remove forging strains. Do not attempt to harden the steel before such annealing.

A great deal of steel is constantly being spoiled by carelessness in the forging operation. The billets may be perfectly sound, but even if the steel is heated to a good forging heat, and is hammered too lightly, a poor forging results. A proper blow will cause the edges and ends to bulge slightly outwards—the inner-most parts of the steel seem to flow faster than the surface. Light blows will work the surface out faster; the edges and ends will curve inwards. This condition in extreme cases leaves a seam in the axis of the forging.

Steel which is heated quickly and forging begun before uniform heat has penetrated to its center will open up seams because the cooler central portion is not able to flow with the hot metal surrounding it. Uniform heating is absolutely necessary for the best results.

Figure 16 shows a sound forging. The bars in Fig. 17 were burst by improper forging, while the die, Fig. 18, burst from a piped center.

Figure 19 shows a piece forged with a hammer too light for the size of the work. This gives an appearance similar to case-hardening, the refining effect of the blows reaching but a short distance from the surface.

While it is impossible to accurately rate the capacity of steam hammers with respect to the size of work they should handle, on account of the greatly varying conditions, a few notes from the experience of the Bement works of the Niles-Bement-Pond Company will be of service.



For making an occasional forging of a given size, a smaller hammer may be used than if we are manufacturing this same piece in large quantities. If we have a 6-in. piece to forge, such as a pinion or a short shaft, a hammer of about 1,100-lb. capacity would answer very nicely. But should the general work be as large as this, it would be very much better to use a 1,500-lb. hammer. If, on the other hand, we wish to forge 6-in. axles economically, it would be necessary to use a 7,000- or 8,000-lb. hammer. The following table will be found convenient for reference for the proper size of hammer to be used on different classes of general blacksmith work, although it will be understood that it is necessary to modify these to suit conditions, as has already been indicated.



Diameter of stock Size of hammer 3-1/2 in. 250 to 350 lb. 4 in. 350 to 600 lb. 4-1/2 in. 600 to 800 lb. 5 in. 800 to 1,000 lb. 6 in. 1,100 to 1,500 lb.

Steam hammers are always rated by the weight of the ram, and the attached parts, which include the piston and rod, nothing being added on account of the steam pressure behind the piston. This makes it a little difficult to compare them with plain drop or tilting hammers, which are also rated in the same way.



Steam hammers are usually operated at pressures varying from 75 to 100 lb. of steam per square inch, and may also be operated by compressed air at about the same pressures. It is cheaper, however, in the case of compressed air to use pressures from 60 to 80 lb. instead of going higher.

Forgings must, however, be made from sound billets if satisfactory results are to be secured. Figure 20 shows three cross-sections of which A is sound, B is badly piped and C is worthless.

PLANT FOR FORGING RIFLE BARRELS

The forging of rifle barrels in large quantities and heat-treating them to meet the specifications demanded by some of the foreign governments led Wheelock, Lovejoy & Company to establish a complete plant for this purpose in connection with their warehouse in Cambridge, Mass. This plant, designed and constructed by their chief engineer, K. A. Juthe, had many interesting features. Many features of this plant can be modified for other classes of work.



The stock, which came in bars of mill length, was cut off so as to make a barrel with the proper allowances for trimming (Fig. 21). They then pass to the forging or upsetting press in the adjoining room. This press, which is shown in more detail in Fig. 22, handled the barrels from all the heating furnaces shown. The men changed work at frequent intervals, to avoid excessive fatigue.



Then the barrels were reheated in the continuous furnace, shown in Fig. 23, and straightened before being tested.

The barrels were next tested for straightness. After the heat-treating, the ends are ground, a spot ground on the enlarged end and each barrel tested on a Brinell machine. The pressure used is 3,000 kg., or 6,614 lb., on a 10-millimeter ball, which is standard. Hardness of 240 was desired.

The heat-treating of the rifle blanks covered four separate operations: (1) Heating and soaking the steel above the critical temperature and quenching in oil to harden the steel through to the center; (2) reheating for drawing of temper for the purpose of meeting the physical specifications; (3) reheating to meet the machine ability test for production purposes; and (4) reheating to straighten the blanks while hot.

A short explanation of the necessity for the many heats may be interesting. For the first heat, the blanks were slowly brought to the required heat, which is about 150 deg.F. above the critical temperature. They are then soaked at a high heat for about 1 hr. before quenching. The purpose of this treatment is to eliminate any rolling or heat stresses that might be in the bars from mill operations; also to insure a thorough even heat through a cross-section of the steel. This heat also causes blanks with seams or slight flaws to open up in quenching, making detection of defective blanks very easy.

The quenching oil was kept at a constant temperature of 100 deg.F., to avoid subjecting the steel to shocks, thereby causing surface cracks. The drawing of temper was the most critical operation and was kept within a 10 deg. fluctuation. The degree of heat necessary depends entirely on the analysis of the steel, there being a certain variation in the different heats of steel as received from the mill.

MACHINEABILITY

Reheating for machine ability was done at 100 deg. less than the drawing temperature, but the time of soaking is more than double. After both drawing and reheating, the blanks were buried in lime where they remain, out of contact with the air, until their temperature had dropped to that of the workroom.

For straightening, the barrels were heated to from 900 to 1,000 deg.F. in an automatic furnace 25 ft. long, this operation taking about 2 hr. The purpose of hot straightening was to prevent any stresses being put into the blanks, so that after rough-turning, drilling or rifling operations they would not have a tendency to spring back to shape as left by the quenching bath.

A method that produces an even better machining rifle blank, which practically stays straight through the different machining operations, was to rough-turn the blanks, then subject them to a heat of practically 1,0000 for 4 hr. Production throughout the different operations is materially increased, with practically no straightening required after drilling, reaming, finish-turning or rifling operations.



This method was tested out by one of the largest manufacturers and proved to be the best way to eliminate a very expensive finished gun-barrel straightening process.



The heat-treating required a large amount of cooling oil, and the problem of keeping this at the proper temperature required considerable study. The result was the cooling plant on the roof, as shown in Figs. 24, 25 and 26. The first two illustrations show the plant as it appeared complete. Figure 26 shows how the oil was handled in what is sometimes called the ebulator system. The oil was pumped up from the cooling tanks through the pipe A to the tank B. From here it ran down onto the breakers or separators C, which break the oil up into fine particles that are caught by the fans D. The spray is blown up into the cooling tower E, which contains banks of cooling pipes, as can be seen, as well as baffies F. The spray collects on the cool pipes and forms drops, which fall on the curved plates G and run back to the oil-storage tank below ground.

The water for this cooling was pumped from 10 artesian wells at the rate of 60 gal. per minute and cooled 90 gal. of oil per minute, lowering the temperature from 130 or 140 to 100 deg.F. The water as it came from the wells averaged around 52 deg.F. The motor was of a 7-1/2-hp. variable-speed type with a range of from 700 to 1,200 r.p.m., which could be varied to suit the amount of oil to be cooled. The plant handled 300 gal. of oil per minute.



CHAPTER VI

ANNEALING

There is no mystery or secret about the proper annealing of different steels, but in order to secure the best results it is absolutely necessary for the operator to know the kind of steel which is to be annealed. The annealing of steel is primarily done for one of three specific purposes: To soften for machining purposes; to change the physical properties, largely to increase ductility; or to release strains caused by rolling or forging.

Proper annealing means the heating of the steel slowly and uniformly to the right temperature, the holding of the temperature for a given period and the gradual cooling to normal temperature. The proper temperature depends on the kind of steel, and the suggestions of the maker of the special steel being used should be carefully followed. For carbon steel the temperatures recommended for annealing vary from 1,450 to 1,600 deg.F. This temperature need not be long continued. The steel should be cooled in hot sand, lime or ashes. If heated in the open forge the steel should be buried in the cooling material as quickly as possible, not allowing it to remain in the open air any longer than absolutely necessary. Best results, however, are secured when the fire does not come in direct contact with the steel.

Good results are obtained by packing the steel in iron boxes or tubes, much as for case-hardening or carbonizing, using the same materials. Pieces do not require to be entirely surrounded by carbon for annealing, however. Do not remove from boxes until cold.

Steel to be annealed may be classified into four different groups, each of which must be treated according to the elements contained in its particular analysis. Different methods are therefore necessary to bring about the desired result. The classifications are as follows: High-speed steel, alloy steel, tool or crucible steel, and high-carbon machinery steel.

ANNEALING OF HIGH-SPEED STEEL

For annealing high-speed steel, some makers recommend using ground mica, charcoal, lime, fine dry ashes or lake sand as a packing in the annealing boxes. Mixtures of one part charcoal, one part lime and three parts of sand are also suggested, or two parts of ashes may be substituted for the one part of lime.

To bring about the softest structure or machine ability of high-speed steel, it should be packed in charcoal in boxes or pipes, carefully sealed at all points, so that no gases will escape or air be admitted. It should be heated slowly to not less than 1,450 deg.F. and the steel must not be removed from its packing until it is cool. Slow heating means that the high heat must have penetrated to the very core of the steel.

When the steel is heated clear through it has been in the furnace long enough. If the steel can remain in the furnace and cool down with it, there will be no danger of air blasts or sudden or uneven cooling. If not, remove the box and cover quickly with dry ashes, sand or lime until it becomes cold.

Too high a heat or maintaining the heat for too long a period, produces a harsh, coarse grain and greatly increases the liability to crack in hardening. It also reduces the strength and toughness of the steel.

Steel which is to be used for making tools with teeth, such as taps, reamers and milling cutters, should not be annealed too much. When the steel is too soft it is more apt to tear in cutting and makes it more difficult to cut a smooth thread or other surface. Moderate annealing is found best for tools of this kind.

TOOL OR CRUCIBLE STEEL

Crucible steel can be annealed either in muffled furnace or by being packed. Packing is by far the most satisfactory method as it prevents scaling, local hard spots, uneven annealing, or violent changes in shape. It should be brought up slowly to just above its calescent or hardening temperature. The operator must know before setting his heats the temperature at which the different carbon content steels are hardened. The higher the carbon contents the lower is the hardening heat, but this should in no case be less than 1,450 deg.F.

ANNEALING ALLOY STEEL

The term alloy steel, from the steel maker's point of view, refers largely to nickel and chromium steel or a combination of both. These steels are manufactured very largely by the open-hearth process, although chromium steels are also a crucible product. It is next to impossible to give proper directions for the proper annealing of alloy steel unless the composition is known to the operator.

Nickel steels may be annealed at lower temperatures than carbon steels, depending upon their alloy content. For instance, if a pearlitic carbon steel may be annealed at 1,450 deg.C., the same analysis containing 2-1/2 per cent nickel may be annealed at 1,360 deg.C. and a 5 per cent nickel steel at 1,270 deg..

In order that high chromium steels may be readily machined, they must be heated at or slightly above the critical for a very long time, and cooled through the critical at an extremely slow rate. For a steel containing 0.9 to 1.1 per cent carbon, under 0.50 per cent manganese, and about 1.0 per cent chromium, Bullens recommends the following anneal:

1. Heat to 1,700 or 1,750 deg.F. 2. Air cool to about 800 deg.F. 3. Soak at 1,425 to 1,450 deg.F. 4. Cool slowly in furnace.

HIGH-CARBON MACHINERY STEEL

The carbon content of this steel is above 30 points and is hardly ever above 60 points or 0.60 per cent. Annealing such steel is generally in quantity production and does not require the care that the other steels need because it is very largely a much cheaper product and a great deal of material is generally removed from the outside surface.

The purpose for which this steel is annealed is a deciding factor as to what heat to give it. If it is for machineability only, the steel requires to be brought up slowly to just below the critical and then slowly cooled in the furnace or ash pit. It must be thoroughly covered so that there will be no access of cool air. If the annealing is to increase ductility to the maximum extent it should be slowly heated to slightly over the upper critical temperature and kept at this heat for a length of time necessary for a thorough penetration to the core, after which it can be cooled to about 1,200 deg.F., then reheated to about 1,360 deg.F., when it can be removed and put in an ash pit or covered with lime. If the annealing is just to relieve strains, slow heating is not necessary, but the steel must be brought up to a temperature not much less than a forging or rolling heat and gradually cooled. Covering in this case is only necessary in steel of a carbon content of more than 40 points.

ANNEALING IN BONE

Steel and cast iron may both be annealed in granulated bone. Pack the work the same as for case-hardening except that it is not necessary to keep the pieces away from each other. Pack with bone that has been used until it is nearly white. Heat as hot as necessary for the steel and let the furnace cool down. If the boxes are removed from furnace while still warm, cover boxes and all in warm ashes or sand, air slaked lime or old, burned bone to retain heat as long as possible. Do not remove work from boxes until cold.

ANNEALING OF RIFLE COMPONENTS AT SPRINGFIELD ARMORY

In general, all forgings of the components of the arms manufactured at the Armory and all forgings for other ordnance establishments are packed in charcoal, lime or suitable material and annealed before being transferred from the forge shop.

Except in special cases, all annealing will be done in annealing pots of appropriate size. One fire end of a thermo-couple is inserted in the center of the annealing pot nearest the middle of the furnace and another in the furnace outside of but near the annealing pots.

The temperatures used in annealing carbon steel components of the various classes used at the Armory vary from 800 deg.C. To 880 deg.C. or 1,475 to 1,615 deg.F.

The fuel is shut off from the annealing furnace gradually as the temperature of the pot approaches the prescribed annealing temperature so as to prevent heating beyond that temperature.

The forgings of the rifle barrel and the pistol barrel are exceptions to the above general rule. These forgings will be packed in lime and allowed to cool slowly from the residual heat after forging.



CHAPTER VII

CASE-HARDENING OR SURFACE-CARBURIZING

Carburizing, commonly called case-hardening, is the art of producing a high-carbon surface, or case, upon a low carbon steel article. Wrenches, locomotive link motions, gun mechanisms, balls and ball races, automobile gears and many other devices are thereby given a high-carbon case capable of assuming extreme hardness, while the interior body of metal, the core, remains soft and tough.

The simplest method is to heat the piece to be hardened to a bright red, dip it in cyanide of potassium (or cover it by sprinkling the cyanide over it), keep it hot until the melted cyanide covers it thoroughly, and quench in water. Carbon and nitrogen enter the outer skin of the steel and harden this skin but leave the center soft. The hard surface or "case" varies in thickness according to the size of the piece, the materials used and the length of time which the piece remains at the carburizing temperature. Cyanide case-hardening is used only where a light or thin skin is sufficient. It gives a thickness of about 0.002 in.

In some cases of cyanide carburizing, the piece is heated in cyanide to the desired temperature and then quenched. For a thicker case the steel is packed in carbon materials of various kinds such as burnt leather scraps, charcoal, granulated bone or some of the many carbonizing compounds.

Machined or forged steel parts are packed with case-hardening material in metal boxes and subjected to a red heat. Under such conditions, carbon is absorbed by the steel surfaces, and a carburized case is produced capable of responding to ordinary hardening and tempering operations, the core meanwhile retaining its original softness and toughness.

Such case-hardened parts are stronger, cheaper, and more serviceable than similar parts made of tool steel. The tough core resists breakage by shock. The hardened case resists wear from friction. The low cost of material, the ease of manufacture, and the lessened breakage in quenching all serve to promote cheap production.

For successful carburizing, the following points should be carefully observed:

The utmost care should be used in the selection of pots for carburizing; they should be as free as possible from both scaling and warping. These two requirements eliminate the cast iron pot, although many are used, thus leaving us to select from malleable castings, wrought iron, cast steel, and special alloys, such as nichrome or silchrome. If first cost is not important, it will prove cheaper in the end to use pots of some special alloy.



The pots should be standardized to suit the product. Pots should be made as small as possible in width, and space gained by increasing the height; for it takes about 1-1/2 hr. to heat the average small pot of 4 in. in width, between 3 and 4 hr. to heat to the center of an 8-in. box, and 5 to 6 hr. to heat to the center of a 12-in. box; and the longer the time required to heat to the center, the more uneven the carburizing.

The work is packed in the box surrounded by materials which will give up carbon when heated. It must be packed so that each piece is separate from the others and does not touch the box, with a sufficient amount of carburizing material surrounding each. Figures 27 to 31 show the kind of boxes used and the way the work should be packed. Figure 31 shows a later type of box in which the edges can be easily luted. Figure 30 shows test wires broken periodically to determine the depth of case. Figure 28 shows the minimum clearance which should be used in packing and Fig. 29 the way in which the outer pieces receive the heat first and likewise take up the carbon before those in the center. This is why a slow, soaking heat is necessary in handling large quantities of work, so as to allow the heat and carbon to soak in equally.

While it has been claimed that iron below its critical temperature will absorb some carbon, Giolitti has shown that this absorption is very slow. In order to produce quick and intense carburization the iron should preferably be above its upper critical temperature or 1,600 deg.F.,—therefore the carbon absorbed immediately goes into austenite, or solid solution. It is also certain that the higher the temperature the quicker will carbon be absorbed, and the deeper it will penetrate into the steel, that is, the deeper the "case." At Sheffield, England, where wrought iron is packed in charcoal and heated for days to convert it into "blister steel," the temperatures are from 1,750 to 1,830 deg.F. Charcoal by itself carburizes slowly, consequently commercial compounds also contain certain "energizers" which give rapid penetration at lower temperatures.

The most important thing in carburizing is the human element. Most careful vigilance should be kept when packing and unpacking, and the operator should be instructed in the necessity for clean compound free from scale, moisture, fire clay, sand, floor sweepings, etc. From just such causes, many a good carburizer has been unjustly condemned. It is essential with most carburizers to use about 25 to 50 per cent of used material, in order to prevent undue shrinking during heating; therefore the necessity of properly screening used material and carefully inspecting it for foreign substances before it is used again. It is right here that the greatest carelessness is generally encountered.

Don't pack the work to be carburized too closely; leave at least 1 in. from the bottom, 3/4 in. from the sides, and 1 in. from the top of pots, and for a 6-hr. run, have the pieces at least 1/2 in. apart. This gives the heat a chance to thoroughly permeate the pot, and the carburizing material a chance to shrink without allowing carburized pieces to touch and cause soft spots.

Good case-hardening pots and annealing tubes can be made from the desired size of wrought iron pipe. The ends are capped or welded, and a slot is cut in the side of the pot, equal to one quarter of its circumference, and about 7/8 of its length. Another piece of the same diameter pipe cut lengthwise into thirds forms a cover for this pot. We then have a cheap, substantial pot, non-warping, with a minimum tendency to scale, but the pot is difficult to seal tightly. This idea is especially adaptable when long, narrow pots are desired.

When pots are packed and the carburizer thoroughly tamped down, the covers of the pot are put on and sealed with fire clay which has a little salt mixed into it. The more perfect the seal the more we can get out of the carburizer. The rates of penetration depend on temperature and the presence of proper gas in the required volume. Any pressure we can cause will, of course, have a tendency to increase the rate of penetration.

If you have a wide furnace, do not load it full at one time. Put one-half your load in first, in the center of the furnace, and heat until pots show a low red, about 1,325 to 1,350 deg.F. Then fill the furnace by putting the cold pots on the outside or, the section nearest the source of heat. This will give the work in the slowest portion of the furnace a chance to come to heat at the same time as the pots that are nearest the sources of heat.

To obtain an even heating of the pots and lessen their tendency to warp and scale, and to cause the contents of the furnace to heat up evenly, we should use a reducing fire and fill the heating chamber with flame. This can be accomplished by partially closing the waste gas vents and reducing slightly the amount of air used by the burners. A short flame will then be noticed issuing from the partially closed vents. Thus, while maintaining the temperature of the heating chamber, we will have a lower temperature in the combustion chamber, which will naturally increase its longevity.

Sometimes it is advisable to cool the work in the pots. This saves compound, and causes a more gradual diffusion of the carbon between the case and the core, and is very desirable condition, inasmuch as abrupt cases are inclined to chip out.

The most satisfactory steel to carburize contains between 0.10 and 0.20 per cent carbon, less than 0.35 per cent manganese, less than 0.04 per cent phosphorus and sulphur, and low silicon. But steel of this composition does not seem to satisfy our progressive engineers, and many alloy steels are now on the market, these, although more or less difficult to machine, give when carburized the various qualities demanded, such as a very hard case, very tough core, or very hard case and tough core. However, the additional elements also have a great effect both on the rate of penetration during the carburizing operation, and on the final treatment, consequently such alloy steels require very careful supervision during the entire heat treating operations.

RATE OF ABSORPTION

According to Guillet, the absorption of carbon is favored by those special elements which exist as double carbides in steel. For example, manganese exists as manganese carbide in combination with the iron carbide. The elements that favor the absorption of carbon are: manganese, tungsten, chromium and molybdenum those opposing it, nickel, silicon, and aluminum. Guillet has worked out the effect of the different elements on the rate of penetration in comparison with steel that absorbed carbon at a given temperature, at an average rate of 0.035 in. per hour.

His tables show that the following elements require an increased time of exposure to the carburizing material in order to obtain the same depth of penetration as with simple steel:

When steel contains Increased time of exposure 2.0 per cent nickel 28 per cent 7.0 per cent nickel 30 per cent 1.0 per cent titanium 12 per cent 2.0 per cent titanium 28 per cent 0.5 per cent silicon 50 per cent 1.0 per cent silicon 80 per cent 2.0 per cent silicon 122 per cent 5.0 per cent silicon No penetration 1.0 per cent aluminum 122 per cent 2.0 per cent aluminum 350 per cent

The following elements seem to assist the rate of penetration of carbon, and the carburizing time may therefore be reduced as follows:

When steel contains Decreased time of exposure 0.5 per cent manganese 18 per cent 1.0 per cent manganese 25 per cent 1.0 per cent chromium 10 per cent 2.0 per cent chromium 18 per cent 0.5 per cent tungsten 0 1.0 per cent tungsten 0 2.0 per cent tungsten 25 per cent 1.0 per cent molybdenum 0 2.0 per cent molybdenum 18 per cent

The temperature at which carburization is accomplished is a very important factor. Hence the necessity for a reliable pyrometer, located so as to give the temperature just below the tops of the pots. It must be remembered, however, that the pyrometer gives the temperature of only one spot, and is therefore only an aid to the operator, who must use his eyes for successful results.

The carbon content of the case generally is governed by the temperature of the carburization. It generally proves advisable to have the case contain between 0.90 per cent and 1.10 carbon; more carbon than this gives rise to excess free cementite or carbide of iron, which is detrimental, causing the case to be brittle and apt to chip.

T. G. Selleck gives a very useful table of temperatures and the relative carbon contents of the case of steels carburized between 4 and 6 hrs. using a good charcoal carburizer. This data is as follows:

TABLE 15.—CARBON CONTENT OBTAINED AT VARIOUS TEMPERATURES

At 1,500 deg.F., the surface carbon content will be 0.90 per cent At 1,600 deg.F., the surface carbon content will be 1.00 per cent At 1,650 deg.F., the surface carbon content will be 1.10 per cent At 1,700 deg.F., the surface carbon content will be 1.25 per cent At 1,750 deg.F., the surface carbon content will be 1.40 per cent At 1,800 deg.F., the surface carbon content will be 1.75 per cent

To this very valuable table, it seems best to add the following data, which we have used for a number of years. We do not know the name of its author, but it has proved very valuable, and seems to complete the above information. The table is self-explanatory, giving depth of penetration of the carbon of the case at different temperatures for different lengths of time:

- Temperature Penetration - 1,550 1,650 1,800 - - - - Penetration after 1/2 hr. 0.008 0.012 0.030 Penetration after 1 hr. 0.018 0.026 0.045 Penetration after 2 hr. 0.035 0.048 0.060 Penetration after 3 hr. 0.045 0.055 0.075 Penetration after 4 hr. 0.052 0.061 0.092 Penetration after 6 hr. 0.056 0.075 0.110 Penetration after 8 hr. 0.062 0.083 0.130 -

From the tables given, we may calculate with a fair degree of certainty the amount of carbon in the case, and its penetration. These figures vary widely with different carburizers, and as pointed out immediately above, with different alloy steels.

CARBURIZING MATERIAL

The simplest carburizing substance is charcoal. It is also the slowest, but is often used mixed with something that will evolve large volumes of carbon monoxide or hydrocarbon gas on being heated. A great variety of materials is used, a few of them being charcoal (both wood and bone), charred leather, crushed bone, horn, mixtures of charcoal and barium carbonate, coke and heavy oils, coke treated with alkaline carbonates, peat, charcoal mixed with common salt, saltpeter, resin, flour, potassium bichromate, vegetable fibre, limestone, various seed husks, etc. In general, it is well to avoid complex mixtures.

H. L. Heathcote, on analyzing seventeen different carburizers, found that they contained the following ingredients:

Per cent Moisture 2.68 to 26.17 Oil 0.17 to 20.76 Carbon (organic) 6.70 to 54.19 Calcium phosphate 0.32 to 74.75 Calcium carbonate 1.20 to 11.57 Barium carbonate nil to 42.00 Zinc oxide nil to 14.50 Silica nil to 8.14 Sulphates (SO3) trace to 3.45 Sodium chloride nil to 7.88 Sodium carbonate nil to 40.00 Sulphides (S) nil to 2.80

Carburizing mixtures, though bought by weight, are used by volume, and the weight per cubic foot is a big factor in making a selection. A good mixture should be porous, so that the evolved gases, which should be generated at the proper temperature, may move freely around the steel objects being carburized; should be a good conductor of heat; should possess minimum shrinkage when used; and should be capable of being tamped down.

Many "secret mixtures" are sold, falsely claimed to be able to convert inferior metal into crucible tool steel grade. They are generally nothing more than mixtures of carbonaceous and cyanogen compounds possessing the well-known carburizing properties of those substances.

QUENCHING

It is considered good practice to quench alloy steels from the pot, especially if the case is of any appreciable depth. The texture of carbon steel will be weakened by the prolonged high heat of carburizing, so that if we need a tough core, we must reheat it above its critical range, which is about 1,600 deg.F. for soft steel, but lower for manganese and nickel steels. Quenching is done in either water, oil, or air, depending upon the results desired. The steel is then very carefully reheated to refine the case, the temperature varying from 1,350 to 1,450 deg.F., depending on whether the material is an alloy or a simple steel, and quenched in either water or oil.



There are many possibilities yet to be developed with the carburizing of alloy steels, which can produce a very tough, tenacious austenitic case which becomes hard on cooling in air, and still retains a soft, pearlitic core. An austenitic case is not necessarily file hard, but has a very great resistance to abrasive wear.

The more carbon a steel has to begin with the more slowly will it absorb carbon and the lower the temperature required. Low-carbon steel of from 15 to 20 points is generally used and the carbon brought up to 80 or 85 points. Tool steels may be carbonized as high as 250 points.

In addition to the carburizing materials given, a mixture of 40 per cent of barium carbonate and 60 per cent charcoal gives much faster penetration than charcoal, bone or leather. The penetration of this mixture on ordinary low-carbon steel is shown in Fig. 32, over a range of from 2 to 12 hr.

EFFECT OF DIFFERENT CARBURIZING MATERIAL



Each of these different packing materials has a different effect upon the work in which it is heated. Charcoal by itself will give a rather light case. Mixed with raw bone it will carburize more rapidly, and still more so if mixed with burnt bone. Raw bone and burnt bone, as may be inferred, are both quicker carbonizers than charcoal, but raw bone must never be used where the breakage of hardened edges is to be avoided, as it contains phosphorus and tends to make the piece brittle. Charred leather mixed with charcoal is a still faster material, and horns and hoofs exceed even this in speed; but these two compounds are restricted by their cost to use with high-grade articles, usually of tool or high-carbon steel, that are to be hardened locally—that is, "pack-hardened." Cyanide of potassium or prussiate of potash are also included in the list of carbonizing materials; but outside of carburizing by dipping into melted baths of this material, their use is largely confined to local hardening of small surfaces, such as holes in dies and the like.

Dr. Federico Giolitti has proven that when carbonizing with charcoal, or charcoal plus barium carbonate, the active agent which introduces carbon into the steel is a gas, carbon monoxide (CO), derived by combustion of the charcoal in the air trapped in the box, or by decomposition of the carbonate. This gas diffuses in and out of the hot steel, transporting carbon from the charcoal to the outer portions of the metal:

If energizers like tar, peat, and vegetable fiber are used, they produce hydrocarbon gases on being heated—gases principally composed of hydrogen and carbon. These gases are unstable in the presence of hot iron: it seems to decompose them and sooty carbon is deposited on the surface of the metal. This diffuses into the metal a little, but it acts principally by being a ready source of carbon, highly active and waiting to be carried into the metal by the carbon monoxide—which as before, is the principal transfer agent.

Animal refuse when used to speed up the action of clean charcoal acts somewhat in the same manner, but in addition the gases given off by the hot substance contain nitrogen compounds. Nitrogen and cyanides (compounds of carbon and nitrogen) have long been known to give a very hard thin case very rapidly. It has been discovered only recently that this is due to the steel absorbing nitrogen as well as carbon, and that nitrogen hardens steel and makes it brittle just like carbon does. In fact it is very difficult to distinguish between these two hardening agents when examining a carburized steel under the microscope.

One of the advantages of hardening by carburizing is the fact that you can arrange to leave part of the work soft and thus retain the toughness and strength of the original material. Figures 33 to 37 show ways of doing this. The inside of the cup in Fig. 34 is locally hardened, as illustrated in Fig. 34, "spent" or used bone being packed around the surfaces that are to be left soft, while cyanide of potassium is put around those which are desired hard. The threads of the nut in Fig. 35 are kept soft by carburizing the nut while upon a stud. The profile gage, Fig. 36, is made of high-carbon steel and is hardened on the inside by packing with charred leather, but kept soft on the outside by surrounding it with fireclay. The rivet stud shown in Fig. 37 is carburized while of its full diameter and then turned down to the size of the rivet end, thus cutting away the carburized surface.

After packing the work carefully in the boxes the lids are sealed or luted with fireclay to keep out any gases from the fire. The size of box should be proportioned to the work. The box should not be too large especially for light work that is run on a short heat. If it can be just large enough to allow the proper amount of material around it, the work is apt to be more satisfactory in every way.

Pieces of this kind are of course not quenched and hardened in the carburizing heat, but are left in the box to cool, just as in box annealing, being reheated and quenched as a second operation. In fact, this is a good scheme to use for the majority of carburizing work of small and moderate size. Material is on the market with which one side of the steel can be treated; or copper-plating one side of it will answer the same purpose and prevent that side becoming carburized.

QUENCHING THE WORK

In some operations case-hardened work is quenched from the box by dumping the whole contents into the quenching tank. It is common practice to leave a sieve or wire basket to catch the work, allowing the carburizing material to fall to the bottom of the tank where it can be recovered later and used again as a part of a new mixture. For best results, however, the steel is allowed to cool down slowly in the box after which it is removed and hardened by heating and quenching the same as carbon steel of the same grade. It has absorbed sufficient carbon so that, in the outer portions at least, it is a high-carbon steel.

THE QUENCHING TANK

The quenching tank is an important feature of apparatus in case-hardening—possibly more so than in ordinary tempering. One reason for this is because of the large quantities of pieces usually dumped into the tank at a time. One cannot take time to separate the articles themselves from the case-hardening mixture, and the whole content of the box is droped into the bath in short order, as exposure to air of the heated work is fatal to results. Unless it is split up, it is likely to go to the bottom as a solid mass, in which case very few of the pieces are properly hardened.



A combination cooling tank is shown in Fig. 38. Water inlet and outlet pipes are shown and also a drain plug that enables the tank to be emptied when it is desired to clean out the spent carburizing material from the bottom. A wire-bottomed tray, framed with angle iron, is arranged to slide into this tank from the top and rests upon angle irons screwed to the tank sides. Its function is to catch the pieces and prevent them from settling to the tank bottom, and it also makes it easy to remove a batch of work. A bottomless box of sheet steel is shown at C. This fits into the wire-bottomed tray and has a number of rods or wires running across it, their purpose being to break up the mass of material as it comes from the carbonizing box.

Below the wire-bottomed tray is a perforated cross-pipe that is connected with a compressed-air line. This is used when case-hardening for colors. The shop that has no air compressor may rig up a satisfactory equivalent in the shape of a low-pressure hand-operated air pump and a receiver tank, for it is not necessary to use high-pressure air for this purpose. When colors are desired on case-hardened work, the treatment in quenching is exactly the same as that previously described except that air is pumped through this pipe and keeps the water agitated. The addition of a slight amount of powdered cyanide of potassium to the packing material used for carburizing will produce stronger colors, and where this is the sole object, it is best to maintain the box at a dull-red heat.



The old way of case-hardening was to dump the contents of the box at the end of the carburizing heat. Later study in the structure of steel thus treated has caused a change in this procedure, the use of automobiles and alloy steels probably hastening this result. The diagrams reproduced in Fig. 39 show why the heat treatment of case-hardened work is necessary. Starting at A with a close-grained and tough stock, such as ordinary machinery steel containing from 15 to 20 points of carbon, if such work is quenched on a carbonizing heat the result will be as shown at B. This gives a core that is coarse-grained and brittle and an outer case that is fine-grained and hard, but is likely to flake off, owing to the great difference in structure between it and the core. Reheating this work beyond the critical temperature of the core refines this core, closes the grain and makes it tough, but leaves the case very brittle; in fact, more so than it was before.

REFINING THE GRAIN

This is remedied by reheating the piece to a temperature slightly above the critical temperature of the case, this temperature corresponding ordinarily to that of steel having a carbon content of 85 points, When this is again quenched, the temperature, which has not been high enough to disturb the refined core, will have closed the grain of the case and toughened it. So, instead of but one heat and one quenching for this class of work, we have three of each, although it is quite possible and often profitable to omit the quenching after carburizing and allow the piece or pieces and the case-carburizing box to cool together, as in annealing. Sometimes another heat treatment is added to the foregoing, for the purpose of letting down the hardness of the case and giving it additional toughness by heating to a temperature between 300 deg. and 500 deg.. Usually this is done in an oil bath. After this the piece is allowed to cool.

It is possible to harden the surface of tool steel extremely hard and yet leave its inner core soft and tough for strength, by a process similar to case-hardening and known as "pack-hardening." It consists in using tool steel of carbon contents ranging from 60 to 80 points, packing this in a box with charred leather mixed with wood charcoal and heating at a low-red heat for 2 or 3 hr., thus raising the carbon content of the exterior of the piece. The article when quenched in an oil bath will have an extremely hard exterior and tough core. It is a good scheme for tools that must be hard and yet strong enough to stand abuse. Raw bone is never used as a packing for this class of work, as it makes the cutting edges brittle.

CASE-HARDENING TREATMENTS FOR VARIOUS STEELS

Plain water, salt water and linseed oil are the three most common quenching materials for case-hardening. Water is used for ordinary work, salt water for work which must be extremely hard on the surface, and oil for work in which toughness is the main consideration. The higher the carbon of the case, the less sudden need the quenching action take hold of the piece; in fact, experience in case-hardening work gives a great many combinations of quenching baths of these three materials, depending on their temperatures. Thin work, highly carbonized, which would fly to pieces under the slightest blow if quenched in water or brine, is made strong and tough by properly quenching in slightly heated oil. It is impossible to give any rules for the temperature of this work, so much depending on the size and design of the piece; but it is not a difficult matter to try three or four pieces by different methods and determine what is needed for best results.

The alloy steels are all susceptible of case-hardening treatment; in fact, this is one of the most important heat treatments for such steels in the automobile industry. Nickel steel carburizes more slowly than common steel, the nickel seeming to have the effect of slowing down the rate of penetration. There is no cloud without its silver lining, however, and to offset this retardation, a single treatment is often sufficient for nickel steel; for the core is not coarsened as much as low-carbon machinery steel and thus ordinary work may be quenched on the carburizing heat. Steel containing from 3 to 3.5 per cent of nickel is carburized between 1,650 and 1,750 deg.F. Nickel steel containing less than 25 points of carbon, with this same percentage of nickel, may be slightly hardened by cooling in air instead of quenching.

Chrome-nickel steel may be case-hardened similarly to the method just described for nickel steel, but double treatment gives better results and is used for high-grade work. The carburizing temperature is the same, between 1,650 and 1,750 deg.F., the second treatment consisting of reheating to 1,400 deg. and then quenching in boiling salt water, which gives a hard surface and at the same time prevents distortion of the piece. The core of chrome-nickel case-hardened steel, like that of nickel steel, is not coarsened excessively by the first heat treatment, and therefore a single heating and quenching will suffice.