Steel: Carburising and Quenching

Carburising of steel
Carburising is an ancient heat treatment process which consists in increasing the carbon content at the surface of the steel in order to modify its mechanical properties. Originally, this process was used to improve the hardness and wear resistance of metal parts made from low carbon steels.
In principle, carburising is based on a simple idea: exposing the steel to a source of carbon at a high temperature for a sufficiently long time so that carbon atoms can gradually penetrate the material. This phenomenon, known as diffusion, progressively transforms the structure of the metal at the surface, making it capable of being quenched afterwards.
This treatment was widely used in Antiquity and during the Middle Ages for the manufacture of tools and weapons. It made it possible to obtain a good balance between external hardness and internal toughness.
Three main types of carburising are generally distinguished. Solid carburising. Liquid carburising. Gaseous carburising.
In our workshop, we use neither liquid carburising nor gaseous carburising, as these methods require the use of complex, sometimes toxic, chemical products, as well as specific industrial installations. These techniques are therefore unsuited to an artisanal, experimental, small scale practice such as ours.

Solid carburising
Solid carburising is the only method used in our workshop, as it is part of ancient practices, simpler to implement and more consistent with our historical approach.
It consists in placing the steel part in a closed container in the presence of a carbon source such as charcoal or bone charcoal. The whole assembly is then brought to a high temperature for several hours. Under the effect of heat, the carbon contained in the charcoal slowly diffuses into the surface of the metal. This migration progressively transforms the structure of the steel in its superficial layer.
For a steel to undergo quenching, it must reach a minimum carbon content. Carburising therefore makes it possible to transform a mild steel into one whose surface becomes capable of hardening abruptly during quenching.
This process allows the following results to be obtained. An extremely hard outer layer, resistant to wear, friction and repeated shocks. A softer, more flexible inner core, capable of absorbing stresses without breaking.
This contrast in properties between the exterior and the core of the material is particularly interesting for mechanical parts, miniature tools and elements subjected to repeated stresses.
In the case of very small parts, such as those produced in our workshop, it is even possible, by extending the heating time, to increase the carbon content right to the core of the material. This makes it possible to obtain a steel hardened throughout its entire mass, if the final use of the part requires it.
This capacity for adaptation makes solid carburising a particularly relevant method for work on a miniature scale.

Problems related to carburising
The main challenge of carburising today lies in the fact that it is a practice that has become rare, gradually abandoned in favour of faster and more standardised industrial processes.
Several reasons explain this disappearance. Technological advances in modern steel production now make it possible to obtain alloy steels with precise characteristics directly, making carburising less necessary in many cases. The complexity of the process, which requires a long preparation time, constant monitoring and extended heating cycles, sometimes lasting several days or even several weeks on an industrial scale. The limitation of the treatment to the surface of the metal for parts of large section, which prevents thick or massive parts from being treated efficiently. The need for specific know how, which is now rarely passed on and is often absent from modern training focused on contemporary technologies.
However, in the context of an artisanal and experimental workshop, carburising regains its full meaning. Thanks to the small size of the parts being worked, the treatment time can be considerably reduced. In our case, only a few hours may be enough to obtain a satisfactory result, whereas industry would require much longer times.
This method therefore makes it possible to reconcile tradition and experimentation, efficiency and respect for ancient techniques, precision and historical coherence.

Towards quenching
Carburising is only a preparatory stage. It opens the way to another essential phase of heat treatment: quenching, which consists in rapidly cooling the heated part in order to lock the carbon enriched structure in place and make it extremely hard.
The quality of the carburising directly influences the effectiveness of the quenching. For this reason, great care is given to each stage: the type of charcoal used, the duration of heating, the temperature reached, the size of the part and the thermal environment.
These parameters, carefully adjusted, make it possible to obtain precise results, adapted to the specific requirements of each miniature object.