X-ray diffraction, optical microscopy and microhardness measurements were used to study the structure formation in low carbon steel containing 0.04% C, 0.3% Mn, and 0.2% Si, 0.2% Al was subjects to laser irradiation in air with a KVANT-18 M setup using discrete 8-ms pulses. The resulting spots are either isolated (up to 2 mm in diameter) or overload, forming a track up to 40 mm long. The density of the irradiation energy was varied between 2 and 16 J/mm² to provide treatment conditions changing from heating into the α and γ fields (without melting) to the surface melting of the samples. Dislocation density defined on the value of a physical broadening of X-ray reflexes.

    The laser treatment of low carbon steel results in a nonmonotonic distribution of dislocation density both through the laser-affected zone depth and over the diameter of an isolated laser spot in the surface layer. The maximum dislocation density was observed at a about 50 μm and in the center of the laser spot. In this case, the dislocation density observed at this depth exceeds that observed in the surface layer of the center of the laser spot. In all cases, the degree of strengthening (microhardness) is consistent with the dislocation density.

    A series of structure types from dendritic and martensitic structures to fragmented grain structure with a high dislocation density inside the fragments can coexist in the laser spot and over the laser-affected zone depth. The nonmonotonic character of the microhardness distribution is mainly determined by the distribution of dislocation density. Additional contributions to the strengthening of the material come from the structure refinement (the grain refinement, the refinement of their internal structure, and the formation of martensite).

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