Induction Hardening of gears

Induction hardening is a very popular process of heat treatment. This process involves giving heat selectively to the surface region of a gear by induction coils. Induction heating is able to heat up a steel's surface layers into the austenite region. After cooling (quenching), higher hardness structures like bainite and / or martensite are formed that has typically more hardness. The process of induction hardening has the ability to produce a wide range of case depths. Some of the popular design approaches for induction heat treating of gears are.

• CSFC: (conventional single-frequency concept)
• PSFC: (Pulsing single-frequency concept)
• PDFC: ( Pulsing dual-frequency concept)

CSFC (conventional single-frequency concept)
CSFC or the conventional single-frequency concept is applied for hardening gears that has small to mediumteeth. Though the CSFC approach is quite appropriate for medium-size gears and some times for lage gears also. Let's see an example of this approach.

(The image above depicts an induction gear hardening machine using the CSFC approach)

The particular gear being heat treated here is an automotive transmission component. It has a helical teeth on its inside diameter (ID) and a large teeth on its outside diameter (OD). Please note that there is need for induction hardening of both the parts. Inside as well as the outside diameters. A Higher frequency is required to harden the inside diameter of the gear teeth as compared to outside (OD) There is no doubt that CSFC is best for small to medium-size gears, but as stated earlier they can also be applied for large gears. This image is an example of a large gear getting induction hardened that has major diameter of 701 mm, a root diameter of 617 mm and thickness being 79 mm.

( Image of a multi-turn encircling induction equipment for a large gear employing the CSFC approach)

PSFC(Pulsing single -frequency concept)
PSFC or the Pulsing single- frequency concept Came into being to overcome the drawbacks or limitations associated with the CSFC approach in dealing with obtaining a contour-hardened profile of gears.

The PSFC provides the required heat flow towards the root of a gear without the problem of tooth tip overheating. A crisp and proper hardened profile which follows exactly the gear contour can be had by using high power density during the final heating stage.

A typical approach of PSFC (Pulsing single- frequency concept) comprises the following steps in a process cycle.

• Moderate-power preheat,
• Soaking stage
• Short high-power final heat
• Quench, followed by a low power heat for temper.

PDFC(Pulsing dual -frequency concept)
PDFC or the Pulsing dual -frequency concept is the third approach in the induction hardening process of gears. The PDFC or pulsing dual-frequency concept has been in use for a long time. PDFC concept utilizes two different frequencies for producing a desired contour pattern.. A successful approach of PDFC lies in obtaining a contour hardening profile for two very important gears like helical and straight spur.

In the PDFC approach there is preheating of gear within an induction coil to a specific temperature. This temperature is determined by process features. Mostly the temperature is around 100 to 350 ° Centigrades below the critical temperature.

PDFC can be completed using a single-shot mode or a scanning mode. Based on application, double coil design arrangements are used when applying the single-shot mode or scanning modes. There are generally two arrangements for hardening of gears.

• Single coil and double power supplies:
  The process is illustrated below. But this is not very reliable and involves complexities.
  
  
  
  
  
• Double coil and power supplies:
  One coil does the preheating, while the other final heating. Both coil works in tandem in case of scanning mode. While in case of single-shot mode, a two-step index-type approach is generally applied.

Tooth By Tooth Induction Hardening
Tooth by tooth induction hardening is very well suited to coarse pitch gears. The gears case hardened this way produces full tooth contoured hardness pattern.

During an induction heating process, as the inductor is made to pass through the tooth space, it hardens specifically the areas that are tooth wear affected. The open air process ensures that there is hardening to proper depths with minimal heating of adjacent teeth. Distortion of any type is controlled by using precision scanning preferebly on CNC controlled equipment. Tooth by tooth induction hardening process offers an excellent medium for heat trating vis-a-vis a typical furnace process like carburizing.