ABSTRACT
Laser surface melting was used to produce a sound surface layer of modified and
hard structure for a nodular cast iron. The high hardness of surface is an important
requirement for the high wear resistance. The treatments were carried out using
CO2 laser of 6 kW. Nitrogen was used as a shielding gas. The effect of the
processing power on the structure and hardness of the nodular cast iron have been
studied. The microstructures of the treated zones and base metal were investigated
using optical and scanning electron microscopes. Three main zones were resulted;
the melted and solidified zone, hardened one (heat affected zone: HAZ) and the
base metal. In the melted and solidified zone, high percentages of graphite nodules
were completely dissolved and fine dendrites of austenite surrounded by cementite
and some martensite were produced. Graphite nodules and martensitic structure
have been observed in the HAZ. In the three conditions of treatment (700, 1000 and
1500 W at beam travelling speed of 500 mm/min.), no cracks were observed in the
two zones; melted and solidified zone and the HAZ. Remarkable improvements
(420-470%) in microhardness values for the melted and solidified layers were
achieved. The depth of this zone is increased with the increase of processing power.
The maximum melted depths for the hardest layers were 0.8, 1.2 and 1.4 mm in the
cases of 700, 1000 and 1500 W respectively. HAZ of High hardness was also
obtained.