Laser marking stainless steel,
with nothing removed.

Annealing heats the surface in a controlled pattern to a temperature below melting. The metal at the surface oxidizes, and the resulting film reads as a deep black. Nothing is vaporized: there is no recess, no burr, and no recast edge to trap soil or start a crevice. Run a fingernail across an annealed mark and there is nothing to feel.

The MOPA source is what makes this repeatable. The JPT M7 gives independent control of pulse width, from 2 to 500 ns, and of pulse frequency, so heat input can be set high enough to form a complete oxide layer and low enough to avoid melting the surface. The same control produces stable interference colors (blue, gold, bronze) where a drawing calls for color coding. Capabilities lists the equipment and the other mark types.

Etching and engraving remain available when the application calls for them: a gray-to-white etch for cosmetic branding, or a recessed engraving on tooling and hardware that will be blasted, coated, or worn. Both remelt or remove material, so on corrosion-critical parts they are the exception. The material marking guide compares the processes side by side.

Stainless resists corrosion because of a thin, chromium-rich passive film. A mark that melts or removes the surface disturbs that film, and etched areas can show rust bloom in service. An annealed mark keeps the surface intact, which is why it is the standard mark on parts passivated to ASTM A967 or AMS 2700. It still puts heat into the surface, so parameters matter. Too much energy and the mark can rust after passivation or sterilization; too little and the oxide is thin enough for the acid bath to fade it.

A fully formed annealed mark generally survives citric and nitric passivation. Marking can sit on either side of passivation in the router. Mark first, and the bath restores the passive film over the whole part, mark included. Mark last, and the code is verified in its final condition. Follow the drawing or process specification where it fixes the order, and state on the RFQ what happens to the parts after they leave us so the parameters are tuned for it. Post-passivation verification is available on request.

Reusable surgical instruments add repeated cleaning and steam sterilization, and the UDI Data Matrix has to stay readable throughout. That is the use case annealing was adopted for. See medical device marking for UDI content and quality for how codes are graded.

A black annealed mark on bright stainless gives high contrast, and Data Matrix readability is excellent on most finishes. Finish matters more than grade. Bead-blasted, satin, and ground surfaces scatter light evenly and grade consistently. Mirror-polished and electropolished surfaces are specular: the mark is just as black, but a reader sees glare unless the lighting is angled, so module size is chosen with margin. Annealed marks also shift slightly in apparent darkness with viewing angle. That is a property of thin oxide films, not a defect.

Every serialized code is read back and graded per ISO/IEC 15415, or AIM DPM-1-2006 for direct part marks, before shipment. To size a symbol for an instrument shank or a fastener head, use the MIL-STD-130 mark size calculator. To mock up the code for a drawing, use the Data Matrix generator.