Abstract
There has been a great deal of work on the formation of hard white-etching regions in conventional bearing steels such as 1C-1.5Cr wt% when subjected repeatedly to rolling contact stresses. The regions are a consequence of localised mechanical attrition across microcrack faces and mixing, which refine the local structure and force cementite to dissolve. This white-etching matter is often associated with brittle phenomena because the hardness can exceed 1100 HV. In contrast, carbide-free mixtures of bainitic ferrite and retained austenite when subjected to the same loading have been unexpectedly found not to develop the characteristic patches of hard material and to show instead signs of ductility in the attrited regions. The work presented here shows that the white-etching areas that develop in carbide-free bainite are softer than their surroundings, whether they are in hard nanostructured bainite destined for bearing applications or when the steel is designed for the manufacture of rails. Advanced characterisation tools were used for the first time to understand carbon redistribution during its formation. The deep interest about soft white-etching matter originates from the idea that it could lead to the reduction in premature failure of wind turbine gearbox bearings and white-etching layer grinding of rails.
Highlights
Previous studies have highlighted the potential of nanostructured bainite, which consists of fine plates of bainitic ferrite in a matrix of stable austenite, as a bearing steel due to its hardness (600e670 HV), toughness (30e40 MPa m1/2), and rolling-sliding wear resistance [1e5]
It is important to emphasise that the white-etching matter (WEM) that causes the greatest damage is that which is harder than the surrounding matrix, the additional hardness having its origin in the state of deformation and because cementite is taken into solution by mechanical alloying [9,15,16]
It is worth mentioning that a similar approach was followed when determining the microhardness of the WEM formed around cracks in martensitic 1C-1.5Cr steel where the WEM turned out to be 10% harder than the matrix, corroborating the validity of the results found here and eliminating the doubt that the softness is caused by crack proximity [19]
Summary
Previous studies have highlighted the potential of nanostructured bainite, which consists of fine plates of bainitic ferrite in a matrix of stable austenite, as a bearing steel due to its hardness (600e670 HV), toughness (30e40 MPa m1/2), and rolling-sliding wear resistance [1e5]. The voids grow and coalesce into larger entities that lead to fracture This is in contrast to the well-studied mechanism of brittle crack initiation at inclusions in standard quenched and tempered 1C1.5Cr wt% bearing-steels. Due to the brittleness of these alloys [8] and the hardness of the white deformed regions, these white-etching cracks (WECs), or butterfly cracks eventually link up into larger networks that lead to spalling [11]. This mechanism is referred to as “white structure flaking” [10,14]. It is important to emphasise that the white-etching matter (WEM) that causes the greatest damage is that which is harder than the surrounding matrix, the additional hardness having its origin in the state of deformation and because cementite is taken into solution by mechanical alloying [9,15,16]
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