The annealing process involves applying heat to the surface of an uncoated metal workpiece. An oxide layer is created on the processing area, which feels smooth to the touch. Steel and titanium are well suited to annealing, as a high-contrast black oxide layer is formed on these metals.
In order to mark — rather than engrave — the material, the laser beam must be defocused so that a lower amount of energy is applied to its surface. What's more, slower processing speeds are needed to produce the oxide layer.
Due to its adjustable pulse frequency, a MOPA fibre laser can mark steel in a variety of colours such as blue, red, green, and purple. The markings disappear, however, when the metal is heated to approximately 200°C.