Pioneering laser scanning system for hardening of turbine blades

This paper deals with a new system for laser beam surface hardening of low pressure steam turbine blades. The system consists of a prism for beam dividing and two scanning mirror systems. A new technology allows the simultaneous hardening of both suction and pressure side of leading edges of turbine blades in a single pass with only one CO2 laser beam source. Based on physical models process parameters (i.e. traveling speed, spot dimensions, laser power, beam dividing ratio) can be calculated. A new technology was developed to control the intensity distribution of the scanning laser beam. For this purpose, numerical algorithms were developed for calculation of the power density distribution and optimization of the free oscillation function of the scanning mirror. As an result now almost any power density distribution can be generated on the surface of a flat or curved work piece or a work piece with one edge using only one laser optics. Experiments were carried out for investigation of the geometry of the hardened layer depending on the irradiation conditions and for investigation of the phase transformations in test pieces made of the German steel grade X20Cr13. The measured hardness distributions were correlated with the local temperature- time- cycles. Using the new scanning system and the software package for theoretical optimization of process parameters turbine blades were hardened successfully.