Parts are heated to the required austenitizing temperature in less than one second producing high-quality parts at high production rates.

Accelerated austenitizing using simultaneous dual frequency induction heating

Accelerated austenitizing can be defined as heating to a higher than "normal" austenitizing temperature range in less than one second. Key benefits are improved product quality, lower part distortion and higher production rates. Compared with conventional induction heating processes, the process is capable of producing the required metallurgical characteristics to achieve higher surface hardness with less part distortion.

It has long been known that achieving an accelerated austenitizing process by means of high flux-intensity heating produces a very fine grain microstructure. Further progress has been made by Electroheat into producing high-quality metallurgical characteristics at faster processing cycles and with lower part distortion by applying accelerated austenitizing using simultaneous dual frequency (AA/SDF) induction heating compared with results using conventional heat-treating processes. Furthermore, using two frequency ranges (medium and high frequencies) has expanded the use of high flux-intensity heating by making it possible to achieve uniform heating of nonlinear surfaces such as those found on gears, sprockets, camshafts and other nonlinear complex shapes.

(Fig. 1) Uniformly case hardened automotive balance shaft sprocket using a heating time of 0.22 seconds and 87 kW power

Process advantages

Other benefits to the accelerated austenitizing process have been identified. One of the newly discovered advantages is the phenomenon of mass, or self-, quenching. In the past, mass quenching of complex surfaces was only possible in laser-heating applications. This was possible because the laser is capable of heating the surface to the required temperature very rapidly by means of intense energy input to the part surface for a very short period of time. Rapid high-intensity heating allowed the mass of the part below the heated surface to rapidly remove the heat through natural heat flow conduction. This is only accomplished by the rapid acceleration and heat transfer of high intensity energy. However, for various reasons, laser heat treating has had only limited successful application in the heat-treating industry.

Accelerated austenitizing using simultaneous dual frequency can provide the same benefits realized using laser hardening without the high capital and maintenance costs associated with laser equipment. The AA/SDF process uses extremely short heating times (in the range of 0.2 to 0.3 seconds) without the need for part preheating. During the short heating cycle, the entire surface intended to be heat-treated is heated to the optimum austenitizing temperature, then mass quenched. The mass-quenching process eliminates the need for liquid quenching systems and also eliminates all the variables associated with conventional quenching practices. That is, control and monitoring of quenching variables to maintain process integrity are no longer required. This effectively reduces the process quality assurance matrix by as much as 50%, thereby simplifying the heat-treating process and subsequent monitoring.

Other benefits associated with the AA/SDF process include lower part distortion and an increase in residual compressive stresses than what can be achieved using conventional induction heating processes. Lower distortion makes it possible to machine the green part to final dimensions before heat treating, thereby eliminating the need for dimensional finishing operations.

(Fig. 2) Simultaneous dual frequency power curve of a 75-kW medium frequency/75-kW high frequency universal power supply

Increased circumferential residual compressive stresses have been verified in tests conducted by Climax Research Services (Wixom, Mich.) using x-ray diffraction analysis. Consistent measurement values reflect the uniformity of the induction heating process. Results of tests conducted on a powder metallurgy camshaft sprocket show 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. In addition, surface hardnesses of about 840 HK100, or 64.5 HRC, were measured. The core material had a hardness of about 280 HK100, or 29 HRC.

(Fig. 3) Small CF-53 alloy gear uniformly case hardened in 0.12 seconds

Microstructural benefits

Austenitizing temperatures for the AA/SDF process are higher than those typically used for conventional induction heat-treating. The higher temperature is required to get most materials into solution in the short heating time, and therefore, produce a "clear" microstructure consisting of fine martensite. Microstructural analysis verifies the nonexistence of any retained austenite or grain coarsening that could result due to the higher austenitizing temperatures.

(Fig. 4) Ductile-iron cam lobe hardened in 0.30 seconds and mass quenched

In cases where the process requires longer heating times (more than 0.4 seconds, for example), it might then be necessary to use a quench medium to augment the mass-quenching effect. The need for longer heating times usually is due to power restrictions, the use of progressive heating methods such scan heating as opposed to single shot heating and the use of a preheating cycle. Part mass considerations also are a deciding factor. If there is an insufficient mass cross section to rapidly conduct the heat behind the area being heated, then quench augmentation will be necessary. Even in this situation, the short heating times mean an improved quenching action for most materials.

(Fig. 5) Typical high power density inductor used for accelerated austenitizing of an automotive cam sprocket

Successful results using the AA/SDF process have been achieved for gears (fine and coarse pitch), sprockets and camshafts. Future work will be conducted on crankshafts in 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, powder metallurgy grades and ductile and malleable iron. This wide range of materials has been successfully hardened using the accelerated austenitizing process. Materials that typically require longer heating times to get into solution when applying conventional induction heating methods are processed using a higher concentration of simultaneous dual frequency energy to produce higher than normal austenitizing temperatures.

(Fig. 6) Automotive transmission internal gear hardened in 0.62 seconds using the SDF process

Tooling considerations

Induction tooling requirements are similar to those required for conventional induction heat-treating with the exception of using flux concentrators to maximize the efficiency of the process. Both polymer-bound iron concentrators (Fluxtrol Manufacturing Inc., Auburn Hills, Mich.) and iron laminations have been used successfully in simultaneous dual-frequency applications. Without exception, the material used to conduct the current in the inductor is oxygen-free electronic copper. This material ensures optimal induction heating characteristics and inductor efficiency. Machining is used to obtain the inductor design in all cases, as opposed to a bent-tube configuration. The machined inductor concept ensures close inductor-to-part coupling capability to obtain the most efficient, uniform heating possible.

The short heating time of less than 0.4 second means there is minimal conduction of heat into the part. The hardened pattern is a result of actual induced current flow in the part. This means the coil dimensions become more critical to maintain uniform heating. It also should be realized that the short heating time involved with the AA/SDF processing technique means there are fewer part rotations in the inductor during the heating process. Therefore, coil-flux uniformity and rotation speed become more relevant in providing a uniform circumferential heating profile.

Now is a good time to evaluate the quality and economics of your heat-treating processes. Accelerated austenitizing could provide the competitive edge needed in today's economic climate.