HEAT TREATMENTS

Heat treatments used in watchmaking are particularly done on steel. There are three main ones, annealing, hardening and tempering.

Annealing consists of heating, for steel to about 600 degrees Celsius, and letting the metal cool naturally to room temperature. This heat treatment will remove any stresses the metal may have received and the metal will become much more malleable. A concrete case could be a steel bar from which we will remove the stresses received during its rolling. Another case could be a steel box between two stamping strokes, we will remove the tension of the previous stroke and we will make it malleable for the next stroke. It is very easy to recognize a bar after annealing because it twists much more easily than before it was annealed.

Quenching is practised on steel and consists in changing its molecular structure by a thermal shock. The technique consists of heating the steel to a dark cherry red colour, which corresponds to about 800 degrees Celsius, and then immediately dipping the hot part in cold oil. The small insulated pieces can be heated on a piece of coal to avoid burning them, that is to say, decarburizing them. If one wishes to have very hard steel, one will cool the piece in salt water, which will make it even harder but also more brittle. This saltwater technique is sometimes used for certain types of chisels, especially pitting chisels. So the harder the metal is the more brittle it is.

To make hardened steel less brittle there is a third treatment that exists, tempering. This involves reheating the steel to a temperature ranging from 210 degrees Celsius to 300 degrees Celsius, depending on what you want to achieve, and letting the part cool naturally to room temperature. Hardened steel parts such as counter plates, rackets, rosettes, and column wheels can be returned to 210 degrees Celsius. Springs such as a regulator spring sometimes called a gooseneck, will need to be tempered to 300 degrees to make a perfect spring steel that will regain flexibility after quenching but will not fatigue, which would have been the case if it had not been tempered.

The more the piece is heated during the tempering process, the less brittle it will become, but also the softer it will become again. A part tempered at 210 degrees Celsius will be harder and more brittle than a part tempered at 300 degrees Celsius which will be less brittle but also less hard than the first one. This being said a piece of steel tempered at 300 degrees Celsius remains harder than steel that has not been tempered.

To estimate the tempering temperature we can use the colour of the surface oxidation which will always remain at the highest temperature colour reached. This oxidation will be straw yellow at 210 degrees Celsius and will then turn brown, purple, and dark blue at 300 degrees Celsius, and then light blue, grey and finally dark red which will turn to lighter red and finally to fusion. In order to observe the colour of the temper, it is necessary to whiten the piece that will come out blackened from the quenching. The blued needles are due to the tempering technique.