Induction Heating Applications

Accelerated Austenitizing with 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 Accelerated 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.

Charted data of residual compressive stress measurements 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 progressive 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 Manufacturing 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.