PHASE-TRANSITIONS AND MICROSTRUCTURE OF A LASER-INDUCED STEEL SURFACEALLOYING

Alloying the surface of AISI 1045 steel with CrB by laser irradiation causes partial dissolution of the chromium boride in the melt and the formation of different borides of Pe and Cr in the treated layer. At a low laser scan velocity (0.01 m/s), the dissolution of CrB is almost complete, and the microstructure and properties of the top layer are u niform. At a higher scan velocity (0.05 m/s), a large number of CrB pa rticles remain undissolved in the layer, and its properties are hetero geneous. The matrix consists of columns of iron boride, with up to 20 pct Cr dissolved in it, and between them a eutectic containing alpha-F e and chromium-boride with dissolved Fe. Iron boride grows on a transi tional layer of Cr2B coating the surface of residual CrB particles, wh ich-cannot serve as nucleation sites because of the incompatibility of their crystal structure with that of (Fe,Cr)(2)B.