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

Highlights: • Depths of fusion and heat-affected zones in laser glazed cast iron computationally predicted and experimentally verified. • Rapid melting and solidification in fusion zone completely dissolved graphite. • Site-specific transmission electron microscopy revealed unusual sequences of liquid-solid and solid state phase transitions. • Fusion zone underwent either congruent solidification or direct eutectic decomposition. • Conversion of graphite into cementite and fine scale martensite in fusion zone made it extremely hard and wear resistant. 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 H_V0.2, 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.