Surface Microstructure and Properties of Nodular Cast Iron Rapidly Solidified by Laser Surface Melting

Ferrite-matrix nodular cast iron has been modified by a laser surface melting process to develop its microstructure and to improve the surface hardness. A YAG laser beam was irradiated on a substrate and the microstructure of the melted layer was investigated as a function of the pulse energy at a constant specimen travel speed. The surface of the specimen is melted and then rapidly solidified up to a depth of 100 µm order. The melted depth increases with increasing pulse energy. In addition, the ferrite-phase matrix around the spheroidal graphite in cast iron preferentially melts because several alloying elements are segregated at the ferrite/graphite interface. The solidified layer consists of three distinctive parts: first, a martensite phase appears in the vicinity of the melted/unmelted substrate interface, then single-phase austenite crystallized on the martensite phase, and finally a ledeburite-austenite hybrid structure unidirectionally solidified from the substrate towards the surface. A cooling rate from 0.3 to 2.4 × 104 K/s is estimated from the austenite primary dendrite arm spacing under our experimental conditions. The micro-Vickers hardness was also examined in relation to the area fractions of the ferrite, austenite and cementite phases. The Vickers hardness varies from 600 to 900 HV in the solidified layer, whereas the initial substrate shows 200 HV. This tendency for the hardness to increase is estimated from the hardnesses and volume fractions of the soft austenite and hard ledeburite.