HARDENING AND HEAT-TREATING

One of the principal uses of induction heating is in the hardening of steel parts, particularly where only localized surfaces require treatment.  As with any other use of this means of heating, however, the shape of the part and the area to be hardened must be suited to this method of heat transfer.  Parts requiring hardening all over do not usually lend themselves to induction treatment 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 treatment has definite advantages.  These are rapid heating with large production possibilities, uniform control so that rejections are minimized or eliminated, and, 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 warpage, so common with other forms of treatment, usually are eliminated.  There is also, of course, no formation of scale which often eliminates cleaning costs.

With high frequency generators it is customary to have a hardening table with proper provisions for handling the quench, whether submerged or spray quenching is to be carried out.  Two station 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 that one side of the table can be use while the other is being set up for another operation.

There are two rotary driven work holding spindles which 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.  In gear work, for example, it is possible to provide for the necessary heat cycle, which can be followed by a slight delay is desirable, and finally to include the necessary duration of spray quench.  Such a timer can also be used for the operation of solenoid devices or electromagnets, so that the heated part may be dropped for immersion quenching.

These hardening table is also equipped with an initial timer, so that the heating cycle required for a part can be recorded.  For example, in setting up for 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 is a simple matter to adjust the master timer so that 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 change over switch located at the center of the upright panel, so that either hardening station can be operated as desired.

A fixture and coil for the hardening of clutch teeth is located on a stud, positioned at the end of the fixture, and provided with a flange, so that the surface of the work to be heated is held at correct relation to the pancake type multiturn coil.  Current is then applied, and the heat pattern follows the contour of the clutch teeth, as represented by the light bank around them.  This photograph was taken during the heating cycle and is representative of the heat layer obtained on a part of this type.  At the completion  of the heating cycle a spray quench is automatically engaged, by means of a timer and a solenoid operated valve, so that the heated area immediately becomes hardened.  In handling heating operations for parts of this kind, it is obvious that the part which becomes heated is the only area that is hardened.  When automatic heating and quenching are applied,  as in this example, uniformity of hardening is assured.  The hardening operation requires 14 sec., of with 8 sec., represents the heating portion of the cycle.

Another example of an induction hardened part which is spray quenched is a gear made of S.A.E. C-1141 steel.  Three separate hardening operations have been 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 hardening of the eccentric surface, while another may be seen standing at the right of the fixture.  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 sec., for the eccentric; 16 sec., for the gear teeth; and 13 sec., for the clutch teeth.  A case depth of 1/16 in., is provided for the clutch teeth and the eccentric, while a depth of about 3/64 in., 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.

Hardening of machine centers on which only the ends are required hard.  Two parts are placed on the elevating fixture, which, in turn, is raised to the heating position so that the tapered ends of the center enter the work coils.  After obtaining the necessary heating, 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 is necessary to elevate the work into a coil, and where loading can be done more conveniently somewhat remote from the coil.

A vertical type induction heating equipment unit for the hardening of cam surfaces, gear, and bearing eccentric.  In this setup two camshafts 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 heating position, where the operation is repeated.  The surface hardened follow 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.  These, however, are eliminated as a result of selective hardening made possible by high frequency generators.

An induction hardening setup for treating the bearing surfaces on automotive axle shafts.  The heating inductors are arranged in series one above the other, so that two shafts are treated simultaneously.  These parts formerly required the use of inner hardened bearings, which were pressed on to the shaft; but by means of the induction heating equipment method it is now possible to localize harden the shafts themselves so they act as their own inner bearings, thus eliminating 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 bearings is used.  A change of this kind often results in increased strengths of 50 pep cent or more.

Advantage of being able to apply localized heat to a restricted surface which shows a standard form of thread ring gauges.  Gauges of this type usually are 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, and after the proper heat has been attained the part can be dropped into an oil or water quench.  Another advantage in hardening such a part by induction heating equipment is the elimination of the usual heavy scale prevailing in other means of heat treatment.

Many other tools and gauges are suitable for induction hardening, and it is 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 is preferable to use progressive heating methods arranged so that only a small area of the surface is heated at one time.  The work is mounted on an elevating platform, which feeds downward as the surface is progressively heated.  Directly below the internal type coil is located a pressure spray quench ring, which applies a water quench to the surface immediately after it is heated.  Rings for this purpose should be made of a nonmetallic material, such as plastics.  Since the heat is localized at only one portion of the work, there is little change of distortion and warpage, which ordinarily might be experienced on parts such as those heated by other methods.  In providing devices of this kind, it is desirable to include a variable feed mechanism so that 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 is to make a wooden pattern around which is cast a mixture of plaster of Paris.  With the pattern removed, after setting, the plaster forms a mold into which the liquid resign can be poured.  A core, preferably made of metal, is assembly in the mod, so that a shell-type ring is produced.  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 the other for applying the cover.  Commercial liquid plastics are available, which can be cured in 4 to 5 hr., at a baking temperature of 180 to 200˚F.  Some liquid plastic air harden in 24 to 48 hr., but the low temperature baking usually is preferred.  The re-baking of the shell, for molding the cover plate to it, does not affect the previously cured member.

Sometimes it is necessary to harden long bars, where progressive heating is required.  The bar to be heated travels on rolls located on each side of a sink, and is fed through a heating coil and finally through the spray-quench rings at the center.  The bar is fed by the rollers which are power driven.  By varying the speed rate of these rollers it is possible to obtain different feed rates in proportion to the size of the bar being 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.  A Simplified form of setup in which the work piece is suspended in a coil by means of a magnet.  The magnet is connected to a timer operating the control cycle, so that the current is released as soon as the heating cycle has been completed.  The work, which in this case is a small spline shaft, then immediately drops into the quench tank, located directly unit it.

Gear Hardening.  High frequency induction heating equipment is excellent for the 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 effect on the types of steels used for the making of gears.  Heretofore there is been a broad use of alloy steels, usually as a means of obtaining a specified hardness. However, the indications are that a 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 induction hardened gears, and surface hardnesses 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 sq. in., and a carbon content of from 0.45 to 0.50 per cent, 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, a steel with higher carbon content usually can be substituted for a carburizing steel, thus eliminating carburizing.  A 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 f a 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 saving of 43 per cent 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 per cent for 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 sec., using a generator having 20 kW output.

Gear-Hardening Methods.  In applying high frequency induction heating equipment for the hardening of gears and sprockets, where only localized heating and hardening is required, either of two methods may be considered.  One is the immersion quenching into a tank of the heated pieces, and the other is the spray quenching of 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 controlled by timers, which release the part after a predetermined heat cycle has been concluded.  Solenoid operated trips and releases also can be utilized in connection with automatic timers, dropping the work into a quench as may be desired.

With spray quenching it is 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 is often necessary to draw a part back to the required hardness.

Assuming a 3-in. diameter, 0.50 carbon steel gear requires hardening on the teeth only, a water spray quench 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 wanted, 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 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.

Gear Fixtures.  A representative type of spray-quench fixture for hardening a spur gear for this method a table with a built in sink and suitable drain is required.  The gear is mounted on a stud which, in turn, is fitted into a spindle.  Preferably the spindle should be power driven  and rotate during the heating portion of the cycle.  Rotation insures a ore uniform heat pattern and, naturally, 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 where a convenient method of turning is possible.

A multiturn induction coil surrounds the gear, and around this is located the spray-quench ring.  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 set up 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 in., hole, or one slightly smaller, will be found satisfactory.  The induction coil should be of the flat type and would so that 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 that 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 of 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 that the gear assumes the position.  At the same time the water quench is engaged by connections of the solenoid valve, which normally is 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.

In the hardening of gear teeth it is 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 is 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 is 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 multiturn 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 is a choice between oil-hardening and water hardening steels, it is 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 possible also 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 is necessary to heat the work and then drop it into an oil bath.

Heat Penetration.  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, with the result that the upper portion becomes deeply heated and the heat pattern resembles the 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 that 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 is 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 wanted, this can be accomplished by a double heat.  For this the teeth are heated somewhat deeper than merely on the surface, then a delay period is allowed.  Following this, the surface is reheated, then quenched so that a transition zone is obtained.  A cycle of this kind can be made fully automatic by the use of a multi-stage timer.

Going back to the larger tooth form, such as the 4 pitch referred to, it is 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 is 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 in high frequency hardening is the possibility of shaving the teeth before hardening.  Usually shaving is applied to heat treated gears having a hardness of 32 to 38 Rockwell C, with no subsequent heat treatment after shaving.  With the induction hardening equipment method, however, it is 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.  Typical shaving setup for gear teeth of a double cluster gear, which will be induction hardened on both sets of teeth as the next and final operation.  A typical tape reading of a gear before shaving, after shaving, and after induction hardening.  Each horizontal line represents 0.001 in., 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 hardening equipment is the elimination of cleaning after hardening.  Usually a scale is formed when gears are hardened, so that a cleaning operation is required.  With induction hardening equipment, however, practically no scale is formed beyond a discoloration of the surface.

Hardening-time Cycles.  As  a comparison of the time required for hardening standard types of spur gears by high frequency induction heating generators operating at about 200 to 400 kc., and with an output power of about 20 kW., a 3-in. gear can be heated in about 8 sec., and the quench will required 5 sec.  A 5-in., gear will require a heating cycle of 12 to 14 sec., followed by a quench of about 7 sec.  For a 7-in., gear the heating time will be 20 to 25 sec., followed by a quench of 10 to 12 sec.  All these estimates are based on the use of a 0.40 to 0.50 carbon steel of the water hardening type, using a closely coupled heating coil.

Another process change which has 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 well 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.  An example of a 6-in. diameter gear with a bushing assembled in the hole before the induction hardening operation.  The teeth of this gear are cut after the bushing has been assembled, which assures concentricity.

One of the outstanding advantages of high frequency heating for the hardening of gears is the possibility of heating 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 is no advantage in hardening the entire parts, it is possible, with induction heat, to harden the teeth only.

A double cluster gear of the same type made in one piece.  In processing this part each gear is hardened separately, so that two operations are required for hardening.  It would be possible to harden both gears simultaneously by making a double type induction coil, but problems involved in spacing the coil with relation to the work, as well as in compensating for the differential in the diameter of the gears, might cause complications.

After all, the 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 in., in diameter, is heated in 7 sec. with a quench of 4 sec., whereas the larger one, which is 4-in, in diameter, is heated in 13 sec. and quenching in 7 sec.  From this it will be seen that the total hardening time, aside from loading is 31 sec. 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 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 multiturn copper tube induction coil.  The gear measures 2 1/2 in. in diameter, and is heated in 10 sec., followed by a 6 sec. 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 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.

In hardening 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 section and, regardless of the king 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 high power  and high frequency, so that 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.

Fig. 98 - Various types of gears which lend themselves to induction hardening of the teeth only.

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 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 that 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 gears is 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 is 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 is likely to be some closing in of the hole.

By having full knowledge of what is needed to harden gears successfully by the high frequency induction process, it is possible to incorporate these requirements in the original design.  Generally speaking induction hardening can be applied to, perhaps, 90 per cent 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

Hardening one of the gear sections of the triple cluster gear.  Here the smallest gear is usually hardened first, next the medium sized, and finally 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 hardening the large gear last, it is obvious that the heat will be confined to the place where it is wanted and will have not effect on the other gears.

Hardening Bevel Gears.  In hardening bevel gears by means of high frequency heat, the same general procedure as for spur gears is followed. The induction coil is would 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 is 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 is 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 quench 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.

A bevel gear hardened by means of high frequency induction heating, which might offer some difficulties under usual procedures.  The design includes a hub that extends in front of the gear face, which naturally had to be assembled after the gear had been cut.  If the gear were hardened first and then brazed in place, the heat of brazing would draw the temper from the teeth.  With high frequency heating, however, it is possible to braze the shaft into the hub of the gear first, then follow with the hardening operation, localizing heat to the teeth only.  The gear is 3 1/4 in., in diameter, and requires a heating time of 12 sec., followed by a quench of 6 sec.

In hardening bevel gears by the high frequency induction heating process, the work should be rotated by power manually, in order to ensure uniform heating.  While it would be possible to construct and induction coil that would heat all portions of the gear teeth uniformly, this is not always the easy.  Therefore, by turning the work, it is 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 potion of the cycle averages only about one third of the total floor to floor time.  The balance 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 be 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 the average gear would run through 50 to 60 pieces per hour, so that the average current cost per gear would be less than 1/2 cent.

In Fig. 99 is shown 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, many be seen 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

Fig. 99 - A variety of gears which lend themselves to induction hardening, including spur, bevel and cluster types.

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.

Hardening Large Gear.  For the hardening of a large tractor gear, for which an 800-hp., high frequency inductor generator is used, and which is referred to in this chapter describing induction heating equipment.  This gear is made of S.A.E. 1045 steel and is 25.7 in., pitch diameter, with a 5-in. face.  The gear is preheated to 600˚ F before induction hardening of the teeth, and then placed on the fixture, which includes a platform mounted on a vertical spindle.  The spindle then is lowered for a distance of about 6 in., so that the gear is brought into proper relation with the surrounding heating coil, which has a coupling of 5/16 in.  The gear is rotated to assure uniform heating and after a 90-sec. heating period, during which time the profile of the teeth reaches a red heat, the current is turned off and a short time allowed for the heat to soak deeper into the case.  During this delay the gear is lowered to the quenching position and then quenched.

The final treatment of the gear comprises a tempering at 300˚F for approximately 1 hr., which produces a surface hardness of approximately 58 to 60 Rockwell C on the surface to lower readings.

Prior to induction hardening, the gear formerly 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 of the teeth are carried out with the 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 also is a considerable saving in alloy material, running into several pounds per gear.

Another application for induction heating is 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 in. in diameter, and 3 7/8 in. long, with a 1/8 in thick flange.  The test piece is machined all over, then inserted in the heating coil, being suspended from the flange.  The bar is heated with a minimum formation of scale to the proper temperature, then quench from the end by water flowing through a pipe having a 1/2 in. orifice, 1/2 from the end of the specimen, and with sufficient pressure to rise to a height of 2 1/2 in. 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 in., after which Rockwell hardness readings are taken at intervals of 1/16 in 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 of a surface are required, it is 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.  This coil has two tube for cooling, located between an outer and an inner sleeve, all 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 usually to small tools with thinner sections it is quite possible that developments will take place in both metallurgy and induction heating, so that its field will be widened.

Some test conducted with high speed steel 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 is often necessary to compromise to some intermediate grain size for most cutting applications.  In conventionally hardened high speed steel, there is about 7per cent of un-dissolved alloy carbides.

The microstructure of the induction heated high speed steel, rapidly heated for hardening the grain size is smaller and the degree of carbide solution 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 to convert bottom is represented the microstructure of under-heated high speed steel.  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 is expected that induction heating equipment will be more widely used in the future to treat all types of tool steel.

Hardening Crankshafts.  One noteworthy application for induction hardening is the treating of 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 is necessary only to harden the actual surfaces which are subjected to wear and at the same time, use a simplified form of carbon steel, such as S.A.E. 1050.  The operation which is of the vertical type, three or four bearing surfaces are hardened progressively.  The crankshaft then is processed through another induction hardening machine 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 is possible for one operator to service them consecutively and thus obtain a relatively high rate of production.  This method of hardening is carried out on large diesel engine crankshafts, in which 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.

In hardening such parts as crankshaft, it is necessary to use a split type or hinged inductor, made of two pieces.  The parts is located from two surfaces into 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, as may be seen.

Air Hardening.  In some cases it is possible to surface harden certain types of parts without the usually 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 sec., and then turning off the current, the surface heat dissipates into the cold mass underneath fast enough to create a so-called "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 a 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.  In spray quenching metal parts, it is possible to use a light oil instead of water when them metal or the steel so requires, a self contained oil system for this purpose.  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 bigger pieces a submerged quench is to be preferred.  The thing to watch is flashing of the oil.  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.  Or 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 usually is preferred, since it offers better temperature control.  Such units are available complete with automatic controls and can be installed directly on the oil tank.

Inasmuch as high frequency current can be made to operate in a submerged oil bath, it is often possible to carry out hardening operation advantageously by heating the oil.  Inasmuch as the part is not subjected to the atmosphere, there is 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 for general hardening setups, for which other methods are more effective.  In 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 which can be heated exceptionally fast should be considered.

Semiautomatic Fixtures.  An unusual type of hardening operation, using a 50 kW. 9,600 cycle generator the part is a large sprocket, 24 in. in diameter, having 36 teeth, which are mounted in pairs into a fixture.  The sprockets are 3/4 in. thick and the teeth are 3 to 4 in. deep and made of S.A.E. 1045 steel.  Four pairs of teeth are hardened simultaneously in a total time of 20 sec., which includes 10 sec. for heating, 4 sec. for quenching, and 6 sec. for indexing.  Four complete sprockets are hardened in 6 min.

After the parts are loaded, the operation is entirely automatic.  A hydraulically operated cylinder advances the fixture so that the teeth to be hardened enter the induction coil.  Upon completion of 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 through heating of small symmetrical parts to be heated in this case are small projectiles, although a variety of other parts can be similarly handled.  The pieces are placed in the magazines and pass through to the heating inductors, which are multiturn copper tube coils connected in series.  A cam actuated plate holds the parts in 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, when 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 also is 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.