Costly shutdowns and maintenance programs are frequently the norm for heavily loaded bearings and other PT components subject to wear. In the quest for longer wear life of these components, companies are searching for more effective case hardening techniques.
One of the most promising tools for case hardening bearing surfaces is the multiuse laser. Though lasers have many uses, few manufacturers are aware of their case-hardening abilities. This is despite the fact that CO2 lasers have been used to harden metal surfaces for at least 20 years.
LIght Beam Technology Inc., a small industrial services company in Wolf Lake, Ind., has been laser-treating bearing seats, shafts, gear teeth, and splines for 6 years. Company president Larry Boyd says that a 5-kW CO2 laser can heat treat specific areas with pinpoint accuracy and well-controlled depths and hardnesses. “We have customers who were scrapping a lot of parts with induction and flame heat treating,” he reports. “They were amazed that we could get the same or better results without distorting parts.”
The laser can heat treat different types and shapes of bearing elements. The most commonly treated elements are shafts that mate with bearings, especially heavily loaded bearings. Heat treating these shafts reduces the likelihood of galling when the bearing is pressed onto the shaft. Also, for bearings that require frequent replacement because they operate in hostile environments, it reduces the risk of bearing seizure, which would otherwise damage the shaft.
Normally, rollers, balls, and other small bearing components are not good candidates for laser heat treating. But the surfaces on which the bearings run can be hardened in many applications.
Creating a hardened case on a steel part requires heating the surface to a critical temperature at which the material begins to change its atomic structure. After heating, the metal must cool rapidly to transform the surface into a harder structure. In conventional heattreating methods, this high cooling rate is typically achieved by quenching the part with oil, water, or forced air.
Common methods for case hardening parts include furnace, flame, and induction heating, Table 1.
Laser heat treating works on the same principal. A laser beam 1/8 to 5/8-in. wide is focused on the surface of the part. Concentrated light energy emitted by the laser converts into heat energy when it is absorbed by the metal. This causes rapid heating at the surface, and the part must be moved slowly under the beam to avoid heating the area to the melting temperature.
rapidly and the part moves under the beam, there is very little heat input into the part. In most cases, the material “self-quenches” or cools in ambient air. The heated area usually cools enough to touch within seconds after being treated.
The rapid heating and cooling rates achieved by the laser provide several benefits for the treated component. First, because heat input is low and the area heated at any given time is small, there is little risk of warping or distortion. This usually eliminates post-treatment machining. Second, the laser can harden an area as small as 1/4 in. square. Third, this method produces a fine grain structure, which causes a slightly tougher, better wearing surface than with other methods.
The type of material being treated affects the hardness, says Mr. Boyd. As with other hardening methods, laser hardening works best if the metal being treated has a minimum of 0.4% carbon content. In most steels containing 0.4 to 0.7% carbon, the laser achieves a case hardness of 58 to 62 Rockwell C for a depth typically ranging from 0.010 to 0.080 in. Deeper cases are generally not possible with laser hardening because of the risk of melting the surface.
Shaft and bearing applications
In a typical application, lasers heat treat bearing areas and tapered seating areas on arbor shafts used in coiler and recoiler mandrels. These bearing areas, which support the inner races of rollingelement bearings, are highly susceptible to wear because of heavy shaft loading. The tapered areas provide seats for nonrotating elements that support heavy compression loads. Frequently, these areas can’t be treated by other methods because of possible shaft distortion.
These arbors range up to 15-ft long and 15 to 18-in. diameter at the tapered area. They are usually made from AISI 4140 or 4145 steel, which contains at least 0.4% carbon.
In another example, an Ohio manufacturer of steel slitting equipment uses laser heat treating of bearing components. To extend the wear life of bearing seats on their small leveler rolls (2-in. diameter and 80-in. length), lasers harden the 5/8 to 3/4-in. diameter seat areas to depths ranging from 0.020 to 0.030 in. Most of these rolls are made of AISI 1060 steel, and the bearing seats are hardened to 58-62 Rockwell C. Because of the low heat, maximum runout on the seats is held at 0.001 in. or better.
Aside from the reduced risk of distorting a component during heat treating, and the ability to precisely treat a specific wear location, prolonged service life and better performance are two primary benefits of laser heat treating bearing seat areas. Mr. Boyd says his customers report that this reduces the chance of line shutdowns caused by wear problems, and the related maintenance requirements.