Production of Niobium Carbide Layers on High-Strength Bainitic Steels by Thermochemical Treatment of Thermoreactive Diffusion Followed by Austempering

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Steel Research International

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The development of third-generation advanced high-strength steels aims for the production of low-alloy steels with a high strength-to-weight ratio through innovative heat treatments. Thermochemical treatments that eliminate reheating steps are an excellent alternative for the surface hardening of this type of alloy. Herein, the incorporation of a thermoreactive diffusion (TRD) thermochemical treatment along with austempering treatment to obtain carbide-coated high-strength bainitic steels is evaluated. For this purpose, samples of bainitic steels containing ≈1.5 wt% silicon are immersed in an austenitizing bath composed of ferro-niobium and sodium borate for 2, 4, and 6 h at 1100 °C. Subsequently, they are held isothermally in a 360 °C salt bath for 1 h. After treatment, the samples are characterized by optical microscopy, scanning electron microscopy, X-ray diffraction, dispersive energy spectroscopy, Knoop microhardness, and microadhesive wear tests. Microstructural analysis shows the formation of niobium carbide layers at the surface of a substrate composed of granular and plate bainite, characteristic of austempered high-silicon steels. These layers show thickness in the range of 4−9 μm, hardness from 2333 to 2599 HK, presenting hardness up to 6 times higher than the substrates, and wear resistance up to 8 times greater.


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