SUMMARY

Cost-effective machining of hardened steel (>60 HRC) components such as a large wind turbine bearing poses a significant challenge. The thesis investigates a new hybrid machining approach to machine hardened AISI 52100 steel parts more efficiently and cost effectively. The approach consists of a two step process involving laser-induced tempering of the hardened workpiece surface followed by conventional machining at higher material removal rates using low cost ceramic tooling to efficiently cut the laser tempered material. The specific objectives of this work are to (a) study the characteristics of the laser-based hybrid process, and (b) to model the laser tempering aspect of the process. The model will aid in selecting suitable laser scanning conditions that will lower the hardness of a pre-defined volume of material and thereby enable faster material removal rates in the subsequent machining step. First, the laser scanning parameters that yield a tempered surface and subsurface in hardened 52100 steel are identified through laser scanning experiments and their influence on tool life and cutting forces evaluated through actual cutting tests. A thermal model to predict the temperature during laser scanning of hardened steel and a hardness model to predict the resulting hardness are developed. These models are used to select laser scanning conditions that yield the desired hardness reduction at the maximum depth. At this depth, cutting experiments using Cubic Boron Nitride (CBN) tools and low cost ceramic tools are conducted to compare the tool lives and cutting forces in the laser-treated hybrid process and the conventional hard turning process.