Infrared Camera Analysis of Laser Hardening

The improvement of surface properties such as laser hardening becomes very important in present manufacturing. Resulting laser hardening depth and surface hardness can be affected by changes in optical properties of material surface, that is, by absorptivity that gives the ratio between absorbed energy and incident laser energy. The surface changes on tested sample of steel block were made by engraving laser with different scanning velocity and repetition frequency. During the laser hardening the process was observed by infrared (IR) camera system that measures infrared radiation from the heated sample and depicts it in a form of temperature field. The images from the IR camera of the sample are shown, and maximal temperatures of all engraved areas are evaluated and compared. The surface hardness was measured, and the hardening depth was estimated from the measured hardness profile in the sample cross-section. The correlation between reached temperature, surface hardness, and hardening depth is shown. The highest and the lowest temperatures correspond to the lowest/highest hardness and the highest/lowest hardening depth. 1. Introduction Laser hardening is one of the modern possibilities of improving surface properties of the metallic materials. It is a characteristic of the laser surface hardening process that the material is rapidly heated to a high temperature by guiding the laser beam across the workpiece surface. Short times of interaction between the laser beam and the workpiece surface cause that the absorbed energy is concentrated only at the workpiece surface, whereas the core remains cold. The removal of heat from the surface layer to the colder unaffected metal of a machine part permits rapid cooling and formation of a hardened microstructure required at the workpiece surface. Parts of hardened surface can be manufactured separately each with different final properties. The final hardness and hardening depth depend primarily on the beam irradiance on the surface of the workpiece, the processing rate, and the thermophysical properties of the material. A low interaction time and a high power result in a shallow hardening depth whereas the converse results in a deeper hardening depth [1]. The parameter that can affect the final properties is the amount of energy accepted from the incident laser beam. This can be changed by corrections of optical properties of the surface absorptivity. The influence of different types of absorbing coatings on specimen surfaces and measurement of IR radiation with a photodiode was presented in [2]. The depth of