Hardening with Induction Heating

Many types of steel are treated with heat to increase toughness and resistance to wear. The effectiveness of this process depends on the steel's carbon content. When the steel is heated above its transformation temperature (720°C), the carbon changes the steel's crystalline structure to an austenite. The harder, more brittle steel is then quickly cooled or quenched .

To make the steel less brittle and more usable, another process called tempering is used; the steel is slowly heated to just below the transformation temperature and then slowly cooled.

There are two general types of hardening processes: through- hardening treats the entire part, while case-hardening generally treats the part surface area and some of the interior area, according to the depth of hardening requirements for a specific application.

Modern induction heating provides reliable, repeatable, non-contact and energy-efficient heat in a minimal amount of time. For hardening, induction provides the necessary control and accuracy to focus the heat to a specific area of the part. Solid state systems are capable of heating very small areas within precise production tolerances, without disturbing individual metallurgical characteristics.

For case-hardening, typical frequencies are 450 kHz for case-depths of 0.030" to 0.080"; thicker case-depths of 0.100 to 0.150" are typically hardened at 10 kHz. It is important to heat the part quickly with high power density, and then quench the part rapidly to prevent the inside of the part from exceeding the transformation temperature. Through hardening is generally defined more by the time required to heat through the part than the by the frequency of the power supply

Typical RF power supplies for hardening range from 5 to 120 kW, depending on the material and application requirements.

The equipment shown here is a four station hardening machine. Each of the four stations has a 4 way horizontal turret with dedicated fixturing. The machine was designed for a leading manufacturer to allow them to induction harden any one of 16 different components to a predetermined specification without operator intervention other than for loading and unloading. One induction heater provides the heating energy to the work stations on a non-simultaneous basis. All process variables for all components are stored in a microprocessor allowing the equipment to be operated by non skilled personnel.

This photograph shows a typical vertical shaft hardening machine. The high frequency induction heater can be seen on the left and the control system that stores all process variables for some 200 component types can be seen on the right (the water and quench cooling unit is out of view behind the hardener). This equipment is used for hardening machine tool parts.

This Rotary Index hardening machine  is used for hardening a range of agricultural products. The control panel on the far left is for the water and quench cooling/pumping unit that is just to the right of it. The main body of the machine has a vertical opening access door to allow the loading/unloading of components. The control console (foreground) has a touch sensitive screen with visual display of the components to be processed and graphic indication of the areas to be hardened or annealed.

This photograph shows a section through a hardened shaft. The steel shaft is about 8mm (5/16") in diameter. The section has been polished and etched and clearly shows the hardened zone to be around 1mm (3/32"). This particular shaft is for an electric drill and has been processed on one of our High Frequency 4 index twin spindle rotary shaft hardening machines.