The principles of laser surface heat treatment are in essence the same principles of metallurgy that are applied to traditional steel thermal phase transformation. It is still necessary to satisfy the following three conditions in order to thermally transform steel from austenite to martensite:
  • Sufficient carbon present in solution at the surface of the steel to transform to the phase of austenite
  • Appropriate austenitizing temperature must be accomplished in order to transform the steel surface to austenite
  • Appropriate cooling rate to transform the now-austenitized surface by rapid cooling into martensite
The depth of hardening will be determined by the steel’s hardenability and the laser power selected as well as the type of laser being used. The principle of laser heat treatment lies in the fact that the focal length of the laser-generated beam can be adjusted to create a thermal band across the workpiece surface in relation to the laser-beam diameter.

The power density of the laser beam is usually given in watts per square centimeter and will generate a sufficiently high heat pattern at the steel surface in relation to the core of the material (which will remain cold). It is the fact that the laser heat energy is absorbed into a narrow band (width of the laser beam) that will generate the heat necessary to transform that band into the phase of austenite.

The transformation of the austenitic phase through to a fresh martensite phase will be dependent on the mass (weight) of the body of steel being laser heat treated. In other words, a heat-sink pattern within the steel will pull the generated surface heat into the body of the steel. The principles of metallurgy are still applicable to transform the austenite into martensite by rapid cooling. In this case, however, the cooling is accomplished by the steel itself.

The depth of hardening is generally accomplished as a result of the hardening response of the material being treated. Steels that are low to medium carbon and plain carbon steels will not produce very high surface-hardness values, whereas the medium and high carbon steels that contain both sufficient carbon and alloying elements will respond favorably to produce high hardness values. The advantages of laser heat treatment are usually seen in the following:
  • Elimination of complex quenching equipment
  • Reduced scrap rate
  • Reduced thermal-process time
  • Reduced equipment surface-area requirements
  • No pollution
  • Lower energy costs
  • Fast turnaround times
  • Reduced material handling
  • Reduced distortion
  • Less residual stress relief
  • Less risk of surface cracking (providing that all of the appropriate operational conditions have been met)
The disadvantage of laser heat treatment is the depth of formed case, which is shallow (approximately 0.100 inch maximum). Of course, the high capital equipment cost is also an issue. Laser heat treatment including surface hardening is used in many applications, including: cutting, engraving, thermal phase transformation and gun-barrel breech slides.