Simultaneous dual frequency applied to a single inductor. Plot shows coil voltage  wave shape of medium frequency with superimposed high frequency; coil voltage is    30% high frequency and 70% medium frequency. Photo shows uniform heating of       gear profile.

 
Simultaneous dual-frequency induction heating adds a new dimension to the variables that control the induction heating process.  The process requires less energy and    space than earlier dual-frequency induction heating systems, as well as atmosphere furnace treating systems.  It also can be more easily incorporated into an automotive manufacturing line or flexible work cell.  In addition, the process provides a cost     benefit by allowing the use of high-carbon steels, which are ideally suited for induction hardening, compared with more expensive high alloy steels used with thermo-chemical processes.  The reduced overall heating associated with simultaneous frequency heat-treating technique improves the microstructure of the hardened area, has minimal    effect on material core properties and, in most cases, eliminates the need for finish machining to meet dimensional specifications after hardening.

 

Initial roadblocks

From the very beginning of the industrial application of induction heating to heat       treat gears, engineers have attempted to obtain a uniform cast-hardened surface         on the tooth profiles of gears.  Case hardening increases both the abrasion resistance   of the tooth face and the fatigue properties of the root of the tooth without through hardening.  However, early induction heating approaches were limited to what could      be achieved using a single-frequency power supply, or two power supplies operating independently at different frequencies to sequentially heat the gear plus the     mechanics necessary to switch power and move the part.

Limitations of a single-frequency power supply and the limited frequency range of      early induction equipment only allowed achieving one heat-treating requirement at a time.  For example, the strength of the root of the tooth could be achieved (Fig. 1)   using a frequency of 10 kHz, the classical medium frequency available using motor-generator sets.  However, the abrasion resistance of the tooth face also was     increased a the same time at the cost of through hardening the tooth, which          made the gear susceptible to fracture.   

The tooth face could be hardened without through hardening using high frequency     (200 to 450 kHz) generated using vacuum-tube oscillators.  However, high       frequency was  Not suitable to harden the root of the tooth because the tooth            tip was overheated and the tooth eventually was through hardened as a result of        the longer heat time required to reach the root area.

Fig 1 Image of the root of the tooth hardened using a medium frequency (10 kHz).    Root-circle diameter: 10 mm (0.40 in.); tooth-tip diameter: 20 mm  0.785in.)                                                                                          

Fig 2 Image of the root of the tooth hardened using a high frequency (120 kHz).      Root-circle diameter: 10 mm (0.40 in.); tooth-tip diameter: 20 mm (0.785in.).                                                                                         

Fig 3 Image of a tooth hardened using the simultaneous dual-frequency (10 kHz          and 120 kHz) method. Root circle diameter: 10 mm (0.40 in.), tooth tip diameter:             20 mm0.785in.)                                                                                                        

Mechanical approaches

No generators were available to produce a frequency between 10 and 200 kHz, and     the available medium and high frequencies were too low too high, respectively, to achieve the ideal case characteristics.  Therefore, induction heating system manufacturers attempted to solve the problem using mechanical methods, leading to development of a dual-frequency process.

In the process, both medium and high frequency ranges are applied separately, one   after the other, to the same work-piece.  This requires one medium and one high-frequency power supply; each power supply has its own inductor, which is spatially separated from the other.  The root of the tooth is austenitized in the first inductor  using the medium frequency circuit.  The workpiece then is indexed into the second inductor using the   high-frequency circuit to austenitizing the tooth tips and tooth faces.  The gear is then quenched.  To avoid through heating of the tooth, the final austenitizing operation must be very short (<1 second), which requires very high   indexing and power switching speeds.  Therefore, the process never became popular because of very high demand on mechanical component life and accuracy.

 

Conventional approaches

The development of solid-state, high-frequency generators in the range of 20 to         200 kHz made it possible to improve the dual-frequency induction case hardening process.  The workpiece is heated to a temperature below the austenitizing     temperature using low frequency, or a low proper density, then the skin is      austenitized using a high-frequency pulse, and the parts is quenched.

The hardness contour achieved using this method closely approximates the           desired contour (Fig. 2).  However, the maximum hardness depth still is achieved         at the tooth tip where no hardening is necessary, and the hardness depth at the       root is marginal in most instances.

Generally, the gear is over-heated at the tooth tip and underheated in the root        area.

A refinement of the dual-frequency method uses one common inductor, which is         first incorporated into a medium-frequency power supply circuit and then switched       into a high-frequency circuit.  The time required for switching is about 0.5 second.  Overall heating time typically is about 10 to 12 seconds.  Quench time varies from            10 to 15 seconds depending on material and residual heat in the part.  Combines       heat and quench cycles range from 20 to 27 seconds.  Part transfer time is added        to determine total processing time.
 

Further developments

A further refinement to the dual-frequency method eliminates the need for          switching frequencies.  In the process, a common inductor is simultaneously supplied  with medium and high-frequency energy.  The frequency mixture at the inductor   consists of a high-frequency oscillation superimposed on a fundamental medium frequency.  The amplitude of both oscillations can be controlled separately.  In this    way the power component of both frequencies, and, therefore the case hardening   depth in the root of the tooth and in the tooth face can be adjusted and set   separately depending on the requirements for the part.

Simultaneous dual-frequency heating achieves a hardened, uniform case tooth        profile without influencing the core in a heating time of less than 0.5 second            (Fig. 3).  Quench time is reduced to 1 second or less, many gears are self-        quenched due to the rapid, shallow heating characteristics of the process.

Automotive transmission gears require not only good abrasion and fatigue properties,    but also must have low distortion.  The amount of distortion directly relates to the    level of noise generated by the transmission;  the noise level increases with       increasing distortion.  There is a direct relationship between the amount of         distortion and the volume of material heated;  the more material heated, the         greater the distortion.  Longer heat times at lower frequencies used in the pre-       heating stage of conventional processes results in deep heat penetration, with a    greater potential for distortion.  The shorter heating time of simultaneous dual-   frequency induction hardening at a given case hardening depth requires less          thermal dissipation resulting in less distortion.

Accelerated Austenitizing  Simultaneous Dual Frequency Induction Heating

There hasn't been a better time than now to evaluate the quality and economics of    your heat-treating processes. Accelerated Austenitizing may provide you with the competitive edge you need in today’s economic climate.

Higher production rates, improved quality and lower distortion are the key benefits         of Accelerated Austenitizing. Recent developments in applying Austenitizing with Simultaneous Dual Frequency induction heating has provided new inroads into      producing higher quality microstructure at faster processing cycles and lower distortion results than conventional heat-treating processes. With the advent of using two frequency ranges, medium and high frequencies, has expanded the use of high flux intensity heating through uniform heating of nonlinear surfaces such as gears      sprockets camshaft and other nonlinear complex surfaces. High flux intensity heating     to produce the Accelerated Austenitizing process have long been known to produce a very fine grain microstructure. The process is credited for producing the metallurgical characteristics needed to produce higher surface hardness with less distortion than conventional induction heating processes.

Recent developments at Electroheat Technologies have found other benefits to the Accelerated Austenitizing process. One of the new advantages is the phenomena of  mass quenching. Mass quenching of complex surfaces was only possible in the past   with laser heating applications. The reason being the laser was able to heat the surface temperature very rapidly with intense heat for a very short period of time. This rapid   high intensity heating allowed the part mass below the heated surface to rapidly    remove the heat through natural heat flow conduction. This is only accomplished by  rapid acceleration and heat transfer of high intensity energy.

For various reasons, which are beyond to scope of this report, laser heat-treating has had limited successful applications in the heat-treating industry. Accelerated  Austenitizing with Simultaneous Dual Frequency can provide the benefits realized        with laser hardening without the high capital and maintenance costs associated with  laser equipment. The AA/SDF process uses extremely short heat times in the range        of 0.2 to 0.3 seconds, without any part preheating. During the short heat cycle the  entire surface intended to be heat-treated is heated to the optimum austenitizing temperature then “auto” or mass quenched. The mass quench process eliminates the need for liquid quenching systems and does away with all the variables associated with conventional quenching practices. That is, quench variables normally controlled and monitored to maintain process integrity is no longer required. The process quality assurance matrix has been effectively reduced by as much as 50% thereby simplifying  the heat-treating process and subsequent monitoring.

There are other benefits associated with the AA/SDF process; lower distortion and an increase in residual compressive stresses than what can be achieved with conventional induction heating processes. Lower distortion allows for post heat-treating, that is the green part is machined to finish dimensions before heat-treating thereby eliminating the need for dimensional finishing operations. Further benefit of increased circumferential residual compressive stresses and more consistent measurements have been verified in tests using X-ray defraction analysis. Tests conducted on a powered metal camshaft sprocket yielded a gain of more than 34% in residual compressive stresses. This benefit  is extremely advantageous for gear and sprocket tooth strength and bending fatigue  life. Surface hardness of about 840 HK100 or about 64.5 HRC were recorded. The core material was 280 HK100 or about 29 HRC.

Sample Source          Residual Stress Ksi       Confidence (+/-)
AA/SDF  -01                    -63.2                       3.0
AA/SDF – 02                    -67.1                       5.9
Conventional  -01             -47.3                       4.2
Conventional – 02             -12.2                       4.6

Charted data of residual compressive stress measurement taken on a powdered metal cam sprocket.

Austenitizing temperatures for the AA/SDF process is higher than normally used with conventional induction heat-treating. The higher temperature is needed to get most materials in solution with the short heat time, less than 1 second, and as a result a   clear microstructure with fine martensite is realized. Microstructure analysis has also verified the nonexistence of any retained austenite or grain coarsening that might have been present due to the higher austenitizing temperatures.

In cases where the process requires longer heat times, more than 0.4 seconds, then      it may be necessary to use a quench medium to augment the mass quenching effect.  The longer heating times are usually due to power restrictions or the use of pro-  gressive heating methods such scan heating as apposed to single shot heating or  the use of a pre-heating cycle. Mass considerations are also a deciding factor. If there is insufficient cross section of mass behind the area being heated to rapidly conduct the heat, then quench augmentation will be necessary. In any event the short heating    times mean an improved quenching action for most materials. Successful results with  the AA/SDF process have been achieved with gears (fine and coarse pitched),   sprockets and camshafts. Future work will be conducted on crankshafts for fillet and  non-fillet  hardening applications. AA/SDF looks very promising for providing superior crankshaft  fillet and journal hardening. Materials used to date include but have not   been limited to SAE 4140, SAE 1050 Mod, SAE 5046, CF-53, powdered metal, ductile   and malleable iron. This wide range of materials have all been successfully hardened    with the Accelerated Austenitizing process. Materials usually requiring longer heat    times when applying conventional induction heating methods in order to get into   solution are processed using  a higher concentration of SDF energy in order to produce higher than normal austenitizing temperatures.

Induction tooling requirements are similar to what is used for conventional induction  heat-treating with the exception of using flux concentrators to maximize the efficiency of the process. Both polymer bound iron concentrators, supplied by Fluxtrol Manu-facturing Inc., and iron laminations have been used successfully with simultaneous      dual frequency applications. In all cases the material used to carry the current in the inductor is oxygen free electronic copper. This selection of material assures the optimum in induction heating characteristics and efficiency of the inductor.

In all cases the inductor is a machined designed inductor as apposed to bent tube configuration. The machined inductor concept assures close inductor to part coupling capability to obtain the most efficient and uniform heat as possible. The short heat durations, less than 0.4 second, means there is minimal conduction of heat in the part. The hardened pattern is a result of actual induced current flow in the part. This means the coil dimensions become more critical in order to maintain uniform heating. It must  also be realized that the short heat time involved with the AA/SDF processing technique means there are fewer part rotations in the inductor during the heating process, so coil flux uniformity and rotation speed become more relevant in providing a uniform circumferential heating profile.

The future looks excellent for Accelerated Austenitizing using simultaneous dual  frequency heating for induction heat-treating applications. The application of this  process for induction heat-treating is only limited by our imagination.

 

For more information:  Mike Hammond is President, Electroheat Technologies, LLC, 3578 Van Dyke, Almont, MI 48003; tel: 810-798-2400; fax: 810-798-2402; e-mail: mhammond@electroheat-technologies.com