1,771,208 research outputs found
Preferential weld corrosion of X65 pipeline steel in flowing brines containing carbon dioxide
Get PDFThe aim of this research was to investigate the cause of the severe localised
corrosion that sometimes occurs at welds in carbon steel pipelines carrying
hydrocarbons and inhibited brines saturated with carbon dioxide. A rotating
cylinder electrode (RCE) apparatus was designed so that electrodes machined from
the weld metal, heat affected zone (HAZ) and parent material of welded X65
pipeline steel could be galvanically coupled and tested in high shear stress
conditions. The galvanic currents flowing between the weld regions were recorded
using zero-resistance ammeters and their self-corrosion rates were found by
polarisation resistance measurements. The total corrosion rate of each weld
region was obtained from the sum of the self-corrosion and galvanic
contributions. In uninhibited conditions, the weld metal and HAZ were both
cathodic to the parent material and localised corrosion was prevented. However,
when an oilfield corrosion inhibitor was present a current reversal took place,
which resulted in accelerated weld corrosion. Electrochemical impedance
spectroscopy (EIS) showed that the inhibitor film had lower electrical
resistance and was less protective on the weld metal than on the parent
material. At the highest shear stress, a second current reversal could occur
when the inhibitor was removed from all regions of the weld and there was a
return to the original galvanic behaviour. It was concluded that preferential
weld corrosion was caused by unstable conditions in which the inhibitor film was
selectively disrupted on the weld metal but remained effective on the other weld
regions
Effect of torch angle on arc properties and weld pool shape in stationary GTAW
No full textIn this paper, a three dimensional numerical simulation is performed on a stationary arc to study the effect of torch angle in gas tungsten arc welding (GTAW) of SS304 stainless steel. A comparison has been made to investigate 90o and 70o torch angles and analyze the effect on arc and weld pool shape. Current density, heat flux and gas shear stress are calculated in the arc region and are used as input to the workpiece to determine the weld pool. Buoyancy and Marangoni shear also affect the weld pool shape and are taken into account. The computed and experimental results are observed symmetric for 90o torch angle. For 70o torch angle, current density and hence the heat flux due to electron contribution is found the maximum behind and heat flux due to conduction and convection is found the maximum ahead of the electrode tip in the welding direction. This makes the maximum of total heat flux symmetric along the arc center. Heat flux due to conduction and convection decreases as the torch angle decreases resulting in a shallow weld pool. The nonsymmetric “w” shaped weld pool is developed by the combined effect of the gas shear and Marangoni convection. It is found that for 70o torch angle, the weld pool becomes non-symmetric, shallow and wide ahead of the electrode tip in the welding direction. The numerical weld pool shapes are verified through experiments
An evaluation of weld metal nitrogen retention and properties in 316NL austenitic stainless steel
No full textA series of tests were conducted using varying levels of nitrogen and helium in a conventional argon shielding gas when welding 316LN austenitic stainless steel. The outcome was that a 15 per cent nitrogen addition to the argon shielding gas had the most significant effect on increasing the weld metal nitrogen. Subsequent additions of helium to the argon 15 per cent nitrogen shielding gas had very little overall benefit. Increasing the nitrogen content of the weld metal had the consequential effects of decreasing the ferrite content and the hardness. As a result of solid solution strengthening, the yield strength increased with increase in nitrogen content. There was an increase in impact toughness as the nitrogen content increased. This was related to the decreased ferrite content associated with the strong austenetizing potential of nitrogen. It was also shown that an almost fully austenitic weld metal could still have very good toughness. In combination with these effects there was no loss in corrosion resistance. The addition of nitrogen to a conventional argon shielding gas presents attractive cost and quality benefits over the established requirement to over alloy the weld filler material with expensive alloys such as nickel
Arc pressure and weld metal fluid flow whilst using alternating shielding gases Part 2 : arc force determination
No full textThe transient variation of the shielding gas present in the alternating shielding gas process produces a dynamic action within the liquid weld metal. Flow vectors opposite in direction have been reported due to the various forces acting on the weld metal when argon and helium are present, however no data has been provided to substantiate this claim. This part of the study evaluates the various forces acting on the liquid weld metal when using argon and helium and their effects discussed. It was determined that argon produces a greater vertically downward force in the central region than helium for both the arc force and Lorentz force. While helium produces a greater radially outwards force at the pool surface than argon due to plasma shear stress and Marangoni convection. In addition, the buoyancy force, i.e. the vertically upward force in the central portion of the weld metal, was greater for helium
Flow accelerated preferential weld corrosion of X65 steel in brine
Get PDFPreferential weld corrosion (PWC) remains a major operational challenge that
jeopardizes the integrity of oil and gas production facilities. It is the selective
dissolution of metal associated with welds, such that the weld metal (WM) and / or the
adjacent heat-affected zone (HAZ) corrode rather than the parent metal (PM). Corrosion
inhibition is conventionally used to mitigate this problem however several indications
suggest that some corrosion inhibitors may increase PWC. Furthermore, it is not
possible to detect systems that are susceptible to PWC and or to understand the apparent
ineffectiveness of some corrosion inhibitors at high flow rates. Consequently, the aim of
this research is to assess the suitability of submerged jet impingement method to study
flow accelerated preferential weld corrosion, which is critical to safe and economic
operations of offshore oil and gas facilities.
In this research, a submerged jet-impingement flow loop was used to investigate
corrosion control of X65 steel weldment in flowing brine, saturated with carbon dioxide
at 1 bar, and containing a typical oilfield corrosion inhibitor. A novel jet-impingement
target was constructed from samples of parent material, heat affected zone and weld
metal, and subjected to flowing brine at velocities up to 10 ms-
1
, to give a range of
hydrodynamic conditions from stagnation to high turbulence. The galvanic currents
between the electrodes in each hydrodynamic zone were recorded using zero-resistance
ammeters and their self-corrosion rates were measured using the linear polarisation
technique. At low flow rates, the galvanic currents were small and in some cases the
weld metal and heat affected zone were partially protected by the sacrificial corrosion of
the parent material. However, at higher flow rates the galvanic currents increased but
some current reversals were observed, leading to accelerated corrosion of the weld
region. The most severe corrosion occurred when oxygen was deliberately admitted into
the flow loop to simulate typical oilfield conditions. The results are explained in terms
of the selective removal of the inhibitor film from different regions of the weldment at
high flow rates and the corrosion mechanism in the presence of oxygen is discussed
Effect of Microstructure on Mechanical Properties of High Strength Steel Weld Metals
Get PDFThe effects of variations in alloying content on the microstructure and mechanical properties of high strength steel weld metals have been studied. Based on neural network modelling, weld metals were produced using shielded metal arc welding with nickel at 7 or 9 wt. %, manganese at 2 or 0.5 wt. % while carbon was varied between 0.03 and 0.11 wt. %. From mechanical testing, it was confirmed that a large gain in impact toughness could be achieved by reducing the manganese content. Carbon additions were found to increase strength with only a minor loss to impact toughness as predicted by the modelling. The highest yield strength (912 MPa) in combination with good impact toughness (over 60 J at –100 oC) was achieved with an alloying content of 7 wt. % nickel, 0.5 wt. % manganese and 0.11 wt. % carbon.
Based on thermodynamic calculations and observed segregation behaviour it was concluded that the weld metals solidify as austenite. The microstructure was characterised using optical, transmission electron and high resolution scanning electron microscopy. At interdendritic regions mainly martensite was found. In dendrite core regions of the low carbon weld metals a mixture of upper bainite, lower bainite and a novel constituent—coalesced bainite—formed. Coalesced bainite was characterised by large bainitic ferrite grains with cementite precipitates and is believed to form when the bainite and martensite start temperatures are close to each other. Carbon additions were found to promote a more martensitic microstructure throughout the dendrites.
Mechanical properties could be rationalised in terms of microstructural constituents and a constitutional diagram was constructed summarising microstructure as a function of manganese and nickel contents
Enhanced Weld Penetrations In GTA Welding with Activating Fluxes Case studies: Plain Carbon & Stainless Steels, Titanium and Aluminum
Get PDFFlux applications prior to the convention Gas Tungsten Arc Welding (GTAW) is known to improve weld penetrations and improve process competitiveness. This paper summarizes the investigations on aluminum, plain carbon steels, stainless steels and titanium. The importance of flux composition, homogeneity and profile of its application are shown to be primordial in determining the weld depth to width ratio of weld pools. The mechanisms that lead to improved penetrations along with some industrial case studies are presented
Evaluation of the intrinsic crack growth rates of weld joints
Get PDFA method is presented for evaluating weld intrinsic fatigue crack growth rate
(FCGR) by excluding the influence of residual stresses. The method is
potentially useful for predicting crack growth lives of structural components
using measured FCGR data from coupon specimens. Calculation procedures are
developed and demonstrated via an example of crack growth across a longitudinal
weld subjected to both constant amplitude loading and constant applied stress
intensity factor ranges. Trends of intrinsic FCGR in different weld regions are
identified. The methodology can also be used for establishing intrinsic FCGR
laws for cracks propagating within and parallel with the weld joint
Derivation of forces acting on the liquid weld metal based on arc pressure measurements produced using alternating shielding gases in the GTAW process
No full textAs part of an ongoing process to fully evaluate the effects of an alternating shielding gas supply on the gas tungsten arc and gas metal arc welding processes, a comparison between arc pressures produced using argon, helium, alternating gases and GTAW-P has been conducted. The alternating shielding gas process is reported to create a dynamic stirring action within the liquid weld metal as a result of three independent phenomena: a) variation in weld pool fluidity, b) arc pressure variation, and c) arc pressure peaking. These effects have been the basis of previous advantages associated with the process, however these phenomena have not previously been verified and are based solely on theoretical assumptions. Arc pressure measurements are presented which allowed for the numerical derivation of various forces acting on the liquid weld metal in order to estimate the flow vectors present when each shielding gas is present
Hydrogen distribution and redistribution in the weld zone of constructional steels
No full textThis thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.The invention of electric arc welding revolutionized the steel construction industry, but also brought some problems when the welded region has inferior properties compared to the plate metal. A major cause of brittle failure was identi ed as hydrogen embrittlement of the weld zone, although a comprehensive understanding of this phenomenon is not, even now, available.
Hydrogen in solution in the weld zone is found in arc welds, due to the intense conditions in the welding arc. There is invariably a sufficient source in the form of moisture and hydrocarbon residue to give a few parts-per-million (ppm) by mass of hydrogen in the weld pool, which is a sufficient concentration to bring the possibility of hydrogen cold cracking in
the completed weld.
Hydrogen is significantly mobile in steels at room temperature, which is certainly why
a few ppm of hydrogen can concentrate on a microscopic scale and initiate cracks, but
also means that on a macroscopic scale there is hydrogen dispersion, which can relieve the
cracking risk or place hydrogen in hydrogen cracking susceptible regions. The understanding of solubility and mobility of hydrogen in steels of different compositions and microstructures is therefore paramount.
The question investigated in this work is whether the characteristics of the weld hydrogen cracking tendency can be explained by the features of weld hydrogen transport, especially when steel selection is a variable. Plate steel ranging from a 0.22%C pearlitic steel to a 0.05%C thermo-mechanically controlled-rolled and accelerated-cooled (TMCR-AC) high strength low alloy (HSLA) steel with no pearlite, plus a 0.4%C non-plate steel, were included
in the experimental program. Welds were made with rutile ux-cored-wire (R-FCW) at two
hydrogen levels, together with rutile shielded-metal-arc (R-SMA) welds.
In order to investigate the di usion rates, a novel experiment has been devised. The
welded plate has been milled away at an angle from the underside of the weld to provide increasing distances between the fusion boundary and the plate under-surface. The formation of hydrogen bubbles in glycerol enabled the measurements of the time dependent diffusion distances. The results clearly show a square root time correlation, as expected from the Fickian
mechanism and enabled the calculation of diffusion coefficients for different steels. A nearly four fold difference was found between the steels, with the fastest hydrogen movement in the TMCR-AC steel.
To reveal the initial distribution of hydrogen some samples were frozen immediately after welding and machined under liquid nitrogen. This test ruled-out any signi cant hydrogen dispersion during the deposition of the weld and during the cooling down period.
The experimental data were interpreted using a new numerical computer model, based on
random jumps of hydrogen between equivalent lattice sites. It is shown that this numerical model gives identical results to the analytical Fickian approach, but has the advantage that it can be used for any boundary shape.
When this model has been applied to the experimental data, some unexpected features
have been found. The amount of hydrogen emerging at surfaces distant to the weld was
higher than expected from a concentration-driven mechanism; suggesting that a di erent
transport mechanism should be applied. The numerical model has also indicated a discontinuity in the hydrogen concentration at the fusion boundary. It is a consequence of the model that hydrogen solubilities and di usivities are inversely related properties of the metal; a feature which is supported by experimental evidence. The tendency of hydrogen cracking to appear in the weld metal rather than in the heat-a ected-zone (HAZ) can thus be explained
by higher di usivity of hydrogen in the plate metal. It appears that there is a relationship between the diffusivity and the microstructure, particularly when the content and form of carbon is considered
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