Induction hardening heat treatment

Among the heat treatments of steel or cast iron, particularly significant is induction hardening, used to increase the surface hardness of mechanical parts that are subject to important wear. With induction hardening their resistance and fatigue limit are greatly improved allowing a substantial lengthening of their life cycle.
Induction hardening is preferable, compared to other heat treatments with oven heating such as gas nitriding, due to:

  • yield efficiency;
  • speed;
  • energy saving.

In fact it is possible to reach similar mechanical characteristics by means of a more convenient treatment in terms of time and costs, with the advantage of no shape/dimensions restrictions for the pieces to be treated and applicable on single unit up to large volumes. Moreover, there is a considerable energy saving resulting from the immediate heating (thanks to the induction) of a specific and limited area of the component to be treated.

Induction hardening vs nitriding

Going on into the comparison with nitriding, induction hardening allows to reach absolutely much greater hardening depths, up to 20-25 mm, and without requiring specifically dedicated steel types, as it is based on the structural transformation of the material and not consequent to a chemical reaction as it happens with gas nitriding (in this case triggered by the forced absorption of nitrogen).
Moreover, the exclusive speed of induction hardening makes it a “light” manufacturing phase compared to other heat treatments and mechanical machining: in a few steps, based on the study of the drawing and the technical requirements, it is possible to estimate a quotation defining supply conditions and delivery terms, the latter generally in the order of a few days but, when possible for urgent cases, even only 1 day!
As Termostahl we offer an induction hardening service matching all these benefits: thanks to many installed plants and our production flexibility it’s possible to assign specific priorities and planning the hardening treatment on the first available machinery, facilitating customer’s request as much as possible.
For particular situations or mechanical parts difficult to transport, taking advantage by the installation / dimensional features of some of our systems, we can also perform hardening treatment “on site” at the customer’s factory (premises), moving the hardening plant and the necessary equipment. The know-how we have acquired, the possibility to support the customer in the cost/benefit analysis during the project phase, accurate feasibility checks and dedicated inspections, allow us also to reach this, rare but useful, capability.

Induction hardening: technical details

The heat treatment based on induction hardening consists in:

  • a) the instant heating of the area to be reinforced, at a temperature above the austenitizing point of steel Ac3 (900 ° – 950 °);
  • b) the maintaining of heating at this temperature for an appropriate time of homogenization of the material;
  • c) the subsequent fast cooling (with jets of water or other coolants), in order to allow the desired transformation.

The temperature at which the steel is heated, and the residence time at this temperature, must allow the starting structure to become completely austenitic without exceeding criticality thresholds. Therefore the cooling speed must be particularly high and controlled, to avoid reaching temperatures that trigger unsolicited and not advantageous transformations.
The induction hardening heat production is carried out by means of currents, as the name implies, “induced” into the body to be heated when it is subjected to the action of an alternating magnetic field.
The magnetic field is generated by an inductor coil which transfers the energy to the body to be heated by electromagnetic means and therefore without direct contact between the elements.
The induced currents, also called “Foucault currents”, give rise to losses due to the Joule effect directly into the material to be heated, which constitute the heat sources necessary to increase the body temperature.
The magnetic excitation field is obtained applying an alternating current, characterized by specific intensity and frequency, through an induction coil generally made of a copper tube internally cooled by means of water circulation
Exploiting this characteristic of the conducting bodies subjected to magnetic field, that induces electrical currents in the neighboring conductors hooked on the excitation field frequency, it is possible to obtain different hardening depths just varying the current frequency.
Specifically, HIGH FREQUENCY systems, based on currents between 100 – 1000 KhZ, are used for induction hardening heat treatments in order to obtain medium/low hardening depths, usually on small mechanical parts.
MEDIUM FREQUENCY induction hardening plants, between 3 – 100 KhZ and with power from 10 – 1.000 Kw, are used to obtain greater depth of hardening on medium-large pieces.
LOW FREQUENCY induction hardening systems, between 50 – 500 hZ, are used to obtain big (high) depths (up to 20-25 mm).

Induction hardening - Applications

The advantages of induction hardening makes it very useful for a wide range of mechanical products related to different industrial sectors. Here are some applications:

  • Automotive: crankshafts /camshafts, transmission and steering parts, dies and cutting dies (used to obtain doors, bonnets, panels, etc.), cutting blades, nuts and bolts.
  • Railway: multiple components of rolling stock, exchange boxes, various linkages.
  • Naval: transmission shafts, gears, components for military applications.
  • Oil & Gas: precision shafts, toothed wheels, pinions typically used in off-shore platforms.
  • Aeronautical: pins, shafts, different gears.
  • Metallurgical: cold milling rolls, work rolls, back-up rolls, toothed crowns, worm screws.
  • Various sectors (metal sheet / paper / chains / machine-tools / food): special bearings, rolls for plate and tension levelers, spiral rollers, embossed rolls, plates, bushings, cams, big screws, linear guidance shafts, wheels for bridge cranes, tye-bars, etc.

As Termostahl we have been working in induction hardening for more than 60 years, in all industrial sectors where this treatment is applied, particularly with Italian customers in the north/central Italy areas due to logistic reasons, as transport costs could significantly affect. When requested we also work with companies in southern Italy and in neighboring European countries such as France, Switzerland, Germany, Slovenia.
Our range of action extends from Turin to Udine and up to Bologna-Florence, including all the major manufacturing districts of the provinces of Milan, Brescia, Bergamo, Verona, Vicenza, Padua, Venice, Pordenone, Parma, Modena, Genoa.
We also actively collaborate with research centers and university laboratories for the engineering of specific production processes, as many times the design phase requirements need particular optimization and fine-tuning before being widely and completely safe applied.