No description available.

In this work, temperature profiles, velocity profiles and other parameters such as output rate, pressure, and power consumption were investigated under various processing variables/conditions. The measurements were carried out using an apparatus attached to the output end of an extruder. The effect of processing variables such as screw speed, barrel and apparatus temperatures, die types and die geometries were studied in this work. An investigation was also carded out to study the effect of a breaker plate on the parameters mentioned above. The measurements were also carried out at various points along the length of the apparatus. The technique called Modified CST (Cooled-Stainless Tube), coupled with a temperature measurement sensor (thermocouple mesh) was, used to simultaneously measure the temperature and velocity profiles at multiple points across the diameter of the duct. The measurements were taken using high speed data acquisition equipment (ADU) which was connected to a person"! l computer. The technique used was evaluated and discussed, and it was found that the technique used gives accurate and repeatable measurements at low cost. In order to assist the interpretation of the data generated, rheological and thermal characterisations were carded out on the material used (LDPE10OBWExxon Chemical). Rheological characterisation was carried out using a capillary rheometer and a high shear viscometer. The melt density at a range of temperatures was measured using a melt index tester. The relationship between pressure, density and temperature was studied using a sample of fixed density inside a chamber of fixed volume at various temperatures. Thermal conductivity was measured using a modified Lee's Disc apparatus. Other thermal properties such as melting temperature, the degree of crystallinity, specific heat were determined using Differential Scanning Calorimetry (DSC). Using the specific heat value generated from DSC, combined with thermal conductivity and density value, thermal diffusivity was calculated. Experimentally, it has been found that the temperature profiles varied with the processing variables used. The profiles were related to the melting process inside the extruder, shear heating effects, conduction effects, residence time of the melt inside the equipment and the thermal/rheological characteristics of the polymer used. The velocity profiles, however, did not directly correlate with the temperature profiles, the profiles being related to the effect of the screw speed, screw tip and die entrance. Apart from screw speed, the power consumption was found to be determined by the power required during melting process. It was also found that the flow inside the apparatus duct was steady and laminar, the temperature and velocity profiles developing over the length of the duct.

Gas turbines in power generation systems use both nickel and cobalt-based superalloys for vanes, blades, discs and combustion chamber components. Cobalt-based superalloys have the advantage over nickel based superalloys in that they have a higher thermal conductivity and hence a greater thermal shock resistance, a greater resistance to thermal fatigue and also a better corrosion resistance. However, in some applications coatings have to be applied to such alloys because of their poor oxidation resistance. The creep strength of cobalt-based superalloys depends primarily on solid solution strengthening and the interaction between the hard carbides and alloy defects, such as dislocations and stacking faults. There is a need to develop an understanding of the microstructural changes that occur in cobalt-based superalloys, with both time and temperature, for life prediction, refurbishment and failure investigations.

No description available.

The work described in this thesis was designed to extend the investigation of Dr.S.Bhattacharya, who showed that the creep of a variety of metals, at room temperature and under constant stress, could be satisfactorily described by the equation:- l)::; a tk. It was hoped to be able to determine the effect of stress on this equation. In addition to this, preliminary work on creep under fluctuating stresses is described in Appendix 1. Previous work pertinent to the investigation is reviewed. It is concluded that, of the two most common equations, the Andrade equation and the power-law, the latter is the more generally applicable. Metallographic evidence seems to support the common association of each of the two terms of the Andrade equation with a recognisable mechanism of creep. It is suggested, however, that such an association involves certain theoretical difficulties.

In this investigation, the fluidity test was adopted to study (a) the flow behaviour of liquid metals at high velocity in thin channels and (b) the maximum fluidity for two important casting eutectics: grey iron and Al-12% Si. The effects of section thickness, pouring temperature, mould binder (for cast iron) and the design of filtering system were also investigated. In the first step of studies, some fluidity tests were carried out with cast iron and the results and the quality of the fluidity strips were examined. The quality of the fluidity strips revealed that due to the high velocity of liquid during casting, the liquid suffered from air entrainment and the flow washed sand inclusions into the stream. SEM microscopy studies indicates the presence of a surface film on liquid cast iron. This film was a carbon film when the melt was poured in resin-bonded moulds. In the atmosphere of sodium silicate moulds a complex silicate film was detected. It seems that, in comparison with the silicate film, the carbon film was strong enough to prevent the liquid flow from disintegration at high velocities of about 2 mls. In the next step, investigations were carried out by the use of real-time x-ray radiography to design an improved filter system to incorporate in the fluidity channel. Among five different filter systems, the one that produced the best quality casting was chosen. Another series of fluidity tests incorporating this filter system was carried out. In addition, two software packages (MAGMAsoft and FLOW3D) were used to simulate the fluid flow and heat transfer conditions inside the filter system. The measurements from both the real-time x-ray radiography technique and the simulation results indicate that a mixed laminar and inertial dependant flow dominates the flow inside the filter system. In this step, the fluidity of cast iron and Al-Si alloy liquids and effects of superheat and modulus were studied and results were in fairly good agreement with theoretical predictions. The results revealed that the fluidity of metal at high velocities, similar to the fluidity at normal velocity, increases with superheat and is proportional to the square of the modulus of the fluidity channel. In the final stage of this investigation, the flow of liquid in the fluidity channel was studied to find out the mechanism of flow endurance in the channel in both alloys. The mechanism was the formation of a flow channel and the remelting of solid particles at the flow front. Also, quantitative image analysis on the shape and distribution of phases was carried out to evaluate the possible arresting mechanism of the fluidity samples. The evidence indicates that in cast iron with low cooling rates, accumulation of eutectic particles in the flow tip is the main feature that acts to stop the flow. At high cooling rates, the mechanism of flow arrest in liquid cast iron is the formation of vein at some distance behind the flow tip. When this vein is closed due to the growth of grains from walls, the flow will stop. In AI-Si alloy stream, it seems that a-AI dendrites and/or primary silicon particles are carried down stream. Their accumulation is the reason for the arrest of flow.

This thesis presents a study about weld residual stresses created in Esshete 1250 stainless steel cylinders and the impact of the residual stresses on fracture of the Esshete cylinders. A series of residual stress measurements in welded Esshete cylinders using both Deep Hole Drilling and incremental Deep Hole Drilling techniques are presented. These measurements are compared against independent measurements using other techniques such as Neutron Diffraction and the Contour Method on almost identical cylinders. This is to obtain a confidence on residual stress measurement results using different techniques currently available for steel. There is another stainless steel material which is studied in this thesis. That is 316L stainless steel. Residual stress measurements were also carried out on welded cylinders made of 316L material and compared with independent results. The author of this thesis carried out only the Deep Hole Drilling and incremental Deep Hole Drilling measurements. Measurement results using other techniques were obtained from other researchers such as those from the Open University. The study on the impact of the residual stresses on fracture of the welded cylinders was however done only on Esshete 1250 material. The Esshete welded cylinders were in two conditions: as-welded and thermal aged. The thermal aged condition was obtained by putting the Esshete cylinder to a furnace for 10,000 or 20,000 hours at 650°C and this is to simulate the thermal ageing effect from service at a power station. The residual stress measurements were carried out on both conditions to find out residual stress before and after thermal ageing. The conclusions were that at as-welded condition, the peak tensile value weld residual stress can exceed the yield stress of Esshete material at room temperature; repair welds increased membrane stress in weld residual stress in both axial and hoop directions; and thermal ageing relaxed significantly the weld residual stress in both girth and repair welds. The 316L cylinders are in as-welded condition only and one conclusion was that the repair weld in 316L welded cylinders also increased the membrane stress of the weld residual stress field in both axial and hoop directions. Highly tensile residual stresses were measured in a repair weld of an Esshete 1250 cylinder and motivated an investigation into how these residual stresses affect the fracture of the Esshete cylinder and how they relax with plasticity. This is because the weld residual stress has a high membrane stress of 350MPa in the axial direction and 410MPa in the hoop direction. The repair weld residual stress is found to be almost constantly tensile at 350MPa in the axial direction and 410 MPa in the hoop direction with a peak tensile stress in Esshete steel welds was found to be approximately 550MPa. The highly tensile residual stresses present in the repair welded Esshete cylinder are detrimental to the integrity of the cylinder as a crack subjected to this residual stress field is more likely to grow under external loading than if the residual stress is less tensile or the membrane component is less tensile. As a result, when an engineer accesses the structural integrity of a welded structure, tensile residual stress with a significantly tensile membrane component will require much attention and may lead to a redesign. The influence of such a highly tensile residual stress field is investigated in this thesis using Finite Element analyses. A conclusion from this study is that the repair weld residual stress has a significant effect on the fracture of the Esshete welded cylinder. The presence of tensile residual stresses were shown to promote the onset of yielding in the cracked cylinder earlier than would be the case if residual stresses were not present. However, for higher applied loads the simulations with and without residual stresses converged which indicated that the accumulated plasticity has relaxed the residual stresses completely. This is indeed a shortcoming of current structural integrity assessment codes which is the inability to predict relaxation of residual stresses with plasticity. As an example, a widely used industrial code for structural integrity assessments is R6 [1]. It is a well developed tool to predict fracture or failure of a predefined structure such as a pipe with a circumferential crack or an axial crack. It is also able to include the presence of residual stress in assessments and predict conservatively fracture or failure of a structure with the presence of residual stress. R6 is unable to predict how a residual stress field would relax with plasticity. The axial residual stress in an Esshete repair welded cylinder was also found to have similar features to a long-range residual stress field based on mapping results presented in Chapters 5 and 6. As one focus of the investigation on residual stress impact on fracture of the Esshete cylinder is how the residual stresses relax with plasticity during fracture, an idealised long-range residual stress model along with an improved experimental rig were employed to illustrate how a long-range residual stress relaxes with plasticity and the relationship of that relaxation with elastic follow-up. The long-range residual stress model and experimental rig were concluded to provide an excellent representation of how the imposed long-range residual stress relaxes with plasticity. Measured residual stress relaxation was also compared against predictions provided by the idealised model. The main part of this thesis study on residual stress impact on fracture of a welded Esshete cylinder comes from a large-scale four point bending experiment. A 600mm long Esshete steel cylinder containing a repair weld with a part-circumferential through thickness crack introduced underwent the large scale four point bending. Simulations for this four point bending experiment were carried out, with and without residual stresses. Prior to these simulation results, techniques to map measured residual stresses into 3D finite element models were developed and are explained in this thesis. The FE simulations with and without residual stresses illustrated how residual stresses influence the fracture of the long cylinder. Axial residual stress relaxation was shown using a crack affected zone concept. This concept also allows estimates of elastic follow-up created in a cracked cylinder, ,and was found to be about 1.3. This value corresponds to rapid residual stress relaxation. The relationship between the elastic follow-up of a crack affected zone with the relaxation of the axial residual stress is also discussed.

No description available.

Ferritic plasma nitrocarburising of iron and steels has been carried out using a pilot scale industrial plasma processing unit. Atmospheres containing nitrogen, hydrogen, carbon dioxide, and in some cases methane were employed during the investigation. The nitrocarburised compound layers formed as a result of this glow discharge thermochemical treatment have been examined using a variety of metallographic and surface analysis techniques (including NRA, EPMA, AES, GDOES). Following the characterisation of the resultant compound layers, falex wear tests were performed on selected specimens to evaluate the wear resistance of the various nitrocarburised surfaces.

No description available.