This thesis presents a theoretical and experimental investigation into the modelling of the electro-fusion process, applied to welding polymer pipes. The theoretical background of the transient heat transfer between the fitting and pipe, including variable interface thermal resistance and involving consecutive changes of phase (melting and re-solidification) and the problem of the thermo-mechanical induced stresses in the joint, are fully discussed. Three 2D axisymmetric models of the EFW process, with different degrees of complexity, have been developed, refined and validated by comparison with experimental data: a finite element coupled model, with both temperature and displacement degrees of freedom, and two sequential heat transfer models, finite element and finite difference based. The effect of the melt movement into the fitting-pipe initial clearance is discussed and has been modelled by a 'virtual material movement' method. For the sequential models a 'gap evolution model' has been developed to describe the closure of the initial fitting-pipe gap through the process. Results from the simulations of the electro-fusion welding process performed using all three models, which give an exceptionally good insight into the temperature, displacement and stress fields within the joint, are fully discussed and validated through comparison with experimental data.

The overall aim behind this work was to gain and apply an understanding of the mechanical effects of welding, in order to mitigate distortions that had previously hampered uniaxial testing of tungsten inert gas (TIG) butt-welded, thin-section, nickel-base superalloy INCONEL 718 (IN718). With this aim achieved, better test data could be obtained, leading to more reliable material data for IN718 welds, which can be used for more accurate life assessments for shell structures and components. ABAQUS finite element (FE) simulations of the arc welding process were used to understand the development of welding-induced distortions and reduce their prevalence. An efficient, distortion-mitigating welding procedure, devised using the numerical simulations, was used to manufacture TIG butt-welded specimens that were free of distortion. Uniaxial tensile, creep and high temperature (620°C) fatigue tests were performed on both welded and non-welded IN718 specimens. It was found that IN718 welds are significantly less ductile than the parent material, and although welded IN718 exhibits comparatively little loss of tensile strength, its creep and high temperature fatigue properties are severely compromised. The lower performance of the welded specimens was attributed to the microstructural differences when compared with the non-welded material. The mechanical properties of the IN718 weld material were calculated and verified using FE analyses, based on the test data obtained. A continuum damage mechanics approach was employed for creep modelling and the necessary material constants were determined. A Smith, Watson and Topper (SWT) strain range parameter was proposed for fatigue life assessment, based on validation obtained using the test data. Three-dimensional, quasi-static elastic-plastic and creep FE analyses were used for a life assessment case study of an IN718 generic spoke structure. The effect of including a weld in a critical region was investigated for both constant (creep) and cyclic loading conditions at 620°C.

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No description available.

A detailed study has been made of commercially produced, powder metallurgy, BT42 grade high speed steel (H.S.S.) indexable cutting tool inserts coated with a thin layer of titanium nitride (TiN) by chemical vapour deposition (C.V.D.). The characteristics of the coating before and after the obligatory substrate heat treatment were first ascertained using various techniques including X-ray diffraction, Auger electron spectroscopy, fractography, scanning and transmission electron microscopy, microhardness testing, profilometry, optical microscopy and scratch-adhesion testing. It was found that the characteristics of the TiN coating were not significantly affected by the substrate heat treatment and that they were comparable to those presented in the literature for similar C.V.D. coatings on cemented carbide inserts. The cutting properties of the fully characterised TiN coated H.S.S. inserts were then compared to those of uncoated inserts in a series of turning tests on 817M40 workpiece material, carried out under conditions of built-up edge (b.u.e.) formation at cutting speeds in the -1 range 30-60 m min. Comparison of the inserts with regard to tool life, tool wear, tool forces, apparent coefficient of friction, b.u.e. formation and workpiece surface finish, revealed that the TiN coated inserts had significantly improved cutting properties relative to the uncoated inserts. Additional tests showed that increasing coating thickness led to further improvements in cutting performance. The role of the TiN coating was studied from metallographic sections through worn inserts, and the reasons for its significant effect on the cutting properties of tbe H.S.S. inserts are discussed. Similar inserts coated with TiN by physical vapour deposition (P.V.D.) were also characterised and their cutting performance determined. The results obtained are compared to those for the C.V.D. TiN coated inserts. Differences in the cutting performance are related to the different characteristics of the two coatings arising from the processes used for their deposition.

Welding has been one of the most extensively used joining processes for engineering applications and is the most frequently used process in nuclear power plants. Welding involves complex thermal, mechanical and metallurgical phenomena, affecting the microstructure of the material and generating internal or residual stresses and distortions in the process. Residual stresses are locked up stresses resulting from the thermal and/or mechanical processing of the parts. Residual stresses are inevitable and usually detrimental to the service life of a component often resulting in collapse or total structure failure. Numerous welding techniques have been developed over the past decades with the aim to reduce the residual stresses and enhance the performance of the component. These techniques need to be thoroughly studied and understood before implementing them in actual service. Electron beam welding and laser beam welding are two emerging techniques that are most promising because of many favourable features, including narrow fusion width to depth ratio, high welding speeds and capability to join metals that are dissimilar without any filler material. However to understand the full capability of these methods, it is essential to study the processes and their consequences on the joint. This dissertation presents the development of numerical and experimental approach to analyse electron beam welding and laser beam welding in a modified 9Cr-lMo (P91) butt welded plate. Modified 9Cr-lMo steel is used in nuclear power plants because of its high desirable properties such as strength and creep resistance at high temperatures. A number of simulation procedures using sequentially coupled thermo-mechanical analysis of the welding process are developed to study the welding process and the generation of residual stresses. The model incorporates the sol id-state phase transformation, exhibited by P9l steel during rapid cooling stage, which is the critical factor in the final residual stress field. The finite element models are validated using neutron diffraction measurements. The validated models are then used to study the influence of material properties, hardening models, annealing temperature and the boundary conditions on the final residual stress distribution . Also post-weld heat treatment used for relaxing the residual stresses due to welding is simulated and the extent of relaxation is studied. Uniaxial cross-weld creep tests are conducted on electron-beam welded samples to investigate the creep life. With the experience gained from modelling electron beam welding on P91 plates, an attempt has been made to develop a finite element model to simulate the electron beam welding of dissimilar metal welds in a butt plate made of P91 and AISI 316LN SS steels. The developed model is evaluated based on neutron diffraction experiments. Significant amount of effort has been directed towards developing an accurate and reliable numerical model to simulate the complex phenomena and severe non-linearity associated with welding processes such as temperature dependent material properties, hardening models, boundary conditions and solid-state phase transformation, which is the main purpose of this research. The residual stresses are predicted successfully. It is shown that the major contributor towards the residual stress profile is the volume change associated with the solid-state phase transformation during the cooling stage. Other factors such as temperature dependent thermo-mechanical properties, material hardening properties and boundary conditions have relatively less influence on the residual stresses.

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No description available.

No description available.

This dissertation details the findings of experimental investigations of welds made between ferritic creep resistant steels that differ in chromium content. Analysis of the microstructural evolution during the application of post weld heat treatments is reported. Particular attention was paid to the key alloy strengthening mechanisms and the manner in which they were affected by carbon redistribution which takes place when these welds are exposed to high temperatures. The fusion interface regions of transition joints, made between P91 parent alloy and P22, P23 and P24 type weld consumables, were analysed in as received and post weld heat treated conditions. Carbon redistribution from the low to higher alloyed material, which resulted in its depletion from weld alloy adjacent to the fusion line, was confirmed in all weld systems subsequent to post weld heat treatment (PWHT). The effect of tempering treatments, carried out at 730°C for two and eight hour durations, on carbide populations in partially decarburised weld alloy was explored. The consequential microstructural changes, which were affected by the dissolution of M23C6 and M7C3 carbides, were compared to those observed in regions of weld alloy unaffected by carbon depletion. High resolution transmission electron microscopy (HRTEM) and field emission scanning electron microscopy (FESEM) were used extensively in the analysis of weld metal and heat affected zone (HAZ) microstructures. Electron diffraction and x-ray energy dispersive spectroscopy were exploited in the crystallographic and chemical characterisation of precipitates. Their evolution as a function of thermal exposure is presented for each alloy. Chemical signatures for each precipitate species, which enabled their identification, were determined for carbides in the different alloys. However, due to variations in the compositions of fusion interface M23C6 carbides, some permutations of which overlapped with compositions of M7C3, satisfactory identification demanded classification of their crystal structure. A significant difference between the microstructures of P23 and P24 alloys, in the weld specimens tested, was observed. Although vanadium and niobium carbonitrides (MX) were identified in both alloys, their distributions were not the same. Retention of carbonitride particles within partially decarburised P23 and P24 weld materials, subsequent to 8 hours post weld heat treatment, has been substantiated. Diffraction intensity distributions in Debye-Scherrer ring patterns, which were generated from MX precipitation, indicated lattice parameters varied. Microanalysis revealed that MX precipitates were present over a wide range of compositions. A combination of the composition analysis and diffraction studies indicated that MX precipitation was stable over a range of compositions in the carbon depleted regions of P24 alloy. Recrystallisation of the bainitic P22 weld alloy adjacent to the fusion line, which was accompanied by a loss of material hardness, was observed in 2 and 8 hour PWHT P91/P22 welds. It has been shown that the microstructural stabilisation of carbon depleted T/P23 and T/P24 alloys was conferred by a dispersion of MX precipitates. Retention of these stable particles, which in many cases are less than 10 nm in diameter, in carbon depleted material, resulted in the complete avoidance of any recrystallisation in 2 hour post weld heat treated T/P23 and T/P24 welds and only isolated occurrences in 8 hour tempered specimens. Subgrain size distributions were determined from electron channeling contrast images of various regions of the dissimilar metal welds. Results showed that, although recrystallisation of MX forming alloys did not occur, destabilisation of lath boundaries, due to the dissolution of M23C6 and M7C3 carbides, results in a coarser subgrain microstructure in carbon depleted P24 weld alloy. The loss of resistance to plastic deformation as a result of recrystallisation, which has been shown to take place in decarburised P22 alloy, was not observed in the alloys which precipitated the MX phase.