Laser hardening heats up

High power direct diode lasers have advantages over CO₂

Lasers have been used in heat treating for over 30 years; however, various practical and cost related limitations have acted to keep this a niche application. Now, high-power direct diode lasers offer a solution that overcomes the drawbacks of prior technology, promising to expand this market in a substantial way.

Conventional heat treating

Heat treating, or case hardening, is a process used in many industries to improve the wear characteristics and extend the lifetime of steel parts such as cutting tools and shaft bearing surfaces. Heat treating involves heating and then rapid cooling (called quenching) of a part. This transforms the steel's crystalline structure by incorporating carbon in a way that makes the lattice harder than normal at room temperature. For industrial case hardening, the goal is to harden only a thin outer layer. The bulk material retains its original crystalline form, which is more flexible, less brittle, and more ductile. Because hardening is typically performed after a part has been dimensionally formed, ideally it should not introduce any physical distortion of the part shape. (Editors' note: Readers are encouraged to read "Laser Heat Treatment Simplified," a Featured Article on the ILS web site, www.industrial-lasers.com.)

In general, the various traditional (non-laser) techniques can be divided into two broad categories, namely, diffusion methods and selective hardening methods. In diffusion techniques, such as carburizing, nitriding, and carbonitriding, a low carbon steel is bulk heated while in contact with an external supply of carbon or other elements. These diffuse into the surface layer, and then the part is rapidly quenched in liquid.

Selective hardening is typically performed on steels that already contain enough free carbon to produce the desired hardness when integrated within the steel crystal lattice. In this case, localized surface heating is applied to the part, typically using a flame or electrical induction, so as to raise the temperature of the desired area, followed by quenching.

Laser techniques

Laser heat treating is another selective hardening technique in which a spatially well-defined beam of laser light is absorbed near the surface, causing rapid heating. This heating is limited to the illuminated area, and penetration into the bulk material is limited. Often the bulk material acts as a heat sink for the extraction of heat from the surface, therefore enabling self-quenching.

The ability to precisely define the illuminated area, together with the short timescale of energy transfer into the material, give rise to the main benefits of laser heat treating. Specifically, the benefits include rapid processing, precise control over case depth/application location, and minimal part distortion.

Laser hardening eliminates some of the drawbacks of traditional techniques. For instance, flame hardening is limited by poor reproducibility, poor quench characteristics, and environmental issues. As a result, flame hardening is most suitable for medium to large size components. Induction hardening typically produces deeper thermal penetration, thus requiring an active water quench, both of which can lead to undesirable and uncontrollable distortion. The laser heat treating process is also much simpler to design and maintain than induction hardening due to its ability to easily limit heating to the irradiated area and to reach mechanically inaccessible areas. This enables the laser to be readily applied for treating a wide range of part sizes and shapes without the need for special coils custom-designed for each part geometry.

Given these advantages, why haven't lasers penetrated the heat treating market more deeply? Most laser hardening has been performed with CO₂ lasers. While these are excellent tools for a wide range of tasks requiring very intense extremely localized energy – e.g. cutting, welding, and drilling – they are often not well matched to the needs of heat treating. One problem is that the output wavelength of 10.6 µm is not well absorbed by virtually any steel or other metal, for that matter. As a result, surfaces for heat treating with a CO₂ laser must first be "painted" with an absorptive coating. In addition, the infrared CO₂ output cannot be fiber delivered, limiting access.

Direct diode lasers

FIGURE 2. The top and bottom surface of a piston ring groove have been selectively heat treated to enhance wear resistance.

The beam shape and size of direct diode lasers is another advantage. For the majority of laser hardening applications, the laser beam illuminates an area that is smaller than the total area to be processed. Thus, either the work piece or the beam is translated in order to achieve total coverage. Diode lasers naturally output an extended beam shape that is well matched in size and intensity distribution with many hardening tasks and that can be readily reshaped to match the dimensional requirements of a specific task. Furthermore, the near infrared output of the direct diode laser can be readily fiber delivered, thereby enabling tremendous process flexibility.

The electrical efficiency (conversion of input electrical energy to useful light output) of the diode laser is about three to four times higher than that of the CO₂ laser. This translates directly into lower operating cost. Additionally, the diode laser has instant "on" capability so there is no standby power consumption. An even larger savings results from reduced maintenance costs, which are orders of magnitude smaller for the diode laser compared to CO₂ lasers. Maintenance downtime is also minimized because the physically compact diode laser can be more rapidly replaced than bulkier CO₂ lasers, and replacements can even be shipped via overnight courier services.

The diode laser also offers simplified integration. Their small size facilitates the integration of diode lasers into CNC machining equipment so that the hardening process can be performed immediately after machining, even within the same combi-machine.

Applications

Titanova (St. Louis, Missouri) is a contract manufacturer that specializes in non-cutting services based on direct diode lasers, in particular heat treating, cladding, remanufacturing, and conduction mode welding. "Direct diode laser heat treating is particularly useful when the part has a specific, limited surface area that needs to be case hardened, like the top of a gear tooth, or if the area to be hardened is difficult to access, such as the bottom of a narrow cylinder," explains Titanova founder and president John Haake. "The process is also especially cost effective with high precision parts that would be negatively impacted by the mechanical distortion experienced with traditional heat treating methods."

FIGURE 3. The direct diode laser is compact and easy to deploy, enabling it to process large parts or difficult to access areas. Here it is heat treating the bearing contact area on a wheel spindle of a large vehicle.

A typical example of the latter are the drive shafts used in a variety of machines. Often, various bearing areas on these shafts must be hardened to improve wear characteristics, but it is critical that part shape be maintained; otherwise the shaft will be out of balance when spun. Titanova has successfully performed selective heat treating on drive shafts for a manufacturer of tire balancing equipment. Formerly, this was done using induction hardening, and required subsequent grinding and bending of the shaft to restore shape.

Titanova also heat treats engine piston ring grooves for a manufacturer of construction equipment. This was formerly performed with CO₂ lasers; however, it was found that once the black coating burned off the part, the CO₂ beam would reflect and produce some heat treating on the wrong area of the part. This problem doesn't occur when using near IR diodes, which are much more readily absorbed by the material.

Titanova has several customers that require heat treating of relatively low value parts in small to moderate quantities (less than 50,000 per year). Examples are various machine tools and engine components. It some cases, it was simply not cost effective to heat treat these products in the past using other methods due to the labor required for preparation (painting) or post processing. Haake notes, "High-power, direct diode lasers are expanding the available market in these instances by making heat treating a cost-effective and technically feasible process where there was none before."

Keith Parker, Sr. is business development manager, direct diode systems, at Coherent Inc., email [email protected].