Structure Peculiarities of the Surface Layers of Structural Steel under Laser Alloying

The article considers laser alloying technologies of surface layers of samples made of structural steel using powder filler add-on materials based on tungsten carbide and chromium. Structural transformations, concentration changes of chemical elements, grain and subgrain structures, dislocation density, size and volume fraction of phase precipitates, causes of crack formation in treated surface layers under various alloying regimes have been studied by the methods of optical, analytical scanning, and transmission electron microscopy. It was found that the tendency to crack formation of samples after laser alloying is due to structural (crystallite size, shape factor) and concentration changes associated with redistribution of elements, in particular, chromium. Noticeable changes in the structural-phase state of the treated surfaces are observed for an optimal mode of laser alloying. This laser alloying regime is characterized by a uniform distribution of dislocations with a general decrease in their density. The sizes of phase precipitates in the surface layers are more dispersed and evenly distributed in the internal volumes of grains. These are nanoparticles with a size of 40…100 nm and larger phase precipitates (up to 1 $\mu$m).