Evaluation of the Possibility to Obtain Nanostructured Bainite in High-Carbon and High-Silicon 9XC Bearing Steel
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JMEPEG (2020) 29:5329–5336 https://doi.org/10.1007/s11665-020-05038-8
Evaluation of the Possibility to Obtain Nanostructured Bainite in High-Carbon and High-Silicon 9XC Bearing Steel Dominika Baran and Aleksandra Kro´licka (Submitted March 30, 2020; in revised form June 18, 2020; published online August 17, 2020) The main aim of this research is the evaluation of the possibility of obtaining a nanostructured bainite after isothermal heat treatment of 9XC (according to GOST standard) bearing steel. It was found that the annealing at low temperature (250 °C) allows to obtain the microstructure of nanobainite consisting of nanometric-scale bainitic ferrite laths and austenite with a dominant film-like morphology. In the case of higher isothermal treatment temperature (300 °C), a microstructure was slightly coarser and the proportion of austenite with blocky morphology increased. The identification of a highly refined microstructure was performed using TEM and SAED methods. Besides, isothermal annealing temperature significantly affects hardness. The hardness difference between annealing at 250 °C (630.0 ± 4.4 HV1) and 300° C (517.3 ± 1.1 HV1) was over 100 HV. Such a substantial decrease in hardness was caused by the fraction and morphology of the occurred phases. Due to the high silicon content in the tested steel (1.54%), the carbide precipitation was delayed, but locally the cementite precipitates were confirmed. Based on the research, it was found that the commercial 9XC steel is appropriate for nanostructurization in the low ranges of bainitic transformation. Designing of further heat treatment should be focused on the lowest possible temperature of isothermal annealing (even below 250 °C). Keywords
9XC steel, bainite transformation, bearing steels, nanobainite, Si-rich steels, TEM study
1. Introduction Bhadeshia and Edmonds (Ref 1) were the first to introduce methods for designing steels with high mechanical properties that are characterized by a microstructure consisting of nanometric-scale bainitic ferrite laths and austenite with filmlike morphology. To obtain nanostructured bainite (NB), the heat treatment included low-temperature long-term annealing is performed (Ref 2-6). The chemical composition of steel is also an important factor in designing nanobainitic steel. Elements such as carbon, silicon and manganese significantly enhance the austenite mechanical stability (Ref 7). Sufficient amount of Si (more than 1.5 wt.%) retards the precipitation of carbides into bainite plates (Ref 4, 8-10). The nanobainitic steels exhibit high tensile strength (over 2000 MPa), ductility (up to 14%) and fracture toughness (40 MPa m1/2) (Ref 3, 9, 11-13). Besides, steel after isothermal heat treatment is less prone to quench cracking and generation of undesirable tensile residual stresses at the surface (Ref 10, 13). Due to the high mechanical properties, NB steels are widely used in industrial areas and applied in roller bearings (Ref 2, 11, 14), ballistic shields (Ref 15) and railways (Ref 12, 14). Moreover, in 2009 one
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