Graphitauflösung im Laserschmelzbad

During the laser remelting of cast iron the graphite inclusions are dissolved to an extent depending on temperature and time. The surface tension gradient driven melt bath convection leads to an almost uniform carbon concentration in the melt. Carbon diffusion into the solid decreases the melting temperature and, therefore, facilitates melting. The dissolution kinetics of spherical graphite inclusions in nodular cast iron during laser remelting is analyzed for the resting and the well-stirred melt using a semi-analytical approach to the moving boundary problem. Carbon transport from the dissolving particle proceeds close to its surface by diffusion but farther away by convection, the size of the diffusion zone depending on the particle's speed and hence on the forces acting on it as well as on its current size. A particle of initial radius 20 micrometers is found to dissolve in an eutectic melt within less than about 0.1 s at 2000 Cel and about 0.6 s at 1500 Cel. Lower dissolution rate s are possible if particles cluster together in regions with comparatively low convective fluxes. The computed dissolution kinetics is in satisfactory agreement with particle size and distribution observed in the melting zone of laser remelted nodular cast iron GGG 40.