Quantitative X-ray diffraction phase analysis was utilized to investigate the structure of a Fe-3.5 wt% C alloy after laser melting. The measurements were taken on surfaces modified by a series of equidistant laser tracks with overlappings k = 0.25, 0.50 and 0.75, and the results were extrapolated to the non-interfering tracks. On the same samples, the microstructure was observed by scanning electron microscopy. The structure of the rapidly solidified surface layers consists of alpha-Fe (ferrite and martensite), gamma-Fe (retained austenite) and cementite. The results of X-ray diffraction point to a very high amount of carbide indicating a non-stoichiometric composition of cementite. Besides, the volume fraction of alpha-Fe increases with increasing coefficient of overlapping k. At the same time, the volume fraction of cementite decreases while the content of gamma-Fe remains nearly constant. The changes are attributed to decomposition of the metastable carbide due to tempering of the former tracks by the following laser passes. This thermal effect was quantified on the base of the heat-transfer model of moving Gaussian beam.