Effekte des Laserumschmelzens auf das Gefüge und die Härte von Grauguss mit Lamellen- und mit Kugelgraphit

Abstract

The present report depicts the impacts of laser beam remelting on the microstructure and hardness of grey cast iron containing lamellar and globular graphites. The surface layer is remelted by means of a cw CO₂ laser the nominal beam power of which is 4.5 kW. The beam is formed by a beam integrator which generates a square focal cross-over point 9 mm in edge length on the sample surface. Transverse specimens are sampled from the laser trace produced for use in the metallographic examinations and hardness measurements. Since the molten surface layer quenches itself because of the cold base material, the melt will solidify, and phases will be transformed, into a solid state under non-equilibrium conditions. The result is that meta-stable phases of an increased hardness will form within the fine-grained remelted zone: cementite, residual austenite, martensite. The remelted zone is joined by the heat-affected zone the microstructure of which is governed by the peak temperature reached, and an accelerated cool-down. The effect of laser beam remelting is that hardness increases significantly in the surface layer, which can be attributed to a grain refining process in the microstructure and the existence of the hard, metastable phases within the laser-beam treated surface layer.