Microstructure of surface layer of T1 and D2 steels after laser melting

Abstract

The aim of this study was to investigate the in-depth structure transformations of the laser-melted layers of the steels mostly used in tool production – T1 and D2 steels. The experiments of laser surface melting of T1 and D2 steels were done with continuous wave CO2 laser (wavelength λ = 10.6 μm). The sample’s microstructure was investigated, and the EDX analysis was made by SEM JEOL JMS-35C. Our results show that the microstructural changes of the laser-melted layer of T1 steel were determined by the process of directed crystallisation from the bottom layer to the surface. There is considerable inhomogeneity, followed by the predominantly ongoing process of chemical liquation during crystallisation. Cellular structure, consisting high-carbon martensite and significant amount of retained austenite, prevails in the melted pool bottom. Austenite–carbide eutectic with possible presence of lamellar martensite is situated along the dendrites’ boundaries (in inter-dendrite spaces). The austenite–carbide quasieutecticum with carbide-type M6C possesses ‘skeletal’ morphology in these regions. In the laser melting of D2 steel, the phase transformations in the transition zone (liquid–solid state) were defined by running of reverse eutectic reaction at the places of overheated and partially melted carbide phases. During cooling of these regions, the inhomogeneous melt goes through changes that involve precipitation of oversaturated with carbon and chrome austenite and disperse microquasieutectic with carbides of the M7C3 type.