LASER SURFACE HARDENING of ferrous materials is an established process used to enhance the mechanical properties of highly stressed machine parts, such as gears and bearings. Surface hardening increases the wear resistance of the material, and under favorable circumstances, increases the fatigue strength caused by residual compressive stresses that are induced in the workpiece surface by the transformation hardening process. The surface hardening process is not fundamentally different from conventional through hardening of ferrous materials. In both processes, increased hardness and strength are obtained by quenching the material from the austenite region to form hard martensite. Surface hardening differs from conventional through hardening in that only a thin surface layer is heated to austenitization temperatures prior to quenching, leaving the interior of the workpiece essentially unaffected.
Because ferrous materials are fairly good heat conductors, it is necessary to use very intense heat fluxes to heat the surface layer to austenitization temperatures without unduly affecting the bulk temperature of the workpiece. This heat input is commonly obtained by the use of very hot flames or by high-frequency induction heating. By selectively heating the workpiece surface to austenitization temperatures, desired surface hardening is obtained by application of a quench medium to the hot surface, or by self-quenching. Self-quenching occurs when the cold interior of the workpiece constitutes a sufficiently large heat sink to quench the hot surface by heat conduction to the interior at a rate high enough to allow martensite to form at the surface.
In recent years, industrial lasers have become available for metalworking uses, including surface hardening. A laser can generate very intense energy fluxes at the workpiece surface, and the resulting temperature profiles in the workpiece usually can be made steep enough to negate the need for external quench media. The laser beam is a beam of light, which is essentially independent of the workpiece, easily controlled, requires no vacuum, and generates no combustion products. However, laser surface hardening also has some disadvantages that might limit its practical use in a heat-treating shop. These disadvantages may include the need for complex optics and the use of coatings because of the low infrared absorption characteristics of steel. In Europe, work has focused on an alternate technique using a Brewster-angle treatment without coatings.
ELECTRON BEAM HARDENING TREATMENT (EBHT) is a short surface hardening procedure for martensitically hardenable ferrous materials. Austenitizing occurs through the energy transferred by electron beams. Precise application of the energy with respect to workpiece location and elapsed time using a focused and deflectable electron beam makes it the process of choice, especially for the partial hardening of highly stressed surface regions in components. The austenitizing process advances from the surface toward the inner core regions of the component via heat conduction, thus allowing for a defined adjustment of the hardness penetration by selecting a suitable energy transfer duration. Typical hardening depths obtained by the EBHT process range from 0.1 to 1.5 mm (0.004 to 0.006 in.). The rapid cooling of the austenite required for martensite formation occurs through a self-quenching process that is dependent on the thermal conductivity and starts after the energy transfer has ceased. Depending on the material selected, the workpiece thickness required should be at least 5 to 10 times the austenitizing depth.
Although developed in 1970, this technique has only recently found practical application in the metals industry. Because it offers the advantages of extremely low hardening distortion and relatively low energy consumption, electron beam hardening provides the metallurgist with an additional option to conventional hardening techniques. For certain specialized applications, electron beam hardening is competitive with both case hardening and induction hardening processes in the heat-treating marketplace.