Shelf Energy Research Articles

Algorithms to Estimate the Ductile to Brittle Transition Temperature, Upper Shelf Energy, and Their Uncertainties for Steel Using Charpy V-Notch Shear Area and Absorbed Energy Data

Toughness and the transition from ductile to brittle behavior are long-standing concerns for applications of ferritic steel such as line-pipe. Three algorithms have been developed to fit a hyperbolic tangent curve to any Charpy V-notch dataset and estimate the uncertainty for (1) the 85% shear appearance area transition temperature and (2) the upper shelf absorbed energy. To fit the hyperbolic tangent curve to the data the (I) first algorithm relied on iterative estimation of four-parameters; (II) the second algorithm on two parameters (after simplification based on physical assumptions); and (III) the third algorithm on only one parameter (after further simplification). The algorithms were written using the open-source programing language, R. The minimum input requirements for the algorithm are experimental data for shear appearance area and absorbed energy from at least four temperatures for the four-parameter algorithm, two temperatures for the two-parameter algorithm, and one temperature for the one-parameter algorithm. The test temperatures and quantity of tests at each temperature can vary. The algorithms are described in detail and demonstrated using a data set of 12 Charpy test results (shear area and absorbed energy) from one API-5L grade X52 pipe with 4.5 mm thick Charpy bars. A future paper will test and compare the algorithms using a wide variety of Charpy V-notch data sets to clarify their applicability and possible limitations.

Effect of Hot-Rolling on the Microstructure and Impact Toughness of an Advanced 9%Cr Steel

A 9%Cr martensitic steel with Ta and B additions was subjected to thermo-mechanical treatment (TMT) including rolling in the range of metastable austenite at 900–700 °C followed by water quenching and tempering at different temperatures. Applied TMT with tempering at T ≥ 700 °C substantially improved the impact toughness. The application of the TMT with subsequent tempering at 780 °C decreased the ductile–brittle transition temperature from 40 to 15 °C and increased the upper shelf energy from 300 to 380 J/cm2 as compared to the normalized and tempered (NT) condition. The microstructural observations with scanning and transmission electron microscopes showed the precipitation of fine Ta-rich MX carbonitride and M23C6 carbide during TMT and subsequent tempering. The analysis of the cleavage facets and the secondary cracks with electron back-scattered diffraction (EBSD) revealed that the brittle fracture occurred via cleavage cracking along {100} planes across the laths, while the high-angle boundaries of martensite blocks and packets were effective barriers to the crack propagation. The increased impact toughness of the tempered TMT steel sample was attributed to enhanced ductile fracture owing to the uniform dispersion of the precipitates and favorable {332}⟨113⟩ crystallographic texture.

EFFECT OF TANTALUM ON THE IMPACT RESISTANCE OF 12% CR STEELS SUBJECTED TO THERMOMECHANICAL TREATMENT

The effect of tantalum on the impact resistance of 12% Cr steels with different tantalum content (12CrTaNb and 12CrNb) was investigated in impact tests in the temperature range from -40 to +120°C with determining the brittle-ductile transition temperature as the temperature in the middle between the upper and lower shelf energies. Both 12% Cr steels were thermomechanically treated by alternating 1050°C annealing and forging in addition to standard normalizing treatment with high-temperature tempering. It was found that for Ta-alloyed 12% Cr steel, the entire fracture toughness versus temperature curve is 30-50 J/cm2 higher over the entire temperature range, including the upper and lower shelf energies, and the brittle-ductile transition temperature is 10°С lower. The main structural parameters of Ta-alloyed 12% Cr steel which can have a beneficial effect on toughness are a smaller initial austenite grain size, a larger average size but lower density of M23C6 carbide particles along low-angle martensite lath boundaries, and a larger volume fraction of VX carbonitrides.

Impact toughness of a 12% Cr martensitic steel: Conventional heat treatment vs. thermo-mechanical processing

The impact toughness together with determination of the ductile–brittle transition temperature for the 12 %Cr steel with the low N and high B contents subjected to conventional heat treatment and thermo-mechanical processing have been investigated. The thermo-mechanical processing includes the interim hot forging and long-term annealing at 1050 °C with the aim to dissolve the delta-ferrite and coarse W-rich particles, as well as to decrease the particle number density of M23C6 carbides along the lath boundaries and to precipitate V-rich MX phase. The temperature dependence of Charpy energy curves including upper and lower shelf energy is found to be shifted in a side of higher energies. The ductile–brittle transition temperature decreases from 70 °C to (45–50)°C.

Effect of radio-frequency plasma spheroidization on the microstructure and mechanical properties of 10Cr ferritic oxide dispersion-strengthened steel

Reducing the porosity of irregularly shaped mechanically alloyed oxide dispersion-strengthened (ODS) steel powders is critical for their application at cryogenic temperatures. Although the plasma rotating electrode process has been widely used as a suitable method for producing spherical powders, its high cost and low powder yield impede its application. Thus, we investigated the feasibility of applying plasma spheroidization on ODS steel particles, to reduce their porosity, by investigating the influence of radio-frequency plasma spheroidization on the microstructure and mechanical properties of 10-Cr ODS steel. The ODS steel powder was ball-milled and then spheroidized prior to consolidation. The prepared samples were analyzed using X-ray diffraction, electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) as well as subjected to tensile and impact tests. The EBSD analysis revealed that the grains in the plasma-spheroidized sample were coarser than those in the non-spheroidized sample, and thus, the room- and cryogenic-temperature Charpy impact strengths of the latter were superior to those of the former sample. TEM analysis showed that the average particle size of plasma-spheroidized ODS steel was slightly higher than that of the non-plasma-spheroidized ODS steel. Furthermore, the room-temperature yield strength of the non-spheroidized ODS was higher than that of the spheroidized ODS because of the finer grains of the former. In the future, plasma spheroidization can be combined with other techniques to obtain completely dense ODS steel with increased upper shelf energies for various advanced and military applications.

Correlation of Charpy-V notch and different Charpy-U notch impact specimen energies for structural metals

The most common mechanical test to evaluate the fracture resistance of a structural metal is the Charpy-V impact test. Nearly all structural integrity assessment procedures use it. The preferred notch geometry is the V-notch, but other geometries are or have been sometimes used. The U-notch has normally a root radius of 1 mm, compared to the V-notch which has a root radius of 0.25 mm. Even though the V-notch specimen is today the main geometry, many different specimen versions are still being used to some extent. More importantly, vintage data corresponding to something else than V-notch data may be the only information available for older structures, still in use. For such cases, it is imperative to have a correlation between different notch geometries. Here simple correlations for the upper shelf energy and the 35 J/cm2 transition temperatures are developed for four different U-notch geometries.

Effects of Si on the Ductile‐to‐Brittle Transition Behavior of As‐Cast Advanced High‐Strength Steels

High Si concentrations in third‐generation advanced high‐strength steels (AHSS) are known to cause cracking in continuous cast slabs during cooling at temperatures below 300 °C. To investigate the mechanism connecting Si to this embrittlement, impact toughness tests are conducted on as‐cast Fe–0.2C–3.0Mn steels with 0.5, 1.5, and 3.0 wt% Si at temperatures between 100 and 400 °C. The propagation path of the cracks through the microstructure of the specimens is examined. The ductile‐to‐brittle transition (DBT) behaviors of the three steels are compared. Higher Si concentrations raise the DBT temperature of the steels. The influence of Si on both solid‐solution strengthening and autotempering during cooling likely contributes to this by increasing the hardness of these as‐cast microstructures. In addition, the precipitation of pro‐eutectoid ferrite (αPE), which is promoted by higher Si concentrations, significantly lowers the upper shelf energy of the DBT curve. The αPE phase also alters the propagation path of brittle fracture in the specimens, potentially contributing to the increase in DBT temperature. The increase in DBT temperature and decrease in upper shelf energy may contribute to the as‐cast AHSS slab embrittlement at low temperatures observed in the industry.

Numerical prediction of the effect of irradiation on the Charpy Upper Shelf Energy of Reactor Pressure Vessel steels

Neutron irradiation affects the mechanical properties of Reactor Pressure Vessel (RPV) steels used in Light Water Reactors (LWR). Fracture properties are particularly monitored based on surveillance programs, mostly through Charpy impact tests. The evolutions of Ductile Brittle Transition Temperature (DBTT) and Upper Shelf Energy (USE) with irradiation are evaluated and used for structural integrity assessment depending on regulation procedures. Semi-empirical correlations have been proposed to predict DBTT and USE shifts as a function of material chemical composition and fast neutron fluence, based on extended experimental database. In this study, numerical simulations of Charpy impact tests are performed in order to predict quantitatively the evolution of USE with irradiation. In a first part, the mechanical properties of an A508 Cl3 RPV steel are presented, including tensile tests on flat and notched samples and Charpy impact tests, for various irradiation levels and test temperatures. USE values are found to be within the bounds of correlation curves. In a second part, the experimental data are used to calibrate constitutive equations describing the material behavior up to high strain levels. Charpy impact test finite element simulations are then detailed, including ductile fracture modelling. In a third part, the ductile fracture model is calibrated based on the experimental data, and numerical simulations are used to predict the evolution of USE with irradiation. The decrease of USE with irradiation is recovered, and the numerical results are found to be in quantitative agreement with the correlation curves. Additional simulations confirm the agreement for different reference unirradiated USE and chemical composition, as well as using a simplified ductile fracture model. These results indicate that numerical simulations of Charpy impact test relying on calibrated constitutive equations can be used to predict USE and required experimental characterization are finally discussed.

Influence of W concentration on the Charpy impact property and corrosion behavior of reduced activation ferritic-martensitic steels for fusion reactor

Fe-9Cr-0.4Mn-0.2V-0.1Ta-0.02Ti-0.1C-W (all in wt%) alloys with different W additions of 1 and 2 wt% were prepared to investigate the effects of W content on the mechanical properties and corrosion behavior of reduced activation ferritic–martensitic steels. The base steel samples were made through a heat treatment involving normalizing and tempering after hot rolling. When the W content was 1 wt%, the microstructure of the base steel consisted of tempered martensite with two types of precipitates, intergranular Cr-enriched M23C6 carbide and Ta/Ti-enriched MC carbide, while δ-ferrite additionally formed in an alloy containing W at 2 wt%. The results of Charpy impact tests showed that the ductile-to-brittle transition temperature slightly increased with an increase in W concentration without degrading the upper shelf energy, implying that the impact property slightly deteriorated. In addition, potentiodynamic polarization tests showed that the resistance to pitting corrosion also decreased when the W concentration was increased from 1 to 2wt%. This unexpected degradation of the impact property and corrosion resistance in alloy containing 2 wt% W was caused by the formation of δ-ferrite. That is, the δ-ferrite provided a relatively easy path for crack propagation during the impact tests. Also, since the δ-ferrite is a W-enriched phase, the matrix adjacent to the δ-ferrite was electrochemically active and acted as a local anode and the δ-ferrite formation then decreased the resistance to pitting corrosion.

Effect on impact properties of adding tantalum to V-4Cr-4Ti ternary vanadium alloy

Four V-Ta-4Cr-4Ti quaternary alloys containing different quantities of Ta were investigated to determine the effect of Ta content on the Charpy impact properties. Five button-shaped ingots of the V-4Cr-4Ti ternary alloy and V-xTa-4Cr-4Ti quaternary alloys (x = 3, 9, 15, and 22 wt.%) were fabricated on a laboratory scale by using non-consumable arc-melting in an argon atmosphere. Charpy impact tests were conducted at temperatures ranging from 77 K to 293 K using an instrumented impact tester. Both the upper shelf energy and the ductile–brittle transition temperature increased with increasing Ta content. The addition of 3 wt.% Ta resulted in solid solution strengthening without any degradation of the Charpy impact properties. Thus, the addition of 3 wt.% Ta (V-3Ta-4Cr-4Ti) is an appropriate amount to use in blanket structural materials for nuclear fusion reactors. The spectra of TEM-EDS for V-3Ta-4Cr-4Ti quaternary alloy indicate that there is no significant enrichment of Ta in the matrix as compared with that in the precipitate. However, thermal aging may result in the formation of the Laves phase, causing the degradation of Charpy impact properties. The characterization of precipitates, thermal aging, and creep tests of the V-3Ta-4Cr-4Ti quaternary alloy need to be investigated to determine the optimum Ta content.

Direct Observation of Cu Clusters and Dislocation Loops by Cs-Corrected STEM in Fe-0.6wt%Cu Alloy Irradiated in BR2

The neutron irradiation of Fe-based fusion and fission reactor materials leads to an increase in ductile-to-brittle transition temperature with a decrease in upper shelf energy. It is well known that Cu content has a strong influence on the embrittlement phenomenon, as Cu-rich clusters (CRPs) are thought to be directly responsible for embrittlement. In contrast, mechanical property studies for steels with different Cu levels exhibit dominant matrix defects in the embrittlement of both low-Cu steels and high-Cu steels at high fluences. To determine the effects of dislocation loops and CRPs on radiation hardening in those steels, neutron irradiation was conducted on Fe-0.6wt%Cu alloy. The neutron irradiation was performed in BR2 at 290 °C up to a dose of 4.1 × 1024 n/m2. After irradiation, the microstructure was observed and analyzed by spherical aberration-corrected transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) combined with X-ray energy-dispersive spectroscopy, using a JEOL ARM200FC. This technique enabled simultaneous observation of ~10 nm CRPs and dislocation loops. Additional high-voltage electron irradiation was performed at room temperature, and the dislocation loops were identified as interstitial-type dislocation loops. Radiation-induced hardening due to neutron irradiation was estimated by measuring the size and density of the dislocation loops and the CRPs. These results suggest that simultaneous observation of dislocation loops and CRPs using the Cs-corrected STEM with EDS analysis is essential for the study of radiation-induced hardening in Fe-based alloys.

Influence of Graphite-Phase Parameters on the Mechanical Properties of High-Silicon Ductile Iron

High silicon solid solution-strengthened ductile iron exhibits advantageous combinations of static strength and ductility due to elevated silicon contents. However, the Charpy impact toughness decreases rapidly with increasing silicon content. In particular, the transition temperature between ductile and brittle fracture behaviour is shifted significantly to higher temperatures. Therefore, the optimum adjustment of the graphite phase is of major importance in order to achieve sufficient toughness properties. So far, little is known about the influence of the graphite phase on the toughness properties of high-silicon ductile iron. In the present study, the graphite particle distribution was therefore systematically varied for ductile iron grade EN-GJS-500-14. The effect of the graphite nodule count in the range from 77 to 273 1/mm2, and the influence of the average particle size and particle distance on the Charpy impact energy and the characteristics of the resulting transition curves were investigated by means of statistical analyses. The results that were obtained indicate that the ductile-to-brittle transition temperature in Charpy impact tests can be reduced by about 22 °C by increasing the average nodule count by 100 1/mm2. Additionally, both the lower and upper shelf energies are raised by increasing the nodule count by 100 1/mm2. Based on these results, it becomes apparent to producers that the inoculation technology for toughness-stressed components needs to be specifically considered and adapted.

Effects of Carbon and Phosphorus on Microstructure and Mechanical Properties of Friction Stir Welded Weathering Steels

Weathering steels with the increased concentration of carbon (C) and phosphorus (P) that tend to be difficult to join without defects by fusion welding were fabricated by friction stir welding (FSW), and the effects of C and P contents on the microstructure and mechanical properties of FSWed joints were investigated. P segregation formed during solidification was present in the base material (BM) of the steels. In the stir zone (SZ) of FSWed joints, P segregation was reduced under the condition of higher welding temperature (above A3). In the tensile test, both the addition of C and P increased tensile strength. On the other hand, the increase in C content impaired the uniform elongation, while the increase in P content did not deteriorate it. It is because the work hardening rate was improved by the P addition presumably due to the suppression of cross slip of dislocations. In the Charpy impact test, the increase in C and P contents shifted the DBTT to the higher temperatures and simultaneously decreased the upper shelf energy. EPMA analysis performed on the cross-section of the ductile fracture surface of BM of 0.3 wt%P steel revealed that voids were formed at the boundary between P segregated area and the non-segregated area. Therefore, P segregation was inferred to be the main factor determining the upper shelf energy. Since the P segregation of the joint FSWed above A3 was improved, the upper shelf energy of the SZ was the highest among BM and SZ FSWed below A1.

Synergy of strengthening and toughening of a Cu-rich precipitate-strengthened steel

A high strength steel with a combination of ∼930 MPa yield strength and excellent low temperature toughness with an upper shelf energy of above 200 J and ductile brittle transition temperature (DBTT) of lower than −90 °C is developed. The strengthening and toughening mechanisms are investigated systematically based on the detailed characterization on microstructures including the matrix and precipitates. The results indicate that the steel is composed of a fine lath martensite with rod-like Cu precipitates. The high strength is achieved by a combination of solid-solution strengthening, dislocation strengthening, grain boundary strengthening and precipitation strengthening of Cu-precipitates. The instrumented Charpy impact results further indicate that the crack propagation is the main factor affecting DBTT while the dislocation density has an obvious effect on both crack initiation and propagation. The fine lath structure of the low carbon martensite enhances the crack resistance and delays the rapid unstable crack propagation at low temperatures. Both the strengthening and toughening are thoroughly discussed in details.

Effect of coiling temperature on impact toughness of hot rolled ultra-high-strength multiphase steel strips

The tensile and Charpy impact properties of four strip samples from two different B-added low-C ultra-high-strength steel strips (Al-treated and Ti-treated), coiled at two different temperature ranges (360–380 °C and 450–460 °C), have been evaluated and correlated to the microstructural parameters, dislocation density, and the intensity of high-angle boundaries. The effects of coiling temperatures on the microstructural evolution and mechanical properties have been discussed. The volume fraction of the individual phase constituents (namely, granular bainite, upper bainite, lower bainite and tempered martensite) and their hardness, local deformation response and the strain-hardening ability, as determined from nanoindentation testing, influenced the bulk properties such as hardness, tensile properties (strength and ductility), Charpy impact properties (upper shelf energy, USE, and ductile-to-brittle transition temperature, DBTT) and strain-hardening abilities under both quasi-static and dynamic loading conditions. The dominance of granular bainite and upper bainite (75–90 %) reduced the strength (670–722 MPa yield strength), improved ductility (16.7–19.5 % elongation to failure) and USE (35–42 J) in the samples coiled at the higher temperatures. In contrast, a higher fraction of tempered martensite and lower bainite (78–82 %) significantly increased the strength (808–814 MPa), reduced ductility (13.0–14.5 %) and USE (19–29 J) in the lower temperature coiled samples. The DBTT showed a complex trend with the microstructural parameters. It depended on the USE level, as well as on the ‘effective grain size’ of the matrix.

Side-grooved Charpy impact testing: Assessment of splitting and fracture properties of high-toughness plate steels

The fracture properties and susceptibility to crack-divider delamination (or splitting) of three commercially produced high-toughness X70 pipeline steels are evaluated with Charpy impact test samples modified to incorporate side grooves. Temperature-dependent impact data are compared with standard Charpy V–notch (CVN) and drop weight tear test data (DWTT). It is shown that the modified geometry prevents the accumulation of plastic deformation at upper shelf energy temperatures and improves the accuracy of impact properties measurements. It also promotes splitting, mirroring the splitting behavior assessed with DWTT samples. To demonstrate the effects of splitting on fracture characteristics and impact energies, steels with similar tensile properties but different splitting susceptibilities are considered. Splitting severity is maximum close to the ductile–brittle transition temperature. However, the effect of splitting on impact energy is minimum at such temperature, as this type of delamination increases energy absorption at lower temperatures and decreases it by a similar extent at higher temperatures. This finding is discussed by examination of force–displacement curves from the instrumented impact tests.

Progress of the Oil Spill Risk Analysis (OSRA) Model and Its Applications

The Bureau of Ocean Energy Management (BOEM) is responsible for managing the development of US Outer Continental Shelf (OCS) energy and mineral resources. Because oil spills may occur from offshore oil and gas activities, BOEM conducts oil spill risk analysis (OSRA) prior to oil and gas lease sales. Since the 1970s, BOEM has developed and applied the OSRA model to evaluate the risk of potential oil spills to environmental resources. This paper summarizes some of the OSRA model progress and applications in the past decade: (1) calculation of the risk of catastrophic oil spills (with a volume over one million barrels), which concludes that the return period of a catastrophic oil spill in OCS areas is estimated to be 165 years; (2) a more efficient way to estimate the probability of oil spill contact to environmental resources in the Gulf of Mexico; (3) weathering calculations in OSRA, which enhances the accuracy of the OSRA model results; and (4) application of OSRA to the Ixtoc I oil spill as an example of how the OSRA model simulates large oil spills for oil spill preparedness and response.

Additive manufacturing technologies for EUROFER97 components

By uncoupling the manufacturability from the design process, additive manufacturing of the baseline material EUROFER97 can open significant design freedom for divertor and breeding blankets in fusion technology. As additive manufactured components are known to possess unique microstructures compared to EUROFER97 from standard technologies, the aim of this paper is to investigate additive manufactured EUROFER97 components and the influence of post processing steps on their microstructure and mechanical properties from a materials science point of view.This paper covers the technological fabrication process of EUROFER97 by selective laser melting (SLM), including the production of pre-alloyed EUROFER97 powder, an SLM-parameter study and the design and production of custom-build thin walled test components by SLM. In the initial state after fabrication, SLM-EUROFER97 components exhibit a bimodal, anisotropic microstructure with large ferritic grains. The fraction of ferritic grains increases with decreasing wall thickness. A heat treatment including austenitization, quenching and tempering, allows to achieve a fully martensitic, uniform microstructure for all wall thicknesses. Therefore, there is no influence of wall thickness on mechanical properties of EUROFER97 produced by SLM to be expected, as long as the SLM-part is submitted to an appropriate heat treatment.Further, the comparison of the initial state after fabrication and after post processing reveals the necessity of both hot isostatic pressing and heat treatment to improve the performance. While all material conditions lead to sufficient tensile properties, the Charpy impact properties of SLM-EUROFER97 are inferior in comparison to conventionally produced EUROFER97. A heat treatment alone only improves the ductile-to-brittle transition temperature, whereas hot isostatic pressing reduced the residual porosity of the SLM parts and a subsequent heat treatment improved the ductile-to-brittle transition temperature as well as the upper shelf energy.

Preliminary study on the fabrication of 14Cr-ODS FeCrAl alloy by powder forging

A simple powder forging process was presented herein to fabricate an Fe-14Cr-4.5Al-2W-0.4Ti-0.5Y2O3 ODS FeCrAl alloy. The forged alloy exhibits a high density that exceeds 97 % of the theoretical density. The ODS alloy was investigated in terms of the residual porosity, morphology and phase structure of oxide nanoparticles, impact toughness and tensile properties. It was found that refined grains were obtained during powder forging. A residual porosity less than 1.1 % has no impact on the precipitation of oxide nanoparticles. The average diameter of the oxide particles is 7.99 nm, with a number density of 2.75 × 1022 m−3. Almost all of the oxides are identified as orthorhombic YAlO3 particles. The refined grains and uniformly distributed oxide nanoparticles enable the alloy to show excellent mechanical strength and ductility below 700 °C, and enable the ductile-to-brittle transition temperature to be close to room temperature. However, a slight decrease in strength at 1000 °C and the Charpy upper shelf energy has been suggested to be due to the residual porosity. These results indicate that powder forging can be used as a promising technique for the fabrication of ODS alloys.

Modelling and Analysis of Offshore Energy Hubs

Clean, multi-carrier Offshore Energy Hubs (OEHs) may become pivotal for efficient offshore wind power generation and distribution. In addition, OEHs may provide decarbonised energy supply for maritime transport, oil and gas recovery, and offshore farming while also enabling conversion and temporary storage of liquefied decarbonised energy carriers for export. Here, we investigate the role of OEHs in the transition of the Norwegian continental shelf energy system towards zero-emission energy supply. We develop a mixed-integer linear programming model for investment planning and operational optimisation to achieve decarbonisation at minimum costs. We consider clean technologies, including offshore wind, offshore solar, OEHs and subsea cables. We conduct sensitivity analysis on CO$_2$ tax, CO$_2$ budget and the capacity of power from shore. The results show that (a) a hard carbon cap is necessary for stimulating a zero-emission offshore energy system; (b) offshore wind integration and power from shore can more than halve current emissions, but OEHs with storage are necessary for zero-emission production and (c) at certain CO$_2$ tax levels, the system with OEHs can potentially reduce CO$_2$ emissions by 50% and energy losses by 10%, compared to a system with only offshore renewables, gas turbines and power from shore.