A comparative study of the boride layer morphology and diffusion kinetics on low alloy steels

Abstract

This work focuses on the low alloy steels (EN19, EN24, and EN41B).These steels have wide applications in the automotive sectors. They contain carbon (between 0.35-0.45 wt.%)and other minor alloying elements like Mn, Si, Mo, Ni and Al. Boronizing treatment provides the high load-bearing capacity, hardness, corrosion resistance, and wear resistance to the surface of steels. Metallurgical aspects of the boronizing of the steels were studied in this work. The boronizing treatment was carried out using the pack method. Pre-conditioned stainless-steel container with the powders of BoropakTM, SiC, and Al2O3was used for the boronizing treatment. The steels were boronized at 850, 950 and 1050°C for 2, 4 and 6 h. Boronized layer was analysed using optical microscopy, scanning electron microscopy (SEM), electron probe microanalyser (EPMA), Vickers micro-hardness tester, and X-ray diffraction (XRD). The columnar Fe2Bwas formed in the boronized layer. Nature of the boronized layer was dependent on the chemistry of the steels. The extent of zig-zag appearance of the interface between the boronized layer and core was more for the EN19 than the EN24 and EN41B steels. Unlike the EN19, small islands of borides were observed between the columnar morphology of Fe2B for the EN24 and EN41B steels.The effect of temperature and time on the kinetics of the boronizing wasstudied for these steels. The kinetics of boronizing was strongly dependent on the temperature, time and chemistry of the steels.The parabolic growth of the boronized layer with time was observed. Equations for the thickness of the boronized layer were determinedfor the steels using the obtained results. This study also focused on the corrosion behaviour of the boronized EN41B steel. Boronizing improved the corrosion resistance of the steel. Further, the boronizing-capability of the used pack-mixture was determined with and without the addition of activators. The appreciable thickness of the boronized layer was observed due to the addition of activators in the used pack-mixture.