Rationalizing surface hardening of laser glazed grey cast iron via an integrated experimental and computational approach

Grey cast iron, a widely used inexpensive alloy, typically exhibits inferior and spatially inconsistent hardness and wear behavior. Using laser glazing, the surface of grey cast iron has been uniformly hardened to 1000 HV0.2, an eight-fold increase from the base alloy. This paper clearly demonstrates that the exceptional increase in the surface hardness is the consequence of complex multi-scale graded microstructures, resulting from novel far-from equilibrium phase transformation pathways, occurring during laser surface melting followed by inherent rapid solidification and solid-state cooling. The fusion zone of this graded layer exhibits complete dissolution of graphite flakes in the liquid which undergoes two distinct types of solidification: a) congruent solidification of austenitic dendrites, supersaturated with carbon and b) direct eutectic solidification of austenite + cementite lamellae. In the heat-affected zone, the pearlite matrix transforms into austenite without significant dissolution of graphite flakes during solid-state heating. These experimentally observed far-from equilibrium phase transformation pathways are rationalized based on the local temperatures and very high heating and cooling rates, predicted using thermo-kinetic models. Coupling multi-physics computational modelling with detailed multi-scale microstructure characterization, provided novel insights into these phase transformation pathways, and the potential for exploiting them in surface-engineering as well as more broadly during additive manufacturing. Keywords: Laser treatment, Cast Iron, Phase transformations, Rapid solidification