The effect of age-hardening mechanism on hydrogen embrittlement in high-nitrogen steels

The effect of age-hardening regime on peculiarities of hydrogen-assisted fracture and tensile properties in two high-nitrogen Fe-23Cr-17Mn-0.1C-0.6N and Fe-19Cr-22Mn-1.5V-0.3C-0.9N steels was studied. A large number of intergranular (austenite/austenite) and interphase boundaries (austenite/coarse particle) provides high fraction of trapping sites for hydrogen atoms in V-alloyed steel. This leads to a change in fracture regime from transgranular brittle mode in coarse-grained V-free steel to intergranular brittle fracture of hydrogen-assisted surface layers in fine-grained V-alloyed steel with coarse (V,Cr)(N,C) particles. The formation of cells (Cr₂(N,C) particles and austenite) along the grain boundaries due to discontinuous precipitate-hardening reaction facilitates predominantly interphase hydrogen-assisted fracture for both steels. The complex reaction of the particle-strengthening mechanisms including discontinuous precipitation with formation of austenite/Cr₂(N,C)-plates interfaces or homogeneous nucleation of coherent (V,Cr)(N,C) particles in austenite (age-hardening regime 700 °C, 10 h) promotes mainly transgranular cleavage-like fracture mode under hydrogen-charging. The structural scheme is proposed to describe a change in hydrogen-assisted fracture micromechanisms and tensile properties of the steels with different density and distribution of interphase and intergranular boundaries.     

The influence of intergranular and interphase boundaries and δ-ferrite volume fraction on hydrogen embrittlement of high-nitrogen steel

We study the effect of grain size of austenitic and ferritic phases and volume fraction of δ-ferrite, which were obtained in different solution-treatment regimes (at 1050, 1100, 1150 and 1200 °C), on hydrogen embrittlement of high-nitrogen steel (HNS). The amount of dissolved hydrogen is similar for the specimens with different densities of interphase (γ-austenite/δ-ferrite) and intergranular (γ-austenite/γ-austenite, δ-ferrite/δ-ferrite) boundaries. Despite, the susceptibility of the specimens to hydrogen embrittlement, depth of the hydrogen-assisted surface layers, hydrogen transport during tensile tests and mechanisms of the hydrogen-induced brittle fracture all depend on grain size and ferrite content. The highest hydrogen embrittlement index I_H = 32%, the widest hydrogen-affected layer and a pronounced solid-solution hardening by hydrogen atoms is typical of the specimens with the lowest fraction of the boundaries. Even though fast hydrogen transport via coarse ferritic grains provides longer diffusion paths during H-changing, the width of the H-affected surface layer in the dual-phase structure of the HNS specimens is mainly determined by the hydrogen diffusivity in austenite. In tension, hydrogen transport with dislocations increases with the decrease in density of boundaries due to the longer dislocation free path, but stress-assisted diffusion transport does not depend on grain size and ferrite fraction. The contribution from intergranular fracture increases with an increase in the density of intergranular and interphase boundaries.     

Thresholds for failure of high-burnup LWR fuels by Pellet Cladding mechanical interaction under reactivity-initiated accident conditions

ABSTRACT

To contribute to the future updating on the Japanese safety criteria for pellet/cladding mechanical interaction (PCMI) failure of light water reactor fuels under reactivity-initiated accident (RIA) conditions, this paper summarizes the recent important outcomes from research programs with the Nuclear Safety Research Reactor (NSRR). Applicability of current criteria, which are defined as a function of fuel burnup and possibility of introducing another parameter for new criteria were evaluated based on the results of the RIA-simulated pulse irradiation tests, post-test examinations, and supporting analytical work, such as the reevaluation of fuel enthalpies in earlier NSRR experiments. Failure-threshold curves based on cladding hydrogen content as a primary measure of fuel degradation have been proposed as a possible alternative that can be used to judge the occurrence of PCMI failure to ensure conservativeness in a more pertinent manner.     

The effect of Acetobacter sp. and a sulfate-reducing bacterial consortium from ethanol fuel environments on fatigue crack propagation in pipeline and storage tank steels

This paper evaluates the effects of microbiologically influenced corrosion (MIC) on fatigue-crack growth of candidate materials useful in expanding bio-ethanol usage, including a storage-tank steel (ASTM A36) and two pipeline steels (API 5L X52 and X70). The microbiological species sampled and cultivated from an ethanol fuel production stream are responsible for both acetic acid and hydrogen sulfide production that lead to significant increases in fatigue-crack growth rate across a wide range of stress-intensity-factor amplitudes (ΔK). The mechanism for increased fatigue damage is hydrogen uptake through adsorption into the steel, which embrittles material ahead of the growing fatigue crack.     

Hydrogenation method based on electrodeposited layers controlling the hydrogen desorption rate

An innovative hydrogenation method to investigate the hydrogen embrittlement of metals and alloys is hereby presented. The benefits of electroplating samples with copper and nickel prior to gaseous hydrogenation at mid-range temperatures are quantified. It is showed that these electrodeposited layers allow to control the hydrogen desorption rate occurring after hydrogenation, during the cooling of the hydrogenated specimen. The present study demonstrates the capability of the method to control the introduced total hydrogen concentration within a margin of 0.2 wt.ppm. The applicability of the described method to further investigations into hydrogen concentrations effects on hydrogen embrittlement of ferritic alloys by the means of mechanical tests is evaluated.     

Evaluation of stress corrosion cracking and hydrogen embrittlement in an API grade steel

Stress corrosion cracking (SCC) and hydrogen embrittlement (HE) of pipeline steels in contact with soil was investigated. Different soils were prepared in order to determine their physical, chemical and bacteriological characteristics. Slow strain rate testing was carried out by using aqueous extracts from soil samples and NS4 standard solution. Stress vs. strain curves of API 5L grade X46 steel were obtained at different electrode potentials (E_corr, 100 mV below E_corr and 300 mV below E_corr) with 9 × 10⁻⁶ s⁻¹ and 9 × 10⁻⁷ s⁻¹ strain rate. In addition, the hydrogen permeation tests were carried out in order to evaluate the susceptibility of hydrogen penetrates into theses steels. The results demonstrated the incidence of cracking and their dependence on the potential imposed. In that case, cracking occurred by stress corrosion cracking (SCC) and the hydrogen embrittlement (HE) had an important contribution to cracking initiation and propagation. Cracking morphology was similar to the SCC reported on field condition where transgranular cracking were detected in a pipeline collapsed by land creeping. It was important to point out that even under cathodic potentials the material showed the incidence of secondary cracking and a significant reduction of ductility.     

Electrochemical hydrogen discharge of high-strength low alloy steel for high-pressure gaseous hydrogen storage tank: Effect of discharging temperature

Hydrogen discharge technique of high-strength low alloy steel for high-pressure gaseous hydrogen storage tank was developed by using an electrochemical technique. The electrochemical hydrogen discharge of high-strength low alloy steel were investigated in a deaerated borate buffer solution (0.3 M H₃BO₃ + 0.074 M N₂B₄O₇, pH = 8.4). By applying a potential of +630 mV_SCE which is higher than the hydrogen equilibrium potentials and lower than the pitting potential, the oxidation reaction of metal (Fe → Fe²⁺ + 2e⁻) is limited and oxidation reaction of the hydrogen (H₂ + 2OH⁻ → 2H₂O + 2e⁻) was induced simultaneously. Thus, the pre-charged hydrogen inside the specimen was eliminated effectively without any damage to the specimen. The electrochemical hydrogen discharge method was performed at 25 °C, 50 °C and 75 °C. The efficiency of hydrogen discharge was accelerated with increasing temperature because the exchange current density of hydrogen is increased with temperature.     

压力容器用钢辐照脆化评估方法比较

虽然经过了30多年的努力,在脆性转变温度区域预测压力容器用钢的断裂韧性仍然存在较大的不确定性.通过比较现有的评价材料断裂韧性的方法,对近些年发展起来的特征曲线法的理论基础提出质疑,指出传统的参照脆性转变温度的断裂韧性曲线法是建立在较坚实的物理基础之上,虽然常被认为过于保守,却是可靠的.通过对单相组织和多相组织断裂韧性试验结果的分析,强调在理论和实践上解决解理断裂韧性不确定性的出路在于对解理断裂物理过程的认识和评价方法的改进.     

电沉积Zn-Fe合金的渗氢行为研究

用双电解池测氢法研究氯化物镀液体系中电沉积Zn-Fe合金过程中的渗氢行为.结果表明:在所考察的工艺参数中,镀液pH值对渗氢电流的影响最大.用慢应变速率拉伸试验法(SSRT)考察了电镀Zn-Fe合金试样的氢脆特性,并用扫描电子显微镜(SEM)对断口的微观形貌进行了表征.所有Zn-Fe合金镀层的试样均显示出不同程度的氢脆敏感性.