Metallographic investigation of laser-treated ductile iron surface with different laser heat inputs

In the current work, the impact of various laser heat inputs on the final microstructure of the nodular graphite cast iron as well as its microhardness values were evaluated. A proper processing window of four different laser powers (600, 1000, 1400, and 1800 W) with four corresponding different laser scanning speeds (1.667, 4.1667, 12.5, and 20.8 mm s−1) was accomplished in this study. Laser hardening, partial melting, and melting processes were achieved as a result of applying different laser heat inputs. The aim was to reach the optimum condition that obtained a free crack microstructure with high hardness values. The microstructure was examined in detail through scanning electron microscopy. Chemical analysis of different zones in the microstructure was analyzed utilizing energy dispersive X-ray (EDX). The results revealed that at low laser heat input (laser hardening) the microstructure consisted of large needle-shaped martensite with retained austenite and undissolved graphite nodules while, at medium laser heat input (partial laser melting), the microstructure contained eutectic ledeburite structure, retained austenite and plate-shaped martensite phase. Whereas, at high heat input (complete laser melting), the structure showed a remarkable refinement of cementite phase and interdendritic eutectic carbides beside the needle-shaped martensite. The heat-affected zone and the overlapped areas have been thoroughly examined as well. The hardened layer measured 250 μm at the lowest value of 9.6 J·mm−2, whereas with increasing the heat input to 144 J·mm−2 the thickness of the deepest melted layer reached ∼1650 μm. The hardness of the modified microstructure significantly increased by almost six-fold higher than that recorded for the as-cast substrate.