Heat Treatment of Metals in Tool- and Mould-Making
Precision heat treatment of metals can be realized with the help of diode lasers in a more flexible, precise, and often more economical way than with other laser beam sources or other tools like gas flames, infrared rays, and induction coils, e.g. for the selective hardening of gripping tools or mold surface treatment at particularly stressed areas.
At surface hardening of steelwork components, laser systems are reputed to have high energy consumption, which is why they have not caught on in all production areas. High-power diode lasers, however, can overcome this prejudice since they allow for the energy-efficient hardening of even complex component geometries and thus offer an economical, interesting alternative to induction hardening.
Heat treatment is a familiar method for increasing the wear and fatigue resistance of steel components. In this case, by means of a heat source, the areas close to the surface of the workpiece are heated up to a range of 900 to 1500°C, so much so that the steel structure, which is ferritic at room temperature, changes to austenite. Afterwards, it cools down to temperatures under 100°C. During this process, carbon atoms are deposited in a high-temperature lattice structure and cannot diffuse outward anymore because of the quick cool-down. Hard martensite is the result. An established method for this process is laser-beam hardening. Once a niche application, it is being successfully applied today because of its high precision at hardening, and yet the method has not caught on everywhere. In many cases, alternative heat treatment methods such as induction hardening are still used. At this application method, the component is usually in a copper coil, where an alternating current runs through in a certain frequency. Together, coil with the component complete an oscillating circuit. This process leads to a high-frequency remagnetization, which again leads to heating of the workpiece. The incoupling of the magnetic field significantly depends on the geometry of the component and can possibly take place in areas that usually should not be heated at all or at least not that much. When the local energy inputs are too high at induction hardening, this can lead to undesired material distortion, e.g. at very fine structures, which can only be balanced by additional post-processing. Furthermore, the components have to be cooled down after heating either by buzzing off with or diving in inti a quenching liquid. For this, a water based quenching media is normally used, but its circulation and cooling would cost additional energy. Why then is induction hardening still often preferred despite its high processing effort? The reason can be found in the fact that lasers are, because of their high energy consumption, uneconomical while inductive methods are therefore — despite the extra work — the more cost-efficient option. However, this assumption originated from a time when CO2 lasers were the only commercially available high power sources. With an electrical efficiency of about 10 percent and an optical incoupling in steel of 40 percent maximum (without the additional absorption-increasing coating), this type of laser actually only delivers a low energy efficiency. Besides, it is expensive. Induction hardening, however, reaches efficiencies between 15 and 63 percent, depending on the component geometry. Thus, the additional effort for post-processing and cooling was estimated to be lower than the investment and operation costs for laser systems. So, it is understandable that many users preferred inductive hardening over laser hardening so far.
In the last few articles of our little series, we presented some key areas for the application of diode lasers. Now we focus on a further application field: heat treatment. Heat treatment is used for hardening machine components, tools, accessory and commodities, but also for the softening of high-strength materials.
The heat treatment of metal, such as the hardening of steel, is one of the oldest industrial processes. Its roots go back to antiquity. Even today's laser heat treatments still follow the classical objectives: The targeted application of heat to defined surface areas alters material properties and protects components from wear and corrosion.
By the way, diode lasers can also be used for drying printing inks. The color layers are heated with a laser beam, and because of temperature induced increased viscosity they are more quickly absorbed by the printing material. The modular and compact components of the laser can be directly integrated into the systems of offset printers.
This shows clearly why heat treatment is one of the main application fields for diode lasers — compared to other methods they are simply ahead. Thanks to accurate heating, the lasers are more flexible, more precise, and even more economical than induction, gas flames, or infrared radiators. And that’s not all: This universal tool called diode laser can also be used for brazing, which our next blog article will further explain.