The advantages of laser hardening are less refinishing work and the ability to process irregular, three-dimensional workpieces. Costs related to refinishing work is reduced or eliminated entirely.
Laser hardening is a surface hardening process. It is used exclusively on ferrous materials suitable for hardening including, steels and cast iron with a carbon content of more than 0.2 percent.
To harden the workpiece, the laser beam usually warms the outer layer to just under the melting temperature (about 900 to 1400 degrees Celsius). Once the desired temperature is reached, the laser beam starts moving. As the laser beam moves, it continuously warms the surface in the processing direction. The high temperature causes the iron atoms to change their position within the metal lattice (austenization). As soon as the laser beam moves away, the hot layer is cooled very rapidly by the surrounding material in a process known as self-quenching. Rapid cooling prevents the metal lattice from returning to its original structure, producing martensite. Martensite is a very hard metal structure. The transformation into martensite yields greater hardness.
The laser beam hardens the outer layer, or case, of the workpiece. The hardening depth of the outer layer is typically from 0.1 to 1.5 millimeters. On some materials, it may be 2.5 millimeters or more. Greater hardening depth requires a larger volume of surrounding material to ensure that the heat dissipates quickly and the hardening zone cools rapidly enough.
Relatively low power densities are needed for hardening. At the same time, the hardening process involves treating extensive areas of the workpiece surface. That is why the laser beam is shaped so that it irradiates an area that is as large as possible. The irradiated area is usually rectangular. Scanning optics are also used in hardening. They are used to move a laser beam with a round focus back and forth very rapidly, creating a line on the workpiece with a power density that is virtually uniform. This method makes it possible to produce hardened tracks up to 60 millimeters wide.