Laser Marking Stainless Steel With a Black Annealed Effect

Stainless steel is an excellent material for laser engraving at the 1µm wavelength where a range of different effects can be produced such as ablation (engraving), thelaser marking of various degrees of shading from light to dark and even a limited range of colours can be produced too.

One of the most visually appealing of laser marked effects is dark annealed stainless steel where the near jet-black effect of the laser mark contrasts beautifully with the clean and crisp surface of the stainless steel finish.

This type of mark is high in contrast and is often used to increase the perceived value of products that are marked in this way.

The colour change occurs by using the laser to heat the metal and create within it an oxide layer. Colours and shading will vary according to the amount of heat generated during the laser marking process.

A laser marking machine with a higher laser power can produce these types of mark at a much faster rate than a lower power laser marking machine.

For a laser engraver with a lower power, the feed rate (speed of beam delivery) will need to be lowered in order to expose the material to the laser for longer and generate enough heat. Even more heat can be added by closing the distance between the hatch lines in the graphic thereby multiplying the degree of overlapping of the focal point from hatch line to hatch line. Even more heat can be generated by delivering the hatch in a localised way; hatching small connected areas at a time.

The smoothest, most uniform results are achieved by deliberately marking the material out of focus (defoucused). This provides for a much larger, softer focal point to add even more heat. The required degree of defocus will vary depending on the configuration of the machine. How far to defocus in order to achieve the best result is initially a trial and error process but once the ‘sweet spot’ is found the process is easily repeatable.

If using a lens with 163mm f/l (110mm work area) the sweet spot is usually found between 3-5mm defocus. When using a lens with 254mm f/l (180mm work area) the sweet spot is usually found between 7-9mm defocus.

The oxidised layer of annealed material will only be approx 20-30µm in depth. Therefore, this is not a particularly hard-wearing form of laser mark. To produce a harder wearing mark it is possible to fist ablate (engrave) the steel then to anneal the material to further darken it. While this type of mark is certainly more resilient to wear and tear it is however not a visually smooth and lustrous as a mark annealed without ablation. Using a machine with a Z-axis that is controllable by software is essential for this process.

Even further enhancement can be achieved by first lightening the surface of the material with the laser delivered at an alternative angle, such as 45 degrees for example, then annealing over it with further passes. This procedure can produce a near to black on white effect for maximum contrast.

The laser annealing process can raise the stainless steel to nearly 600c, so this is not a suitable process to apply over large areas and/or to thin material as to do so can cause the material to deform. Materials that do not have a flat surface are not suitable for this process.

At Lotus Laser Systems we manufacture a wide range laser marking and engraving solutions ideally configured for laser cutting, laser marking and laser engraving all types of materials. Our experts would be happy to recommend which configuration best suits your application.