Heat Treatment in Tool and Mold making
Heat treatment of metals can be undertaken in a more flexible, precise, and often more economical way with the help of diode lasers 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 for particularly stressed areas.
When it comes to surface hardening of steel components, laser systems are said to have a high energy consumption, which is why they have not caught on in every production area. High-power diode lasers, however, can overcome this prejudice since they allow energy-efficient hardening of even complex component geometries, thereby offering an economical, appealing alternative to induction hardening.
Heat treatment is a common method for increasing the wear resistance and fatigue resistance of steel components. In this case, a heat source is used to heat up the areas close to the surface of the workpiece 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 is quenched to temperatures below 100°C. During this process, carbon atoms are deposited in a high-temperature lattice structure and cannot diffuse outward because of the quick cool-down: resulting in hard martensite. The laser-beam hardening process is an established approach. Once a niche application, it is being successfully applied today because of its high precision in terms of hardening, and yet the method has not caught on everywhere. In many cases, alternative heat treatment methods, such as induction hardening, are still used. With this application method, the component is usually placed within a copper coil, with an alternating current running through the coil at a certain frequency. Together, the coil and the component complete an oscillating circuit. This process leads to high frequency remagnetization, which, in turn, leads to the workpiece being heated up. The coupling of the magnetic field significantly depends on the geometry of the component and can potentially take place in areas that usually should not be heated at all, or at least not all that much. With induction hardening, if the local energy inputs are too high, this can lead to undesired material distortion, e.g. of very fine structures, which can only be balanced out by additional post-processing. Furthermore, the components have to be cooled down after heating, either by being buzzed off or being immersed. A water-based quenching medium is normally used for this, but its circulation and cooling requires additional energy. Why, then, is induction hardening still often preferred despite its high processing effort? The reason can be found in the widespread belief that lasers are uneconomical because of their high energy consumption, 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 coupling in steel of a maximum of 40 percent (without the additional absorption-increasing coating), this type of laser does indeed only deliver a low energy efficiency. Besides, they are expensive to buy. Induction hardening, conversely, reaches efficiencies of between 15 and 63 percent, depending on the component geometry. As a result, the additional effort for post-processing and cooling was estimated to be lower than the investment and operation costs for laser systems. Consequently, it is understandable that many users have preferred inductive hardening over laser hardening to date.
Over the last few articles of our little series, we revealed some key areas for the application of diode lasers. Now, we are focusing on a further field of application: heat treatment. Heat treatment is used for hardening machine components, tools, accessory and commodities, but also for softening high-strength materials, too.
Heat treatment of metal, such as hardening of steel, is one of the oldest industrial processes, with its roots stretching back to antiquity. Even modern-day laser heat treatments still share these traditional objectives: the targeted application of heat to defined surface areas alters the material’s properties and protects components from wear and corrosion.
Incidentally, diode lasers can also be used for drying printing inks. As part of this process of “setting”, the ink 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, compact components of the laser can be directly integrated into the systems of offset printers.
This clearly demonstrates why heat treatment is one of the main fields of application for diode lasers — compared to other methods, they are simply streets ahead. Thanks to their targeted heating, the lasers are more flexible, more precise, and even more economical than induction, gas flames, or infrared radiators. And that’s not all: the universal tool of the diode laser can also be used for brazing, which our next blog article will explain further.