Evolution mechanism and precipitation kinetics of carbides in 50Cr6Ni2Y alloy steel by direct laser deposition

Carbides as main reinforcing phase play a significant role in direct laser deposition (DLD) 50Cr6Ni2Y alloy steel with high hardness and wear-resistant. The 50Cr6Ni2Y alloy steel samples were prepared with different laser energy average densities (EAD) from 90 J/mm2 to 110 J/mm2. Subsequently, the precipitation and evolution mechanism of carbides were studied by X-ray diffraction (XRD), scanning electron microscope (SEM), Transmission electron microscope (TEM), etc. The results indicate that the DLD samples were mainly composed of fine bainite ferrite (BF) and nano-sized (Fe,Cr)23C6 and (Fe,Cr)7C3 carbides. Herein, the dispersed nano-sized carbides attributed to the precipitation strengthening, which could improve the mechanical property of the samples. The carbides nucleated and grew at the intersection of the BF steps, and part of the carbide particles were surrounded by the fast-growing BF during the growth process. With the increase of EAD, more C dissolve into BF instead of forming carbides, which causes the carbide fraction to drop from 11.5 +/- 1.8% to 2.1 +/- 0.5%. The parameter k of carbide precipitation kinetic equation was calculated as 16.36443 +/- 0.51, and the modified kinetic equation was obtained, which can be used to control the volume fraction of carbides in DLD 50Cr6Ni2Y alloy steel. The sample with 100 J/mm2 EAD had the best matching performance of strength (hardness of 537 HV and tensile strength of 1294 MPa) and toughness (elongation of 5.5%) because of 5.8 +/- 1.3% fraction carbide precipitation. This research may provide a basis and technical reference for future studies on the precipitation and evolution mechanism of carbides in DLD 50Cr6Ni2Y alloy steel.