Microstructure and mechanical properties of low-carbon steel fabricated by electron-beam additive manufacturing

Abstract

The microstructure and mechanical properties of a billet obtained by electron-beam additive manufacturing (EBAM) using an industrial welding wire of low-carbon steel were studied. After EBAM, the steel billet possesses the phase composition constant in volume (ferrite with carbides). Deformation behavior of samples of steel obtained by the additive methods depends on their position in the billet. In its lower part, which is characterized by a high cooling rate in the EBAM-process, predominantly equiaxed ferrite grains with an average size of 15 μm are formed. The mechanical properties and deformation behavior (the presence of a yield plateau, the stages of plastic flow) of this part of the billet are close to those of normalized low-carbon steel obtained by conventional methods of metallurgy and thermomechanical processing. In the middle and top parts of the billet, the coarse non-equiaxed ferritic grains (hundreds of micrometers) form. The mechanical properties in this part weakly depend on the position of the samples and their orientation relative to the building direction of the billet — the yield plateau disappears; the yield stress, the ultimate tensile strength, the elongation become lower than those of the normalized low-carbon steel obtained by the conventional method. Despite the predominance of ferritic grains (with carbides), a small portion of grains with a lamellar ferrite morphology resembling martensite or bainite is observed in all parts of the billet. The latter is the result of a complex thermal history of the billet, which can undergo multiple phase transformations during successive heating and cooling cycles.