The efficiency of steam reforming depends strongly on the creep resistance of the material used for the reformer tubes. The currently most widely used reformer tube material is HP40 (25 Cr, 35Ni, 40 Fe and 0.4C) austenitic stainless steel. A further improvement in the creep resistance of HP40 is needed for efficiency improvement and for a cost reduction in steam reforming. In order to develop a next generation creep resistant alloy, three HP40-based alloys, namely Alloy A, B and C, with different chemical compositions and/or solidification rate, were studied. Previous tests at 1000 oC and 40 MPa have shown that the creep properties of Alloy C are slightly better than those of Alloy B, both being significantly better than Alloy A. The microstructures of three alloys, under as-cast, crept and heat treated conditions, have been analysed so as to understand their relative creep performance. The small intragranular M23C6 may have contributed significantly to the smaller creep rate, and thus a longer creep life for Alloy B and Alloy C as compared with Alloy A. A microstructure-based climb-glide bypass creep model was described to predict the creep behaviour of HP40. Suggestion on the next generation HP40 alloy has been made.

In recent years, the increasing demand for rare earth magnets for use in motors and machines has led to the need for more efficient use of NdFeB materials. Novel processing techniques have been developed to produce NdFeB materials with excellent magnetic properties without the need for heavy rare earth elements. Two specific novel processes including the Flash Spark Plasma Sintering (FSPS) process and the Hydrogen Ductilisation Process (HyDP) were studied in this project with the focus on the use of microstructural analysis and magnetic measurements to fully understand the mechanisms, hence develop and optimise the processing conditions for production of fully dense anisotropic magnets. The most optimal FSPS sample demonstrates a uniform, fine-grained structure with a high degree of crystallographic alignment, leading to a high coercivity (1438 kA m\(^−\)\(^1\)) and remanence (1.16 T) giving a BH\(_m\)\(_a\)\(_x\) of 230 kJ m\(^−\)\(^3\). While with appropriate s-HDDR and compression conditions, the HyDPed sample exhibits a typical s-HDDR microstructure with a submicron grain size. It is shown in the work that the disproportionated sample shows an enhanced ductility and thus can be shaped at room temperature, and a useful degree of anisotropy is produced within the HyDPed sample during the compression.

Hot isostatic pressing (HIP) is of interest to the aerospace industry due to the ability to produce polycrystalline components free from defects, which typically exhibit comparable or superior properties to their cast counterparts. The capability also exists with powder HIP to produce large parts with complex designs to near netshape (NNSHIP) in a single processing step, which can potentially reduce the buy-to-fly ratio ofhigh temperature gas turbine engine components. Such components, however, are manufactured from Ni-superalloys and certain issues exist with the HIP ofNi-superalloys that require addressing. The research presented in this thesis investigates HIP of a Ni-superalloy CM247LC to assess the viability of the process and the alloy for high temperature turbine and combustor casing components of the future. The influence of powder characteristics, post-HIP heat treatment and modification of the HIP procedure, have been investigated to assess the effects on high temperature properties. HIPped CM247LC can significantly outperform cast CM247LC in terms of hot tensile properties (particularly ductility), suggesting NNSHIP is potentially viable. In the current work, however, poor creep resistance was exhibited which is a concern for high temperature components.

High strength, low alloy, hot rolled strip steels with yield stresses in the range 700 to 1300MPa are required for the Lifting and Excavating product sector. Improved combination of strength and toughness in these high strength steels is desired, requiring a detailed understanding of the relationship between microstructure and mechanical properties. In this work 12mm thick 700MPa yield stress strip steels with fully bainitic microstructures, with different compositions and/or processing conditions, have been studied. The microstructures of the steels were investigated with both optical microscopy and SEM. Micro-hardness and Charpy impact tests (at different temperatures) were carried out to investigate the mechanical properties, followed up with fracture surface analysis and unit crack path (UCP) analysis. Three types of bainitic microstructures were identified and quantified, including upper bainite, lower bainite and granular bainite. The fracture surface and UCP analysis indicating that granular bainite is detrimental to toughness, but cannot necessarily be avoided in the steels investigated; while the presence of a small amount of lower bainite (above 5%) was found to improve the impact toughness significantly. Continuous cooling transformation (CCT) diagrams for steels with different compositions were produced, which allowed investigation of the phase transformation behaviour and selection of optimum chemical composition/coiling temperature for improved properties. The effects of alloying elements (B, Mo and V) on the transformation behaviour have been confirmed through the experiments. An optimised combination of alloying and coiling temperature has been proposed and validated via investigating plant trial products with similar alloying and processing parameters.

Two additive processing scenarios were considered covering manufacturing and repair applications. One scenario required evaluation of three processes for the addition of shaped blocks to a casing. The other involved infilling a shallow hemispherical indent with one of three polycrystalline alloys (alloy 718, waspaloy™ and RR1000) using different deposition strategies. Processinduced discontinuities were characterised and controlled using a design of experiments approach. As distinct from welding, process variables of overlap, Toolpath, bead shape,sequencing and incremental height were established as influential variables Toolpath in particular can give markedly different textures and grain alignment which would be expected to influence mechanical anisotropy. Fine pool processes seem less prone to cracking and more likely to yield fine microstructures.

The research involved the optimisation of various machining operations for the manufacture of automotive turbocharger impellers. Phase 1 work comprised a capability study of the Al-Si C355 impeller machining process within Doncasters Sterling. The results highlighted that both C\(_p\) and C\(_{pk}\) values were generally above the automotive industry standard of 1.66 and that current processes were capable of producing parts to specification. Benchmark testing of TiN coated WC and PCD tooling against uncoated WC was undertaken in Phase 2 experiments. While the PCD tools showed significant improvement, the TiN coated tools were inferior in terms of thrust force, torque and workpiece surface roughness when compared against the uncoated WC tools. Phase 3 research evaluated the performance of advanced diamond like carbon (DLC) coatings against WC when blind-hole drilling C355 and reaming 6082-T6 aluminium alloys. Benefits in terms of extended tool life and reduced material adhesion/BUE were observed when drilling and reaming using the commercial Graphit-iC™ coating. The performance of CVD diamond coated and PCD tools were also assessed with the latter showing lower wear rates with negligible workpiece adhesion/BUE compared to WC in both drilling and reaming experiments. The former however failed catastrophically at high cutting speeds during drilling.

No description available.

Linear friction welding (LFW) is known as an advanced technique to substitute fusion welding of joining Ni-based superalloys. However, due to the complex microstructural development caused by LFW, some unexplained issues still need to be solved, such as: influence of weld parameters (weld pressure, amplitude of oscillation, and frequency of oscillation) on the development of microstructural, microhardness, texture and residual stresses in LFWed IN718-IN713LC; impact of post-weld heat treatment (PWHT) on the microstructural and microhardness development; influence of rapid thermal cycle on the precipitate dissolution of IN718 and IN713LC; formation mechanisms of alumina inclusions (Al-oxides) on the weld interface. The present work aims to investigate these issues.

A major factor limiting the use and longevity of rare earth based magnetic materials is their susceptibility to aqueous corrosion and associated detrimental effects upon the magnetic properties of the material. This process was investigated through a combination of exposure to simulated environmental conditions and hydrogen absorption/desorption studies (HADS) in conjunction with magnetic characterisation. This study utilises NdFeB MQP-B melt-spun ribbon manufactured by Magnequench, in the form of MQI bonded magnets and also in its unbonded state as MQ powder. Specifically, it was concerned with how effective a variety of bonding media (epoxy resin,PTFE, zinc) and surface coatings (PTFE, Qsil, zinc LPPS, Dex-Cool) were at limiting the impact of aqueous corrosion in MQI bonded magnets. To characterise the effect of hydrogen absorption upon the magnetic properties of the MQP-B, hydrogen uptake was induced followed by a series of outgassing heat treatments with subsequent magnetic characterisation accompanied by HADS techniques performed after each outgas. This allowed comparisons to be made between the effects of aqueous corrosion process and hydrogen absorption upon the magnetic properties of the alloy. This study has clearly demonstrated the link between the abundance of environmental moisture and rate of Hci losses in MQI bonded magnets. In addition to this the key mechanism responsible for the degradation of magnetic properties has been identified. These losses have been attributed to the absorption of hydrogen generated by the dissociation of water in the presence of NdFeB during the aqueous corrosion process. It has been shown that the use of a bonding media that is impermeable to water can limit the effects of aqueous corrosion by limiting water access to the Magnequench particles (MQP) and also the positive effects of the use of suitable surface coatings has been shown to be effective for the same reason.

Titanium aluminide alloys are prone to extensive shrinkage porosity, particularly interdendritic layer porosity. This research aims to develop a better understanding of shrinkage porosity in Ti-46Al-8Nb alloy and how to minimize it. Practical measurements in conjunction with computer simulations were carried out in order to study the effects of cooling rate and temperature gradient on shrinkage porosity in Ti-46Al-8Nb investment castings, through investigating the influence of casting geometry, specifically mould taper on cylindrical bars, and preheating mould temperature on the macrostructure and the formation of shrinkage porosity. The interface heat-transfer coefficient of the ceramic shell/surroundings was determined using measured cooling curves obtained in 200 mbar of Argon and in vacuum. Furthermore, the alloy/mould interfacial heat transfer coefficient was estimated and was verified by comparing measured and predicted cooling curves of Ti-46Al-8Nb. The Niyama criterion function was validated to predict shrinkage-related porosity in Ti-46Al-8Nb castings, through either thermal or tilt-filling ProCastTM simulation models. Positions in a casting with Niyama values above 4 (°C. min)0.5cm-1; (3 (K. s)0.5mm-1); could be considered completely sound. The critical value of the Niyama function was 2 (°C. min)0.5cm-1; (1.5 (K. s)0.5mm-1); below which the position tested was susceptible to macroshrinkage porosity.