Laser cladding of Co-based protective coatings on Cr-steel

The direct laser cladding process is applied for production of the cobalt-based stellite SF6 coatings on the chromium steel substrates of a composition designed for the steam turbine blades. A cw 1.2 kW CO2 laser stand equipped with a multi-stream nozzle mounted co-axially with the focussing optics is applied. Experiments are performed for the non-preheated as well as preheated base material. Differences in microstructure, chemical composition, and in the cracking susceptibility between coatings produced on preheated (780-1000 K) and non-preheated substrates are analyzed by means of scanning electron microscopy (SEM) and the Energy Dispersive X-ray Spectroscopy (XEDS). Results indicate a significant improvement of the clad quality and decrease of the micro-cracking susceptibility with the increase of preheating temperature. The presence of Fe in the coatings is ascribed to migration from substrate due to preheating.