The mechanism of formation of nanostructure and dissolution of cementite in a pearlitic steel during high pressure torsion

The microstructure of the surface layer of railway tracks is investigated. It is shown that the surface layer of the rail transforms during exploitation into a nanocrystalline Fe–C alloy. The mechanism of the nanostructure formation is discussed. It is shown that the transformation of pearlite to the nanostructured Fe–C alloy layer is caused by the heavy plastic deformation at the wheel–rail contact zone. The transformation of the microstructure of the surface may take place at rail–wheel contact temperatures less than 230°C and its mechanism is similar to that taking place during mechanical alloying.

Nanostructure formation on the surface of railway tracks

The formation mechanisms and properties of white layers produced in machining of hardened steels are not clearly understood to date. In particular, detailed analysis of their structure and mechanical properties is lacking. This paper investigates the differences in structure and properties of white layers formed during machining of hardened AISI 52100 steel (62 HRC) at different cutting speeds. A combination of experimental techniques including transmission electron microscopy (TEM), X-ray diffraction (XRD), and nano-indentation are used to analyze the white layers formed. TEM results suggest that white layers produced at low-to-moderate cutting speeds are in large part due to grain refinement induced by severe plastic deformation, whereas white layer formation at high cutting speeds is mainly due to thermally-driven phase transformation. The white layers at all speeds are found to be comprised of very fine (nano-scale) grains compared to the bulk material. XRD-based residual stress and retained austenite measurements, and hardness data support these findings.

Analysis of white layers formed in hard turning of AISI 52100 steel

The actual service life of wind turbine gearboxes is often well below the desired 20 years. One of the prevalent failure modes in gearbox bearing raceways is white structure flaking (WSF) by the formation of butterflies and white etching cracks with associated microstructural change called white etching areas. Despite these failures having been observed for two decades in various industries, the detailed reasons and mechanisms for their formation are not fully understood. In this review, white etching area formation mechanisms are discussed, specifically grain refinement, and effects of carbon/carbide in a range of bearing steels of widely differing carbon content. The review also highlights the severe transient, cyclic loading and tribochemical operating conditions of gearbox bearings and explains how these may act as drivers to produce WSF. Much previous research has focused on the detrimental effects of hydrogen, but other work suggests that hydrogen is not the only cause for WSF. Possible meth...

White structure flaking (WSF) in wind turbine gearbox bearings: effects of ‘butterflies’ and white etching cracks (WECs)

The available experimental data and hypotheses concerning cementite decomposition during the cold work of pearlitic steels are reviewed. The results of studies performed using thermomagnetic analysis, Mossbauer spectroscopy, internal friction and APFIM are used to discuss the mechanism governing cementite decomposition. The following features of this phenomenon seem to be important: (i) the fraction of the decomposed cementite increases with the refining of the initial pearlitic structure, i.e. with the increase of the ferrite–cementite interfacial area; (ii) the decomposition effect saturates as strain increases; (iii) carbon–dislocation interaction in ferrite and MeC bonding in cementite have a strong influence on cementite decomposition. The conclusion is made that cementite decomposition is controlled by the transfer of carbon atoms from cementite to dislocations accumulated near the interface during deformation. This is because the binding enthalpy between carbon atoms and dislocations in ferrite exceeds the solution heat of cementite. Some relevant effects of cementite decomposition in practice are discussed.

Decomposition of cementite in pearlitic steel due to plastic deformation

Formation of white-etching layers on rail treads

Surface modification, corrugation and nanostructure formation of high speed railway tracks

Comparison between nanophase formation during friction induced surface wear and mechanical attrition of a pearlitic steel

Nanostructure Formation and Mechanical Alloying in the Wheel/Rail Contact Area of High Speed Trains in Comparison with Other Synthesis Routes

The fine structure and microhardness of the white layer formed at the surface of railroad rails under service conditions were studied. This layer was found to have an ultrafine-grained structure with an average ferrite grain size of 200 nm. The initial cementite framework in the layer is broken, and the hardness increases by a factor of 3.5 as compared to the initial state. The nature of the high-strength state of the white layer is discussed.

G. Baumann

Papers

Formation of white-etching layers on rail treads

Surface modification, corrugation and nanostructure formation of high speed railway tracks

Comparison between nanophase formation during friction induced surface wear and mechanical attrition of a pearlitic steel

Nanostructure Formation and Mechanical Alloying in the Wheel/Rail Contact Area of High Speed Trains in Comparison with Other Synthesis Routes

Nanostructure and hardness of white layer at the surface of railroad rails