Induction heat treatments, including hardening, modify the mechanical properties of steel thanks to rapid and controlled heating, followed by a cooling phase whose speed varies according to the treatment and the desired result.
In this article, we explain how hardening, tempering, and annealing work, how they differ, and why precise temperature control makes a difference in the final result.
In industrial applications, in fact, the success of a heat treatment depends not only on reaching the required temperature but on the ability to repeat it precisely cycle after cycle, keeping all process parameters constant.
We then present the induction heating technologies designed and manufactured by SEIT Elettronica, which offer companies advantages in terms of accuracy, control, and repeatability in industrial processes, including automated ones.
Induction hardening: what it is and how it works in steel heat treatments
Hardening is the heat treatment that increases the hardness of steel. With induction technology, the heating phase takes place in a localized and repeatable manner, a condition that allows reaching the required temperature targets without heating the entire component.
This feature is particularly useful when it is necessary to treat only a specific zone of the component, limiting deformation and preserving the mechanical properties of the parts not affected by the treatment.
What is induction hardening and what is it used for
Induction hardening is a heat treatment that brings steel beyond its upper critical temperature (Ac3), i.e., the threshold at which the structure transforms completely into austenite, and cools it rapidly, so as to transform the structure of the material, making it harder and more wear-resistant. It serves to improve the mechanical performance of components subjected to high loads or continuous friction.
A frequent example is the hardening of industrial springs made of AISI 420 stainless steel. With the induction heating technologies by SEIT Elettronica, the piece, obtained by bending the sheet metal when it is still soft, is brought to 1100 °C in a few seconds and then rapidly cooled: it is this sequence that gives it the hardness and elasticity necessary to withstand repeated loads, maintaining the ability to return to its original shape.
How induction heating works
Induction heating exploits an alternating electromagnetic field generated by an inductor: the induced currents generate heat directly within the conductive material, without contact and without flame. The depth of heating depends on the working frequency, the characteristics of the material, and the geometry of the inductor.
The generator brings the piece to temperature, but the result of the treatment depends on knowing exactly which temperature is reached: for this reason, the thermal data must be detected and transmitted in real time.
Compared to traditional furnace methods, induction hardening offers concrete advantages:
- Localized heating: only the zone that needs to harden is treated, reducing deformations and preserving the properties of the rest of the component.
- Speed and energy efficiency: the heat is generated directly in the piece, so heating times drop to a few seconds and waste is limited.
- Process repeatability: by setting the power, time, and temperature parameters, each cycle replicates the previous one with the same precision, a useful requirement in mass production.
These elements make induction suitable for both manual processing and integration into automatic lines.
In industrial productions, however, the result does not depend exclusively on the induction generator. The inductor design, the choice of working frequency, the temperature control, and the definition of process parameters represent determining elements for achieving a stable and repeatable treatment.
Steel tempering: when it follows hardening
Tempering is a heat treatment that heats already hardened steel to a lower temperature than the so-called lower critical temperature (Ac1), i.e., the threshold beyond which the structure of the steel begins to transform. This process reduces its brittleness while maintaining much of the acquired hardness.
However, tempering does not always follow hardening: this process is carried out only when a hardened piece is too rigid and toughness needs to be restored to it. In these cases, hardening and tempering form a sequence, as occurs for the treatment of many tools and mechanical components subjected to high stresses. In the induction tempering of spindle holder , managed with SEIT technologies, for example, the component is brought to and stabilized at 700 °C to relieve residual stresses left by previous processes, including hardening, improving dimensional stability.
Accurate temperature control during tempering is essential to avoid unwanted variations in the mechanical characteristics of the component.
Steel annealing: an independent process
Unlike tempering, annealing does not presuppose prior hardening: it is a heat treatment by itself that involves heating followed by slow cooling, with the aim of softening the material, relieving internal stresses, and improving its workability.
Induction annealing is often used when it is necessary to intervene only on specific areas of the component, avoiding subjecting the entire piece to a complete thermal cycle. It does not act on a hardened piece but returns the steel to a more ductile condition.
A fairly frequent application case is the annealing of steel heating elements for the refrigeration or air conditioning sector: the curved section of the heating elements is brought to 750 °C to reduce the stresses generated by bending, increasing the reliability and durability of the component.
The difference between tempering and annealing therefore lies in the purpose of each treatment and in the starting conditions: tempering corrects an already hardened steel to reduce its brittleness, while annealing softens the steel with a slow cooling to make it more workable.
Temperature control: SEIT technologies in industrial processes
The quality of all these treatments depends on the ability to reach and maintain precise and well-defined temperature targets. SEIT induction technology allows achieving this type of precision thanks to Power, Time, and Temperature (PTT) control programs, which regulate heating with an accuracy of ±5 °C and save every profile to replicate it at each cycle.
Each PTT program is developed for the customer’s specific application and allows replicating the process while keeping the parameters defined during the testing phase constant.
To measure and transmit thermal data in real time, two instruments are used, chosen based on the type of processing:
- Pyrometer: provides the precise temperature reading at a specific point on the treated piece, a solution suitable when control needs to focus on a circumscribed area.
- Thermal camera: measures the temperature over the entire framed area (thermal window) and identifies heat zones, communicating directly with the generator to modulate the power during the process.
The choice of measuring instrument does not depend exclusively on the temperature to be detected, but also on the component geometry, the surface to be controlled, and the characteristics of the production process.
In both cases, it is the temperature data that governs the action of the generator, to ensure that the treatment remains within the established parameters. The same temperature control makes induction suitable for other precision processes as well, such as induction brazing. The generators of the Platinum HF and Platinum MF lines by SEIT Elettronica are designed for manual use or for insertion into automatic lines, guaranteeing the traceability of each cycle.
The dedicated software manages the heating parameters from an Industry 4.0 perspective and allows storing and recalling programs on multiple generators. Once fine-tuned, in fact, the same treatment can be transferred from one station to another while maintaining identical temperature targets. In this way, with SEIT technologies, induction heat treatments become a documentable and repeatable process on a large scale.
Want to define the most suitable heat treatment for components produced in your company’s industrial processes? Contact SEIT Elettronica: we will evaluate the best induction technology together.
