Kinetics and mechanical properties of borided GCr15 bearing steel

Abstract

The kinetics and mechanical properties of borided GCr15 bearing steel was investigated. The boriding treatment was carried out in a solid medium at 1123, 1173, 1223, and 1323 K for 2, 4, 6, and 8 h. The microstructures and mechanical properties of the boride layer were characterized by optical microscopy, scanning electron microscopy, X-ray diffraction, and Vickers hardness tester, and the growth kinetics characteristics were also studied based on experimental data. The results indicate that the boride layer has a smooth and compact morphology, and the presence of FeB and Fe2B on the steel substrate is confirmed by X-ray diffraction analysis. The thickness and hardness of the boride layer increase with treatment time and temperature, where the thickness ranges from 33.4 to 318.5 μm. The increased hardness is mainly because of the increase in the highly hard FeB phase content. The content of Fe2B phase, which has a low hardness, decreases with an increase of layer thickness. The hardness of the boride layer HV0.1 ranges within 1630-1950, and it is increased by 5 to 6 times compared with the matrix. The hardness test results of the boride layer cross section indicate that there is a wide transition of hardness gradient between the boride layer and the matrix. The kinetic equation based on the experimental data and Arrhenius equation was investigated, the active energy of B element in the GCr15 bearing steel is 188.595 kJ·mol-1, and the derived kinetic equation is verified by experiments. The results indicate that the maximum error between the theoretical derivation and experimental derivation is 4.93%. Therefore, the derived kinetic equation can effectively predict the thickness of the boride layer on GCr15 bearing steel.