The Effect of Laser Beam Overlapping on the Hardness and Microstructure of AISI 414O Steel at different Laser Power

Abstract

Many different types of components require heat treatment to produce desired surface qualities, such as wear resistance. One typical treatment is case hardening, where the surface is hardened to a specified depth. Indus-trial examples include bearing races, cylinder liners, gears, pistons, piston rings, spindles and valve seats. Traditionally used heat treating processes include induct-ion hardening, arc hardening, electron beam hardening, gas carburizing, nitriding and carbonitriding. An alternative process to the traditional methods is laser hardening, also referred to as laser transformation hardening, which is a selective heat treating method. The laser beam has large power intensity that can be shaped to produce a desired power distribution, which rapidly heats the sur-face region of the workpiece. Because the power profile is shapeable, discrete patterns across a workpieces surface can be hardened. The bulk of the material is unaffected and acts as a heat sink to cause rapid quenching, producing large cooling rates within the workpiece. A martensitic surface with a maximum case depth of 2 mm can thus be produced; yielding a surface that can better withstand corrosive environments, high temperatures, and/or wear and have better fatigue properties. At the same time, the bulk of the material is unaffected so that it retains its ori-ginal properties, such as ductility. When compared to the other processes, laser hardening causes little deformation of the part, so that post machining is practically elimi-nated. The energy input is more efficient, because only the portion of the part undergoing treatment is heated. The treatment can be carried out in air with no external quenching media. If the area to be hardened can be seen optically, then it can be hardened with a laser. A wider variety of materials can be hardened; even low carbon steels can be hardened because of the rapid heating and cooling rates caused by the laser. The resultant micro-structure is often better than that produced by other meth-ods, i.e., it is harder and more fatigue resistant.