C. M. Sellars's research while affiliated with The University of Sheffield and other places

Publications (210)

Article
Simple cantilever bend testing to assess mechanical descaling of industrial hot-rolled steel rods has been developed and analysed. This test method has no contact damage to the scale, and produces a range of strains along the rod, which always include those for scale behaviour from crack initiation to complete spallation. A series of methods has be...
Article
The effects of position of specimens in coils of low carbon steel rod, laying temperature, cooling conditions, aging time, relative humidity and temperature on scale cracking and removal behaviour in bending tests of mechanical descalability have been studied. It was found that laying temperature has a larger effect on scale thickness than cooling...
Article
Both numerical analysis based on finite-element (FE) modeling and experimental evidence concerning the secondary oxide-scale failure at entry into the roll gap are presented and reviewed for a better understanding of events at the roll-workpiece interface, in turn, leading to better definition of the boundary conditions for process models. Attentio...
Article
This paper presents a modelling strategy that combines neuro-fuzzy methods to dene the material model with cellular automata representations of the microstructure, all embedded within a nite element solver that can deal with the large deformations of metal processing technology. We use the acronym nf-CAFE as a label for the method. The need for suc...
Article
Since the 1950s, thermomechanical processing in the iron and steel industry has progressed from being skills based, using results from trial and error development work, to being science based, using computer modelling for process optimisation and control, and prediction of product properties. Some of the steps in this evolution are illustrated in t...
Article
During the hot rolling of austenitic stainless steels, complete static recrystallisation is expected between passes unless finishing temperatures are low. Typically progressive refinement takes place to grain sizes in the range 20–50μm. However, most experimental studies of the effect of strain, strain rate, temperature and initial grain size on re...
Article
This study presents the application of the finite elementmethod and intelligent systems techniques to the prediction of microstructural mapping for aluminium alloys. Here, the material within each finite element is defined using a hybrid model. The hybrid model is based on neuro-fuzzy and physically based components and it has been combined with th...
Article
A new model for strain induced precipitation of Nb(C,N) is developed from the existing model for single pass hot deformation. This new model can be extended to multipass deformation to explain the microstructural evolution during the hot deformation of Nb supersaturated high strength low alloy (HSLA) steels. The key feature of this model is the mic...
Article
ompression testing has been used with ramped changes in strain rate during deformation, and with changes in strain rate between double deformations to study their effects on flow stress. No systematic deviations from a mechanical equation of state were found for ramped increase or decrease in strain rate, even at the highest experimental ramping ra...
Article
A model for strain-induced precipitation of Nb(C,N) for single pass hot deformation has been developed with a new approach. The model demonstrates the need for having both growth and coarsening processes taking place concurrently, even at early stages of precipitation. The model predicts time evolution of precipitate radius, precipitate number dens...
Article
Full-text available
The present work describes a hybrid modeling approach developed for predicting the flow behavior, recrystallization characteristics, and crystallographic texture evolution in a Fe-30wt pct Ni austenitic model alloy subjected to hot plane strain compression. A series of compression tests were performed at temperatures between 850°C and 1050°C and st...
Article
Plane strain compression tests have been carried out on Ti stabilised interstitial free steel at 700°0C with constant and changing strain rates. Specimens were annealed in a salt bath at 750°C to determine the effects of changing strain rate on the kinetics of static recrystallisation and on the recrystallised grain size. After relatively slow chan...
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The effect of plastic deformation on the grain boundary surface area per unit volume and edge length per unit volume is examined using two methods. The first by applying homogeneous deformations to tetrakaidecahedra in a variety of orientations and the second by using the principles of stereology. It is shown that the methods produce essentially id...
Article
Finite element analysis has been used to study possible causes of asymmetrical deformation in plane strain compression tests and their consequences. It is shown that U shaped or Z shaped distortion of the specimens can arise from temperature gradients, lateral offset of the tools and non-uniform lubrication, which causes different coefficients of f...
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During the last decade Genetic Programming (GP) has emerged as an efficient methodology for teaching computers how to program themselves. This paper presents research work which utilizes GP for developing mathematical equations for the response surfaces that have been generated using hybrid modelling techniques for predicting the properties of mate...
Article
Full-text available
Dynamic recrystallisation (DRX) is an important aspect for industrial applications in hot metal working. Although DRX has been known for more than thirty years, its mechanisms have never been precisely investigated, in part because it was not readily possible to make local texture measurements. In the present work, the material behaviour during DRX...
Article
The shear strain distributions in two pass forward–forward (FF) and forward–reverse (FR) rolling have been modelled using plane strain finite element models of individual passes and then summing the results from the individual passes. This method gives excellent agreement with the average shear strain, defined by the net shear angle, and with the d...
Article
During hot rolling processes, the material under deformation undergoes different deformation conditions, i.e. temperature changes and strain rates. One particular variable is the change in strain rate which can vary from low to very high values in industrial rolling. Usually empirical models are used for predicting the material characteristics but...
Article
Modelling of the recrystallisation kinetics of Interstitial Free (IF) steel after hot deformation using plane strain compression (PSC) at strain rates from 0.1 to 10 s⁻¹ over a temperature range of 700 to 900 °C has been carried out. The model is based on a hybrid modelling technique combining neuro-fuzzy models and physical equations. Explicitly....
Article
This paper describes good practice for the measurement of hot flow stress in metallic materials. It is applicable to hot (isothermal) uniaxial axisymmetric compression tests at medium to high rates of strain (10−4−102 s−1) at deformation temperatures below the solidus.Guidance is provided on appropriate testpiece geometries and methods of verifying...
Article
Full-text available
This Good Practice Guide is applicable to hot (isothermal) plane strain compression (PSC) tests at medium to high rates of strain (10−3 to 102 s−1) at deformation temperatures below the solidus.Guidance is provided on appropriate testpiece geometries and methods of verifying the temperature distribution along the length of the testpiece. Flow diagr...
Article
This paper describes a novel modelling approach based on a hybrid structure developed for predicting the material properties of aluminium alloys for different deformation conditions. The model is based on physical equations and neuro-fuzzy models. The paper describes the methodology for developing the hybrid model and the validation process which c...
Article
The work is based on the assumption that the flow stress of a material should be insensitive to the method of plastometric testing or to the size of the samples. It is generally observed, however, that various methods of testing yield different values of the flow stress. Endeavours are made to eliminate these differences and various methods of corr...
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Full-text available
Strain induced precipitation is a key phenomenon that controls the microstructure evolution during the finish rolling stages of microalloyed steels. Extensive research has shown that the precipitation of Nb(CN) delays the onset of recrystallisation. This paper presents a model to describe the precipitation kinetics during isothermal holding followi...
Article
A new model of strain induced precipitation in niobium microalloyed austenite is proposed. This is based on the experimental observation of formation of microbands during hot deformation of iron - 30% nickel, which remains austenitic to room temperature. Precipitates are preferentially nucleated on nodes in the dislocation network in the microbands...
Article
In hot rolling, rolling load, torque, motor power, and energy are important measurable process outputs that are influenced by the shape and size of the roll gap. The magnitudes of these output variables are dependent on the extent of inhomogeneous deformation during a rolling pass, which is characterised by the redundant shear strain. A previously...
Article
Numerical models for materials properties prediction require validation to assure the developer and the user that the mechanics and numerical algorithms implemented in the model are correct and consistent with the experimental information available in the literature. Validation of computer models for hot deformation of aluminium–magnesium alloys is...
Article
The objective of the present work was to investigate the thermomechanical behaviour of a commercial Al-Mg alloy and to understand the deformation mechanisms taking place at small strains (<0.2) and high temperatures (>300degreesC) at strain rates relevant to hot working conditions (0.01-10 s(-1)). The experimental approach addresses different effec...
Article
The recrystallization kinetics of 3% Si steel after hot rolling and plane strain compression at strain rates from 0.05 to 5 s(-1) over a temperature range of 800 to 1100 degrees C have been investigated. The flow stress of the ferritic 3% Si steel Indicates that the dynamic recovery rate is about two orders of magnitude faster than in austenite. Re...
Article
Two pass hot rolling experiments have been carried out on 3% Si steel rolled to total strains in the range 0.25 to 1.1 at temperatures of 850 to 1100 degrees C. Retardation of recrystallization after the second pass compared with after a single pass to the same total strain arises mainly from differences in the strain path history when only recover...
Article
The recrystallization kinetics and the recrystallized grain size of 3% Si steel after hot rolling at temperatures of 900 to 1100 degrees C and annealing at the rolling temperature have been investigated for a range of initial grain sizes from 140 to 850 mu m and initial austenite contents from 0 to 16%. It is shown that, in single phase ferrite, nu...
Article
The static recrystallization kinetics and the recrystallized grain size of 3% Si steel after hot rolling at temperatures of 900 to 1100 degrees C to equivalent strains of 0.25 to 1.24 applied in a single pass or by two pass rolling have been investigated. When the annealing temperature was changed for a constant rolling temperature, an apparent act...
Chapter
Volume 14A is the first of two volumes that covers the distinct processes and industries of bulk working and sheet forming. It provides comprehensive coverage and essential technical information on the process-design relationships that are needed to select and control metalworking operations that produce shapes from forging, extrusion, drawing and...
Article
A finite element (FE) model for hot flat product rolling has been developed using the commercial software ABAQUS/ STANDARD version 5.5 to obtain the local deformation history of the material required to predict the local microstructural behaviour, which results in through thickness variations in mechanical properties. Physically based models of rol...
Article
Modelling of the recrystallisation kinetics of Fe3%Si steel after hot rolling and plane strain compression at strain rates from 0.05 to 5s-¹ over a temperature range of 900 to 1100°C has been carriedout. The model is based on a hybrid modelling technique developed for aluminium alloys via a combination of neuro-fuzzy models and physical equations....
Article
Increased prior cold work accelerates the recrystallisation kinetics of B2 ordered Fe–25.2at%Al at 600 and 700 °C with nucleation occurring heterogeneously at grain boundaries leading to an Avrami exponent of 0.5 and an activation energy of ∼290 kJ/mol. Recrystallisation in the D03 ordered state at 500 °C is greatly retarded compared with the B2 or...
Article
Plane strain compression tests to investigate the effects of heterogeneity of deformation on various initial specimen geometries have been carried out. Equations for correction of nominal strain and strain rate to slip line field strain and strain rate have been developed and applied to experimental flow stress-strain data. Investigation of the def...
Article
A series of model stainless steels containing 0, 30, 50, 70 and 100% volume fraction of δ was prepared. In these two phase alloys a duplex structure of equiaxed grains of δ and γ with a uniform distribution was established through hot deformation and heat treatments. The static recrystallisation kinetics were followed in all five alloys after hot t...
Article
For modern metals industries using thermomechanical processing, off-line modelling and on-line control based on physical knowledge are highly desirable in order to improve the quality of existing materials, the time and cost efficiency, and to develop new materials. Neural network and neuro-fuzzy models are the most popular tools, but they do not e...
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Full-text available
This paper considers how data based neurofuzzy modelling techniques for the poorly understood relationships between changing process histories and the evolution of the internal state variables of dislocation density, subgrain size and subgrain boundary misorientation can be combined with physically-based models to investigate the effects of the int...
Article
Two types of approach based on the statistics of extremes have been developed recently to estimate the sizes of large inclusions in clean steel. The first type (termed here “threshold” approaches) includes methods based on the Generalized Pareto distribution (GPD) and the Exponential distribution (EXPGPD). Both of these methods use measurements of...
Article
Oxide failure during mechanical descaling at room temperature has been analysed using laboratory testing coupled with mathematical modelling based on application of the finite element method. A cantilever bending test procedure and two finite element models were applied to investigate scale crack spacing, spalling, and delamination along interfaces...
Article
During flat rolling the metal undergoes complex strain paths due to the interaction of the stock with the rolls. These changes in strain path affect the stored energy and hence the recrystallisation behaviour of the material on annealing. The current paper investigates the effect of the roll pass schedule on the recrystallisation kinetics of type 3...
Article
Microstuctural changes after warm rolling of an interstitial free steel have been studied by annealing deformed specimens in a salt bath in the ferrite temperature range. Static recovery and recrystallisation were followed by Knoop hardness measurements, optical microscopy, and TEM. The results show that the kinetics of the static restoration proce...
Article
Fatigue failures in high strength steel components often originate from large, brittle inclusions. The durability of the components is strongly dependent on the size of the inclusions and the magnitude of the local stresses caused by the applied loads. A successful design must consider both the size and the number of large inclusions as well as the...
Article
During industrial hot working operations, such as rolling, forging and extrusion, regions of the workpieces undergo a change in strain path. A significant effect of change in straining direction in tension/compression deformation, which is the simplest change in strain path, on microstructural evolution and subsequent recrystallisation behaviour of...
Article
Hot rolling tests, using mild steel slabs with thin and thick oxide scales, have been conducted to investigate the cracking and deformation behaviour of the scale during hot rolling to various reductions. The effects of the variables other than temperature on scale cracking have been simulated by cold rolling tests, using lead slabs with brittle la...
Article
A model Fe–30 wt% Ni, 0.1 C, 1.61 Mn, 0.1 Nb microalloyed steel, that simulates conventional microalloyed C–Mn steels, but does not transform from the austenite phase on cooling, is reported. Plane strain compression testing was undertaken at 950°C at a constant true strain rate of 10 s−1. Samples were deformed in a two stage process. An initial tr...
Article
Large and brittle oxide inclusions may initiate fatigue failure in bearing steels. The size of the maximum inclusion m a large volume must be predicted by statistical analysis because only small samples can directly be analysed and there are limitations on non-destructive testing methods. A new method based on the Generalized Pareto distribution (G...
Article
Hot stalling rolling tests using mild steel slabs with thin and thick scale layers, and cold stalling rolling simulation tests using lead slabs coated with brittle lacquer layers, have been conducted to investigate the behaviour of the surface oxide scales before hot rolling. The effects of rolling reduction, temperature, scale thickness and struct...
Article
The present paper reports a comparison of flow stresses obtained by tension, plane strain compression, and axisymmetric compression testing in different machines with or without extensometers in the gage length section. Flow stress curves obtained from different test systems suggest machine and specimen geometry dependence, when the strain is compu...
Article
A detailed finite element analysis of the hot plane strain compression test has been carried out to investigate the effect of various test parameters on the measured response and deformation. This paper (Part 2) reports the results of three-dimensional finite element simulations of the test, to study the influence on spread of specimen geometry and...
Article
Thermomechanically coupled finite element analysis of the hot plane strain compression test has been carried out to investigate the effect of various test parameters on the measured response and deformation of specimens. The results are presented in a series of papers. In this paper (Part 1), the results of two-dimensional simulations are discussed...
Article
Hot rolling of an aluminium–1% manganese alloy has been carried out. Wedge shaped specimens were rolled in two pass schedules, of either two forward passes or a forward and a reverse pass to the same overall net strain. Through thickness marker pins were inserted to allow the investigation of plastic flow during the different rolling schedules. The...
Article
Microbands, as a dislocation substructure formed during hot deformation have been recognized to consist of geometrically necessary dislocations. During rolling or plane strain compression (PSC) deformation, the microbands form parallel plates. The microband orientation in axisymmetric deformation was studied to clarify such dislocation.
Article
Use of numerical predictive methods such as finite element analysis is becoming progressively more common for modelling industrial hot metal working and forming processes. These tools are used not only to predict the thermomechanical behaviour of metals but increasingly to predict microstructural changes by linking them to physical models of recrys...
Article
The Generalized Pareto Distribution (GPD) method has recently been applied to the estimation of the characteristic size of the maximum inclusion in clean steels for the first time. This allows data on inclusion sizes in small samples of steel to be used to predict the size of the maximum inclusion in a large volume of steel, a parameter of importan...
Article
The hot direct rolling (HDR) of thin slabs introduces some new microstructural phenomena with respect to conventional hot rolling of steels. This paper aims to investigate the microstructural changes of as cast austenite under these conditions. Current laboratory techniques for HDR simulation require a freshly cast slab for every experiment and a p...
Article
The effect of β′-, β″-, and β Mg–Si precipitates in the initial microstructure of an AA6063 alloy on the flow stress at elevated temperatures was studied using plane strain compression tests. The stress–strain curves were modelled with a general solute-dependent constitutive equation based on the exponential saturation equation as proposed by Sah a...
Article
The mechanisms of the formation of the columnar grains in hot bands were experimentally studied. Results indicate ultra low carbon steels can produce columnar grains grown from near the surface to the center during hot rolling due to the rapid propagation of the ferrite transformation front triggered by severe roll chilling. A combination of carbon...
Article
Full-text available
Over the past 20 years following on from the international conference Niobium 81, considerable progress has been made regarding the development of mathematical models for thermomechanical rolling, from the point of solidificatio n through to the end of finish rolling. As one would expect, certain areas have received far more attention, and are cons...
Article
The maximum inclusion size in clean steels influences fatigue behaviour and other mechanical properties. Hence, its estimation and the uncertainties associated with the estimation are important issues for steel makers and users. Here, two methods based on the statistics of extremes, one termed the Statistics of Extreme Values (SEV) method and the o...
Article
Predicting the maximum inclusion size in a large volume of clean steel from observations on a small volume is a key problem facing the steel industry. The maximum inclusion size controls fatigue behaviour and other mechanical properties. Recently manufacturers have started using the method evolved by Murakami and co-workers, which is based on the s...
Article
Estimation of the maximum size of inclusions in a large volume of steel is an important issue in relation to clean steels. Statistics of extremes can be used for the estimation, and one such method, developed by Murakami and coworkers, is now used by the steel industry for characterising steel specimens. An alternative method, based on the generali...
Article
For two different steels, the characteristic size of the maximum inclusion in a large volume has been estimated by the generalised Pareto distribution (GPD) method using inclusion data from two measurement procedures: first, optical metallography of polished sections, and second, scanning electron microscopy of the surfaces of cold crucible remelte...
Article
During thermomechanical processing, local region within the stock undergo changes in strain path, which have systematic effects on deformed microstructure and subsequent recrystallisation behaviour. Only limited quantitative data are presently available. This paper presents results for the cumulative effects of repeated strain reversals on the disl...
Article
Constitutive equations which describe the flow stress behaviour of materials during hot deformation are used to model forming processes. Since the flow stress depends on both temperature and strain rate, the Zener–Hollomon parameter which combines these factors, is frequently used to describe the shape of such curves. For materials which dynamicall...
Article
Detailed understanding of the flow stress and microstructural development during plane strain compression testing requires accurate knowledge of the time variation of temperature. A finite difference model of the temperature variations has been developed and implemented in a computer program on a PC, which can rapidly produce results for multidefor...
Article
It is well-accepted that modelling of microstructural evolution of aluminium alloys during thermomechanical processing is highly desirable to predict product properties and/or to design process variables based on requirements for the properties. To do so, having soundly based physical models is of interest for both academic research and industrial...
Article
The recrystallisation and precipitation behaviour of an austenitic stainless steel has been studied with particular emphasis on the role of the experimental parameters in controlling the precipitation mechanisms of sigma phase. The alloy, containing 23 wt% chromium and 23 wt% nickel and with negligible carbon (0.004 wt%), was aged isothermally at d...
Article
A Ti containing interstitial free steel was warm rolled in the temperature range 500–800°C, using wedge shaped slabs to produce a range of strains in a single rolling test. Some plane strain compression tests, under similar conditions, were carried out to obtain accurate stress–strain data. Variations in substructural features including subgrain si...
Article
In the present work an investigation of constitutive equations is made to obtain good fit to experimental data and ensure proper extrapolation out of experimental range of plane strain compression tests to low and high values of Zener-Hollomon parameter, which are required in finite element modelling. A computer program for optimisation of rheologi...
Article
The prediction of the maximum inclusion size in a large volume of clean steel from data on small specimens is a key issue for steelmakers and users. The statistics of extremes has recently emerged as a powerful tool for this purpose. Murakami and coworkers have applied one branch of the theory to steels, based on measuring the maximum inclusion siz...
Article
Models representing material behaviour are now an essential component of the development process for rolled products. Althoug models based on physical parameters are being proposed, most current models employ empirical equations, which assume tha the deformation can be characterized by the strain rate, temperature and the equivalent plastic strain....
Article
Introduction to Deformation processing of metals. A Discussion Meeting held at the Royal Society on 21 and 22 October 1998. Organized and edited by M. F. Ashby, F. J. Humphreys, C. M. Sellars, H. R. Shercliff and M. J. Stowell.The control of microstructures and textures during the deformation and annealing of alloys is of great importance and has b...
Article
Constant and varying strain-rate deformation conditions have been applied to an Al–1% Mg alloy using plane strain compression (PSC) testing. Transmission electron microscope (TEM) investigations have shown significant variations in internal microstructural parameters after different strain-rate histories to a strain of 1.0, even though the final fl...
Article
The recrystallization behaviour of an Al–1% Mg alloy has been investigated following hot deformation under varying strain-rate conditions. Significant differences in recrystallized grain size and recrystallization kinetics are observed following a decreasing strain rate, as compared with constant strain-rate tests. These differences reflect the mic...
Article
The effect of spray forming on the structure and properties of a 17% Cr, 2.5% C white cast iron is described and compared with conventionally cast material of the same composition. Spray forming resulted in a substantial reduction in microstructural scale (eutectic (Cr,Fe)7C3 fields of up to 500 m in conventionally cast material were replaced by di...
Article
The effect of spray forming on the wear properties of a 17% Cr 2.5% C white cast iron is described and compared with conventionally cast material of the same composition. Spray forming resulted in a substantial reduction in microstructural scale (eutectic (Cr,Fe)7C3 fields of up to 500 μm in conventionally cast material were replaced by discrete ca...