Mechanical properties and wear performance of bainitic steels

As a potential candidate for a rail steel, bainitic steels have been studied for several years. Previous research conducted at OGI showed that high wear resistance was achieved in a low carbon (0.04% C) granular bainitic steel. A strong influence of microstructural features on the wear behavior highlighted the need to consider microstructure as well as mechanical properties in evaluating the wear behavior of a steel. This work was initiated to further investigate the wear performance of bainitic steels and establish structure/property relationships over a wider range of compositions.; Six Mo-B alloys were designed by varying additions of carbon, manganese, chromium, and nickel to provide a matrix of steels to investigate. Microstructural characterization was carried out by means of optical microscopy, SEM and TEM. The deterioration performance was investigated in two aspects: wear and deformation. Both wear and deformation tests were conducted in an Amsler machine under rolling/sliding conditions. The mechanical properties of the steels were investigated using tensile testing and Charpy impact testing. For comparison, a pearlitic rail steel and an austenitic manganese steel were also investigated in terms of wear and deformation resistance under rolling/sliding conditions.; The microstructural characterization revealed that granular bainite or carbide-free bainite was obtained in the Mo-B steels in the hot rolled conditions (except the as-received J2 that exhibited lower bainite) and with subsequent heat treatments with air-cooling or water quenching. A faster cooling rate produced more lath ferrite and reduced massive ferrite. The improved wear resistance was mainly associated with an increase of lath ferrite in the microstructure.; It was found that carbon played a significant role in determining microstructure, mechanical properties and wear performance. As carbon content increased, more lath ferrite in bainite was produced, which gave rise to an increased strength and improved wear performance. A decrease in wear rate with an increase in cooling rate was attributed to the microstructural changes and the improved mechanical properties. Carbide-free bainite with high strength and high deformation resistance was beneficial to improving wear resistance. This study confirmed that the wear resistance of the bainitic steels was comparable to high hardness pearlitic rail steels. The steel of 0.26% C could even compete with an austenitic manganese steel in terms of wear and deformation resistance.