Degradation of nanostructured bainitic steel under rolling contact fatigue

The consequences of rolling contact fatigue on carbide-free nanostructured bainite intended for bearing applications are presented for the first time. Tests performed at various intervals followed by mechanical, microscopical, and crystallographic characterisation lead to the conclusion that the degradation mechanism is ductile void formation at interfaces, followed by growth and coalescence into larger voids that lead to fracture along the direction of the softer phase. This is di↵erent from the conventional damage mechanism that involves crack initiation at inclusions and propagation, for example in typical bearings steels such as 52100. The huge density of interfaces in the nanostructure allows the formation of a large dispersion of voids, and ultimately cracks, at depths consistent with the maximum orthogonal shear stress which occurs below the contact surface. This study should prove useful for the eventual usage of nanostructured bainitic steels in rolling bearings.Funding by CONACyT, the Cambridge Overseas Trust, and the Roberto Rocca Education Programme is highly appreciated and acknowledged.This is the accepted manuscript version. The final published version is available from Elsevier at http://www.sciencedirect.com/science/article/pii/S0921509314010715

The effect of vanadium alloying on the wear resistance of pearlitic rails

Due to the increasing demand on pearlitic rails to support heavier loads and higher speeds, grooved rails for tramways and light rail transit (LRT) require strengthening via alloying or heat treatment, i.e. accelerated cooling after hot-rolling. In this study, the wear performance of five different commercial grades is compared using unlubricated twin-disc testing and detailed microstructural characterisation. Results indicate that the vanadium-alloyed steel experiences less than half the wear rate of a heat-treated grade with a similar hardness. This is attributed to the increased stability and toughness of vanadium-containing cementite, as compared to pure cementite. The evidence suggests that vanadium partitions into the cementite lamellae during the formation of pearlite. This improvement in wear resistance due to vanadium could enable an approximately two-fold reduction in the maintenance and life cycle costs relative to heat-treated rail alternatives.regarded as transition-phases relative to mixtures of cementite and ferrite.ArcelorMittal Research Collaboration Agreement number 6085701156

Infusion of hydrogen into nanostructured bainitic steel

The trapping of hydrogen in nanostructured bainitic steel has been investigated using thermal desorption analysis, in order to determine the potency of the ferrite-retained austenite (α/γ) interfaces and retained austenite as trapping sites. Thermal desorption data showed that the volume of retained austenite is more effective in trapping hydrogen than the interfaces. There is a close correlation between the quantity of hydrogen and the retained austenite content rather than the density of interfaces. A local equilibrium model was able to reproduce the hydrogen desorption behaviour of saturated and unsaturated samples considering both retained austenite and α/γ interfaces as the trapping sites. A trap binding energy ranging from 47 to 52 kJ/mol was estimated for retained austenite, suggesting that the observed trapping capacity originates from the austenite lattice sites.The work was supported in part by a grant from Conselho Nacional de Desenvolvimento Científico e Tecnológico, Brazil

Cracks in Martensite Plates as Hydrogen Traps in a Bearing Steel

It is demonstrated that a macroscopically homogeneous distribution of tiny cracks introduced into a martensitic bearing steel sample can provide powerful hydrogen traps. The phenomenon has been investigated through thermal desorption spectroscopy and hydrogen permeation measurements using both cracked and integral samples. The e↵ective hydrogen di↵usion coefficient through the cracked sample is found to be far less than in the uncracked one. Similarly, when samples are charged with hydrogen, and then subjected to thermal desorption analysis, the amount of hydrogen liberated from the cracked sample is smaller due to the trapping by the cracks. Theoretical analysis of the data shows that the traps due to cracks are so strong, that any hydrogen within the cracks can never in practice de-trap and cause harm by mechanisms that require the hydrogen to be mobile for the onset of embrittlement.W. Solano-Alvarez is very grateful for support from the Worshipful Company of Ironmongers, CONACyT, the Cambridge Overseas Trust, and the Roberto Rocca Education Programme.This is the accepted manuscript. The final publication is available at Springer via http://dx.doi.org/10.1007/s11661-014-2680-8

Soft novel form of white-etching matter and ductile failure of carbide-free bainitic steels under rolling contact stresses

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 $\textit{softer}$ 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.W. Solano-Alvarez appreciates and acknowledges funding by CONACYT, the Cambridge Overseas Trust, and the Roberto Rocca Education Programme. Part of this research was financed under EPSRC grant EP/M023303/1 “Designing steel composition and microstructure to better resist degradation during wheel-rail contact” in collaboration with the Rail Safety and Standards Board (RSSB), the Department of Transport, the University of Leeds, and Cranfield University for which we are thankful.This is the final version of the article. It first appeared from Elsevier via http://dx.doi.org/10.1016/j.actamat.2016.09.01

Synchrotron and neural network analysis of the influence of composition and heat treatment on the rolling contact fatigue of hypereutectoid pearlitic steels

A series of experimental hypereutectoid pearlitic steels were tested under rolling contact sliding conditions using a lubricated twin-disc setup to study the influence of different chemical compositions and heat treatments on rolling contact fatigue life. Tested samples were then characterised using microscopy and synchrotron measurements as a function of depth from the contact surface. Results, analysed through neural networks, indicate that the most influential factor in lengthening the number of cycles to crack initiation of hypereutectoid steels is hardness, attained by increasing the cooling rate from the hot rolling temperature, but adequate alloying additions can enhance it further. The harder, fast-cooled samples displayed less plastic flow at the surface than the softer slow-cooled ones. With regard to chemical composition, silicon was found to strengthen the ferrite thus reducing strain incompatibilities with the cementite, preventing in this way the fragmentation and eventual dissolution of the lamellae. This is beneficial since larger depths of cementite dissolution were found in samples with lower cycles to crack initiation for a given cooling rate (hardness). Samples containing vanadium lasted longer and displayed less plastic deformation at the surface than those without, at a similar hardness.The authors are thankful to Dr Andreas Stark from the Institute of Materials Research of the Helmholtz-Zentrum Geesthacht for his help with synchrotron measurements, to Dr Giorgio Divitini of the Electron Microscopy Group in the Department of Materials Science and Metallurgy for his help with TEM/EDS, and to the Phase Transformations Group members Dr Neelabhro Bhattacharya, Ailsa Kiely, and Dr Arunim Ray for their help with synchrotron data conversion and analysis. This research was financed under EPSRC grant EP/M023303/1 “Designing steel composition and microstructure to better resist degradation during wheel-rail contact” in collaboration with the Rail Safety and Standards Board (RSSB), the Department of Transport, the University of Leeds, and Cranfield University Work by M. J. Peet was supported by the Medical Research Council Grant No. U105192715