Design criteria for rolling contact fatigue resistance in back-up rolls.

The demands placed on back-up rolls in hot strip mills have been investigated by a combination of literature and industrial studies. The tribological operating conditions have been established and the maximum local loads and pressure distributions at the work roll/back-up roll interface have been obtained by processing mill and roll schedule data using a computer program (commercial software developed by V AI Industries (UK) Ltd) and applying the theories of contact mechanics. After a study of the responses of the rolls to these demands and possible failure mechanisms, research has centred on surface initiated damage whereby cracks can propagate into the roll substrate potentially reaching the internal residual stress fields and leading to catastrophic failure. A proposed qualitative contact and fracture mechanics model, for the rolling contact fatigue and spalling failure, has been quantified theoretically using published methods for determining the stress intensity factors at the tips of pressurised and water lubricated, inclined rolling contact fatigue cracks. The predictions of the quantitative model in terms of crack directions and lengths have been validated by microscopic observation of the morphologies cracks produced in test discs used in the "SUROS" Rolling-Sliding Testing Machine and also in a sample of material spalled from a back-up roll. The quantitative failure model includes criteria for crack branching either upwards leading to micro spalling or downwards (potentially catastrophic) and the link between these two cases has been related quantitatively to the value of the mode I threshold for the roll material. After linking mechanics to microstructure and quantifying the interactions between wear and rolling contact fatigue in this case, practical quantitative recommendations have been made for the design of bainitic back-up roll materials, back-up roll redressing procedures and the surface roughness of both the work rolls and back-up rolls presented to the mill

Corrosion behaviour of mechanically polished AA7075-T6 aluminium alloy

In the present study, the effects of mechanical polishing on the microstructure and corrosion behaviour of AA7075 aluminium alloy are investigated. It was found that a nano-grained, near-surface deformed layer, up to 400 nm thickness, is developed due to significant surface shear stress during mechanically polishing. Within the near-surface deformed layer, the alloying elements have been redistributed and the microstructure of the alloy is modified; in particular, the normal MgZn2 particles for T6 are absent. However, segregation bands, approximately 10-nm thick, containing mainly zinc, are found at the grain boundaries within the near-surface deformed layer. The presence of such segregation bands promoted localised corrosion along the grain boundaries within the near-surface deformed layer due to microgalvanic action. During anodic polarisation of mechanically polished alloy in sodium chloride solution, two breakdown potentials were observed at −750 mV and −700 mV, respectively. The first breakdown potential is associated with an increased electrochemical activity of the near-surface deformed layer, and the second breakdown potential is associated with typical pitting of the bulk alloy

Establishing a qualitative model for rolling contact fatigue and spalling failure of back-up rolls

Abstract not available

Modelling of oxide scale evolution in hot rolling and descaling

Oxide scale behaviour in thermomechanical processing has been the subject of intensive research for several years that allowed developing a finite element (FE) based model to simulate a range of events of relevance to the process and the surface quality of the hot rolled product. A range of experimental techniques have also been developed, each providing a partial insight. An overview of this research is presented in the sequence of rolling and finishing with descaling. The model has been extended to provide the basis for detailed numerical investigations of the roll/stock interface behaviour during multi-pass hot rolling operations. The modelling techniques have been used for providing design criteria for AISI430 ferritic stainless steel scale failure during bending. Modelling of near surface deformation during rolling of aluminium alloys is under consideration.Michal Krzyzanowski, John H. Beynon, Mike F. Frolish, and Samantha Clow

Modelling of formation of stock surface and subsurface layers in breakdown rolling of aluminium alloy

Simulations of the reheating and breakdown rolling of the Al-Mg-Mn aluminium alloy AA3104 carried out using a two-high laboratory mill were supported by detailed numerical modelling of the stock surface layer formation. The model of the stock/roll interface is usually a micro-part of a more complex macro-finite-element model. Corresponding linking of modelling scales is a necessary stage for numerical analysis of fine mechanisms of the interface formation. This procedure allows for consideration of the fine mechanisms responsible for formation of the scale/metal interface while, at the same time, reducing the number of elements under consideration. It has been shown that a small amount of Mg (as oxides) was intermixed into the subsurface layer of a few microns depth by deformation during rolling. The mechanisms which led to the deformation and mixing of the oxide particles into the subsurface layer arose from slip at the roll/stock interface and the action of roll surface asperities on the stock surface.Michal Krzyzanowski, Michael F. Frolish, W. Mark Rainforth, John H. Beyno

The effect of microstructure and composition on the rolling contact fatigue behaviour of cast bainitic steels

The operating life of back-up rolls used in hot rolling mills is limited by rolling contact fatigue (RCF), and by wear. Bainitic steels are used as they offer the appropriate balance of resistance to RCF and wear. Recently, Frolish et al. have proposed a model for the RCF of back-up rolls, which includes a microstructural parameter σ′y sqrt(over(d, ̄)) that must be optimised, where σ′y is the yield stress of the material and over(d, ̄) is the sheaf width within the bainite. The present work sought to verify the model and thereby optimise the microstructure for this application through testing of bainitic steels with a wide range of microstructure, generated through changes in composition (a base composition of nominal content 0.4% C, 5% Cr, 0.85% Mo was cast, with V contents systematically varied up to 0.35%) and heat treatment. RCF testing was undertaken using a maximum Hertzian contact pressure of 1500 MPa. Samples without V exhibited limited crack initiation and no propagation. All V containing samples failed. No effect of prior austenite grain size was found, in line with the model of Frolish et al. A granular bainitic structure promoted extended time to initiation but rapid subsequent crack propagation and failure, while the lower bainitic structures exhibited the reverse (early initiation, but slow propagation). The effect of bainite dimensions on crack behaviour are discussed. © 2007 Elsevier B.V. All rights reserved.M.R. Green, W.M. Rainforth, M.F. Frolish, J.H. Beyno

Design criteria for rolling contact fatigue resistance in back-up rolls

This research centres on surface initiated damage on back-up rolls whereby rolling contact fatigue cracks can propagate into the rolls potentially reaching the internal stress fields and leading to catastrophic failure and has sought to establish design criteria for avoiding such failures. The project objectives have been achieved by examining field evidence, determining the loading and tribological conditions at the work roll/back-up roll interface and investigating both theoretically and experimentally the mechanisms involved in rolling contact fatigue in this case. The presented rolling contact, fatigue and fracture mechanics model includes criteria for crack branching either upwards (i.e.relative safety) or downwards (i.e.potentially catastrophic) and the link between these two cases is related, quantitatively, to the properties of the roll material. After linking mechanics to microstructure, the influence of work roll test disc surface roughness on both the surface wear of and the interaction between wear and rolling contact fatigue at the surface of back-up roll test discs has been quantified using the results obtained from experimental simulations carried out on a rolling–sliding testing machine. Finally practical quantitative recommendations are made for the mechanical and microstructural design of bainitic back-up roll materials, back-up roll redressing procedures and the surface roughness of both work rolls and back-up rolls presented to the mill

A quantitative model for predicting the morphology of surface initiated rolling contact fatigue cracks in back-up roll steels

A previously presented qualitative model for the rolling contact fatigue and spalling failure of back-up rolls has been quantified in terms of crack lengths and growth directions. The morphologies of surface initiated fatigue cracks have been predicted using published data on the mode I and mode II thresholds in low carbon and roll steels, respectively, and the theoretical determination of the mode I and mode II stress intensity factors at the tips of the inclined surface cracks. The predictions have been validated by using the results of the metallographic examination of rolling contact fatigue cracks produced in test discs used in experimental simulations and the examination of spalled material from a back-up roll