Discrete laser spot hardening of austempered ductile iron

This study investigates the effect of discrete spot laser surface hardening (LSH) on austempered ductile iron (ADI) specimens. ADI surfaces were irradiated with single laser pulses, having different beam powers and diameters with the intent of establishing the parameters that yield a surface with the best combination of maximum hardness and hardened depth, whilst exhibiting minimal distortion and no cracking. A process parameter map was plotted and showed that to produce a hardened microstructure without melting and cracking, one must operate within a very small range of parameters achieving surface hardness values between 700 and 800 HV and depths of approximately 150 μm. The resulting microstructure consisted of martensite with unaltered graphite nodules. After laser hardening, the mean surface roughness increased from 0.15 to 0.43 μm. When applying laser powers higher than 780 W, melting was observed within the laser spot. Laser surface melting (LSM) resulted in a severely distorted surface, with an increase in the mean surface roughness from 0.15 to 1.34 μm. Within the melted zone, transverse cracks and the dissolution of the graphite nodules could be observed. The rapid self-quenching associated with laser melting resulted in an austenitic microstructure having a surface hardness of approximately 400 HV. A martensitic structure with hardness values peaking around 700-800 HV, was observed at a depth 250-300 μm below the molten structure.