Surface heat treatment of steels

Surface heat treatments of ferrous alloy parts are designed to improve their resistance to fatigue and/or wear by hardening critical surface areas with heat inputs that are brief and localized enough to confine structural effects. The associated thermal cycles involve either rapid, localized solid-phase austenitization heating (typically a few seconds above 800°C) followed by natural or forced quenching, or surface remelting followed by cooling to achieve the desired structural evolution (e.g. transformation of grey cast iron into white cast iron).

In addition to these heat-only treatments, surface remelting treatments are also being developed in the presence of a filler material in the form of a pre-deposit or sprayed powder: simple enrichment, coating with a new alloy, inoculation of hard or refractory particles...

In this article, we will only deal with the family of treatments without filler material, focusing primarily on the solid-phase hardening of steels.

In the latter field, which is economically important for the mechanical engineering industries, surface hardening by quenching after heating with electromagnetic induction (subcutaneous thermal source of high-frequency induced currents) is the most widespread.

The growth of this process is linked to its technical and economic advantages:

• cost-effective (yield) and flexible limitation of energy input;

• availability (lead time) and flexibility of the source ;

• fast, non-polluting cycles for production line processing;

• quality of treated parts in terms of reproducibility of hardening, risk of deformation, ease of natural compression of surfaces, low thickness affected by oxidation.

The particular development of these treatments in the automotive industry has led to the development of generators and inductors adapted to high production rates, as well as automated transfers enabling just-in-time industrial management.

Other thermal sources capable of high surface power, such as continuous or pulsed lasers, electron beams and plasma torches, enable even more precise energy localization than induction, and are beginning to be used in industrial surface hardening applications. Despite the fact that investment costs have come down to reasonable levels, they are still mainly used for treatments that are difficult in terms of access and details of the areas to be treated.