Metal hardening process & method
The goal of all metal hardening processes is to ensure a structural transformation of steel and cast iron materials with greater strength. First, the original ferritic-perlitic material structure is austenitized by heating, and then transformed into hard martensite by quenching. Here, hardening with high-power diode lasers has the decisive advantage of making an exclusive locally heat input at stressed areas possible. By this, at complex components, a partial hardening can be realized, while in other zones the ductility of the initial structure can be maintained. With induction hardening, such a local treatment is not possible in many cases and thus has disadvantages compared to laser hardening. As the workpiece is only heated near the surface with a low hardening depth at laser brazing and the heat is discharged very efficiently over the neighboring material, in most cases there is no need for an additional quenching media, which keeps costs low. As the warpage is compared to other hardening techniques in quite a minimal way, additional methods to correct the material deformations can mostly be omitted.
Hardening material surfaces with diode lasers
The processes of surface hardening with laser are generally usable for all materials at which classical hardening methods can be used due to the adequate carbon content. At laser hardening, only the especially stressed areas of the components are locally hardened, e.g. at steels and cast iron in tool manufacturing. The thermal treatment of wear or cyclical stressed component areas – e.g. in job order production – can be realized quite effectively and flexibly in combination with a Laserline OTZ Zoom optic.
Diode lasers are especially suitable for surface hardening of steelwork components. The laser beam heats up areas close to the surface of a workpiece at between 900°C and 1500°C, causing a heat-induced austenitization of the originally ferritic-perlitic steel structure. When the beam moves on, the material on the processed part cools down quickly (self-quenching) and hard martensite is created. For many reasons, this process can clearly be realized more efficiently with laser than with induction coils or gas flames. First, the material physically heats up more quickly than with other methods. Second, hardening relevant zones can be treated more selectively, meaning that by local heat input only certain parts of the workpiece can be heated. And third, the heat input can be dosed in an exact manner by special beam shaping optics with integrated pyrometers or thermal cameras, last but not least with particular focus on local different heat conductions at the same component. Due to these characteristics, the diode laser is extremely well-suited for treating geometric complex structures that require in some zones a hardening; in others, however, they have to keep their ductility. Furthermore, these process advantages allow for cost savings. That is, because of the low deformation and self-quenching of the material, usually no (or only little) actions are required for distortion compensation and cooling. Finally, this is reflected positively in terms of required time and material.
Particularly predestined for this treatment are wear or cyclically stressed components such as camshafts. In every gasoline and diesel engine, steel rubs against steel. Thus, the contact zones must be hardened, otherwise the longevity of the parts would be hard to imagine. At these complex structures, the inductive method does not help much; the construction of modern camshafts (with different shaped cams and shifting gates for the cylinder deactivation or change in the engine control) requires a precise surface hardening, which is only possible with diode lasers. Even in manufacturing of large sheet metal tools, diode lasers have established themselves for a long time now. The oldest hardening plants have here already been in use since 10 to 15 years. As the prices of laser beam sources have continued to decrease for years now, additional to these typical fields of use are new applications that are permanently being explored.
Process advantages of the diode laser
Hardening with diode lasers makes it possible to achieve the respective material-specific maximum hardness at machine components, tools, component parts, and commodities. The temperature regulation during the metal hardening process ensures that the respective optimal process results for each material and application are reached. However, heat treatment can also be used to locally reduce the firmness of high-strength materials to make sure better deformability in those local areas.
Compared to other processes, a diode laser offers many advantages:
- Ideal adjustment of the focus to the hardness contour
- Local heat treatment of defined partial areas
- Integration of the heating process in existing production lines
- Hardening of complex geometries is made possible