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Nanostructured bainitic steels exhibit an optimum strength/toughness combination as a consequence of their extremely fine structure. They have also demonstrated potential for wear-resistance applications. The aim of this work was to develop bearing steels by the multi-scale control of complex ferritic structures, designed using atomic transformation theory and processed by novel heat treatments. Based on the results, the new ball bearings outperformed conventional grades, approaching more expensive material options. the structure comes from the extremely thin platelets of bainitic ferrite, but also dislocation forests and solution strengthening. It is known from X-ray diffraction (XRD) analyses and confirmed by atom probe tomography (APT) that bainitic ferrite contains much more carbon in solid solution than it is consistent with paraequilibrium thermodynamics conditions [12][13][14][15].The different strength-ductility combinations observed in nanostructured bainite are associated with the stress and strain-induced martensitic transformation of retained austenite during tensile testing [9,11,[16][17][18][19][20]. Additionally, the formation of twins in austenite films has been identified as a strain hardening mechanism contributing to ductility in these structures [20].Complex properties, such as wear resistance and fatigue endurance, must be also assessed before the commercialization of novel bainitic steels. Earlier work [4,5] showed that the wear resistance of carbide-free bainitic steels was comparable to that of the best pearlitic grades. In the context of rail applications, Chang [6] investigated the rolling/sliding wear performance of several high silicon bainitic steels, along with detailed microstructural characterization. Results showed lower wear rates in carbide-free bainitic steels and a beneficial effect of the austenite embedded in the sub-micron ferritic structure. In fact, Zhang et al. [21] found that high silicon bainitic steels show better wear resistance than much harder martensite. Likewise, Yang et al. [22] observed that material performance improved when the transformation temperature decreased and a nano-scale bainitic structure was formed.These studies hint at the considerable benefits of nanostructured bainite against several wear and surface damage mechanisms [7,23,24]. Retained austenite is considered to be critical for improving wear resistance in these structures, since this phase provides hardening by transformation into martensite [23,25]. It is well-known that hardness affects the stress needed to deform the material in the rolling/sliding contact, and it is an important factor in decreasing material loss. Wang et al. [26] found that the austenite in the vicinity of a sliding surface decomposes under the influence of high shear strains, resulting in the formation of an extremely fine structure with grains of ferrite only about 3 nm in size.Rolling contact fatigue (RCF) has also been determined to be a key mechanism of material removal in the wear performance of nanostructured bai...

Bearing steels are heat treated to obtain martensitic microstructures that provide high hardness necessary for good rolling contact fatigue performance. For the most common bearing steel, SAE 52100, without specific thermal treatments such as stabilizing tempering, the microstructure consists of unstable phases that can evolve in service leading to detrimental dimensional variations. A previous publication of the 10th ASTM bearing steel conference held in 2014 focused on explaining and modeling the dimensional variations induced by thermal aging of SAE 52100 and the role of retained austenite in the expansion occurring in service. The influence of a few heat treatment parameters on retained austenite content was also briefly discussed based on the literature. Very few publications are available in the literature regarding the influence of heat treatment parameters on dimensional stability. An experimental study was conducted at NTN-SNR to better understand this topic. It was based on the same methodology presented in 2014 but with different initial heat treatments. The influence of austenitizing parameters such as time and temperatures, but also of the cooling conditions during quench, were examined. Different experimental techniques were used to precisely qualify and quantify microstructural evolutions: thermoelectric power measurements, synchrotron X-ray diffraction, and quench dilatometry. Through this study, it was concluded that the widely held belief that the amount of retained austenite is the only parameter to assess expansion in service was too simple. In some cases, heat treatments with similar retained austenite contents led to significant differences in dimensional stability. This article describes such results, depending on the initial heat treatment, and proposes an explanation on the microstructural phenomena leading to these differences.

The evolution of aerospace engines has led to a significant reduction in polluting emissions and fuel consumption, while increasing the demands placed on bearings. Hence, meeting these increasing demands for tomorrow's bearings raises the issue of producing a new material that will ensure the fatigue life and reliability. The research on high-performance material is a good opportunity to promote the introduction of powder metallurgy steel. Powder metallurgy for bearing steel manufacturing was introduced several decades ago, but the effective use in aerospace engines is still very limited. This is probably the consequence of a lack of understanding of how modern powder metallurgy steels could be used to manufacture bearings. The main interest in powder metallurgy is that it is very well adapted to increase hardness after heat treatment as a consequence of the high addition of alloying elements. Due to the fast solidification conditions, there is no specific problem concerning segregation and carbide size. In a previous symposium, we presented some data concerning ASP 2055 and the interesting metallurgy associated with this material. This article will focus on additional results concerning the mechanical and rolling contact fatigue behavior, with emphasis on toughness and microcleanliness. Thermomechanical treatment and the metallurgical evolution after each manufacturing step will be described. The main parameters concerning the formability of ASP 2055 will also be detailed. In this article, the methodology used for the metallurgical characterization will be described in detail. The mechanical results and rolling contact fatigue test results will be discussed in comparison with the well-known reference M50 vacuum induction melting/vacuum arc refining (VIM/VAR). Emphasis will be put mainly on the toughness and cleanliness assessment results. The interest in using new techniques such as microwave microscopy to reveal subsurface defects is highlighted, underlining how promising these techniques are.