Dissertations / Theses on the topic 'Linepipe'

The benefits of high strength steels in terms of reduced material volume due to enhanced mechanical performance have been known for some time. Large diameter transmission linepipe steels of minimum 690MPa ('X1OO') yield strength have been developed throughout the previous decade, and have recently become commercially available. Before these steels are used in linepipe construction projects, fimdamental work regarding their ability to be field welded required undertaking. This thesis presents data arising from girth welding experiments involving a variety of X 100 linepipe steels, welding consurnables and welding processes. Target girth weld mechanical properties thought suitable for a strain-based MOO pipeline design were proposed at the outset of the research. Optimisation of pulsed gas metal arc welding waveforms for the single and tandem wire processes, alongside the establishment of the base material properties formed an early part of the research. An extensive programme of solid wire welding consumable evaluation was then undertaken for single, tandem and dual torch narrow gap welding processes. The majority of equipment and procedures used throughout the work were as close to current field practice as possible, to minimise the time required to transfer the technology to the field situation. Work then focussed on the optimised alloy levels and welding procedure requirements for the production of full girth welds, using a variety of industry pipeline welding standards and supplemental techniques to assess the joint integrity. It has been demonstrated that, subject to careful selection of welding consumable and fairly precise control of welding process variables and parameters, there are no major problems in obtaining weld metal strength levels of at least 120 MPa above the 690 MPa specified minimum yield strength (SMYS) of the parent pipe. This objective has been achieved in welds made usirig all three mechanised process variants examined. The desired target properties of strength and toughness were achieved with a variety of consumables and pipe materials of different composition. Tie-in and repair procedures were also developed during the course of the research, with particular attention focussed on the application of high strength rutile flux cored ýVires. These wires attained strength levels overmatching the pipe specified minimum yield strength (690MPa), but would not reach the guaranteed overmatch level of 81 OMPa. An examination of the thermocycles associated with four mechanised narrow gap welding techniques (single, tandem, dual and dual tandem) was undertaken. The experimental technique developed allowed the solidifying weld bead to be monitored, as well as the cumulative temperature cycles experienced by the underlying layers. Succesful determination of the cooling rates, times and transformation temperatures allowed a comparative evaluation of the four processes, using an optimum weld metal composition suitable for single wire welding of X100. This led to an understanding of the metallurgical history, and its consequent effect on the associated mechanical and microstructural properties. A similar series of experiments was undertaken to examine these effects using variations in preheat with a single wire process. In most cases considerable property variations were attained for'the same weld metal chemistry, joint geometry and arc energy, highlighting the sensitivity of the process and procedure in achieving the required properties. The high cooling rates determined from the thermocycle experiments explained the microstructural and mechanical properties attainable from lean alloying levels. A series of metal cored wires, based around the same alloy as for the thermocycle experiments, was consequently manufactured to examine small changes in weld metal chemistry. The individual wires involved changes in carbon, nickel, molybdenum and chromium to examine potential property variations arising from a highly controlled narrow gap welding procedure. The results again highlighted the sensitivity of the narrow gap welding technique in generating considerable property variation within the weld metal. Tolerance ranges for specific alloying additions to attain the proposed strength levels with a single and tandem wire process were derived from the data.

The final microstructure and resulting mechanical properties in the heat-affected zone (HAZ) of welded linepipes are predominantly determined by austenite decomposition during cooling after welding processes. Thus, a full understanding of continuous cooling transformation of austenite is a key step toward improving the overall performance of linepipes. The main objective of the current study is to investigate the influence of cooling rate, prior austenite grain size and niobium content of austenite on austenite decomposition kinetics and the resulting microstructures for an X80 linepipe steel. To consider the significant effect of the niobium solid solution level on the transformation of austenite, two thermal histories were developed. For the first case, Nb was dissolved in solid solution prior to austenite decomposition. In contrast, the second scenario involved the formation of Nb(C,N) precipitates prior to austenite decomposition, i.e. leaving a low level of Nb in solid solution. Austenite grain growth studies were conducted to obtain grain sizes similar to those observed in the HAZ of the girth-welded steel, i.e. 5-80μm. Furthermore, employing appropriate thermal cycles, continuous cooling transformation (CCT) tests were conducted to examine the effect of niobium condition, austenite grain size and cooling rate on austenite decomposition behavior of the steel. Cooling rates varied in the range of 3−100ºC/s and dilation measurements were utilized to capture the transformation kinetics of austenite upon cooling. The resulting microstructures, which usually consist of ferrite, bainite and martensite-austenite (MA) constituents, were examined using optical microscopy. They were revealed using appropriate etchants and the corresponding phase volume fractions were subsequently measured in accordance with ASTM standards. Hardness measurements were also conducted on CCT samples.

The main aim of this research is to model the internal micro damage accumulated during cold deformation and the degree of reduction of damage that can be achieved by heat treatment of linepipe steel. A set of unified viscoplastic constitutive equations was developed to simulate microstructural evolution and the effect on mechanical properties due to cold deformation followed by annealing. In addition, practical experiments have been performed to validate the constitutive equations. Firstly, the industrial motivation for the project was previewed and damage-modelling techniques were reviewed to identify the research objectives. A group of interrupted uniaxial tensile tests were conducted to study the effect of different annealing times on reducing the degree of damage and improving mechanical properties of a cold formed single phase linepipe steel. From the experimental results, it was observed that healing by subsequent annealing enables a significant improvement in the mechanical properties of the deformed steel that has experienced only light damage, but has no significant effect on heavily damaged steel. Following this, a set of constitutive equations describing accumulation and annihilation of dislocations, grain size evolution, recrystallisation, plasticity induced damage and their effects on viscoplastic flow of materials was developed, for uniaxial stress conditions and by numerical integration experimental results were used to determine material constants for these equations, for the particular steel. Secondly, the constitutive equations were expanded to enable damage nucleation, growth and plastic flow to be predicted for various stress states. The constitutive equations were implemented in a commercial FE solver (ABAQUS) using the VUMAT user-subroutine. The numerical results reproduce the essential features of necking phenomena and provide a physical insight into damage evolution within a tensile testpiece under given necking conditions. Dual phase steel is of greater importance industrially, than a single phase steel, but because of the greater complexity in its microstructure, the development of microstructural constitutive equations for it is very difficult. Thus, in this work, some knowledge of damage initiation in a dual phase steel was obtained by practical investigation of microstructure. It showed that damage is due to ductile cracking characterised by the nucleation of micro-voids near the ferrite-pearlite interface. Using the mesoscopic modelling technique, by simulation, it was possible to determine likely sites for damage initiation.

The principal aim of this work was to determine the role of non-metallic inclusions in the process of hydrogen stepwise cracking (SWC). Additionally, the influence of inclusions upon the notch ductility of hydrogen charged (HC) and uncharged (UN) tensile specimens was examined. To obtain a basis for experiment a series of low carbon-manganese steels were prepared by induction melting. In order to produce variations in the composition, morphology, volume fraction, size and distribution of the inclusions the steel chemistry was adjusted prior to casting by additions of deoxidiser and Ca-Si injection. Sections of each ingot were hot rolled. Metallography, image analysis, mechanical tests and hydrogen SWC tests were then carried out. The volume fraction, morphology, and shape of inclusions influenced the tensile ductility of the steels. Marked anisotropy was found in the steels containing type II MnS inclusions at all rolling temperatures, whereas the fully Ca treated steel was isotropic. It was found that several inclusion parameters (projected length PL, mean free distance MFD, nearest-neighbour distance NND) correlated with fracture strain. An increase in inclusion volume fraction and/or the dimension of inclusions on a plane parallel to the plane of fracture led to a decrease in fracture strain. The inclusion parameters did not correlate with the fracture strains for the HC tensile specimens. However, large or clusters of inclusions acted as the principal sites for crack initiation. `Fisheyes' or areas of `flat' fracture were often found on these fracture surfaces. The criteria for SWC initiation was found to be either large inclusions or clusters of inclusions. As the PL of inclusions increased the probability of large SWCs occurring increased. SWC initiation at inclusions was believed to occur at a critical concentration of hydrogen. Factors which assisted the concentration of hydrogen at inclusions were discussed. None of the proposed mechanisms of hydrogen embrittlement could be identified as the single cause of SWC.

The work is concerned with the development of a multi-stage corrosion fatigue lifetime model, with emphasis on pitting as a precursor to cracking. The model is based upon the quantitative evaluation of damage during the overall corrosion fatigue process. The fatigue response of as-received API 5L X65 linepipe steel has been investigated in terms of the evolution of damage during pit development, pit-to-crack transition and crack propagation. Micro-potentiostatic polarisation was conducted to evaluate role of stress on pit development. Crack growth rate measurements were conducted on pre-pitted specimens, which were tested in air and brine, to evaluate the initiation and propagation behaviour of cracks emanating from artificial pits. Finite element analysis was undertaken to evaluate the stress and strain distribution associated with the pits. A cellular automata finite element model was also developed for predicting corrosion fatigue damage. Pit growth rate was enhanced under stress. It was considered that the strain localisation effect of the pit facilitated strain-assisted dissolution. In air, cracks initiated predominantly from the pit mouth. FEA results indicated that this was due to localisation of strain towards the pit mouth. In corrosion fatigue, cracks tended to initiate at the pit base at low stress and at the pit mouth at higher stresses. Crack initiation lifetimes were shorter in the aggressive environment compared to air and the effect of the environment on crack initiation lifetime was lower at higher stress levels. Crack initiation lifetime for double pits generally decreased with decreasing pit-to-pit separation distance. The microstructure was observed to influence crack growth behaviour in air particularly in the early stages when cracks were short. The acceleration and retardation in crack growth were attributed to the resistance of grain boundaries to crack advance. Cracks sometimes arrested at these barriers and became non-propagating. Introduction of the environment for a short period appear to eliminate the resistance of the microstructural barriers thus promoting re-propagation of the previously arrested crack. The continued crack propagation after the removal of the environment suggests that the influence of the environment is more important in the early stages of crack growth. Crack growth rates were higher in the aggressive environment than in air. The degree of environmental enhancement of crack growth was found to be greater at lower stress levels and at short crack lengths. Oxide-induced crack closure and crack coalescence were two mechanisms that also affected crack growth behaviour.2-D cellular automata finite element simulation results, with and without stress, show good agreement agreed with experiments i.e. pit depth and pit aspect ratio increase with time. Results from 3-D cellular automata simulations of pits are also consistent with experiments. Fatigue lifetimes were significantly shorter (i) in the brine environment than in air and (ii) for specimens with double pits compared to single pits of similar depth. Fatigue strength in air was found to decrease with increasing pit depth. Corrosion fatigue lifetimes predicted based upon the developed model showed good agreement with the experimental lifetimes.

During welding, the heat affected zone (HAZ) of X80 linepipe steel is subjected to very steep spatial variations in temperature and concentration of Nb bearing particles which results in a strongly graded microstructure. Therefore, models on the length scale of the microstructure, i.e. the so-called mesoscale, are useful to simulate microstructure evolution in the HAZ. Among mesoscale models, phase field modelling (PFM) is selected because it is based on diffusional time steps and it is a robust tool to capture complex morphologies, e.g. bainitic ferrite. A PFM is developed for austenite grain growth in 2D and 3D that is applicable to rapid heat-treatment cycles taking the pinning/dissolution effects of Nb bearing particles into account by using an effective mobility concept. In addition, a PFM is developed for the austenite decomposition to predict the simultaneous formation of polygonal ferrite and bainite. PFM is coupled with a carbon diffusion model and an effective interface mobility is introduced to implicitly account for the solute drag effect of Nb. For simplicity, the formation of carbide-free bainite is considered and a suitable anisotropy approach is proposed for the austenite-bainite interface mobility. The model is first applied to a TRIP steel in which ferrite and bainite form separately, and bainite can be considered carbide-free bainite. Then the model is applied to simulate the microstructural evolution in the HAZ of the X80 linepipe steel accounting for the thermal and microstructural gradients and validated with microstructure observations made in a weld trial.
Applied Science, Faculty of
Materials Engineering, Department of
Graduate

Weld solidification cracking is an important issue in fusion welding. If undetected, the cracking defects can act as stress concentration sites which lead to premature failure via fatigue, as well as offer favourable sites for hydrogen assisted cracking and stress corrosion cracking. For welded steel products such as deep sea oil and gas transportation pipes, such defects heighten the risk of catastrophic in-service failures. Such failures can lead to devastating environmental, economic, and social damage. In this thesis, a comprehensive review of literature associated with steel linepipe and solidification cracking defects is first presented. Fluid flow prior to solidification is then observed and quantified in situ using a novel synchrotron X-ray radiography approach. The flow is dynamic at velocities up to 0.52 m/s and primarily driven via Marangoni flow. The relationship between the microstructure and mechanical properties of the welded linepipe are extensively characterised, with a new equation derived to assess fracture toughness based on the size and distribution of carbonitride precipitates. Weld residual stresses are measured both before and after linepipe expansion in the U-forming, O-forming and expansion process for the first time using a neutron diffraction technique. To further understand the fundamental mechanisms of solidification cracking during welding of high strength steels for subsea linepipe, a novel small-scale Varestraint test rig was developed for use in synchrotron X-ray imaging experiments and a Transvarestriant test rig utilised for industrial scale weldability tests. Solidification cracking during the welding of steel is observed in situ for the first time using a micro-radiography approach and the 3D crack network is rebuilt using a micro-tomography technique. It is proposed that solidification cracks nucleate from sub-surface cavities associated with: i) residual liquid high in solute and impurity concentration (hot cracks), ii) Ti (C,N) precipitated during solidification (that induce ductile microvoids). Solidification cracks then propagate via inter-dendritic hot tearing.

Five linepipe type steels were produced in order to study the effect of calcium and magnesium injection on their final properties. Two of these steels were at the extremes of the sulphide range i.e. 0.003 and 0.017% sulphur with no injection attempted; thereby, providing standards to compare with the injected steels. The oxygen level varied from 21 to 63 p.p.m. The cast ingots were controlled-rolled and isothermally rolled in order to study the deformation characteristics of the residual non-metallic inclusions. The structure and cleanliness of these steels was evaluated metallographically using the light microscope, SEM, and image analysis and the results related to their Charpy toughness and HIC resistance. Increasing sulphur levels decreased final properties of the steel. In the untreated state, with as little as 0.003% sulphur, test orientation was highly influential. Modification of sulphur bearing steels was achieved with low modifying element to sulphur ratios provided that the oxygen content was very low. Injection of calcium into steel caused interaction with oxide and sulphide inclusions which was biased toward oxide reduction relative to sulphur removal. Magnesium again reduced oxides and appeared to be linked with aluminia containing inclusions in the final product. It produced improved toughness values relative to a similar sulphur containing calcium treated steel. The results of this work could be extended to establish the mechanism of inclusion modification with magnesium additions to sulphur bearing steels.

In the transportation of oil and gas products from and over remote locations, such as Canada's Arctic environment, pipelines are used. Girth welding to join sections of steel pipelines creates a substantial heat affected zone (HAZ) within the base pipeline steel. While there is significant concern as to the fracture and mechanical properties of the HAZ as whole, detailed knowledge about the mechanical properties of the microstructural constituents is lacking. For this research, measurements of the temperature time profile in the HAZ of single and dual torch welds were made. This was then used to guide heat treatments of X80 steel in a Gleeble simulator to create samples of 8 different bulk microstructures with differing amounts and morphologies of bainite, ferrite and martensite-retained austenite (MA). From the heat treated samples tensile and Kahn tear test specimens were made for testing at ambient, -20⁰C, and -60⁰C. The highest strength microstructure proved to be the finest, lower bainitic microstructure, while the lowest strength microstructure was the coarsest, upper bainitic sample containing a significant amount of MA. This was observed to be true at all testing temperatures. As part of the tensile behaviour investigation, the Bouaziz dislocation based model for work hardening was applied and shown to fit well across all temperatures and conditions. The Kahn tear test, a machine notched, thin-sheet, slow strain rate test, showed all tests failed in a ductile manner. Relative toughness measurement from this test showed that the fine, lower bainitic microstructure was the toughest and the coarse, ferritic microstructure was the least tough. This work presents a novel measurement of dual torch temperature time profiles in a real HAZ, an extensive mechanical testing program of isolated microstructures relevant to the X80 HAZ at potential pipeline operating temperatures, and an applied a robust model to fit the work hardening behaviour for all conditions. This work has the potential for future application in microstructure evolution-property models, and in a combined mechanical model of the different microstructures to further improve understanding of HAZ mechanical responses.
Applied Science, Faculty of
Materials Engineering, Department of
Graduate

Five API grade steels designed for linepipe applications produced using different processing routes and with varying microstructures were studied against differences in work hardening and work softening behaviour obtained from mechanical data. The rolling history and wt % additions of alloying elements will determine how the microstructures perform under reverse deformation schedules commonly seen during large diameter linepipe fabrication as steels can undergo work softening in the reverse direction of deformation, otherwise known as the Bauschinger effect. The Bauschinger effect is known to be dependent on the initial forward pre-strain, volume fraction (VF) of carbo-nitride particles and initial dislocation density. The effects of grain size and solid solution strengthening are a matter of debate in the literature and the combined effects of all five strengthening mechanisms have rarely been quantified. TEM investigations determined the dislocation densities to be between 2.2 x1014 m-2 - 5.8 x1014 m-2 in the as received condition. Observed trends presented and discussed in this body of work have given a greater insight into the influence microstructure has on the mechanical properties across a wide range of HSLA steels of similar strength grades, which are of important consideration for future development of low carbon steels designed for the petrochemical industry.

For the manufacture of higher strength pipelines to be feasible, a better understanding of the effects of welding on toughness is necessary. Bevel submerged arc welds were performed on X80 grade steel. The subsequent Charpy V-notch (CVN) test results indicated that the notch placement in the various heat affected zone regions, and hence the bead geometry, affected the test results. A series of bead-on-plate (BOP) submerged arc welds then were performed on X70 grade steel plate to determine the effects of current, voltage, heat input, polarity, and waveform manipulation (i.e., balance, offset, and frequency) on both single and tandem weld bead geometry. A new bead profile characteristic, the SP ratio, is proposed to describe weld bead geometry, and its relationship with welding parameters is discussed. Sub-size CVN specimens, pulled from many of the BOP weld coupons, were then tested. The greatest subsize CVN fracture energies were achieved when the bead was produced using lower heat input, and when the bead profile possessed a greater SP ratio.
Materials Engineering

碩士
國立海洋大學
船舶機械工程學系
82
An optimized process on the gas metal arc welding for API 5L X52 linepipe steel was pursued among the characteristics of strength, toughness, weldability and welding productivity in the light of increasing welding heat input and filler wire with titanium chemical composition. The weldments was created by single pass welding with 1/2K groove type, filler wires with/ without titanium accompanied by seven different levels of heat input from 0.4 KJ/mm to 6.0 KJ/mm. Hardness distributions on base metal, heat-affected zone, and weld metal were evaluated in Vickers hardness va- lues respectively. Hardness criterion of linepipe steels for sour services, 248HV10, was met except for the heat input less than 0.42KJ/mm. Hard spot distribution had the lowest proba- bility Based on the evidances of this research, welding conditions of Silicon-Maganisum-Titanium filler wire with 2.5KJ/mm heat input will optimize the characteristics of weldment among strength, toughness, weldability and welding productivity for API 5L X52 linepipe steel by gas metal arc welding.

Low temperature cleavage fracture behaviour was investigated using four experimental microalloyed bainitic plate steels. The four plate samples were produced by different thermomechanical processing (TMP) schedules and had yield strengths in the range 540 - 670 MPa. Microstructures were characterized by optical microscopy (OM), scanning electron microscopy (SEM) and electron back scattered diffraction (EBSD). Quantitative data was obtained for prior austenite grain (PAG) size, volume fractions of two bainite types (conventional bainite and acicular ferrite) and EBSD 15° domain size. Charpy impact tests (using two notch orientations) were carried out over a range of temperatures. Cleavage facet sizes were measured on -196°C Charpy samples. The range of TMP schedules produced variations in PAG width, type of bainite and 15° domain size. The effects of these three microstructural features on cleavage crack propagation are discussed. Results indicate that the microstructures are controlled by i) deformation below TNR and ii) accelerated cooling rate. Domain structure reflects TMP. There is no clear correlation between domain size and cleavage facet size.
Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2009-07-30 19:17:01.25

碩士
國立海洋大學
材料工程研究所
87
Abstract The tensile properties and fatigue crack growth of API 5L X65 steel welds were investigated. Constant extension rate tensile(CERT) tests were performed to investigate the effect of hydrogen embrittlement on the tensile properties of various specimens. The specimens after charging in a saturated H2S solution for 48 hours, were used to evaluate the enhanced crack growth induced by hydrogen as compared to the unembrittled specimens. The results indicated the effect of tempering treatment at 600℃ for 2 hours had minor influence on the hardness and tensile properties for steel plate and weld. It could be due to the reprecipitation of banded structure during tempering treatment, resulting in the slight increase in hardness and tensile strength. Meanwhile, the slight drop in strength and drastical decrease in ductility was obtained for the steel plate tested in a H2S solution. For the welds receiving postweld tempering treatment, hydrogen embrittlement susceptibility could be reduced, which were associated with the increase of tensile strength and the decrease of surface cracks. Tensile-fractured specimens revealed ductile dimple fracture was observed for the specimens tested in air in contrast to quasi-cleavage fracture for the specimens tested in a H2S solution. Although the impact toughness of specimens varied significantly with orientations with respect to the rolling direction, however, only minor change in fatigue crack growth rate(FCGRs) of the steel plate had been found. In the as-welded weldments, both the weld metal and heat-affected zone(HAZ) had lower FCGRs than the steel plate at the same ΔK ranges. As the crack propagated across the HAZ into the base metal, the fatigue crack growth characteristics were consistent with the steel plates. However , the FCGRs of the as-welded weldment in the weld metal and HAZ were lower than those of the tempered weldments, which might be attributed to the presence of residual stress. After immersion treatment, obviously enhanced crack growth occurred within the lowΔK ranges for all the specimen, especially for the tempered welds. Tempered welded still had the highest FCGRs among the specimens within the low ΔK ranges. Fatigue fractographies showed the fractured surface consisted of transgranular fatigue and quasi-cleavage mixed fracture for those specimens comprised of embrittled regions. Keywords：banded structure, fatigue crack growth rate, hydrogen embrittlement, heat affected zone, weld metal, quasi-cleavage