Glossary · Technology

Laser marking (annealing)

In marking, the laser only changes the colour of the surface without removing any significant material. In annealing, heat turns stainless steel dark or colourful.

Explained in more detail

Marking is an umbrella term for processes that discolour the surface but hardly remove any material. In annealing, the laser heats the metal surface well below its melting point without melting it. A thin oxide layer grows on the surface and, depending on its thickness, appears yellow, brown, purple, blue or black. The surface stays smooth, which is why annealing is often used for medical technology and tools. Annealing marks need high energy density at slow speed and a small line interval, typically with fibre or IR lasers at 1064 nm. Diode and CO₂ lasers can only mark bare metal with marking spray, which is burnt onto the surface.

Where does the term come from?

Tempering is a term from the heat treatment of steel: hardened steel is heated again, which produces the typical temper colours that smiths used to estimate temperature. Marking comes from the French marquer, to mark.

In practice

For black annealing marks on stainless steel you need a small line interval, usually around 0.01 to 0.02 mm, and a rather slow speed.
Always test new sheets with a grid, as the alloy has a strong influence on the colour.
Clean the metal of grease and fingerprints beforehand, otherwise the marking will be blotchy.
With marking spray, diode and CO₂ lasers also mark stainless steel. Wash off any excess with water afterwards.

Not to be confused

Marking is not engraving: engraving removes material and creates a tangible recess, whereas marking leaves the surface smooth.

Frequently asked questions

Can I mark stainless steel with a diode laser?

Directly only to a very limited extent, because bare metal reflects the blue light. With marking spray or a 1064 nm module, permanent dark marks are possible.

Is an annealing mark food-safe?

Annealing removes no material and applies no foreign substances, which is why it is used for cutlery and medical products. Whether the end product is suitable, however, depends on the material and the regulations for its use.