Hydrogen effect on fracture toughness of pipeline steel welds, with in situ hydrogen charging

The API 5L X70 and X52 pipeline steel weld fracture toughness parameters are measured in a hydrogen environment and compared to the ones in air. The hydrogen environment is created by in situ hydrogen charging, using as an electrolyte a simulated soil solution, with three current densities, namely 1, 5 and 10 mA/cm². A specially designed electrolytic cell mounted onto a three-point bending arrangement is used and hydrogen charging is performed during the monotonic loading of the specimens. Ductility is measured in terms of the J₀ integral. In all cases a slight change in toughness was measured in terms of K_Q. Reduction of ductility in the base metal is observed, which increases with increasing current density. A more complex phenomenon is observed in the heat affected zone metal, where a small reduction in ductility is observed for the two current densities (1 and 5 mA/cm²) and a larger reduction for the third case (10 mA/cm²). Regarding microstructure of tested X70 and X52 base and HAZ metal, it is observed that the hydrogen degradation effect is enhanced in banded ferrite-pearlite formations. The aforementioned procedure is used for calculating the fracture toughness parameters of a through-thickness pipeline crack.     

耐热钢和低合金钢异种钢隔离层焊接接头性能研究

本论文分析了采用低合金钢焊材和隔离层焊接接头结构的SA-387Gr11CL2耐热钢和Q345R低合金钢的焊接接头的常温性能、微观组织和结构。经过焊接工艺试验证明,采用隔离层可以在低合金钢与耐热钢之间获得优良的焊缝,有效地解决低合钢和耐热钢之间的焊接难题,保证产品的焊接质量。     

Non-contact,nondestructive hydrogen and microstructural assessment of steel welds

Nondestructive low-frequency impedance has been developed to determine hydrogen content in operating pipeline steel and weldments through a structural coating. A low frequency impedance measurement is similar to a resistivity measurement with a depth function due to the sensor coil reactance. Resistivity introduces variability in impedance measurements because resistivity is a function of the conductivity of the material, the depth of the measurement, and the alloy content. The conductivity, based on the free electron model, is a function of the electronic effective mass, the electron concentration, and the dominating scattering mechanisms, which is altered by such factors as inclusions, microstructure, temperature, and strain. Each of these variables must be separated out to obtain a hydrogen content measurement in operating pipelines (with a structural coating) using low frequency impedance. Techniques used to separate out the variables associated with operating pipeline steels are presented. The use of real-time low frequency impedance measurements to monitor hydrogen content as it diffuses out of a steel weldment is presented and discussed.     

The effect of microstructure on fracture mechanisms of 2·25Cr1Mo low alloy steel, part B: the influence of carbides

The size and distribution of carbides in 2·25Cr1Mo steel after different heat treatments has been studied by undertaking detailed metallographic examination and hardness testing. As expected, prolonged exposure to elevated temperature resulted in significant coarsening of the metallic carbides present. The effect of these changes in carbide size on fracture behaviour was established by undertaking a programme of miniature disk testing. Analysis of these data demonstrated that ageing increased the tendency for brittle fracture. These effects are compatible with the concept that the critical stress for brittle fracture is inversely proportional to the size of the carbides present.     

Effect of nickel on hydrogen permeation in ferritic/pearlitic low alloy steels

Nickel offers several beneficial effects as an alloying element to low alloy steels. However, it is, in the oil and gas industry, limited by part 2 of the ISO 15156 standard to a maximum of 1 wt% due to sulfide stress cracking resistance concerns.Hydrogen uptake, diffusion, and trapping were investigated in research-grade ferritic/pearlitic low alloy steels with Ni contents of 0, 1, 2 and 3 wt% by the electrochemical permeation method as a function of temperature and hydrogen charging conditions.Qualitatively, the effective diffusion coefficient, D_eff, decreased with increasing Ni content. The sub-surface lattice hydrogen concentration, C₀, decreased with increasing Ni content in all charging conditions while the trend between the sub-surface hydrogen concentration in lattice and reversible trap sites, C_OR, and Ni content varied with the charging conditions. Irreversible trapping, evaluated by consecutive charging transients, was not observed for any of the materials. Lastly, the possible influence of an increasing fraction of pearlite with increasing Ni content is discussed.     

The effect of nickel content on the environmental assisted cracking (EAC) behaviour of low alloy steels in sour environments—A review

This particular review attempts to rationalise the effects of nickel level on the susceptibility to environmental assisted cracking (EAC) of low alloy steels. Historically, nickel has been viewed as a detrimental addition; indeed research studies indicated that the use of more than 1% Ni lowers the low alloy steel's resistance to both sulphide and hydrogen stress cracking. However, in view of nickel's beneficial effects on cryogenic tensile, fatigue, and fracture toughness properties there has been renewed interest in rigorously elucidating nickel's true role in environmentally assisted cracking processes. Recent studies tend to show that with the appropriate heat treatment low alloy nickel steels can have equivalent EAC resistance to nickel free steels. Nickel's effect on low alloy steels appears to be in reducing the Ac1 (thus producing untempered martensite when tempered at high temperatures), decreasing the lattice diffusion of hydrogen, enhancing formation of surface trenches, and reducing the overall corrosion rate.     

The effect of microstructure on fracture mechanisms of 2·25Cr1Mo low alloy steel, part A: the influence of non-metallic inclusions

The effect of variations in the size and distribution of non-metallic inclusions on the fracture behaviour of 2·25 Cr1Mo low alloy steel has been studied by undertaking a programme of disk bend testing at temperatures between 20°C and −196°C. Analysis of load/displacement data in combination with detailed fractographic evaluation indicates that inclusions greater than about 4 μm in diameter significantly increase the susceptibility for the formation of ductile voids. The ease with which ductile voids form then has a direct effect on the load and strain to failure. In contrast, inclusions do not appear to influence behaviour when fracture occurs by cleavage.     

Interfacial ensemble effect of copper nanoparticles and nickel metal-organic framework on promoting hydrogen generation

Ammonia borane is a promising chemical hydrogen storage material to the hydrogen economy in the future. However, efficient and inexpensive catalyst for realizing this process remains challenging. Herein, the interfaces of copper nanoparticles (NPs) and nickel metal-organic framework (MOF) with different Cu/Ni ratios (Cu@Ni_x-MOF) have been constructed by Cu NPs in situ growth in Ni-MOF via a facile one-step solvothermal method. The Cu@Ni_x-MOF composites show much higher catalytic activity than their monometal counterparts and the optimized Cu@Ni₆-MOF catalyst achieves a turnover frequency of 69.1 min^?1 for ammonia borane hydrolysis, superior to most reported Ni-based catalysts. Density functional theory simulation and in situ Raman spectroscopy analysis propose that this remarkable enhanced activity of Ni-based catalysts is induced by a synergetic effect of Cu–Ni²⁺ dual active sites at their interfaces. This study shows that the strategy of dual active sites hold great promise to achieve efficient non-noble metal catalysts toward heterogeneous catalysis.     

Hydrogen effect on a low carbon ferritic-bainitic pipeline steel

Hydrogen effect on an API 5L X65 low carbon ferritic-bainitic steel is investigated, by evaluating the fracture toughness parameters in air and in hydrogen environment. The hydrogen environment is manifested by in situ hydrogen charging of the X65 steel, using the electrolytic solution NS4, which simulates the electrolyte trapped between the pipeline steel and the coating in a buried pipeline. The fracture toughness results of the X65 are compared to two other pipeline steels with different microstructures, namely an X52 and an X70, possessing a banded ferritic-pearlitic and banded ferritic-mixed bainitic-pearlitic microstructure, respectively. The X65 steel exhibits significant reduction of fracture toughness parameter J₀ integral due to hydrogen charging and insignificant variation of fracture toughness parameter K_Q. Comparing the three steels, the lowest reduction of J₀ integral due to hydrogen charging, is met on the X52 and the highest in the X65.     

Threshold fatigue crack extension characteristics of a low alloy steel: The influence of environment and microstructure

The present investigation is aimed at examining the influence of environment and microstructure on the fatigue crack extension behaviour of a low alloy steel. Significant environmental effects were recorded under low R-Ratio testing but not under high R-Ratio conditions, viz., the ΔK_th values recorded in salt solution were significantly larger than those for a dry air environment and a constant value of 10 MPa√m was recorded irrespective of microstructure (yield strength). Such effects were attributed to oxide induced closure effects.

Above threshold, environmental assisted cracking behaviour in the salt solutions was the result of the appearance of transgranular cleavage facets on the fatigue surfaces. The extent of this environmentally enhanced cracking was simply related to the extent of cleavage failure.     

Influence of carbon content and tempering temperature on fatigue threshold characteristics of a low alloy steel

This paper attempts to describe the effect of carbon content on the fatigue threshold characteristics ΔK_th in various heat treated conditions. Essentially it has been shown that a tempering treatment increased ΔK_th while increasing the carbon content of steels from 0·13% to 0·8% significantly decreased the ΔK_th value by over 100%. At intermediate fatigue crack growth rates all the data show a linear relationship with ΔK level.

In terms of yield strength σ_y, the threshold stress intensity level could be given by the expression: ΔK_th = 8·74 − 3·42 × 10⁻³(σ_y).

At near threshold fatigue crack growth levels significant amounts of isolated intergranular failure were observed in the 400°C tempered condition. In the other heat treated microstructures only a flat trans-granular ductile striated failure mode was evident. A maxima in the amount of intergranular facets occurred at ΔK values approaching 15 to 20 MPa√m. It has been shown the existence of intergranular failure resulted from environmentally induced fracture (through the diffusion of hydrogen) that occurred within the crack tip enclave.     

Influence of hydrogen content on the tensile properties and fracture of austenitic stainless steel welds

This study evaluates the effects of internal hydrogen on the tensile properties and fracture behaviours of a tungsten inert gas (TIG) weld and an autogenous electron beam (EB) weld of a 304L steel tube. Tensile specimens were thermally charged with hydrogen gas to achieve three different levels of hydrogen in these materials. Metallographic examination revealed that the TIG weld contained skeletal and lathy ferrite, whilst the EB weld displayed a fine dispersion of skeletal and vermicular ferrite. Average volume fractions of ferrite in these welds were 8% and 1%, respectively. The tensile data showed that hydrogen increased the yield and tensile strength, and caused a significant loss in ductility, particularly for the TIG weld. Fractographic analysis revealed that hydrogen induced a change in the fracture mode of the welds and promoted cracking at or near ferrite–austenite interfaces. The TIG weld was found to exhibit a higher susceptibility to hydrogen embrittlement than that of the EB and base metal.     

Effect of electrotransport treatment on susceptibility of high-strength low alloy steel to hydrogen embrittlement

Electrotransport theory is defined as mass transportation of solute such as hydrogen in metal under the influence of an electrostatic force field. In this study, electrotransport treatment was applied to remove the accumulated hydrogen inside of the high-strength low alloy steel. The effectiveness of the electrotransport treatment was evaluated by hydrogen concentration measurement, slow strain rate test, and fracture surface analysis. The efficiency of electrotransport treatment is improved with increasing applied current and time, and the highest efficiency was obtained as 88.7% at 450 A for 40 min. The ultimate tensile strength and elongation of specimen after electrotransport treatment was enhanced dramatically in comparison with that of specimen under hydrogen charging condition. The brittle fracture mode was observed on the hydrogen charged specimen, but a clear ductile fracture mode was observed on the specimen after electrotransport treatment. These results confirm that the electrotransport treatment is effective to remove the accumulated hydrogen inside of the high-strength low alloy steel.     

Hydrogen compatibility of austenitic stainless steel tubing and orbital tube welds

Refueling infrastructure for use in gaseous hydrogen powered vehicles requires extensive manifolding for delivering the hydrogen from the stationary fuel storage at the refueling station to the vehicle as well as from the mobile storage on the vehicle to the fuel cell or combustion engine. Manifolds for gas handling often use welded construction (as opposed to compression fittings) to minimize gas leaks. Therefore, it is important to understand the effects of hydrogen on tubing and tubing welds. This paper provides a brief overview of on-going studies on the effects of hydrogen precharging on the tensile properties of austenitic stainless tubing and orbital tube welds.     

Fracture toughness of 15X2HMFA steel and its welds

The generalized experimental fracture toughness results of industrial semi-products, made from Cr---Ni---Mo---V steel and intended for VVER-1000 reactor pressure vessel manufacture, are presented. On the base of test results of nine steel heats having a common purity (sulfur to 0.018%; phosphorus to 0.015%; and copper to 0.20%) and of the same number of heats having a higher purity (sulfur to 0.012%; phosphorus to 0.012%; and copper to 0.06%) the fracture toughness temperature dependences were constructed. These differ appreciably from the reference curves for this material according to PNAE G-7-002-86. The temperature dependences K_1c for 15X2HMFA steel welds were also obtained. By this, for welds produced using manual arc welding and the types RT-45A and RT-45B electrodes as well as electroslag welding with Sv-16X2HMFTA wire and OF-6 flux, these dependences were constructed on the limited number of welded samples, as these welding methods are, practically, not used in the manufacture of such reactors. The fracture toughness assessment for welds produced by submerged arc welding was carried out for two variants of production processing (Sv-10XGNMAA wire and AH-17M flux; Sv-08XGHMTA wire and NF-18M flux), which were applied in the fabrication of circumferential welds for a VVER-1000 reactor.     

Hydrogen embrittlement behavior of high strength low carbon medium manganese steel under different heat treatments

Hydrogen embrittlement (HE) behavior was investigated in a low carbon medium Mn steel with three different volume fraction of retained austenite (RA), which was obtained after different heat treatments. The hydrogen permeation test showed a higher permeability for directly water quenched specimen compared to quench-tempered specimens. Melt extraction test showed hydrogen concentration increased with hydrogen charging current density in the order of directly quenched specimen, QLA, quenched with low-temperature annealed specimens and QHA quenched with high-temperature annealed specimens. Slow strain-rate tensile test was employed to examine the HE behavior, the HE indices decreased with the increase of RA irrespective of increased hydrogen concentration. HE susceptibility can be suppressed by raising intercritical annealing temperature because Mn enrichment increases the stability of RA.     

Effect of chromium,manganese and yttrium on microstructure and hydrogen storage properties of TiFe-based alloy

In this paper, we report the microstructure and hydrogen storage properties of TiFe-based alloys containing chromium (Cr), manganese (Mn) and yttrium (Y). Four alloy samples with chemical composition of TiFe_0.9Cr_0.1, TiFe_0.9Cr_0.1Y_0.05, TiFe_0.9Mn_0.1 and TiFe_0.9Mn_0.1Y_0.05 were prepared by arc melting, and the effects of alloying elements Cr, Mn and Y on microstructure and hydrogenation kinetics and thermodynamics were investigated in detail. It was found that all the four alloys have the main phase of TiFe intermetallic compound. A small amount of secondary phase can be also detected in the alloy samples. Cr substituted alloys have larger lattice parameters than that of Mn substituted alloys. Y in the alloys is mainly existed in the form of α-Y phase, and it transform into YH₃ during hydrogenation process. Very sloped equilibrium plateau regions are observed in Cr substituted alloys, while the Mn substituted alloys have flat equilibrium plateaus. Y addition has almost no influence on pressure–composition–isotherm (p–c–T) curves of Cr substituted alloy, but slightly decrease the equilibrium plateaus of Mn substituted alloys. Hydrogen absorption and desorption kinetics strongly depend on the equilibrium plateau pressures. As a result, the Cr substituted alloys with lower equilibrium plateau pressure have faster hydrogen absorption and slower desorption kinetics compared with Mn substituted alloys. The Cr substituted alloys have poor powdering resistance compared with Mn substituted alloys during hydrogenation cycles, which can be ascribed to the higher hardness of alloy matrix caused by Cr substitution.     

Localisation of plastic deformation in friction stir and electron beam copper welds

Optical strain measurement through digital image correlation during tensile testing was utilised to study the effect of strain rate and welding defects on localisation of plastic deformation in friction stir welded and electron beam welded phosphorous-alloyed oxygen-free copper. Welds cut from full-size nuclear waste copper canisters and additionally made defective welds were studied. Optical strain measurement represents a new way of studying deformation of materials and obtaining local material data. The data was used to construct strain maps and estimate local true stress–strain curves. Using Gini coefficients to quantify strain localisation, it appears that the lower strain rate resulted in slightly faster localisation of plastic deformation with lower stress level.     

INCONEL690镍基合金带极电渣堆焊工艺研究

通过试验研究,确定了SA-508Gr.3 CL.2管板采用INCONEL690镍基合金进行带极电渣堆焊的焊接工艺方案,各项试验数据均满足产品制造技术条件要求,说明INCONEL690镍基合金电渣堆焊工艺能满足第三代核电技术蒸汽发生器管板的堆焊要求.     

Weld repair of low alloy creep resistant steel castings without preheat and post-weld heat treatment

During recent years, a weld repair technique for low alloy CrMoV steel castings, using CrMoV weld metal, which dispenses with preheat and post-weld heat treatment, has been developed. This so-called ‘cold’ weld repair technique is economically attractive because of the potential reduction in down time that it offers. However, its adoption has been delayed because of fears of the risk of brittle fracture at start-up under the action of welding residual stresses and thermal stresses.The present report presents fracture toughness data for untempered CrMoV weld metal, deposited by CrMoV electrodes, so that the risk of fast fracture can be assessed. Additional data on creep ductility of the weld metal are presented and discussed with reference to the longer-term integrity of repairs.