Temperature Measurement in Induction Hardening Processes

In order to reach optimal hardness during the induction hardening process, the metal workpiece must be heated rapidly for a defined period of time over a multi-step temperature profile. The quality and mechanical properties of the resultant parts are significantly influenced by the temperature accuracy and stability before the part is quenched.

When hardening the surfaces of steel, cast steel or cast iron components, the following properties have to be achieved: 

• The component has to attain the required hardness on all desired areas
• The desired or required hardness depth has to be achieved
• The component must not be damaged
• The distortion of the component should be minimal
• The process should be as short as possible
• The results have to be repeatable

The monitoring and the control of the temperature are essential in order to reach these targets. While too low temperatures lead to a too low hardness depth, too high temperatures may result in damage to the component. But also the duration of the inductively coupled energy, i.e. temperature, plays a significant role.

The heating process and energy requirements can be optimized through the use of non-contact temperature measurement and adequate controllers. Infrared thermal imagers are capable of measuring a broad temperature profile to provide improved detail of thermal gradients across the workpiece, enabling further insight into the hardening process.

However, pyrometers provide accurate and precise temperature measurements at a single fixed point and enable the reproducibility that is critical in induction hardening processes. In addition, pyrometers are capable of very fast measurement speeds, which is required for the rapid thermal changes of the induction process.

The use of pyrometers, thermal imagers, and fast controllers for temperature monitoring and process control can increase the quality, accuracy, and repeatability of many heat treatment processes.

The temperatures range between 300 and 1200 °C.

Use of pyrometers with a short wave spectral range recommended. Various pyrometers with different optics, spot sizes, and response times are available.

MIKRON Thermal imagers for measuring hardening processes from 600 °C.

•  MCS640 Series

For annealing temperature measurement after Induction hardening.

Various pyrometers with different optics, spot sizes and response times suitable for applications with temperatures from 50 °C.

MIKRON Thermal imagers for measuring hardening processes from 150 °C.

•  MC320 Series

E.g. for objects with unknown or changing emissivity, measuring objects which are smaller than the pyrometers spot size, measuring objects which move during the measurement or measurements through steam, smoke or dust. The use of ratio-pyrometers is recommended, respectively required.

The optical heads of fiber optics pyrometers can be used at ambient temperatures up to 250 C. They are unaffected by electromagnetic interferences, so that they can be used very close to the inductor. Their small design enables their use in locations that are difficult to access. They can be mounted very close to the measuring object. Optical fiber length up to 30 meters.

Ratio Pyrometers:

The induction process and the power of induction generators can e.g. be controlled using digital IMPAC pyrometers in combination with the extremely fast, digital, programmable PID controller PI 6000. This extremely fast controller (sampling time of only 250 µs) was especially designed for applications in which fast temperature measurement and control are required, e.g. in induction hardening processes.

• PI 6000

IMPAC pyrometers excel in following aspects: