Examination of laser hardened steel surfaces using interference microscopy

A diode laser beam in the near-infrared region is absorbed more efficiently by a metal surface than a CO2 or a Nd:yttrium-aluminum-garnet laser beam. Diode laser transformation hardening without an absorptive coating is feasible even in an inert gas atmosphere. Absence of the absorptive coating, and the fact that the formation of an oxide layer is inhibited, makes it possible to study phase transformations occurring during hardening from the surface relief formed to the processed surface. In the present study diamond polished plain carbon and low alloy steel samples have been hardened in an inert gas atmosphere of argon using a high power diode laser. Surface reliefs caused by phase transformations during quenching have been examined using differential interference contrast (DIC) microscopy. Hardness of each sample was measured and the effect of the processing parameters on the surface hardness and microstructure has been established. DIC imaging was found to be a rapid and illustrative tool for examining microstructures in laser hardened surfaces. Extent of martensite, austenite grain size prior to quenching, and martensite lath size are visible in the DIC images.