Productivity issues in diode laser transformation hardening

The productivity in laser transformation hardening is a complex combination of physics and metallurgy. In many laser processes higher productivity is achieved by increasing the laser power together with the process velocity. In laser transformation hardening the material should remain in the solid state and the use of laser power is limited. Metallurgical transformations are based on kinetics and in order to achieve a given hardness the processing speed can be increased only within certain limits. Many parameter combinations can lead to the required hardened depth, but the consumption of electrical power may be increased drastically due to incorrect parameter selection. Three different types of steels were hardened with more than 20 parameter combinations using a high power diode laser. The hardened depth was measured from the cross section of each sample to establish the dependence between the laser energy and the depth. The absorptivity of a diode laser beam to steel surfaces is measured by calorimetry. Surface hardness was measured from each sample and a microstructural analysis was made to study the metallurgical factors affecting productivity. Absorptivity of the steel surfaces is related to oxidation. Although higher values for absorptivity were obtained by using relatively low laser powers and slow traverse rates, the process efficiency in respect to the electrical consumption is maximized by increasing the laser power and the traverse rate. Metallurgical phase transformations during hardening are limited by transformation kinetics and the process optimization depends highly on the microstructure and alloy concentration of the steel.