AbstractIt is expected that surface treatment by CO2laser will be useful for industrial applications in which improved wear resistance is required on selected areas of already accurately machined parts. Precision in the hardening of such surfaces using laser processes is an additional advantage. Moreover, it is possible to obtain these desirable properties for low cost construction steels. The microstructural changes within the laser heated zones of commercial grade medium carbon C45 steel were determined on thin foils from the treated regions using transmission electron microscopy. Components of the substructure are described and the influence of rapid laser heating on austenite formation and its transformation into martensite are discussed.MST/563