Induction Hardening Technical
Induction hardening and heat treating
One of the principal uses of induction heating is in the induction hardening of steel parts, particularly where only localized surfaces require treatment. As with any other use of induction heating, however, the shape of the part and the area to be hardened must be suited to this method of heat transfer. Parts requiring induction hardening all over do not usually lend themselves to induction heat treating unless they are small in size or so shaped that a suitable heating coil can be properly proportioned to the insure uniform heating. On the other hand, intricately shaped parts, which might be difficult to harden by other means, may be ideal for treatment by induction hardening.
There is no question that induction hardening is a rather selective means of heat treatment, and by no means is it intended to take place of existing heat treating techniques. It has its own field and will cover a rather broad range of hardening. Usually where induction hardening can be used, even with a choice of methods, this form of heat treating has definite advantages, such as: 1) rapid heating with large production possibilities, 2) uniform control so that rejections are minimized or eliminated, and, 3) as a rule, economical heating costs, especially where only localized areas are heated. When heating is confined to localized surfaces, which might represent only a small position of a relatively large part, deformation and warping (common with other forms of treatment) are usually eliminated. There is also, of course, no formation of scale, which often eliminates cleaning costs.
When using high frequency generators, it’s customary to have an induction hardening table with proper provisions for handling the quench, whether submerged or spray quenching is to be carried out. This table should be a two station induction hardening table provided with all the necessary features needed in the spray quenching of parts, such as gears, shafts, clutches, cams, and other steel parts. These units have two sets of terminal connections, so one side of the table can be use while the other is being set for another operation.
There are two rotary driven work holding spindles on the table that can be engaged or disengaged as required. The table also is equipped with a multi-station electric timer, which provides for various types of heating, quenching, and time-delay cycles. When performing gear work, for example, it’s possible to provide the necessary heat cycle. The cycle can be followed by a slight delay if desirable, and include the necessary duration of spray quench. Such a timer can also be used for the operation of solenoid devices or electromagnets, so the heated part may be dropped for immersion quenching.
The induction hardening table is also equipped with an initial timer, allowing the heating cycle required for a part to be recorded. For example, in setting up a run of parts, the first piece is usually heated and the time checked for reaching the quenching temperature. With this time recorded on the initial timer, it’s a simple matter to adjust the master timer so subsequent pieces will be carried through in identical time. After the initial timer has served its purpose, it can be disengaged. The table is equipped with a changeover switch located at the center of the upright panel, so that either induction hardening station can be operated as desired.
A fixture and coil for the induction hardening of clutch teeth is located on a stud, positioned at the end of the fixture, and provided with a flange, so the surface of the work to be heated is held at correct relation to the pancake type multi-turn coil. Current is then applied and the heat pattern follows the contour of the clutch teeth, as represented by the light bank around them.
At the completion of the heating cycle a spray quench is automatically engaged by means of a timer and a solenoid operated valve, so the heated area immediately becomes hardened. When handling heating operations for parts of this kind, it’s obvious that the part which becomes heated is the only area that is hardened. When automatic heating and quenching are applied, uniformity of hardening is assured. The induction hardening operation requires 14 seconds of which 8 seconds represents the heating portion of the cycle.
A gear made of S.A.E. C-1141 steel is another example of an induction hardened part that is spray quenched. Three separate induction hardening operations are performed on this part, one each on the gear teeth, the clutch teeth, and the eccentric. One piece is in the fixture, arranged for the induction hardening of the eccentric surface. This is an unusual example of what can be accomplished with induction hardening, since the gear teeth are hardened to 52 Rockwell C, the clutch teeth to between 55 and 57 Rockwell C, and the eccentric to between 60 and 62 Rockwell C.
The floor-to-floor times for the induction hardening cycles are 18 seconds for the eccentric; 16 seconds for the gear teeth, and 13 seconds for the clutch teeth. A case depth of 1/16 inch is provided for the clutch teeth and the eccentric, while a depth of about 3/64 inch is produced in the gear teeth. No additional machining operations are required after induction hardening. The fixture assembly includes a base plate, a quench ring, a locating stud and holder and the heating coil.
For induction hardening of machine centers on which only the ends are required hard, two parts are placed on the elevating fixture, which, in turn, are raised to the heating position so the tapered ends of the center enter the work coils. After obtaining the necessary heat treating, the current is turned off and the water quench is engaged to complete the hardening. The parts are then lowered and removed. This type of setup will be found useful in a broad field of heating applications, particularly where it’s necessary to elevate the work into a coil, and where loading can be done more conveniently somewhat remote from the coil.
With a vertical type induction heating unit for the induction hardening of cam surfaces, gear, and bearing eccentric, the setup includes two camshafts that are hardened at one loading operation. After two of the cam sections have been heated and quenched, the inductor is automatically raised one increment to bring the next two cams into the heat treating position, where the operation is repeated. The surface hardened follows the contour of the cams and a hardness of 60 Rockwell C is obtained. Usually camshafts of this type, when hardened by other methods, require straightening and cleaning operations. Such operations, however, are eliminated as a result of the selective induction hardening made possible by high frequency generators.
When an induction hardening setup is used for treating the bearing surfaces on automotive axle shafts, the heating inductors are arranged in a series positioned one above the other, so two shafts are treated simultaneously. These parts formerly required the use of inner hardened bearings, which were pressed onto the shaft; but by means of the induction heating equipment method, it’s now possible to localize the hardening of the shafts themselves so they act as their own inner bearings. This eliminates the assembling of an additional part. Usually in applications of this kind the shaft can be made slightly heavier in design and still maintain a smaller over all dimension than when a separate bearing is used. A change of this kind often results in increased strengths of 50 percent or more.
The advantage of being able to apply localized heat to a restricted surface is shown in a standard form of thread ring gauges. Gauges of this type are usually split and are likely to go out of shape when subjected to the normal hardening treatment. When this occurs, there is practically no way to straighten them, resulting in a loss due to spoilage. By means of high frequency heat, however, the internal threaded section can be heated locally by a coil. After the proper heat has been attained, the part can be dropped into an oil or water quench. Another advantage of induction hardening such a part through induction heating is the elimination of the usual heavy scale that occurs in other means of heat treatment.
Many other tools and gauges are suitable for induction hardening, and it’s possible that changes in heating practices will develop with time. There will also be many possible changes in the hardening of jig and fixture parts, which in the past have been made of tool steel and hardened all over. Many of these parts can be replaced by those made of plain carbon steel and locally hardened by induction heat on the surfaces only where resistance to wear is required.
When induction hardening internal surfaces, where the length of the hardened area exceeds the diameter, it’s preferable to use progressive heating methods that are arranged so only a small area of the surface is heated at one time. The work is mounted on an elevating platform that feeds downward as the surface is progressively heated. Directly below the internal type coil is a pressure spray quench ring, which applies a water quench to the surface immediately after it’s heated. Rings for this purpose should be made of a nonmetallic material, such as plastics. Since the heat is localized to only one portion of the work, there is little chance of distortion or warping, which might be experienced on parts heated by other methods. When using devices of this kind, it’s desirable to include a variable feed mechanism so the rate of travel can be selected to suit the nature of the part being heated.
Plastic quench rings for setups of this kind can be made of a liquid resin, mixed with a proper amount of accelerator. The procedure requires making a wooden pattern around which a mixture of plaster of Paris is cast. After setting, the pattern is removed and the plaster forms a mold into which the liquid resin can be poured. A core, preferably made of metal, is assembled in the mold to produce a shell-type ring. The cover is molded to the quench ring shell afterwards to form the final section. This is done by setting the shell into another shallow mold in such a way that the edges are submerged into the liquid resin.
From this procedure it will be seen that two curing operations are required: one to set the shell and another for applying the cover. Commercial liquid plastics are available that can be cured in 4 to 5 hours at a baking temperature of 180 to 200˚F. Some liquid plastics air-harden in 24 to 48 hours, but low temperature baking is usually preferred. Re-baking the shell, for molding the cover plate to it, does not affect the previously cured member.
Sometimes it’s necessary to harden long bars. In such cases, progressive heat treating is required. How it works: the bar to be heated travels on rolls located on each side of a sink and is fed through a heating coil then finally through the spray-quench rings at the center. The bar is fed by the power-driven rollers. By varying the speed rate of these rollers, it’s possible to obtain different feed rates in proportion to the size of the bar being induction hardened and the available power of the high frequency generator. An operation of this kind is useful when the outer surface of a bar requires toughening to resist wear and normal scuffing.
The use of solenoids and electromagnets in induction hardening operations is often desirable. This requires a simplified setup in which the work piece is suspended in a coil by means of a magnet. The magnet is connected to a timer that is operating the control cycle, so the current is released as soon as the heating cycle has completed. The work, which in this case is a small spline shaft, then immediately drops into the quench tank, located directly beeath it.
High frequency induction heating equipment is excellent for the induction hardening of gears. Even though restricted to certain types and sizes of gears, the process, when possible, gives exceptionally fast heating, with uniform results.
High frequency induction hardening will have some affect on the types of steels used for the making of gears. Heretofore there has been a broad use of alloy steels, usually as a means of obtaining a specified hardness. However, the indications are that regular carbon steel can be used successfully for a wide variety of gears, and when only hardness to resist wear is desired, the use of alloy steels can possibly be materially reduced. For example, S.A.E. 1045 steel is suitable for the induction hardened of gears, and surface hardness up to 60 Rockwell C can be readily obtained.
Another grade of steel that has proved suitable for induction hardening is S.A.E. C-1141. This steel has free machining qualities and has been used successfully in the manufacture of a wide variety of gears. Other steels are available, which, having a minimum yield point of 100,000 lb. per square inch and a carbon content from 0.45 to 0.50 percent, will prove suitable substitutes for some of the alloy steels.
Induction hardening will also produce some changes in the processing of gears. In the first place, steel with higher carbon content can usually be substituted for a carburizing steel, thus eliminating carburizing. Steel with 0.40 to 0.50 carbon is slightly more expensive then the same type with a low carbon content. Therefore, if surface hardening of the teeth can be accomplished without carburizing, a worth-while saving results. The average cost of carburizing is $0.04 to $0.08 per pound, whereas the cost of steel with higher carbon content is only $0.03 per pound.
A comparison of costs for a gear on which only the teeth are required hard shows a savings of 43 percent through the use of the high frequency induction heating equipment method. The cost of steel for the gear when made of S.A.E. 1020 carburized, as against S.A.E. 1045 induction hardened, was $0.40 against $0.43; carburizing at $0.039 per pound was $0.319 against zero; heat to harden was $0.041 against $0.01; cleaning was $0.02 against zero; and the total cost was $0.78 against $0.44 a saving of 43 percent for the induction hardened part.
The analysis includes the cost of steel in both cases, because this difference is related to the carburizing and induction hardening costs. While there is only a fraction of a cent per pound difference in cost between the high and low carbon steels, the carburizing cost runs about 0.04 cent per pound. The cost of heating differs, too, since only a portion of the part is heated when induction hardening is applied, and the operating cost of the unit is comparatively low. A gear of this type can be heated, ready for quenching, in 17 to 20 seconds using a generator having 20 kW output.
Gear hardening methods
When using high frequency induction heating for the hardening of gears and sprockets, where only localized heat treating and hardening is required, either of two methods may be considered. One involves immersing the heated pieces into a quenching tank, the other requires spray quenching the part while it remains in the heating coil. The choice lies largely in the requirements of the pieces to be heated and sometimes can be made only after making analyses of both methods.
For immersion quenching setups, a variety of methods can be used. Some can be made semiautomatic, while others work best by manual control. Parts can be suspended into an induction coil by means of electromagnets that are controlled by timers, which release the part after a predetermined heat cycle has been concluded. Solenoid operated trips and releases can also be utilized in connection with automatic timers, dropping the work into a quench as desired.
With spray quenching, it’s often possible to obtain the desired hardness more effectively than by other methods, since a slight variation of the heating or quenching times will vary the Rockwell hardness readings proportionately on a given surface. With immersion quenching, this is not possible, and if this method is used it’s often necessary to draw a part back to the required hardness.
Assuming a 3-inch diameter, a 0.50 carbon steel gear requires hardening on the teeth only. If a water spray quench is being used, a hardness of 60 Rockwell C can be obtained easily by applying a normal heating cycle and a suitable quenching period. If only 55 Rockwell C is desired, however, the duration of quench is proportionately reduced to obtain this hardness. The part, likewise, is not thoroughly cooled so that the remaining heat acts like a drawing operation. This difference in high frequency induction hardening practices is mentioned since it will have a direct influence on the method to be used, and consequently on the type of fixture to be provided.
A spray-quench fixture for induction hardening a spur gear often requires a table with a built-in sink and suitable drain. The gear is mounted on a stud that is fitted into a spindle. Preferably, the spindle should be power driven and rotate during the heating portion of the cycle. Rotation insures a uniform heat pattern and compensates for any variation in the coupling between the work and the induction coil. Usually a speed of 20 to 30 r.p.m. is satisfactory. Hand rotation of the work can be substituted when a convenient method of turning is possible.
A multi-turn induction coil surrounds the gear, and around this the spray-quench ring is positioned. The passage of water for quenching is controlled by a timer, actuating a solenoid operated valve, normally closed. For this type of quench ring, a non-metallic material is recommended, since a metal ring would create a magnetic field and absorb heat. A cast phenolic thermo-setting plastic serves well for such an application.
The spacing of the spray holes in the quench ring should be such that a uniform spray is provided. Usually a 1/16 inch hole, or one slightly smaller, will be satisfactory. The induction coil should be of the flat type so the impinging spray can pass through it to the heated surface of the work. Other modifications for this type of fixture are used for a jaw tooth clutch and for a bevel gear.
Electrically controlled fixtures for gear hardening are often used in induction heating equipment setups, so automatically timed cycles can be carried out. An example of this is the method applied to the hardening of gear teeth. The operation includes heating the teeth, lowering the gear to the quenching position, quenching, and returning it to the loading position. These cycles are carried out by a multi-stage timer and the entire operation is automatic except for loading and unloading the gear, which is located on the stud end of a rotary-driven spindle.
With a gear in place, the cycle is started by a push button station. The high frequency heating coil is energized and, at the same time, the spindle is rotated at a moderate speed by a slow-speed shaded-pole motor, having connection to the timer. When heating has been completed, the spindle rotation is stopped and the spindle is lowered instantaneously by the air cylinder, actuated by the solenoid connection so the gear assumes the position. At the same time, the water quench is engaged by connections of the solenoid valve, which is normally closed. After a predetermined spray quench, this valve is closed and the spindle is returned to its upper position, thus completing the cycle. The coil used for heating the gear teeth is of the single-turn type.
When induction hardening gear teeth, it’s necessary to proportion the height of the heating coil to the surface of the teeth. When the heating coil is of greater height than the face of the teeth, there’s a tendency for the top and bottom surfaces of the gear blank to absorb heat. On the other hand, with a coil having a height slightly less than the face of the teeth, the heat pattern will be more evenly distributed.
When using a single-turn coil for gear hardening, it’s possible to make its height at least equal to the face of the teeth; then, after trail, the coil can be trimmed down until the proper heat pattern has been obtained. With multi-turn coils, however, it may become necessary to provide one less turn or, perhaps, to alter the spacing of the coil turns as needed.
When there’s a choice between oil-hardening and water hardening steels, it’s better to use the latter, because of more favorable quenching conditions. With high frequency induction heating equipment, the hardening cycle can be automatically controlled, so that with the use of a fixture and a quench ring a gear can be heated and quenched at one setting. This is also possible with some types of gears requiring oil quenching, in which case an oil reservoir, pump, oil cooler, and solenoid valve are required. In other cases of oil quenching, it’s necessary to heat the work then drop it into an oil bath.
When gears are hardened by high frequency induction heating equipment at frequencies of 100 to 500 kc., the resultant hardness zone will vary somewhat according to the size of the tooth. With gears having small teeth, of about 20 pitch, for which a straight cylindrical coil is used, the entire tooth is usually heated. With a slightly larger tooth, of about 12 pitch, the heat pattern begins to follow the contour of the tooth. On gears having teeth of 8 or 10 pitch, the heat pattern follows the contour of the teeth closely, and a uniform casehardening is obtained. When the tooth is larger, such as the 4 pitch contour, it becomes difficult to throw the heat down to the bottom of the tooth. This results in a situation in which the upper portion becomes deeply heated and the heat pattern resembles that obtained on finer teeth.
From this it will be seen that teeth of 8 or 10 pitch are best, but that a favorable condition also prevails with those of 12 or 14 pitch. Furthermore, it will be seen that the application of high frequency for the hardening of gear teeth in this range offers a variation in heat patterns and hardness zones. For shallow hardness penetration, for example, the surface only is heated then quenched, as has been described. For a deeper heated zone, additional heating time is provided so the penetration of eddy currents will be increased, after which the heating cycle is followed by a quench. If the entire tooth is to be hardened, the heating portion of the cycle is increased that much longer and then followed by a quench.
When gear teeth of 8 to 10 pitch are surface heated and quenched, the transition zone is shallow. There’s no strict line of demarcation, but more of a gradual blending of the hardened area to the core. However, where a deeper transition zone is desired, this can be accomplished by a double heat. For this, the teeth are heated somewhat deeper than merely on the surface, afterward a delay period is allowed. Following this, the surface is reheated then quenched so a transition zone is obtained. A cycle of this kind can be made fully automatic by using a multi-stage timer.
Going back to the larger tooth form, such as the 4 pitch referred to, it’s possible to use a formed or cast heating coil, in which the inner contour conforms approximately to the shape of the gear teeth. This design gives a more uniform distribution of heat around the sides and bottom of the teeth. For exceptionally large gears, however, it’s best to heat and quench each tooth separately, or perhaps in small multiples, depending on their shape, length, and suitability to the power output of the high frequency generator.
Shaving teeth before hardening
An advantage to high frequency induction hardening is the ability to shave gear teeth before hardening. Usually shaving is applied to heat treated gears having a hardness of 32 to 38 Rockwell C, with no subsequent heat treating after shaving. With the induction hardening equipment method, however, it’s possible to shave the gears when soft, and then harden the teeth as a final operation. This procedure has many advantages, outstanding among which are a harder tooth and less wear on the shaving cutters. With a typical shaving setup for gear teeth of a double cluster gear, which will receive induction hardening on both sets of teeth as the next and final operation, a tape reading is taken of the gear before shaving, after shaving, and after induction hardening. Each horizontal line represents 0.001 inch and a comparison will show that no deformation or run-out has resulted from the hardening operation.
Another saving made possible by high frequency induction heating equipment is the elimination of cleaning after hardening. Usually a scale is formed when gears are hardened, which then requires a cleaning operation. With induction heating equipment, however, practically no scale is formed beyond a discoloration of the surface.
As a comparison of the time required for induction hardening standard types of spur gears by high frequency induction heating equipment (operating at about 200 to 400 kc., and with an output power of about 20 kW.) a 3-inch gear can be heated in about 8 seconds and the quench will require 5 seconds. A 5-inch gear will require a heating cycle of 12 to 14 seconds followed by a quench of about 7 seconds. For a 7-inch gear, the heating time will be 20 to 25 seconds followed by a quench of 10 to 12 seconds. All these estimates are based on the use of 0.40 to 0.50 carbon steel of the water hardening type, using a closely coupled heating coil.
Another process change with many advantages is the assembly of bushings and inserts prior to hardening. If a gear is to be provided with a bronze sleeve bushing, this can be assembled before the teeth are cut. In the hardening operation, the heat will not travel so far as the bushing. Formerly it was often necessary to locate the gear from the pitch circle and grind the hole concentric with the pitch diameter, after which the bushing was inserted. For example, for a 6-inch diameter gear with a bushing assembled in the hole before the induction hardening operation, the teeth are cut after the bushing has been assembled, which assures concentricity.
One of the outstanding advantages of high frequency induction heating equipment for the induction hardening of gears is the ability to heat only the surfaces requiring hardening. The upper gear is cut integrally with a shaft, which in turn, is mounted on a ball bearing. Since there’s no advantage to hardening the entire parts, it’s possible, with induction heating, to harden the teeth only.
When processing a double cluster gear of the same type made in one piece, each gear is hardened separately, requiring two operations for hardening. It is possible to harden both gears simultaneously by making a double type induction coil, but problems involved with spacing the coil with relation to the work, as well as compensating for the differential in the diameter of the gears, might cause complications.
After all, the induction hardening operation is handled so rapidly that little time would be gained by trying to combine the two operations. The small gear, which is 2 1/2 inches in diameter, is heated in 7 seconds with a quench of 4 seconds, whereas the larger one, which is 4-inches in diameter, is heated in 13 seconds and quenched in 7 seconds. From this it will be seen that the total hardening time, aside from loading is 31 seconds per piece. If both gears were combined in one heating cycle the total time would be about the same, but the results would probably not be so uniform.
A setup for induction hardening a gear cut integrally with a shaft is mounted in a horizontal type fixture between centers, and the induction coil surrounds only the section to be hardened. The fixture includes a base, a quench ring, and a multi-turn copper tube induction coil. The gear measures 2 1/2 inches in diameter, and is heated in 10 seconds, followed by a 6 second quench.
Distortion of gears
Again, the elimination of straightening is a point in favor of the induction hardening process for gears of this type. For example, with such a gear it’s likely that some warpage or deformation would result if the entire part were heated for the hardening of the teeth only. It would thus be necessary to add a straightening operation, which at times can be troublesome.
With induction hardening of gears by high frequency heat, very little distortion takes place in the average gear. However, there are naturally some designs in which deformation will occur. Referring to Fig. 98, the gear at A is thin in sections and, regardless of the kind of heating applied, there is a likelihood of some distortion; induction heating equipment would be no exception. An advantage in hardening a gear of this type would be in using high power and high frequency, so the surface of the teeth would be heated to quenching temperature before the conduction of heat to the surface directly below could take place. In this way deformation would be greatly minimized.
The gear shown at C, however, is solid and, when hardened by high frequency heat, there will be practically no change in size or shape of the gear. Many tests have been made before and after induction hardening gears of this type, and at the most there might be a slight increase in size at the pitch diameter; but the concentricity will remain unchanged. Usually, if a gear changes one way or the other, it can be quickly detected after one or two tests, so proper allowances can be made in machine the teeth.
The example shown at D is a double-cluster gear, which has a multi-spline machined in the hole. When the larger gear is induction hardened there will be no distortion; but when heat is applied to the small gear the splined hole is likely to close in slightly, depending upon the wall thickness between the hole and the outside of the teeth. However, since only the contour of the gear teeth is hardened, the material around the hole will be unaffected as far as hardness is concerned, and it’s possible to re-broach the hole with a hand broach to remove any deformation that might have occurred.
The example shown at B is a triple-cluster with three gears of different diameters, all of which are hardened separately. Here, again, if the amount of material between the hold and the teeth is thin, there’s likely to be some hole closing.
By having full knowledge of what is needed to harden gears successfully by the high frequency induction process, it’s possible to incorporate these requirements into the original design. Generally speaking, induction hardening can be applied to, perhaps, 90 percent of all gears in the range covered by this method. The few that might give trouble can often be corrected by slight modifications in design, usually in proportioning the amount of steel around the gear so as to prevent deformation.
When hardening one of the gear sections of a triple-cluster gear, the smallest gear is usually hardened first, next the medium sized, then the largest. This process is necessary because the location of the induction coil for the smaller gears is close to the face of the large one, and if it was hardened first, a slight amount of heat might be generated in it when the smaller gears were heated, with a slight drawing effect. By induction hardening the large gear last, the heat will be confined to the place where it’s wanted and will have no effect on the other gears.
Induction hardening bevel gears
When hardening bevel gears by means of high frequency heat, the same general procedure as for spur gears is followed. The induction coil is wound helically to conform to the face angle of the gear. On straight tooth bevel gears, the heat pattern follows the contour of the teeth and uniform surface hardening can be easily obtained. With spiral bevel gears, however, because the teeth lie at different angles to their normal flow, the eddy current lines are disturbed to such an extent that there’s a tendency to obtain more heat on one side of the teeth than on the other.
On some sizes of spiral bevel gears, this can be overcome by applying slightly more heat to ensure hardening of the concave side. On other forms, however, it’s best to carburize the gear after the teeth have been rough cut, then follow with the finish-cutting operation, after which the teeth can be induction hardened by allowing sufficient time to heat the entire tooth. When the gear is quenched, only the carburized surface will become hardened. While the expense of carburizing adds to the cost of manufacture, it offers the decided advantage of lower heating cost and the absence of scale, as well as the elimination of distortion.
When induction hardened a bevel gear by means of high frequency induction heating equipment, which might offer some difficulties under usual procedures, the design includes a hub that extends in front of the gear face. (The hub was assembled after the gear had been cut.) If the gear were hardened first then brazed in place, the heat of brazing would draw the temper from the teeth. With high frequency heating, however, it’s possible to braze the shaft into the hub of the gear first then follow with the induction hardening operation, localizing heat to the teeth only. The gear is 3 1/4 inches in diameter, and requires a heating time of 12 seconds followed by a quench of 6 seconds.
When induction hardening bevel gears by high frequency induction heating equipment, the work should be rotated by power manually, in order to ensure uniform heating. While it would be possible to construct an induction coil that would heat all portions of the gear teeth uniformly, this is not always the easy. Therefore, by turning the work, it’s possible to compensate for any slight difference in spacing between the coil and the work.
The current cost for the induction heating of gears is comparatively low. When a 20 kW output generator is used for heating the various types of gears, the actual heating portion of the cycle averages only about one third of the total floor-to-floor time. The balance of time is consumed in quenching, removing and loading the work, and changing the fixture from one job to another. Assuming the power cost to be $0.02 per kilowatt, the operating cost for current would range from $0.25 to $0.30 per hour. The number of pieces that can be processed in an hour depends on the size of the gears, but an average gear would run through 50 to 60 pieces per hour, so the average current cost per gear would be less than 1/2 cent.
Fig. 99 shows a representative group of gears suitable for high frequency induction hardening of the teeth. Various single-, double- and triple-cluster gears are included. On the large gear in the upper center, as well as on others, one can see the heat bands set up as a result of high-frequency heating, indicating the depth to which the heat travels. The gear shown at the lower center has three surfaces treated inductively, in three separate operations, namely, the gear teeth, the hole, and the clutch teeth, the respective Rc readings being 55, 60 and 52.
Induction hardening large gears
For the induction hardening of a large tractor gear an 800-hp., high frequency inductor generator is used. For this example, the gear is made of S.A.E. 1045 steel and is 25.7 inches pitch diameter with a 5-inch face. The gear is preheated to 600˚ F before induction hardening of the teeth then placed on the fixture, which includes a platform mounted on a vertical spindle. The spindle is lowered for a distance of about 6 inches so the gear is brought into proper relation with the surrounding heating coil, which has a coupling of 5/16 inch. The gear is rotated to assure uniform heating and, after a 90-second heating period, during which time the profile of the teeth reaches a red heat, the current is turned off and a short time is allowed for the heat to soak deeper into the case. During this delay, the gear is lowered to the quenching position then quenched.
The final treatment of the gear comprises a tempering at 300˚F for approximately 1 hour, which produces a surface hardness of approximately 58 to 60 Rockwell C on the surface to lower readings.
Prior to induction hardening, the gear was made of an alloy steel billet, which was hardened and tempered to 40 Rockwell C, after which the machining and cutting of the teeth were carried out. From a production standpoint, the machining was comparatively slow and the cutter cost somewhat high. With the inductive method, however, machining and cutting the teeth are carried out with plain carbon steel when soft, which is more practical from a machine-ability standpoint. Thus, cutter life has been greatly increased and production cost reduced. There is also a considerable savings in alloy material, running into several pounds per gear.
Another application for induction heating equipment lies in the standard end quench harden-ability test, referred to as the Jominy test. With a simple fixture, this test can be carried out quickly by the inductive method. It consists of preparing a normalized bar 1 inch in diameter and 3 7/8 inches long with a 1/8 inch thick flange. The test piece is machined all over then inserted in the heating coil, which is suspended from the flange. The bar is heated with a minimum formation of scale to the proper temperature, then quenched from the end by water flowing through a pipe having a 1/2 inch orifice, 1/2 inch from the end of the specimen with sufficient pressure to rise to a height of 2 1/2 inches when the test piece is not in place. Water is allowed to flow until the sample is practically cold. Flat surfaces are ground on each side to a depth of 0.015 inch, after which Rockwell hardness readings are taken at intervals of 1/16 inch apart.
The rate of quench is very fast on the end of the specimen. Inasmuch as the heat must pass through the sample by conduction, the upper portion of the piece is quenched slowly. This indicates that quenching has been carried out at different rates, which in turn will vary the Rockwell readings. This method of testing makes it possible to predict how various steels will respond to heat treatment.
When heating and quenching a surface is required, it’s sometimes possible to use a tubular coil provided with spray holes and to limit the flow of water to the quenching portion of the cycle. Usually this type of coil is applied only when the heating time is short, since overheating might result.
A quenching coil of the built-up type made with cooling tubes has two tubes for cooling. The tubes are located between an outer and an inner sleeve brazed together. The cooling water is fed through a separate outlet and circulates around both cooling tubes, built into the coil assembly. The arrangement of the cooling tube connections and the two leads to the generator can be modified as needed to suit the high frequency outlet terminals. The inner sleeve is made with a series of holes through its center portion, which usually provides sufficient quench for a surface as wide as the coil is high, since the water is well distributed into both directions from the center when it strikes the surface of the work.
Even though the application of induction heating to high-speed steel is somewhat limited to small tools with thinner sections, it’s quite possible that developments will take place in both metallurgy and induction heating, so that its field will be widened.
Some tests conducted with high-speed steel induction hardening have shown favorable results. The microstructure of conventionally hardened high-speed steel consists of un-dissolved alloy carbides distributed in an austenitic martensitic matrix. The grains have a definite size depending on the hardening temperature employed and the actual time at the hardening temperature. The grain size and degree of carbide solution materially affect the toughness, red hardness, wear resistance, and cutting characteristics of high-speed steel.
In general, better cutting qualities are obtained with increased grain size and increased solution of the carbides; but since the toughness is adversely affected by increasing the grain size, it’s often necessary to compromise to some intermediate grain size for most cutting applications. In conventionally hardened high-speed steel, about 7 percent of un-dissolved alloy carbides exist.
The microstructure of the induction heated high-speed steel, rapidly heated for hardening the grain size, is smaller. Also, the degree of carbide solution is considerably greater than that usually obtained in high-speed steel hardened in the regular manner. High-speed steel rapidly heated by induction is highly austenitic and multiple tempering should be employed. It will be noted that the time of heating is somewhat less than is necessary to dissolve substantially all of the carbides.
The cutting performance of induction hardened high-speed tools has often been found excellent and it’s expected that induction heating equipment will be more widely used in the future to treat all types of tool steel.
Induction hardening crankshafts
One noteworthy application for induction hardening is the treating of the connecting rod and main bearings of crankshafts. The method used up to the advent of induction hardening required the heating and quenching of the entire crankshaft, followed by a drawing operation. Also alloy steel was used. Now it’s only necessary to harden the surfaces that are subjected to wear and use a simplified form of carbon steel, such as S.A.E. 1050.
For the operation, which is of the vertical type, three or four bearing surfaces are hardened progressively. The crankshaft is then processed through another induction hardening machine that is arranged to harden additional bearings. Usually about three machines are required to handle all the surfaces requiring treatment. Since all the units are controlled and timed automatically, it’s possible for one operator to service them all consecutively and thus obtain a relatively high rate of production. This method of hardening is used for large diesel engine crankshafts. In this case, the parts require mechanical equipment usually in the form of a conveyor line, so arranged that one bearing is hardened at each station. On crankshafts of this kind, the surfaces are hardened to 60 Rockwell C, leaving the core tough and ductile.
When induction hardening parts such as crankshafts, it’s necessary to use a split type or hinged inductor made of two pieces. The part is located in V blocks and is radically aligned by the crank bearing, which is not being hardened. When the work is in position, the upper half of the inductor block is swung down then the hinged clamp is brought into place, so that both halves of the inductor can be firmly contacted. The inductor block in this case is made with integral quenching holes.
In some cases it’s possible to surface harden certain types of parts without the usual quench because of the steep temperature gradient set up by rapid heating of the surface, followed by rapid cooling. By heating a thin layer of the surface only, in a matter of, say, 2 or 3 seconds then turning off the current, the surface heat dissipates into the cold mass underneath fast enough to create a quenching action. While this process is limited and requires carefully selected frequencies and power supply in relation to the size of the part being treated, it provides a means of localized hardening with some possibilities.
Linked with this method, however, is the necessity of obtaining steel with air-hardening properties, or with characteristics that offer hardness when quickly cooled. There are limitations to the degree of hardness obtainable and, likewise, control is not as accurate as with the spray quenching method used for carbon steels. The power required for this type of heating may run high, especially where high frequencies, on the order of 1 megacycle or more, may be needed.
Oil spray quenching
When spray quenching metal parts, it’s possible to use a light oil instead of water when the metal or the steel so requires. A self-contained oil system is used for this purpose and an oil tank of sufficient capacity is located adjacent to the operating table. The oil is fed through the circuit by a pump, which normally circulates the oil through a 3-way valve and back into the tank. This valve is solenoid operated, and when switched over to the spray position, the oil passes through the quench ring onto the work and finally back through the drain to the tank. When the solenoid valve returns to its normal position, the spray quench is cut off and the oil circulates through the original course.
Oil quenching is usually limited to parts not subjected to excessive heat, and where the volume of heated mass is not too great. For larger pieces, a submerged quench is preferred and flashing of the oil might occur. Normally, with smaller pieces, the inductively heated area loses its heat so quickly that flashing does not take place. With a large mass heated to quench temperature, however, flashing might exist.
As a rule, an oil spray of this kind will require some means of cooling the oil. A small compressor refrigeration unit usually will meet this need. Temperature control should be provided so that operation of the compressor will be automatic and thus maintain the temperature of the oil within a desired range. Alternately, water cooling tubes may be placed in the oil tank, which also can be arranged with thermostatic control through a solenoid valve. The compressor unit is usually preferred, since it offers better temperature control. Such units are available complete with automatic controls and can be installed directly onto the oil tank.
Inasmuch as high frequency current can be made to operate in a submerged oil bath, it’s often possible to carry out hardening operations advantageously by heating the oil. Inasmuch as the part is not subjected to the atmosphere, there’s less likelihood of scaling, although the oil must be circulated to prevent its becoming overheated.
This type of operation, however, is limited to small pieces and should not be applied to general hardening setups for which other methods are more effective. When hardening a tool such as a tap or small reamer, however, this submerged heating process has certain advantages. For work of this kind, heat is restricted to the teeth which heat rapidly, and as soon as the current is shut off, the part is quenched automatically. Several grades of light quenching oil are suitable for submerged heating, but, as previously mentioned, only small pieces that can be heated exceptionally fast should be considered.
As an example of an unusual type of hardening operation, which uses a 50 kW. 9,600 cycle generator, we’ll use a large sprocket, 24 inches in diameter, having 36 teeth, mounted in pairs into a fixture. The sprockets are 3/4 inches thick and the teeth are 3 to 4 inches deep and made of S.A.E. 1045 steel. Four pairs of teeth are hardened simultaneously in a total time of 20 seconds, which includes 10 seconds for heating, 4 seconds for quenching, and 6 seconds for indexing. Four complete sprockets are hardened in 6 minutes.
After the parts are loaded, the operation is entirely automatic. A hydraulically operated cylinder advances the fixture so the teeth to be hardened enter the induction coil. Upon completion of the hardening cycle, the fixture automatically returns, then indexes, and finally advances again for the next heating cycle. This example of induction hardening is representative of the possibilities offered by efficient tooling methods, which often are the major consideration in this class of work.
Another type of semiautomatic fixture, which is good for the through heating of small symmetrical parts is useful for small projectiles, although a variety of other parts can be similarly handled. The pieces are placed in the magazines and passed through to the heating inductors, which are multi-turn copper tube coils connected in a series. A cam-actuated plate holds the parts in the correct heating position for the predetermined heating cycle, after which the plate is withdrawn sufficiently to permit the five heated pieces to fall through into the quench tank located underneath. The plate then returns to the holding position, where the feeding plate (also cam-actuated and located directly above the coils) is withdrawn sufficiently to permit five more pieces to drop into the heating position. A third cam-actuated plate is also used to control the passage of the work pieces to the intermediate location prior to entering the heating zone.
Many modifications of this type of heating fixture can be provided, and in some cases, the shape of the part permitting, only a single cam-operated trip may be required. In any event, a study of the part in question should be made to determine the best method of handling.