Techniques

HEAT TREATMENTS

1. Introduction

Heat treatments play an essential role in watchmaking. They make it possible to modify the mechanical properties of the Steels and Alloys used. Three main processes are commonly used: annealing, quenching, and tempering.

2. Annealing

3. Hardening

Quenching is carried out after an austenitizing phase, with heating typically brought to between 780°C and 950°C for tool or spring steels commonly used in watchmaking. This temperature allows the metal’s crystalline structure to transform into austenite, a phase that is unstable at room temperature. Cooling must then be extremely rapid in order to freeze this structure into a martensitic form, which is much harder. Depending on the alloy, this cooling is carried out in oil, in water, or sometimes in air for certain special steels with so-called “soft” quenching. Historically, some watchmakers also used molten lead baths for more precise thermal control. Quenching applies almost exclusively to high-carbon steels, the only ones capable of undergoing this martensitic transformation. It is applied in particular to arbors and pinions, springs, mechanical parts subjected to heavy wear or high pressure, as well as certain manufacturing tools. Non-ferrous metals, such as brass or gold, cannot be quenched in this way, as their hardening relies on other principles, such as cold working.

Figure 1

Artisanal quenching of a Steel component (900-950 °C – Light cherry red)

Click on the thumbnail to enlarge it

3. Tempering

Tempering systematically occurs after quenching, on the same Steels that have undergone this martensitic transformation. It involves moderate heating, generally between 150 °C and 320 °C, the exact Temperature depending on the desired compromise between Hardness and impact resistance. In watchmaking, this process is made visible through the phenomenon of Bluing: Screws and certain components take on a shade ranging from straw yellow around 220 °C to deep blue, between 290 °C and 320 °C. This coloring, due to a thin surface oxidation layer, was traditionally used as a visual Temperature Indicator by watchmakers, before the use of precise measuring equipment. The cooling following tempering is simply done in open air, without any particular need for speed. This treatment reduces the excessive brittleness induced by quenching, while retaining sufficient Hardness for mechanical use. Tempering exclusively concerns previously quenched Steels, notably Screws, springs, and various Movement components subjected to repeated mechanical stress.