Measurement of fracture toughness transition behaviour of Cr–Ni–Mo–V pressure vessel steel using pre-cracked Charpy specimens

The potential application of small pre-cracked Charpy specimens for the prediction of the fracture toughness of the 1T-thickness specimens and the reference temperature T₀ has been examined. Transition fracture behaviour of plane sided, side-grooved and 1T SENB specimens, respectively, was investigated over a wide temperature range. The fracture toughness regions with various fracture initiation mechanisms were defined and ductile to brittle transition temperatures denoted. The fracture toughness transition region of small pre-cracked specimens was shifted to lower temperatures as compared with that of 1T SENB specimens. The fracture toughness data of small pre-cracked specimens have been size corrected (weakest link) to 1T thickness and used to establish the reference temperature T₀ and K_Jc(mean) fracture toughness vs. temperature curve. The calculated temperature T₀ has been in consistence with that of the 1T SENB specimen. However, some corrected fracture toughness data lay outside the scatter band of 1T thickness specimens and the shape of the K_Jc(mean) curve has been quite different from the K_Jc(med)(1T) curve. It was found out that the original measured fracture toughness results of corrected data points lying outside the scatter band violated the validity condition b₀R_p0.2/J_c≥30. Bearing in mind the work of Koppenhoefer and Dodds Jr. (Engng Fract Mechanics (1997);58:249–270), and the most recent analysis of Ruggieri et al. (Engng Fract Mechanics (1998);60:19–36), the fracture data of small pre-cracked specimens having the validity parameter lower than 50 have been first constraint adjusted using the cleavage fracture toughness scaling model of Dodds and coworkers (J Testing Evaluation (1991);19:123–134; Int J Fracture (1995); 74:131–161; Engng Fract Mechanics (1997);58:249–270), and only then size corrected. The K_Jc(mean) curve of such treated data was identical with K_Jc(med)(1T).     

Characterisation of as cast model Mn–Si–Cr–Ni steels for reactor pressure vessel applications

Abstract

Initial results are reported from a study aimed to investigate the role and influence of the elements Cr, Ni, Mn and Si on the radiation stability of reactor pressure vessel steels. Twelve as cast model ferritic steels with basic composition typical of those used in Russian WWER-1000 and Western PWR reactor pressure vessel materials were subjected to Charpy impact, magnetic Barkhausen noise (MBN), Vickers hardness tests and SEM examination. Higher Cr content in model steels was found generally to give increased RMS values independent of Mn and Si contents. The ductile–brittle transition temperatures (DBTT) and hardness values of the model steels were found to be independent of composition. Two steels, with low concentration of Ni and high concentration of Cr or vice versa , showed high transition temperatures (?16 and ?42°C respectively). An additional heat treatment to improve the properties is being considered for these compositions. The correlation between DBTT and MBN results has potential for rapid determination of the effect of composition and irradiation on the steel properties. The next stage of the assessment will investigate the effect of irradiation of the model steels to accumulated neutron fluences of ～10¹⁹ cm^?2.     

Study on fracture strain of Cr–Mo steel in high pressure hydrogen

Cr–Mo steel is often used as the material of the hydrogen storage vessel, but its ductility can be deteriorated by high pressure hydrogen, which makes it possible that the local area of strain concentration on the hydrogen storage vessel made of Cr–Mo steel may fail due to excessive plastic deformation. The limit criterion of local strain established according to the study of the fracture strain is the basis for local failure assessment of the vessel. However, the correlation between the fracture strain and the stress state of Cr–Mo steel in high pressure hydrogen is still unclear, so the limit criterion of local strain for hydrogen storage vessel made of Cr–Mo steel has not been established. In this paper, the slow strain rate tensile test (SSRT) of notched specimens with different notch sizes was carried out in air, 45 MPa hydrogen and 100 MPa hydrogen, respectively. Based on the test results, the whole process from tensile to fracture of the specimens was simulated by finite element method. The distribution of stress triaxiality and plastic strain during the tensile process was analyzed, and the correlations between the stress triaxiality and the fracture strain in different environments were obtained. Finally, the limit criterion of local strain for local failure assessment of 4130X hydrogen storage vessel was established.     

Characterization of high strength and high toughness Ni–Mo–Cr low alloy steels for nuclear application

The reactor pressure vessels of PWRs have mostly been made of SA508 Grade 3 (Class 1) low alloy steels which have revealed moderate mechanical properties and a moderate radiation resistance for a 40 or 60 year operation. The specified minimum yield strength of the material is 345 MPa with a ductile–brittle transition temperature of about 0 °C. While other materials, most of which are non-ferrous alloys or high alloyed steels for a higher temperature application, are being developed for the Generation-4 reactors, low alloy steels with a higher strength and toughness can help to increase the safety and economy of the advanced PWR systems which will be launched in the near future. The ASME specification for SA508 Grade 4N provides a way to increase both the strength and toughness by a chemistry modification, especially by increasing the Ni and Cr contents. However, a higher strength steel has a deficiency due to a lack of operating data for nuclear power plants. In this study, experimental heats of SA508 Grade 4N steels with different chemical compositions were characterized mechanically. The preliminary results for an irradiation embrittlement and the HAZ properties are discussed in addition to their superior baseline properties.     

Issues in replacing Cr–Mo steels and stainless steels with 9Cr–1Mo–V steel

Issues are discussed that affect the development, installation, and operation of 9Cr–1Mo–V steel components in power boilers. Typical issues include chemistry effects, fabricability, weldability, fireside corrosion, steamside corrosion, aging, long-time creep, and damage accumulation.     

Studies of fracture processes in Cr–Mo–V ferritic steel with various types of microstructures

In this paper, the authors report on analysis of the influence of microstructure on ductile and cleavage fracture mechanisms. The question investigated was whether microstructure observations alone can provide sufficient information to predict the possible fracture mechanism or change in fracture mechanism. Four different microstructures of ferritic steel were tested after four different heat treatments. The microstructures examined were ferritic, ferritic–pearlitic, ferritic–bainitic, and tempered martensitic types. It was concluded that the ratio (S_C/S₀) of the area covered by carbides to the total area of a ferritic grain (measured by taking into account large carbides) is the only possible quantitative measure that can be used to predict cleavage fracture.     

Prediction of long-term creep rupture data for 18Cr–12Ni–Mo steel

The existing length of the materials development cycle is currently far too long. Validation of a new methodology to reduce this time requires evidence of its applicability to a wide range of materials. This study focuses on 18Cr–12Ni–Mo steel because of its importance to future power generation. The methodology is shown to accurately predict the strengths of numerous batches of this material obtained from different test facilities out to 100,000 h by analysis of short-term tests from just a single batch of this material. This may justify rapid prediction of 100,000 h strengths for newly-developed austenitic products, such as Save25, Sanicro 25 and BGA4, for which the absence of long-term creep design data now prevents their plant adoption.     

Hydrogen permeation in a Cr–Mo–V medium-carbon steel: Effect of the quenching medium and tempering temperature

Hydrogen permeation tests are carried out to evaluate the effect of the quenching medium and tempering temperature on the permeation parameters and density of hydrogen traps, of a Cr–Mo–V low-alloy medium-carbon steel. Three types of steel membranes are tested: 1) in the as-quenched condition, 2) tempered at 235 °C and 3) tempered at 530 °C; each one quenched in two different media: oil or brine. From the as-quenched condition, the apparent concentration of hydrogen and hydrogen flux, tend to decrease as the tempering temperature increases. The membranes in the as-quenched condition and tempered at 530 °C, show lower hydrogen diffusivity and higher density of hydrogen traps than membranes tempered at 235 °C. It is concluded that tempering at 235 °C, promotes hydrogen induced cracking, which is contrary to what has been previously determined. The cracking is related to a higher hydrogen diffusivity and lower density of hydrogen traps.     

Optimisation of post-weld heat treatment of a 1.25Cr–0.5Mo pressure vessel for high temperature hydrogen service

The ASME pressure vessel design code permits a range of post-weld heat treatment temperatures for 1.25Cr–0.5Mo pressure vessels. Studies using analytical transmission electron microscopy, X-ray diffraction, hardness and Charpy toughness testing, were performed on material from a new pressure vessel to understand the effects of various heat treatment schedules allowed within the code. These techniques have enabled an optimum heat treatment temperature and the time to be determined, with a view to minimising the likely susceptibility of the vessel to temper embrittlement and hydrogen attack.     

Formalisation of Wilshire–Scharning methodology to creep life prediction with application to 1Cr–1Mo–0·25V rotor steel

Abstract

Wilshire and Scharning have recently developed a new methodology that delivers accurate long term creep life predictions, and so offers the prospect of cost effective acquisition of long term creep design data. The present paper develops a procedure for formalising and estimating this Wilshire–Scharning model. Not only does this procedure provide the formalisation required for any acceptable international standard towards creep data acquisition, but it also allows for the construction of confidence limits around the Wilshire–Sharning predictions — something which until now has not been achievable. When applied to 1Cr–1Mo–0·25V steel, it was found that if this material was operated at a stress of 20 MPa, but a temperature of 873 K, there is only a 10% chance of failure at or before 157 702 h, and this could be stated with 95% certainty. The formalisation allows calculations to be carried out for any level of risk and for any material.     

High selective and efficient Fe2–N6 sites for CO2 electroreduction: A theoretical investigation

Developing high efficient and cheap electrocatalysts for carbon dioxide reduction reaction (CO₂RR) is the key to achieve CO₂ transformation into clean energy. Herein, a series of transition metal dimer and nitrogen codoped graphene (M₂N₆-Gra, M = Cr–Cu) acting as electrocatalysts for CO₂RR are investigated based on the density functional method. For M₂N₆-Gra (M = Cr, Mn), the selectivity is poor and CO poisoning is serious. Fe₂N₆-Gra is the best CO₂RR catalyst due to the good selectivity and catalytic activity. The calculated overpotential is very small, i.e., 0.03 V for COOH channel, 0.05 V for HCOO channel. Hydrogen evolution reaction is also refrained on the Fe₂N₆-Gra surface, which further supports its high catalytic performance. For M₂N₆-Gra (M = Co, Ni, Cu), the catalytic activity is poor due to large overpotentials. These results indicate that if designed carefully, the transition metal dimer and nitrogen codoped graphene would be good candidate for the high efficient and selective CO₂RR catalyst.     

Effect of Ni–Mo addition on cyclic and isothermal oxidation resistance and electrical behavior of ferritic stainless steel for SOFCs interconnect

Ni and Mo were added to ferritic stainless steel to improve the properties of the oxidation, electrical conductivity and Cr evaporation rate. The purpose of making this alloy was for application in solid oxide fuel cells (SOFCs) interconnect. Therefore, the cyclic and isothermal oxidation tests were performed and the area-specific resistance (ASR) and the Cr evaporation rate were investigated. The microstructure was discussed by scanning electron microscope (SEM). Glow discharge spectrometry (GDS) and X-ray were used for elemental and phase analysis, respectively. The addition of 3 wt% Mo reduced the oxidation rate and the Cr evaporation resistance by the formation of Laves-phase in oxide/metal interface. By reducing the activity of Fe in the steel, Mo was prevented outward diffusion of cations and anion diffusivity. Furthermore, Mo improved the electrical conductivity by filling the gaps and voids in the oxide/metal interface. The addition of 2 wt% Ni reduced the distribution of Laves-phase particles in substrate and the particles were moved to side oxide/metal interface and neighboring areas to it. Ni also improved the oxidation resistance and electrical conductivity in the steel.     

Identification of inelastic material parameters for modified 9Cr–1Mo steel applicable to the plastic and viscoplastic constitutive equations

In this paper, the material parameters of plastic and viscoplastic constitutive equations for modified 9Cr–1Mo steel are developed for various isothermal conditions to support inelastic analysis for a sodium-cooled fast reactor. To do this, the material parameters related with the elastoplastic behaviour are identified with uniaxial cyclic test data by performing computer simulations, which use the combined Chaboche model including the kinematic hardening rule and the isotropic softening rule. The viscous parameters are identified from uniaxial stress relaxation test data through computer simulations with the pre-determined elastoplastic material parameters. Sensitivity studies are performed for the material parameters to investigate cyclic inelastic behaviour and stress relaxation during a hold time. From the comparison between the tests and the simulations, it is expected that the identified material parameters of the plastic and viscoplastic constitutive equations can accurately express the material characteristics of modified 9Cr–1Mo steel sufficiently well to be used for inelastic analysis.     

Verification of 3-D parallel CDM software for the analysis of creep failure in the HAZ region of Cr–Mo–V crosswelds

The finite element-based Continuum Damage Mechanics (CDM) software DAMAGE XXX has been developed: to model high-temperature creep damage initiation, evolution and crack growth in 3-D engineering components; and, to run on parallel computer architectures. The driver has been to achieve computational speed through computer parallelism. The development and verification of the software have been carried out using uni-axial crosswelded testpieces in which the plane of symmetry of the V-weld preparation is orthogonal to the tensile loading axis. The welds were manufactured using 0.5Cr–0.5Mo–0.25V ferritic parent steel, and a matching 2.25Cr–1Mo ferritic steel weld filler metal. The Heat Affected Zones (HAZ) of welds were assumed to be divided into three sub-regions: Coarse grained-HAZ (CG-HAZ); Refined grained-HAZ (R-HAZ); and, the inter-critical HAZ regions (Type IV-HAZ). Constitutive equations and associated parameters are summarised for weld, CG-HAZ, R-HAZ, Type IV-HAZ, and parent materials, at 575, 590, and 600 °C. These are used to make finite element-based predictions of crossweld testpiece lifetimes and failure modes using the newly developed 3-D parallel computer software, and independent 2-D serial software, at an average minimum cross-section stress of 69.5 MPa. Crossweld testpiece analyses, done using the newly developed 3-D parallel software, have been verified using independent results of 2-D serial software; and, of laboratory experiments.     

Interaction between (La,Sr)MnO3 cathode and Ni–Mo–Cr metallic interconnect with suppressed chromium vaporization for solid oxide fuel cells

Interaction between (La_0.8Sr_0.2)_0.90MnO₃ (LSM) cathode and newly developed Ni–Mo–Cr metallic interconnect is investigated at 900 °C under operation conditions of solid oxide fuel cells (SOFCs). The results show that chromium deposition on the LSM cathodes in the presence of Ni–Mo–Cr interconnect is remarkably reduced as compared to that in the presence of a conventional Fe–Cr metallic interconnect (RA446). In contact with the Ni–Mo-Cr interconnect the overpotential, η, for the O₂ reduction reaction on LSM cathode decreased from 529 to 111 mV during the 1200 min current passage at 200 mA/cm². In contrast, η increased from 464 to 561 mV for the reaction in the presence of a RA446 interconnect. The decrease in η clearly indicates that chromium poisoning effect of the Ni–Mo–Cr interconnect is also significantly suppressed as compared to that with conventional Fe–Cr interconnect materials. The suppressed Cr deposition and poisoning effects observed on the LSM cathodes demonstrate promising potential of the Ni–Mo–Cr alloy as new interconnect materials with significant suppressed chromium vaporization and deposition for SOFCs.     

Use of metallic Ni for H2 production in S–I thermochemical cycle: Experimental and theoretical analysis

The gaseous hydrogen iodide decomposition is a thermodynamically limited reaction and subsequently a considerable energy expense for the separation and recirculation of the unreacted species is required. In addition the homogeneous gas phase decomposition of hydrogen iodide has a very low rate and the use of a catalytic system, which is generally highly expensive, is necessary. Hence, with the aim of overcoming the bottleneck represented by the hydrogen releasing step of the Sulphur–Iodine (S–I) cycle in terms of costs and process efficiency, in the present work an alternative version of the HI decomposition section (HI_x section) is proposed. In that alternative configuration the addition of metallic nickel into the heavy phase coming from Bunsen reaction is conceived in order to quantitatively obtain hydrogen at low temperature. A theoretical and experimental investigation has been performed, a new cycle has been conceived and the resulting energy demand assessed.     

Dry reforming of methane over Ni/MgO–Al2O3 catalysts: Thermodynamic equilibrium analysis and experimental application

In this study, dry reforming of methane (DRM) employing a Ni/MgO–Al₂O₃ catalyst was undertaken to evaluate the effects of temperature (650, 700 and 750 °C), weight hourly space velocity (7.5, 15 and 30 L h⁻¹ g_cat⁻¹) and catalyst MgO content (3, 5 and 10 wt%) on catalytic activity and coke-resistance. The catalysts were prepared by the wet impregnation method and were characterized by wavelength dispersive X-ray fluorescence (XRF), N₂ physisorption, X-ray diffraction (XRD), temperature-programmed reduction (TPR-H₂), temperature-programmed desorption (TPD-NH₃), H₂ chemisorption, thermogravimetric/derivative thermogravimetry analysis (TG/DTG) and scanning electron microscopy (SEM). The best conversions of methane (CH₄) and carbon dioxide (CO₂) and lower coke formation were obtained using higher temperatures, lower WHSV and 5 wt% MgO in the catalyst. The H₂/CO molar ratios obtained were within the expected range for the DRM reaction. The experimental yields of H₂ and CO differed from chemical equilibrium, mainly due to occurrence of the reverse water-gas shift reaction. Thermodynamic analysis of the reaction system, based on minimization of the Gibbs free energy, was performed in order to compare the experimental results with the optimal values for chemical equilibrium conditions, which has indicated that the DRM reaction was favored by higher temperature, lower pressure, and lower CH₄/CO₂ molar ratio.     

Chemical compatibility and electrical contact between Ni–Cr–Mo alloy and LaCo0.6Ni0.4O3−δ in intermediate temperature solid oxide fuel cells

The contact resistance and chemical compatibility of LaCo_0.6Ni_0.4O_3−δ (LCN) coated Ni–Mo–Cr alloy are investigated at 750 °C in air for more than 530 h to simulate the contact situation of cathode/contact layer/interconnect in SOFC stacks. With La_0.72Sr_0.18MnO₃ (LSM) as the cathode, the area specific resistance (ASR) of LSM/LCN/Ni–Mo–Cr alloy assembly increases to a maximum of 240 mΩ cm² during the early stage of the test, and decreases after 55 h to a steady level of ∼220 mΩ cm² till the end of the test. The contribution of formed oxide scale on the alloy to the measured ASR is negligible, compared to that of big AgLaMo₂O₈ particles sporadically distributed in LCN matrix. AgLaMo₂O₈ is formed of evaporated Mo from the alloy, Ag from the testing lead and La from the LCN before the oxide scale on the alloy is well developed. This reaction is expected to cease once the oxide scale is fully established.     

Ni–Mo nanoparticles stabilized by ether functionalized ionic polymer: A novel and efficient catalyst for hydrodeoxygenation of 4-methylanisole as a representative of lignin-derived pyrolysis bio-oils

In the present study, nickel-molybdenum nanoparticles stabilized with ether functionalized ionic polymer were synthesized and utilized as a novel and efficient catalyst for hydrodeoxygenation of 4-methylanisole as a representative of lignin-derived bio-oil. The catalytic upgrading process was performed in the presence of hydrogen with a batch reactor at temperature of 80–200 °C, hydrogen pressure of 10–50 bar, reaction time of 0.5–15 h and catalyst loading of 1–5 mol%. The major reaction classes during 4-methylanisole upgrading were hydrodeoxygenation and hydrogenolysis which resulted in production of 4-methylphenol, toluene, phenol and benzene as the main products. The experimental results indicated that the catalytic activity of Ni–Mo (20%–80%) nanoparticles stabilized with ionic polymer is superior to that with low Mo content. Also, it is observed that the selectivity of deoxygenated products including toluene and benzene improves with increasing the Mo content of the catalyst. Finally, regarding to the excellent catalytic activity of synthesized nanocatalyst during upgrading process of bio-oil at mild operating condition, ether functionalized ionic polymer was introduced as an applicable and effective stabilizers for nickel-molybdenum nanoparticles.     

Design of functionalized double-metal MXenes (M2M’C2T2: M = Cr,Mo, M’ = Ti,V) for magnetic and catalytic applications

Two dimensional (2D) materials have demonstrated huge potential in wide applications ranging from nanodevices to energy storage. In this work, we propose a series of double-metal MXenes functionalized with various terminal atoms (M₂M’C₂T₂), including B, N, O, P and S, based on density-functional theory (DFT) calculation. We screen out a series of stable structures and study their magnetic and electronic properties. We find that the magnetism of M₂M’C₂T₂ can be regulated according to different transition metals and terminal atoms. The magnetic moments of Cr₂TiC₂T₂ and Cr₂VC₂T₂ (T = N, O or S) are mainly contributed by chromium, while those of Mo₂VC₂T₂ (T = N, O or S) are originated from vanadium. We also find that these monolayers are metal with spontaneous conductivity, which is favorable for the electrocatalysis. The Gibbs free energies for the adsorption of hydrogen atoms on Cr₂TiC₂S₂, Cr₂VC₂S₂ and Mo₂TiC₂P₂ are close to zero, indicating their high catalytic activity for hydrogen evolution reaction (HER). Our findings suggest that the functionalized double-metal MXenes are promising materials for magnetic nanodevices and electrocatalysts.