Several existing theories of cold-rolling were reviewed with particular attention to Orowan's Homogeneous theory. It was decided to use this theory throughout us it was the most well tried and proved of those available. Many of the other theories are still in a speculative state, and often require parameters which are not well understood. Orowan's theory enabled one to examine the effect of varying basic parameters with the least amount of complication. The effect of roll force on roll flattening was discussed and a simple model was used based on the assumption that the elastically deformed arc of contact was circular (Hitchcock's Formula). This was incorporated in a digital computer program for evaluating roll force and torque. Consequently a hybrid computing method for obtaining rapid solutions of the rolling equations was developed and using this technique results were obtained faster than the equivalent purely digital solution. There is a need for measuring stress distributions in the roll-bite to substantiata the theory, so a thin film manganin pressure transducer was proposed for measuring these stresses. The techniques involved in the manufacture and application of such a transducer are dealt with in detail. Results were obtained using the transducer for surfaces in rollil1g contact, but it was not possible to ohtain satisfactory results when rolling strip,so that this experimental technique was abandoned. Suggestions ar.e made fot future work on this type of transducer. -viii- Orowan's theory lVas modifiad to incorpor£!.te arbitrary roll profiles since it was felt that the assumption of the cir~ular roll surface was an over-simplification. The method using influence functions to obtain roll deformations due to normal loads was reviewed (Jortner). The influence function for point shear loads was derived using a similar technique. A program was writt.m which included the effects of shear stresses on roil deformations. The program calculated roll force and torque more precisely than hitherto; moreover, a coefficient of friction that could vary throughout the roll bite was incorporated. The effect of shear forces on roll deformation was discussed and in certain cases was shown to affect roll force and torque significantly. The use of complex variables j.n stress analysis was reviewed. A novel method for producing a series solution for point loada using Dirac Functions is presented. Finally closed-form solutions were obtained for point shear and point normal loads, and found to tie up with those obtained using elementary stress distributions. The advalltases of this method are that no a priori assumptions need be made, and no complex integrations are required to obtain surface deformations. This method also explains certain results which are obscured using ~he previous approach. The final chapter applied the preceeding theory to explain the so called "speed-effect". The variables in tha rolling theory were examined and the coefficient of friction emerged as the IDOst likely speed-dependent parameter. Using exper.imental data on the - iY.: - speed eff(!ct obt;'linad by H. Ford, theor.etical resulU. predicted the e.:>rre!.!t roll forCl~ and torqu,: for thick a.Ulealed mater.ial. Using the same coefficient of friction, the results obtained when the strip became thin and hard were much lower than those obtained in practi~e. A stress dependent coefficient of friction was proposed to account for this, but it was shown that using the modified OrO'.Jan theory it was not possible to obtain. sufficie.ntly high roll forces. The speed effect behaviour was produced by choosing a coefficient of friction which decreased with the relativ(:~ velocity between the strip and roUs. The limitations of present theory are discu3sed and suegestions for further work are made.

Oxide scale can affect many aspects of hot metal forming operations, such as heat transfer, friction and the final surface finish of the rolled product. Surface oxide scale is always present on the steel slab and sometimes on cold rolls. Therefore, the study of the thermo-mechanical behaviour of oxide scale, particularly under conditions as close as possible to the steel manufacturing process is very important. In order to undertake a detailed study of oxide scale behaviour, several hightemperature testing techniques were applied. First, high-temperature tensile tests were carried out to investigate the brittle fracture and cracking of the surface oxide scales. Second, high-temperature compression tests were developed and the results obtained revealed many different failure mechanisms that are present in the compressed oxide scale. Finally, a tensile-compressive test was developed and performed under thermal conditions which were as close as possible to hot rolling. The best results in the understanding of oxide scale failure were achieved by closely linked combination of laboratory testing and measurements coupled with detailed finite element analysis. A close microstructural examination of the morphology of oxides was carried out after each experiment and finite element modelling was performed. The three-dimensional finite element simulations helped to improve the interpretation of the thermo-mechanical testing and to obtain more accurate heat transfer and stress-strain relationships. In this work the following thermo-mechanical failure modes of the oxide scales were observed and investigated: brittle fracture (through-thickness cracking, blister failure), indications of plastic behaviour (arrested cracks, unbroken top layers of the oxide scale) and a sticking effect (equivalent to the mill pick-up).

The effect of austempering processing variables on the transformation kinetics, microstructure and mechanical properties of the Fe-C-AI family of spheroidal graphite cast irons, in which the silicon is below 0.2wt% has been investigated. These alloys are described as SO Al cast irons. In the preliminary experiments the spheroidisation and inoculation treatments for SO Al were developed and the microstructure and tensile properties in the as cast and normalised SO Al cast iron at 2.2 and 3.2% of Al examined. The results show that the sequence of spheroidisation of SO Al iron differs significantly from those observed in SG Si iron. Whilst the treatment to introduce some Mg into the melt is required in both irons to alter the growth habit of graphite from flake to spheroidal, the behaviour of the irons towards the subsequent inoculation with various inoculating alloys is quite different. A wide range of mechanical properties of SG Al irons can be obtained, similar to SG Si irons, in the as cast or heat treated conditions. The effects of austenitizing temperatures of 850, 900 and 950°C, austempering temperatures of 300 to 450°C and austempering times of 5 to 300 minutes on impact and tensile properties have been investigated. X-ray diffraction has been used to determine the volume fraction, lattice parameter and carbon content of retained austenite and bainitic ferrite produced under different austempering conditions. Optical and scanning electron microscopy were used to analyse the microstructure. It has been shown that the basic mechanisms for the isothermal transformation of austenite to bainite are essentially similar to those of austempered SO silicon cast irons for both lower and upper bainite. This can be attributed to the strong graphitising effect of aluminium which delays the formation of transformation carbides. However, the higher carbon contents of retained austenite in SG Al iron show the stronger carbide inhibiting characteristics of aluminium compared with those of silicon. The first stage starts with the nucleation of bainitic ferrite from the grain boundaries and adjacent to the graphite nodules. The second stage in upper bainite fonnation is the growth of ferrite, during which carbon diffusion occurs ahead of the ferrite-austenite interface thus enriching the surrounding austenite. This is followed in the third stage by carbide precipitation in the high carbon retained austenite. In lower bainite fonnation the nucleating ferrite is supersaturated with carbon, and carbide precipitation occurs inside ferrite plates. The results show that the rate of fIrst stage transfonnation is higher in SO Al than in the corresponding SO Si iron, and the high carbon retained austenite has a longer life time with a high carbon content. Increasing the austempering temperature was shown to increase structural coarseness and retained austenite content, leading to reduced strength and hardness and improved impact properties and ductility. Retained austenite produced in austempered SO Al iron showed more stability at a high austempering temperature of 450°C, compared to SO Si iron. This was attributed to the graphitising potential of aluminium which suppresses the formation of carbide at that temperature. Austenitizing temperature was found to control the carbon content of both the matrix and of the retained austenite. Increasing the austenitizing temperature was shown to increase the matrix carbon content and to slow the bainitic reaction. Microstructures obtained at higher austenitizing temperature revealed two types of retained austenite: one with a film morphology and the other in a blocky form surrounded by ferrite growing in different directions. The length of bainitic ferrite appears to increase with austenitizing temperature. Austenitizing temperature had a greater effect on hardness in the lower bainitic region and on impact values in the upper bainitic region. The results obtained show that the controlling austempering variables for an SO Al iron of a given starting composition and microstructure are closely similar to those of SO Si iron. The transformation mechanisms are also similar, but the transformation kinetics differ and, to a small extent, microstructural differences give rise to differences in properties between the two types of irons.

This thesis is concerned with investigations into various aspects of rolling thick walled plain and profiled rings and also forming of thin walled V-section rings by ring rolling. The work is presented in three parts. Part one deals with rolling of thick walled plain rings. Part two deals with rolling of profiled ring concerning profile filling. Part three deals with forming of thin walled V-section rings. Chapter I contains a brief survey of the application of the process and a comparison with other methods of forming rings. A survey is made of previous work on this subject. Chapter 2 investigates the rolling of plain rings. An existing theoretical model for plain rings based on the slip-line field solution of the indentation of a block by opposed flat indenters is used and further developed to provide an estimate for strain distribution across the section. Predicted values for roll forces and torques are examined and compared with the experimental results. Radial and axial spread is examined experimentally and control of the axial roll speed and also the quality of the products are examined. In chapter 3,the rolling of profiled rings in closed and open pass rolling is investigated and consideration is given to the effect of applying tilt during profile rolling to aid in profile development. Examination of results indicates that. applying tilt during profile rolling aids in profile filling in both closed and open pass rolling •.. In chapter 4 an account is given of the development of a ring rolling process for the manufacture of V sectioned rings from thin-walled plain cylinders. The rings were required in different diameters and sectional dimensions for a specific application and in the absence of any design information an analysis of the forming process was made to provide guidance on the design of the forming rolls. The analysis is shown to predict satisfactorily the conditions needed for smooth development of the profile and the blank size required to produce a given ring size. The transverse curvature during the forming is analysed and compared with the experimental results. It is shown that theoretical values are in good agreement with the experimental results. Some parts of chapter 4 will appear as a paper in "International Journal of Mechanical Science" Vol 7 1988.

No description available.

The thesis is divided into four main parts; a study of static and dynamic oblique indentation by spherical projectiles - and its relevance to the shot-peening process, a study of the mechanics and of the parameters affecting the shot-peening process, multiple impact erosion as related to the shot-peening process and explosive hardening as compared with the shot-peening process. In the first part, force-indentation curves are presented for the penetration of the plane surface of a nominally semi-infinite block by a round body; different metals were penetrated at different inclinations and under static conditions. These experimental results are compared with those Obtained from a simple theoretical model. Experiments were carried out to determine how the shape and extent of the plastically deformed zone varies with the relative obliquity of the metal surface and indenter. A newly devised method was also employed for investigating the obliquity of the load upon the plastically deformed zone in aluminium, alloy. Upper bound solutions were proposed to determine the effect-of the'obli4uity of the load applied: to a cylindrical punch for penetrating soft metals upon the average indentation pressure. A simple slip-line field, was assumed to apply for use in the case of Spherical punch indentation to determine the depth of the plastic zone. The study was extended further to cover the determination of the strain - distribution in soft metals when indented by rigid spherical and cylindrical punches driven obliquely into the flat surface at various angles. These distributions were obtained using a computer aided visioplasticity technique. A finite element mesh which utilizes a quadratic function and uniquely defines the displacement vector within each element was used. An Almansi strain tensor, which uses the instantaneous co-ordinates to determine the' strains was used for calculating large plastic 'strains. In the dynamic indentation, studies., a simple model is presented' for investigating the impact at--different angles of incidence of a rigid sphere against a block of relatively soft' material with a wide range of impacting speeds.

No description available.

The aim of the thesis is the identification and modelling of the dominant failure mode in hot forging tooling, used in the production of nickel based superalloy compressor blades. A preliminary study identified the most frequently occurring part defects that cause tool failure. The forming process was characterised, relevant tooling failure modes were identified and the mechanical properties and microstructure of the tooling was reviewed. Case studies have been performed identifying the observable damage in worn tooling and investigating the conditions that result in tool failure. The investigation made use of finite element analysis, metrology, metallurgical analysis and thermal measurement. The dominant failure mode was associated with the plastic deformation of the tooling. Testing was performed to gather sufficient data to allow for the development of a lifing model. Isothermal tensile and fatigue tests were performed under temperatures and strain rates representative of hot forging tooling conditions. The microstructural instability during these conditions was investigated. A literature review explored state of the art approaches to modelling the plastic flow behaviour of tempered martensitic tool steels for the purpose of tool life prediction. A lifing model was developed to describe the plastic flow behaviour of tempered martensitic steel. A physical based model developed to describe the creep behaviour of 9-12%Cr tempered martensitic steel has been applied to describe the thermo-mechanical fatigue behaviour of 5%Cr tempered martensitic hot work tool steel. The model was verified by comparing model predictions to experimental data not used in the development of the model. The model was then applied to predict the life of tooling examined in the case studies. The life predictions were within a factor of 10 from measured die lives measured from the forge.

No description available.

No description available.