Production and characterization of carbide free bainitic steels

Bainite structure with varying morphologies in steel has attracted considerable research attention. It has been designed and manufactured in automotive, bearing, gear and railway industries. Bainite morphology is closely related to the transformation temperature and alloying elements component. Lower bainite (LB), which is obtained at a low temperature range, is characterized by the precipitation of carbide within the bainitic ferrite plate. Upper bainite (UB), which is obtained at a high temperature range, is characterised by the distribution of carbide between bainitic ferrite plates.3 The carbide-free bainite as a new type is composed of superfine bainitic ferrite plate and retained austenite, which is generally obtained in steel alloyed with silicon (Si) and/or aluminum (Al). Si and Al exhibit very low solubility in cementite, which significantly retards its growth. Hence, the addition of Si plays a key role in suppressing the precipitation of brittle cementite during phase transformation, thereby producing a carbide-free bainitic microstructure and improving the toughness of the steels. In this study, the production of carbide-free steels with bainitic morphology, which are alternative to traditional bearing steels, and their structural, mechanical and wear properties were examined in detail. Phase diagrams, cooling curves and transformation temperatures of steels with four different compositions were calculated with computational material engineering analyses. Two of these compositions were produced by the traditional sand mold casting method, followed by austempering and subzero heat treatments, respectively. Thermal treatment temperatures were determined from isothermal and continuous cooling curves drawn with JMatPro software. After the austempering heat treatment, bainitic steels containing trace amounts of carbide with superior mechanical properties (~60 HRC hardness) and high wear resistance were obtained. Residual austenite in the microstructure of bainitic steels after austempering was largely removed by subzero heat treatment applied to austempered samples. The mechanical and wear properties observed after subzero heat treatment are similar to those observed after austempering. In addition, the mechanical and wear properties of bainitic steels were compared with the mechanical and wear properties of 100Cr6 (SAE 52100) commercial steel, which is currently used as bearing steel. Among the steels investigated, tempered bearing steel has the highest wear resistance with the lowest wear loss. Although the wear loss of austempered and subzero heat treated steels is close to that of tempered bearing steel, it is slightly higher.