The new hybrid laser alloying method: The influence of nano-bainitic steel microstructural condition on the microstructure and hardness of the alloyed surface

The article presents the results of tests on NANOS-BA® class, grade P1 armour steel in two heat treatment states, subjected to a new type of surface treatment, namely hybrid laser remelting assisted by a low-temperature heat source (L-THES). The first variant produced steel with a nanobainitic structure, while the second resulted in a softened state with large carbides in a ferritic matrix. A hybrid surface remelting/alloying method was employed to modify and harden the substrate, introducing W₂C/WC granules into the substrate. Consequently, a significant increase in hardness was achieved for the softened substrate (64 HRC - an increase of over 104 % compared to the initial state) and by approximately 31 % for nanobainitic steel (67 HRC). It was determined that the factor influencing the microstructure and hardness of the laser-melted/alloyed layer is the difference in the temperature gradient, stemming from the varying values of thermal diffusivity between the nanobainitic steel and the softened state. Including an additional low-temperature energy source in the hybrid process intensified the mixing process. This led to the extraction of small fragments of tungsten carbide and a slight increase in hardness for the nanobainitic steel (the effect of buoyancy forces). This phenomenon was not observed in the softened steel, which exhibited a more significant impact of supercooling and temperature gradient. Ultimately, it was concluded that employing the new hybrid laser remelting/alloying method proved to be a highly effective means of enhancing the surface layer's hardness while preserving the high substrate flexibility.