Fatigue in martensitic 100Cr6: Relationship between rolling contact fatigue microstructural transitions and repetitive push testing

Repetitive uniaxial fatigue testing is introduced to reproduce a similar magnitude of compressive stress to rolling contact during bearing operation, and to investigate the associated microstructural transitions. During the test, the strain per cycle responsible for fatigue damage can be measured. The observed hardness increase suggests that the developed residual stress level is similar to that formed on ball-on-rod bearing testing. The suggested methodology would be helpful in determining the strain responsible for plastic deformation in rolling contact fatigue, as well as for appraising the quality of bearing materials employed for bearing elements.This work was supported by SKF Engineering & Research Centre and financed by SKF ABThis is the author accepted manuscript. The final version is available from Elsevier at http://www.sciencedirect.com/science/article/pii/S0921509314008351. © 2014 Elsevie

Rolling contact fatigue in martensitic 100Cr6: Subsurface hardening and crack formation

Rolling contact fatigue tests on 100Cr6 steel were carried out with a ball-on-rod tester. Microstructural damage was manifested by gradual hardness changes under the subsurface, and microcracks formed adjacent to inclusions; both being evidence of plastic deformation. The hardness increase appears to be due to the development of residual stress, while the microcracks form as a result of the concentration of stress around inclusions. The microcrack orientation is suggested to be affected by the stress state, depending on the degree of residual stresses generated. The residual stress development may be a key factor for optimising the bearing element testing methods, by considering its influence on the damage morphology.This work was supported by SKF Engineering & Research Centre and financed by SKF AB.NOTICE: this is the author’s version of a work that was accepted for publication in Materials Science and Engineering: A. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Materials Science and Engineering: A, Volume 607, 23 June 2014, Pages 328–333. DOI: http://dx.doi.org/10.1016/j.msea.2014.03.143. http://www.sciencedirect.com/science/article/pii/S0921509314004365

Hydrogen transport in metals: Integration of permeation, thermal desorption and degassing

A modelling suite for hydrogen transport during electrochemical permeation, degassing and thermal desorption spectroscopy is presented. The approach is based on Fick's diffusion laws, where the initial concentration and diffusion coefficients depend on microstructure and charging conditions. The evolution equations are shown to reduce to classical models for hydrogen diffusion and thermal desorption spectroscopy. The number density of trapping sites is found to be proportional to the mean spacing of each microstructural feature, including dislocations, grain boundaries and various precipitates. The model is validated with several steel grades and polycrystalline nickel for a wide range of processing conditions and microstructures. A systematic study of the factors affecting hydrogen mobility in martensitic steels showed that dislocations control the effective diffusion coefficient of hydrogen. However, they also release hydrogen into the lattice more rapidly than other kind of traps. It is suggested that these effects contribute to the increased susceptibility to hydrogen embrittlement in martensitic and other high-strength steels. These results show that the methodology can be employed as a tool for alloy and process design, and that dislocation kinematics play a crucial role in such design

A model for the microstructure behaviour and strength evolution in lath martensite

Abstract A new model describing the microstructure and strength of lath martensite is introduced. The packet and block size were found to linearly depend on the prior-austenite grain size when introducing relevant crystallographic and geometric relationships of their hierarchical arrangements. A mechanism for the lath boundary arrangement within a block is postulated to ensure complete carbon redistribution to the lath boundaries. Accordingly, the dislocation density is obtained by considering the lattice distortion energy within a lath being equal to the strain energy of the dislocation density at the lath boundaries. Tempering effects are introduced by estimating the extent of carbon diffusing away from the lath boundaries; this mechanism relaxes the Cottrell atmospheres of lath dislocations and coarsens the boundaries. The yield stress as well as the microstructure evolution during tempering are successfully predicted by combining these results. The model is further extended to describe the yield stress in dual-phase steel microstructures by employing the iso-work principle. The model predictions are validated against experimental data in seven martensitic and five dual-phase steels, where the prior-austenite grain size, carbon content, tempering conditions and martensite volume fraction are employed as input. These results cover wide composition, initial microstructure and tempering conditions

Strain-induced martensite decay in bearing steels under rolling contact fatigue: Modelling and atomic-scale characterisation

Martensite decay in bearing steels manifested as dark etching regions (DERs) under rolling contact fatigue (RCF) is modelled. The proposed model is established based on a dislocation-assisted carbon migration mechanism. The proposed model is capable of predicting the progress of DER formation and the corresponding mechanical property evolution with increasing number of cycles, in good agreement with the experimental data reported throughout seventy years. The effects of RCF testing conditions on DER formation are studied and a useful tool, DER% maps, is developed for illustrating the temperature, contact pressure and number of cycles for DER occurrence. Moreover, an atom probe tomography study is carried out, revealing the nature of DER ferrite and obtaining strong evidence supporting the postulated DER formation mechanism. The successful application of the dislocation assisted carbon migration mechanism to DER formation provides a plausible explanation to the phenomenon of martensite decay under rolling contact fatigue

Correlation between vanadium carbide size and hydrogen trapping in ferritic steel

Hydrogen trapping on vanadium carbides (VC) was studied in a low-carbon ferritic steel. Thermal desorption analysis was performed on two conditions with different carbide sizes but identical volume fractions. Smaller carbides with a higher effective surface area trapped significantly more hydrogen. A correlation between carbide size and hydrogen trap density was established, suggesting that trapping is surface-dominant and a scaling law for trap density was derived. The amount of trapped hydrogen was overall much lower than previously reported for VC-containing martensitic steels. It is therefore suggested that in the absence of a dislocated matrix VC traps relatively little hydrogen.We gratefully acknowledge the funding received from the HEmS project (grant number EP/L014742/1) and the EPSRC-Rolls Royce strategic partnership (EPSRC grant numbers EP/H022309/1 and EP/H500375/1). E. I. Galindo-Nava acknowledges the Royal Academy of Engineering for his research fellowship funding

A microstructure sensitive model for deformation of Ti-6Al-4V describing Cast-and-Wrought and Additive Manufacturing morphologies

Microstructural variations affect deformation response of materials and it is not presented in most of plastic flow prediction models. This work presents a unified description for the deformation response of Ti-6Al-4V (Ti-64) that successfully captures the differences in strength between microstructures produced by conventional cast & wrought routes (C&W) and those obtained by Additive Manufacturing (AM), under various deformation conditions. In the developed model the grain morphology, grain size, phase volume fractions and phase chemical compositions have been linked to the mechanical properties of the studied Ti-64 alloys to predict the effect of processing routes on deformation behaviour of the materials. The model performance has been tested on seven different microstructures from C&W to AM processing routs. It has been found that altering the microstructure greatly affects the yield strength of the tested materials. Additionally, the strength of Ti-64 was found to be mostly affected by the relative volume fraction of α β and α′ and their respective morphology. The results showed that the model not only successfully predicts the strength of martensitic structures generated through AM but also those obtained by quenching in conventional C&W processing. The findings from this study also suggest that the model could be extended to other titanium alloys within the α + β family.M.A.G.F. would like to acknowledge the National Council of Science and Technology of Mexico (CONACYT) for the provision of financial support for his PhD studentship. E.I.G.N. would like to acknowledge the Royal Academy of Engineering for his research fellowship funding. P.E.J.R.D.C. is grateful to the Royal Academy of Engineering for financial support

Modelling and design of stress-induced martensite formation in metastable β Ti alloys

The temperature dependence of the stress-induced martensite (SIM) formation in a Ti-10V-2Fe-3Al (Ti-1023) alloy under compressive loading has been studied. At low temperatures, the stress level at which martensite starts to form increases linearly with the deformation temperature, while the stress at which the deformation switches to regular plastic deformation is roughly temperature independent. A thermostatistical model for dislocation evolution is employed to describe deformation twinning in martensite. Combined effects of twinning induced plasticity and solid solution strengthening are considered in terms of temperature variations. The SIM effect disappears on deformation at temperatures beyond ~ 233 ° C, which is close to the predicted Ms temperature of 240°C. The thermostatistical model predicts a transition from twinned martensite to pure slip at 250°C. By providing a model to predict the martensite formation, and by describing deformation twinning, the present work provides a number of tools that may be employed to conceive new titanium alloys combining improved strength and ductility. © 2013 Elsevier B.V

Solute redistribution in the nanocrystalline structure formed in bearing steels

The microstructural alteration in a white etching area (WEA) formed during rolling contact fatigue of a 1C-1.8Cr steel (wt.%) is studied. A uniform nanocrystalline structure with scattered carbide particles is observed. Si and C atoms segregate to different regions in the cell boundaries, implying a repelling behaviour. Carbon-rich clusters were identified but contained less than 25 at.%C. It is suggested that the interaction between Si and C should be considered in the mechanism for microstructural decay. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Grain boundary carbides as hydrogen diffusion barrier in a Fe-Ni alloy: A thermal desorption and modelling study

© 2018 A significant decrease in hydrogen absorption in the presence of grain boundary carbides compared to the carbide-free microstructure in the Ni-based HR6W alloy was measured by thermal desorption analysis (TDA). This novel observation is at odds with numerous existing reports – precipitate-rich microstructures generally absorb more hydrogen due to trapping effects. This discrepancy can only be explained by grain boundary diffusion which is known to be fast in Ni-based alloys. It is proposed that grain boundary diffusion is hindered by carbides, resulting in decreased hydrogen absorption. Further experimental evidence corroborates the hypothesis. In addition, a diffusion model was developed to quantify the experimental results, incorporating trapping, grain boundary diffusion and temperature effects. It was successfully applied to the reported TDA data as well as additional diffusion data from the literature. A parametric analysis showed that hydrogen absorption scales strongly with grain size and grain boundary diffusivity while grain boundary segregation energy has a much lower impact. The results of the study point at grain boundary precipitation as a possible means of hydrogen embrittlement mitigation in Ni alloys and austenitic stainless steels.EPSRC: EP/H022309/1 and EP/H500375/1 Royal Academy of Engineering for Research Fellowship fundin