Laser heat treating fundamentals
Laser heat treating can provide an exceptionally hard surface on certain types of steel that helps resist wear…and the risk of distortion is virtually zero!
In surface heat treating of steel, the surface of a piece of steel is hardened by heating it to a suitable temperature at which it begins to change its internal structure, and then causing it to cool rapidly. This process is referred to as transformation hardening, because the surface of the metal is transformed to another state. In laser heat treating the concentrated light energy emitted by the laser is converted into heat energy when absorbed by the metal, causing the transformation.
The transformation involved when steel is hardened is actually a change in the arrangement of the atoms in the metal. Technical people involved in heat treating sometimes referred to this as a change in the “crystallographic lattice network” of the metal. That may give you a good word picture of what actually happens. This change occurs at critical transformation temperatures both when the metal is being heated and as it is cooled. The transformation temperature is the temperature above, or below which solid state transformation takes place.
The exact temperature depends on the alloy composition of the steel being treated The changes, and the structures they produce in the metal, all take place while the metal is in the solid state. That is. the metal is not actually melted, because transformation occurs before the surface reaches the melting temperature. After the metal surface is heated, it has to cool at the critical cooling rate, which is the rate at which the heated material is cooled from the transformation temperature to a lower temperature .
In conventional heat treating, high cooling rates are achieved after the heating process by chilling, or quenching the work piece with oil, water or forced air. In most laser heat treating applications the material is allowed to “self-quench ” or cool? in ambient air. That is possible with laser heat creating because the area heated at any one instant is so small.
The transformed area of the steel is called martensite, and it is the iron-carbon structure that is produced upon a very rapid cooling to the martensite transformation temperature. Martensite is the structure that produces the high hardness in ferrous materials The greater the degree of martensite formation, the higher the hardness. Typical heat treatments for most industrial applications result in hardness of 56 – 62 points of the Rockwell C scale.
Normally, 0.4 per cent carbon content is the minimum that can be successfully hardened to this range by any heat treating method, and laser heat treating is no exception to this rule The big advantage the laser provides is uniformity and precise control of the depth of hardened area.
