Standard-compliant measurement of the in-plane fracture toughness of metals (i.e., in the SL or ST directions per ASTM standard denominations) is often impossible due to insufficient material in the through-thickness direction to extract a full Single Edge Bending (SEB) or Compact Tension (CT) fracture specimen. In the present work, we propose a new specimen design methodology to overcome this challenge. A W-shaped SEB specimen (called W-SEB) was developed in this study and its topology optimized using finite element simulations. The new specimen design was validated numerically and experimentally on a case study and results showed excellent agreement with stand-ard ASTM E1820 full SEB specimen measurements. In view assessing the anisotropy of the fracture toughness (K Q and CTOD) of pipeline steels susceptible to hydrogen in-duced cracking (HIC), the W-SEB specimen was used on X65 and X42 pipeline steel samples taken from the field. Experimental results show an increase in the maximum CTOD in the SL direction as compared to the LT direction for both steel grades. Such experimental results could lead to important considerations with respect to accurate fit-ness for service assessment of HIC damaged assets.

This paper presents a combined experimental and theoretical analysis focusing on the individual roles of microdeformation mechanisms that are simultaneously active during the deformation of twinning-induced plasticity (TWIP) steels in the presence of hydrogen. Deformation responses of hydrogen-free and hydrogen-charged TWIP steels were examined with the aid of thorough electron microscopy. Specifically, hydrogen charging promoted twinning over slip–twin interactions and reduced ductility. Based on the experimental findings, a mechanism-based microscale fracture model was proposed, and incorporated into a visco-plastic self-consistent (VPSC) model to account for the stress–strain response in the presence of hydrogen. In addition, slip-twin and slip–grain boundary interactions in TWIP steels were also incorporated into VPSC, in order to capture the deformation response of the material in the presence of hydrogen. The simulation results not only verify the success of the proposed hydrogen embrittlement (HE) mechanism for TWIP steels, but also open a venue for the utility of these superior materials in the presence of hydrogen.

This paper presents a study of hydrogen diffusion for a spiral weld pipe considering the effect of weld residual stress. The results show that the hydrogen mainly gathers at heat-affected zone (HAZ). HAZ is the weakest zone where hydrogen-induced cracking (HIC) occurs. The effect of helix angle on the hydrogen diffusion is also discussed. It shows that different helix angles generate different hydrogen concentrations. As the helix angle increases, both the hydrogen concentration and residual stresses decrease. As the helix angle increases from 40 deg to 50 deg, the equivalent pressure stresses reduce a little, resulting in the change of hydrogen concentration being small. The smaller the helix angle is, the larger the diffusion rate is. The most suitable helix angle should be optimized at 40–50 deg.

In a “very high-temperature reactor” (VHTR), the Nb–V-modified 9Cr–1Mo creep strength enhance the ferritic (CSEF) steel which is the chosen material for fabrication of reactor pressure vessels and piping because of its excellent high temperature thermal and mechanical properties. In such CSEF steel weldments, the hydrogen-induced cracking (HIC) is a critical issue. In the present work, the different levels of hydrogen have been induced in P91 CSEF weld metal to study their effect on HIC. The HIC susceptibility of P91 steel welds has been studied by carrying out the tensile test and flexural test for the different level of diffusible hydrogen. The hydrogen levels in deposited metals have been measured by using the mercury method. The fracture tensile and flexural test samples have been characterized based on the field-emission scanning electron microscope (FE-SEM). It is concluded that higher the level of diffusible hydrogen in deposited metal, more is the susceptibility of P91 steel to HIC. The minimum flexural and tensile strength are 507.45 MPa and 282 MPa, respectively, for 12.54 ml volume of diffusible hydrogen in 100 g of deposited weld metal.

This study investigated the hydrogen embrittlement (HE) cracking behavior produced by local contact loading of high-strength steel. When a spherical impression was applied to a hydrogen-absorbed high-strength steel, HE induces contact fracture, where radial cracks are initiated and propagated from the indentation impression. The length of the radial crack was found to be dependent on the hydrogen content in the steel as well as the applied contact force. A combined experimental/computational investigation was conducted in order to clarify the mechanism of hydrogen-induced contact fracture. In the computation, crack propagation was simulated using a cohesive zone model (CZM) in finite element method (FEM), in order to elucidate stress criterion of the present HE crack. It was found that the normal tensile stress was developed around impression, and it initiated and propagated the HE crack. It was also revealed that the hydrogen content enhanced contact fracture damage, especially the resistance of crack propagation (i.e., threshold stress intensity factor, Kth). The findings may be useful for countermeasure of contact fracture coupled with hydrogen in high-strength steel. Such phenomenon is potentially experienced in various contact components in hydrogen environment.

The internal friction Q − 1 and the Young’s modulus E of NiTi based alloys have been measured as a function of temperature after various thermomechanical and hydrogen-doping treatments given to the materials. Hydrogen is found to play a major role introducing tall damping peaks associated with Snoek-type and H-twin boundary relaxations. Levels of Q − 1 as high as 0.08 have been detected, which are among the highest to date measured in metal alloy systems. For appropriate alloy compositions, these peaks occur at around room temperature (for acoustical frequencies), thus providing a good opportunity to reduce machinery vibrations and noise pollution. In the paper, the conditions are highlighted under which maximum efficiency can be reached in the conversion of mechanical energy into heat.

Experimental evidence indicates that nickel-base alloys fail in the presence of hydrogen by ductile intergranular fracture. The degradation mechanism involves void nucleation at grain boundary carbides and grain boundary decohesion. In this study, a micromechanical model is suggested to understand the interaction of void nucleation and growth with the failure of the grain boundaries. The analysis is carried out at a unit cell comprising an elastic particle imbedded in a ductile matrix, a grain boundary along a plane of symmetry of the cell, and loaded in plane strain perpendicularly to the grain boundary. A phenomenological model for hydrogen-induced decohesion calibrated at the fast-separation limit of the decohesion theory of Rice [1], Hirth and Rice [2], and Rice and Wang [3] was used to describe the hydrogen effect on the cohesive properties of the particle/matrix interface and grain boundary. The finite element results indicate that hydrogen embrittlement of the alloy 690 is controlled by hydrogen assisted void nucleation at the carbides. The effect of hydrogen on grain boundary cohesion is almost negligible. The grain boundary decohesion, which proceeds almost instantaneously upon initiation, is caused by normal stress elevation due to the interaction of the void with the applied load. Lastly evaluative statements are made on the quantitative effect of hydrogen on the fracture toughness of the alloy 690.

The paper reports and discusses the results of an experimental and numerical activity, aimed to the characterization of the influence of hydrogen on the mechanical properties of a few high chromium martensitic steels which are candidates for fusion reactor and chemical applications. Experiments were conducted with the Disk Pressure Test technique, according to which a circular thin specimen is loaded up to rupture by an uniform pressure. As a detailed analysis of the stress/strain distributions in the specimen was not available, this kind of test being mainly used to obtain comparative information about different materials, a nonlinear numerical (Finite Element) model of the specimen was set up, by which stress and strain could be accurately evaluated as a function of the applied pressure. This model was employed both for interpreting experimental results and to achieve a more general understanding of the capabilities of the Disk Pressure Test for the characterization of hydrogen embrittlement effects. The calculated strain at failure showed the typical dependence on hydrogen content, falling to very low levels as a threshold concentration is exceeded.

The influence of hydrogen embrittlement on the tensile failure of 316L stainless-steel notched bars is phenomenologically modeled in this paper. Tensile tests of notched samples suffering hydrogen embrittlement show that hydrogen damage consists in multicracking in the area surrounding the notch, but the macromechanical behavior of the specimens remains ductile. This suggests two different ways for modeling the damage in order to explain its effect on the tensile failure load. The Notch Extension Model (NEM) considers that damage intensity around the notch is high enough to cancel out the mechanical resistance of this multicracked zone, so it assumes that the hydrogen effect is equivalent to a geometric enlargement of the notch. In the Notch Cracking Model (NCM), it is assumed that high intensity damage is concentrated at the notch root and causes this area to behave as a macroscopic crack that extends the original notch. Experimental values from tests and calculated values from models indicate that the notch extension model describes well the influence of hydrogen on the tensile notch behavior of 316 L stainless steel.

Repeated heat-quench cycles of Al-1100 samples resulted in increased surface roughness and corresponding shifts in the temperature-time cooling curve towards shorter overall quench periods. Three different types of initial surface roughness were applied to the test samples: polished, particle blasted, and milled finishes. For each of the three test surfaces, cooling curve shifts during repeated heat-quench cycles were accompanied by surface roughening, the shift was smallest with the milled sample. The surface roughness was examined with the aid of scanning electron microscopy, surface contact profilometry, and X-ray photoelectron spectroscopy. Surface profiles obtained via the profilometer revealed, on a relative basis, significant changes in surface roughness on the polished and particle blasted surfaces but not on the milled (roughest) surface. The roughening was the result of (a) hydrogen diffusion associated with oxidation, (b) oxidation buildup, and, to a lesser extent, (c) expulsion of impurities along dendrite boundaries. The hydrogen diffusion caused localized pressure buildup within the surface and along grain boundaries resulting in the formation of both microscopic (1 to 10 μm) features on the polished and particle blasted surfaces and relatively large (20 to 1000 μm) bumps and blisters on the particle blasted surface. It is shown how these wide spectrum surface roughness features affect cooling rate by (a) raising the Leidenfrost temperature separating the film and transition boiling regimes, (b) increasing the number of boiling sites on the quenched surface, and (c) altering the impact dynamics of the spray drops.

A study of fatigue behavior has been carried out using a set of commercial surfacing chromium steels in the state of deposits postheated at 350°C. The set consisted of various filler metals with different contents of chromium, nickel, carbon, and carbide-forming elements. It has been found that all the deposits examined are characterized by very close values of their fatigue crack growth threshold, ΔK th . In each material, as the stress intensity factor range is varied, three different propagation kinetics are observed, probably triggered by the presence of trapped hydrogen. Fracture toughness and, even though less strictly, subcritical growth by fatigue depended on the vanadium or columbium contents of each material, rather than on the other alloying elements. By adding vanadium or columbium and increasing the columbium content, one obtains a sharp decrease in plane-strain fracture toughness. No interdependence has been found between the values of the fracture toughness and the impact toughness of each material. This apparent discrepancy between the two kinds of toughness can be explained by using a distance criterion for failure at the tips of sharp cracks.

Fatigue crack propagation has been investigated in a NiCrMoV structural steel in air or in electrolytic hydrogen charging environments. The behavior of this steel containing internal trapped hydrogen absorbed during the steelmaking processes was also considered. Hydrogen, both internal and adsorbed by the environment, causes accelerated crack growth over the entire stress-intensity factor range. As the loading conditions are varied, two different damage mechanisms, triggered by hydrogen, are observed, and are separated by a transition zone where the fatigue crack growth rate is constant. The results of the fatigue tests and of a fractographic analysis suggest that the phenomenon is controlled by the stress distribution at the crack tip, and that a transition occurs when the cyclic plastic zone size at the crack tip is larger than the prior austenite grain size.

Slow strain rate tensile tests were performed on annealed and cold drawn Monel 400 and Monel R405 at room temperature in air, mercury, and electrolyte hydrogen. Hydrogen and mercury caused embrittlement with the fractures having the same specific features. Crack initiation was largely intergranular but an increasing proportion of transgranular cracking occurred subsequently, especially in the presence of hydrogen and for Monel R405. It is believed that the decreased cohesive strength and enhanced shear models of embrittlement apply to the intergranular and transgranular crack modes respectively.

Intergranular Stress Corrosion Cracking (IGSCC) in Type 304 stainless steel in high temperature high purity water requires the simultaneous presence of sensitized material, high tensile stress and oxygen. Laboratory and in-reactor stress corrosion tests have shown the benefits of adding hydrogen to the boiling water reactor feed-water to reduce the dissolved oxygen concentration and thereby reduce the chemical driving force for IGSCC. The purpose of this program was to verify the benefit of hydrogen additions on the stress corrosion crack behavior. The program investigated the fatigue and constant load crack growth behavior using fracture mechanics specimens in hydrogen water chemistry (HWC). Isothermal heat treatments were used to sensitize Type 304 stainless steel. Additionally, full size pipe tests containing actual welds were used to evaluate crack initiation, as well as propagation of cracks, to verify the results of the fracture mechanics tests. These pipe tests were performed under a trapezoidal loading cycle. The results of the small specimen tests show that HWC inhibits IGSCC in Type 304 stainless steel. The effects of cyclic loading at slow frequencies which promote IGSCC were also evaluated. Computer aided methods were used in the collection and interpretation of the high temperature crack growth data. The full-size component tests substantiated the benefit of HWC in both crack initiation and growth. All results presented were compared to baseline test data to put the results into perspective.

Dual phase steels are used in many applications for which electroplating is required. Hydrogen absorption and consequent loss of strength and ductility is therefore possible. Tensile tests and subsequent fractographic analysis of charged and uncharged specimens of a dual phase steel showed that hydrogen absorption resulted in deterioration of tensile properties. The deterioration was attributed to weakening of interfaces between ferrite and other constituents.

The creep-rupture behavior of six candidate Stirling engine iron-base superalloys was determined in air. The alloys included four wrought alloys (A-286, Alloy 800H, N-155, and 19-9DL) and two cast alloys (CRM-6D and XF-818). The specimens were tested to rupture for times up to 3000 h at 650° to 925°C. Rupture life (t r ), minimum creep rate (ε˙ m ), and time to 1 percent creep strain (t 0.01 ), were statistically analyzed as a function of stress and temperature. Estimated stress levels at different temperatures to obtain 3500 h t r and t 0.01 lives were determined. These data will be compared with similar data being obtained under 15 MPa hydrogen.

A new high strength, nonmagnetic, iron-base alloy has been developed for large retaining ring application. This alloy is based on the development efforts recently completed in a previous collaboration between EPRI and the University of California, but takes advantage of the fact that tantalum, whose price is subject to fluctuations, can be successfully replaced partially or fully by niobium. Among the niobium modified versions evaluated, the alloy containing 1Nb + 1Ta provided the optimum combination of desired properties. This alloy, which has the approximate composition Fe-34.5Ni-5Cr-3Ti-1Nb-1Ta-0.5A1-1.0Mo-0.3V-0.01B, has demonstrated the capability to reach yield strength levels of around 200 ksi (1379 MPa), following controlled hot working, modest cold expansion and a double aging heat treatment. Compared to the currently used 18Mn-5Cr alloy, the new alloy exhibited superior stress corrosion resistance (in humid air and 3.5 percent NaCl) and comparable toughness in air and dry hydrogen. Physical properties such as density, magnetic permeability, thermal conductivity and coefficient of thermal expansion were found to be compatible with the requirements for retaining ring application. It is anticipated that successful achievement of similar properties in large-scale heats and subsequent commercialization can result in improved generator reliability, size capability and efficiency. This paper is a progress report on this development.

A hydrogen-induced fracture process in the HAZ of a QT-steel weldment is studied numerically. Two-dimensional models are used to estimate the transient distributions of stress and hydrogen during the cooling and during the subsequent crack propagation. The distributions of hydrogen in front of a blunted crack are studied for different void volumes in the fracture zone corresponding to brittle and ductile fracture mechanisms. It is tentatively concluded that the incubation time at slow crack growth is not controlled by the hydrostatic stress. A ductile fracture mechanism leading to a relative void volume of 0.02-0.05 in the fracture zone is found to imply long incubation times resulting in a low average crack propagation speed.

An automated, computer-controlled K-decreasing technique was used to determine the threshold, ΔK th , and low-rate fatigue crack growth of a NiMoV rotor steel. A more conventional K-increasing technique was also used. Excellent agreement between results obtained from both techniques was observed. For the material and environment studied, no crack arrest was observed for crack growth rate down to 2.5 × 10 −8 mm/cycle (10 −9 in./cycle). As such, an operational definition of ΔK th was defined as the stress intensity factor range corresponding to a crack growth rate of 2.5 × 10 −8 mm/cycle (10 −9 in./cycle). In room temperature air environment, ΔK th was found to be 6.2 and 4.0 MPa m (5.6 and 3.6 ksi in. ) for R = 0.1 and R = 0.5, respectively. At the same ΔK level, crack growth rate was found to increase with increasing stress ratio. The influence of stress ratio on crack growth rate, however, decreases with increasing ΔK. By raising temperature to 93° C (200°F), ΔK th was found to be suppressed to 4.4 and 2.9 MPa m (4.0 and 2.6 ksi in. ) for R = 0.1 and R = 0.5, respectively. Stress ratio effect on crack growth rate is the same as at room temperature, but is less significant. Temperature was found to influence crack growth rate in the threshold region for both stress ratio studied, with higher crack growth rate at 93°C (200°F) than at room temperature. Temperature sensitivity was found to be less for R = 0.5 than R = 0.1. The existence of hydrogen was found to have little effect on ΔK th and low-rate fatigue crack growth behavior of this NiMoV rotor steel.

As part of an ongoing program to examine subcritical flaw growth in candidate steels for proposed coal gasifier pressure vessels, an initial study is made of characteristics of ultralow growth rate fatigue crack propagation in thick-section, normalized 2 1/4 Cr-1Mo pressure vessel steel (ASTM A387, Class 2 Grade 22). Crack propagation data are generated over a wide range of growth rates, from 10 −8 to 10 −2 mm/cycle, for load ratios between 0.05 and 0.80 at ambient temperatures in low pressure environments of moist air, dry hydrogen gas and dry argon gas. Particular emphasis is placed on behavior at near-threshold growth rates, below 10 −6 mm/cycle, approaching the so-called threshold stress intensity for fatigue crack growth, ΔK 0 . Near-threshold growth rates, in addition to showing a marked sensitivity to load ratio, are found to be significantly enhanced in gaseous hydrogen compared to air. Similar environmentally-enhanced growth is observed in argon gas. To account for such results, previous models of threshold behavior based on environmental factors (e.g., hydrogen embrittlement) are questioned, and a new approach is presented in terms of the role of oxide debris from moist environments in promoting crack closure. This oxide-induced closure model is found to be consistent with most experimental observations of near-threshold fatigue crack propagation behavior and is proposed as a mechanism for environmental effects at ultra-low growth rates.