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The study is devoted to pack-powder boriding of ASTM A709 steel in a powder mixture containing boron carbide, potassium fluoroborate and silicon carbide. This surface hardening process is conducted between 1123 and 1273 K for from 2 to 8 h. The formed diiron boride layers are studied by different techniques (scanning electron microscopy, x-ray diffraction (XRD) analysis, glow discharge optical emission spectroscopy (GDOES), Vickers microhardness testing, surface profilometry, Rockwell-C indentation cohesion and pin-on-disc tests). Kinetically, an integral-method-based approach is applied to calculate the boron diffusivities in Fe2B. The activation energy of the process is deduced and compared to reported data. The same model is ultimately verified by comparing the empirical values of the thickness of the Fe2B layers obtained at 1223 K and 1273 K for 9 h and the predicted values. [ABSTRACT FROM AUTHOR]

For this work, hot-rolled plates of medium carbon steel (0.5%C) were prepared with different sulphur contents (0.023% to 0.065%). Two steels were alloyed with mischmetal (0.01%) as inclusions modifier to evaluate its effect on fatigue and tensile properties. A detailed study of the type, size, shape and distribution of the sulphide inclusions was carried out by optical and SEM microscopy in the as-cast and hot rolled conditions and correlated with their mechanical properties. The mischmetal treated steels developed a globular, small (~2µm) and well distributed MnS inclusions, producing an improvement in tensile strength (840 MPa and 19% elongation) and fatigue behavior (375 MPa), which was not evident on the untreated steels (741 MPa tensile strength and 300 MPa fatigue limit). The fatigue behavior was tested under the E466 standard, by using nine samples per composition in a R.R. Moore -- rotating beam fatigue testing machine. The higher fatigue limit was obtained for the mischmetal-treated steel with 0.05%S. These results were discussed and correlated to the small size and globular shape of the MnS inclusions. However, it was observed that, additions of 0.01% of mischmetal are less effective as inclusion modifier when sulphur content is higher than of 0.05%, but it is still good enough to obtain a better fatigue behavior compared with the untreated steel containing 0.023% sulphur. [ABSTRACT FROM AUTHOR]

Abstract: In this study, new data about the growth kinetics and indentation properties of cobalt boride layers formed on the surface of the ASTM F-75 biomedical alloy were estimated. The boron diffusion at the surface of the biomedical alloy was conducted using a powder-pack boriding process at temperatures of 1223–1273K with different exposure times for each temperature. Two mathematical approaches were proposed to determine the boron diffusion coefficients of the CoB and Co2B layers in the range of boriding temperatures, in which the experimental results of the kinetics of the cobalt boride layers were compared with those estimated using diffusion models. Finally, the mechanical characterization of the cobalt boride layers was evaluated by indentation techniques with applied loads of 0.98N and 50mN, respectively. According to the nanoindentation tests, a maximum hardness value (30GPa) and a maximum Young's modulus (380GPa) were obtained near the surface region (5μm) of the borided cobalt alloy. [Copyright &y& Elsevier]