Processing parameters in Laser Transformation Hardening have a great effect on microstructures produced via phase transformations. In order to establish the effects for a wide range of processing parameters a large amount of specimens has to be processed and examined by microstructural analysis from polished and etched samples. The process is time consuming and therefore the number of processed samples is often minimized. In the presented study polished surfaces of low alloy steel and plain carbon steel samples have been hardened in an inert gas atmosphere using a HPDL (High Power Diode Laser) beam. Surface relief formed during phase transformations has been photographed using DIC (Differential Interference Contrast) imaging. The effect of processing parameters on austenitization, microstructure and extent of martensitic transformation has been investigated. The hardness of each sample is measured. Experiments show that DIC imaging is a rapid and illustrative tool for microstructural analysis of iron-based material processing.