Embrittled State Research Articles

Radiation and thermal embrittlement of RPV steels: the links of embrittlement mechanisms, fracture modes and microcrack nucleation and propagation. Part 2. Strength and plasticity properties

The uniaxial tension test results are represented over wide temperature range for smooth round bars of 2Cr–Ni–Mo–V steel and A533 steel used for RPVs of WWER and PWR types. These steels are studied in the following states: (1) the initial (as-produced) state; (2) the thermally embrittled state modelling hardening mechanism of embrittlement; (3) the thermally embrittled state modelling non-hardening mechanism of embrittlement; (4) the irradiated state. The true stress-strain curves are determined over wide temperature range that is required for calculation of the stress-and-strain fields for various specimens. The true stress-strain curves for the investigated steels in the initial and thermally embrittled states are obtained when using standard mechanical characteristics. For the irradiated steels the true stress-strain curves are obtained when using the data of the digital video recording under continuous in-testing monitoring of the cylindrical parts of tensile bars. For the irradiated materials the procedure based on standard characteristics cannot be used as it is connected with very small strain. The procedure based on the digital video recording data is verified by comparison of the stress-strain curves obtained for the initial and thermally embrittled states on the basis of the digital video recording data and standard characteristics.

Estimation of hydrogen embrittlement process using statistical analysis method

We propose a statistical analysis method to estimate the progress of the hydrogen embrittlement process by obtaining the transition probability λi that represents the average transition rate of the hydrogen embrittlement state. The estimation results are verified by Monte Carlo simulation. As an application of the proposed method, we evaluate the temperature dependence of the hydrogen embrittlement process and find that the threshold of the fracture decreases as temperature decreases. We also suggest that the rate-limiting process of the hydrogen embrittlement can be estimated by evaluating the apparent activation energy Ea of the state transition using the proposed method.

PDMS material embrittlement and its effect on machinability characteristics by cryogenic abrasive air-jet machining

Abstract The use of cryogenic abrasive jet machining (CAJM) technology for polydimethylsiloxane (PDMS) materials is increasing rapidly due to its high material removal rate and high surface quality. However, how the material properties of PDMS affect the machining properties under cryogenic conditions is still an unsolved problem. Firstly, the thermodynamic and mechanical properties of PDMS materials are studied by dynamic thermo-mechanical analysis and universal material testing. The results show that the PDMS material’s embrittlement temperature is 126 K. Secondly, evaluation of cooling temperature of PDMS during CAJM shows that the machined area has reached embrittled state. Thirdly, the effect of PDMS material’s embrittlement on the machining behaviour is studied. The experimental results show that the PDMS material exhibit increased abrasive erosion at medium-angle under cryogenic conditions and shows a semi-brittle wear mechanism. Furthermore, the erode surface’s roughness is evaluated. The result shows that the roughness decreases with decreasing air pressure at the same impact angle; with increasing impact angle, the roughness shows a trend of increase initially and then decreases. This work shows that the PDMS material’s embrittlement and the erosion response behaviour during CAJM are due to the combined effects of thermo-mechanical properties and material removal mechanisms, providing a better understanding of CAJM technology.

Estimation of the fracture toughness of tungsten fibre-reinforced tungsten composites

Tungsten fibre-reinforced tungsten composites (Wf/W) have been developed to overcome the inherent brittleness of tungsten, which is a promising candidate for the plasma-facing material in a future fusion power plant. As the development of Wf/W evolves, the fracture toughness of the composite is in the focus of interest for further component design. In this contribution fracture mechanical tests on two different types of chemical vapour deposited (CVD) Wf/W are presented. Three-point bending tests according to ASTM E399 as a standard method for brittle materials were used to get a first estimation of the toughness. A provisional fracture toughness value of up to 241 MPa m1/2 was calculated for the as-fabricated and of up to 20.5 MPa m1/2 for a heat-treated and thus embrittled state. As the material does not show a brittle fracture in the as-fabricated state, the J-Integral approach based on the ASTM E1820 was additionally applied for this state. A maximum value of the J-integral of 7.5 kJ/m2 (57.6 MPa m1/2) was determined. A detailed post mortem investigations was used to obtain the active mechanisms.

Discussion on the characterisation of hydrogen embrittlement based on eddy current signals

A non-destructive testing (NDT) method for evaluating mechanical performance has been studied based on the analysis of eddy current signals. Low-alloy steel samples were tested under conditions of tension with the aim of quantifying hydrogen embrittlement (HE). The mechanical responses of samples were investigated after electrochemical hydrogen charging. Eddy current signals were gathered to evaluate the hydrogen embrittlement state using a differential probe. Numerical analysis of hydrogen concentration distribution in material was performed to investigate the response mechanism of the signal. The effect that hydrogen has on the mechanical performance of low-alloy steel has been discussed. The experimental results show that the eddy current signal has a good correlation with the hydrogen-induced plasticity loss index.

Developing an Acoustic Method for Determining the Degree of Hydrogenation in Structures Made of Titanium Alloys

We present results of acoustic studies of hydrogenated samples of PT-7M and PT-3V titanium alloys, aimed at establishing correlations between acoustic parameters and the mass fraction of hydrogen in the samples. Relevant metallographic studies have been carried out, which make it possible to describe the nature of structural changes occurring under hydrogen saturation. The transition to an embrittled state in the alloys under study has been demonstrated by microhardness measurements.

Characterization of Hydrogen Embrittlement in 2.25cr-1mo-0.25v Steel by Eddy Current Method

ABSTRACTEddy current method has been recently developed to characterize mechanical properties of materials and assess internal hydrogen content of high strength low alloy steels. The application of eddy current testing in evaluating hydrogen embrittlement state of 2.25Cr-1Mo-0.25V ferritic steel has been investigated using tensile test and electrochemical hydrogen charging test. The relationship between the embrittlement index and eddy current signal is well established. It found that there is a good linear relation between the hydrogen-induced plasticity loss and the eddy current signal. This shows eddy current testing is an effective method for evaluating hydrogen embrittlement state of 2.25Cr-1Mo-0.25V steel.

Flaw sensitivity in rate-sensitive high strength alloys: An assessment and future research directions

The tensile strength of metallic alloys may be sensitive to the presence of short cracks, particularly in the embrittled state (such as phosphorous or sulphur segregation to the grain boundaries in a high strength steel). Experimental evidence reveals that cleavage accompanied by plasticity can occur when the net section stress is on the order of the yield strength. If the solid were mildly strain hardening but rate-insensitive, then no cleavage would be predicted as the crack tip tensile stress would not attain the local cleavage value. In the present study, the role of rate sensitivity is assessed by placing a tensile cohesive zone at the tip of an edge crack within a visco-plastic solid, and the crack is subjected to remote tension. Thereby, crack initiation and growth is predicted from short flaws in the presence of bulk plasticity. The crack growth resistance curve for long flaws is also determined. Implications of the predictions are discussed for hydrogen embrittlement, and the significance of rate effects in elevating the stress level adjacent to the crack tip is quantified.

Influence of mineral filler on the low-temperature cohesive strength of asphalt mortar

The effects and influence mechanisms of different filler-to-bitumen ratios on the low-temperature cohesive strength of asphalt mortar were studied using quantitative testing technology. Two control variables, namely filler-to-bitumen ratio (the quality of the mineral filler/quality of asphalt) and temperature, were selected for this experiment. The first variable was divided into six groups. The cohesive strength of each group was then measured under different low-temperature conditions. Results show that the optimization of filler-to-bitumen ratio was related to the cohesive strength of the asphalt matrix, which was in turn associated with the change on the surface between the asphalt matrix and the strength of the mineral power fillers generated from asphalt embrittlement. Therefore, the optimal filler-to-bitumen ratio should be determined by the local low-temperature conditions. Moreover, the cohesive strength of the asphalt matrix, its embrittlement state and the contact surface strength effect of asphalt filler should be comprehensively considered in determining the optimal filler-to-bitumen ratio.

Low-temperature embrittlement and fracture of metals with different crystal lattices – Dislocation mechanisms

The state of a low-temperature embrittlement (cold brittleness) and dislocation mechanisms for formation of the temperature of a ductile-brittle transition and brittle fracture of metals (mono- and polycrystals) with various crystal lattices (BCC, FCC, HCP) are considered. The conditions for their formation connected with a stress-deformed state and strength (low temperature yield strength) as well as the fracture breaking stress and mobility of dislocations in the top of a crack of the fractured metal are determined. These conditions can be met for BCC and some HCP metals in the initial state (without irradiation) and after a low-temperature damaging (neutron) irradiation. These conditions are not met for FCC and many HCP metals. In the process of the damaging (neutron) irradiation such conditions are not met also and the state of low-temperature embrittlement of metals is absent (suppressed) due to arising various radiation dynamic processes, which increase the mobility of dislocations and worsen the strength characteristics.

Advanced tungsten materials for plasma-facing components of DEMO and fusion power plants

Tungsten is the major candidate material for the armour of plasma facing components in future fusion devices. To overcome the intrinsic brittleness of tungsten, which strongly limits its operational window, a W-fibre enhanced W-composite material (Wf/W) has been developed incorporating extrinsic toughening mechanisms. Small Wf/W samples show a large increase in toughness. Recently, a large sample (50mm×50mm×3mm) with more than 2000 long fibres has been successfully produced allowing further mechanical and thermal testing. It could be shown that even in a fully embrittled state, toughening mechanisms as crack bridging by intact fibres, as well as the energy dissipation by fibre-matrix interface debonding and crack deflection are still effective. A potential problem with the use of pure W in a fusion reactor is the formation of radioactive and highly volatile WO3 compounds and their potential release under accidental conditions. It has been shown that the oxidation of W can be strongly suppressed by alloying with elements forming stable oxides. WCr10Ti2 alloy has been produced on a technical scale and has been successfully tested in the high heat flux test facility GLADIS. Recently, W-Cr-Y alloys have been produced on a lab-scale. They seem to have even improved properties compared to the previously investigated W alloys.

Sensitivity of ultrasonic nonlinearity to irradiated, annealed, and re-irradiated microstructure changes in RPV steels

The planned life extension of nuclear reactors throughout the US and abroad will cause reactor vessel and internals materials to be exposed to more neutron irradiation than was originally intended. A nondestructive evaluation (NDE) method to monitor radiation damage would enable safe and cost-effective continued operation of nuclear reactors. Radiation damage in reactor pressure vessel (RPV) steels causes microstructural changes that leave the material in an embrittled state. Nonlinear ultrasound is an NDE technique quantified by the measurable acoustic nonlinearity parameter, which is sensitive to microstructural changes in metallic materials such as dislocations, precipitates and their combinations. Recent research has demonstrated the sensitivity of the acoustic nonlinearity parameter to increasing neutron fluence in representative RPV steels. The current work considers nonlinear ultrasonic experiments conducted on similar RPV steel samples that had a combination of irradiation, annealing, re-irradiation, and/or re-annealing to a total neutron fluence of 0.5–5×1019n/cm2 (E>1MeV) at an irradiation temperature of 290°C. The acoustic nonlinearity parameter generally increased with increasing neutron fluence, and consistently decreased from the irradiated to the annealed state over different levels of neutron fluence. Results of the measured acoustic nonlinearity parameter are compared with those from previous measurements on other RPV steel samples. This comprehensive set of results illustrates the dependence of the measured acoustic nonlinearity parameter on neutron fluence, material composition, irradiation temperature and annealing.

Fatigue fracture toughness of metals

ABSTRACTThe paper considers the peculiarities of fatigue crack propagation and final fracture of metals under cyclic loading. It is shown that the value of the fatigue fracture toughness of steels in an embrittled state is appreciably lower than that of the fracture toughness under static loading. A model of the transition from stable to unstable fatigue crack propagation is justified.

Application of a new cleavage fracture criterion for fracture toughness prediction for RPV steels

ABSTRACTIn the present paper, approaches for prediction of cleavage fracture proposed by authors over recent years are briefly reviewed. A new local criterion of cleavage fracture and its application are considered.Local criterion of cleavage fracture formulated in a probabilistic manner consists of cleavage microcrack nucleation condition and condition for their start and propagation through various barriers such as microstresses, grain and dislocation substructure boundaries. As distinct from universally accepted criterion in which microcrack nucleation occurs on the plastic deformation beginning (σeq=σY), cleavage microcrack nucleation according to the formulated condition depends on the maximum principal stress, plastic strain and temperature.On the basis of the proposed criterion, a probabilistic model for fracture toughness prediction (known now as Prometey model) was developed and verified by application to reactor pressure vessels (RPV) materials in the initial, moderately irradiated and highly embrittled states. The Prometey model and other applications of the proposed criterion are briefly considered. The predicted results are compared with test results and the Master Curve.

WWER-1000 base metal reference steel and its characterisation

The surveillance programmes of western power reactors include, in many cases, standard reference materials in addition to actual pressure vessel steels. These are specimens cut from standard steel plates (Heavy Section Steel Technology, JRQ, etc.) that are similar in composition and heat treatment to the base material in the respective reactor pressure vessels, and are supposed to serve as a monitor by comparing the radiation embrittlement of the plant-specific material to the reference material, and to detect anomalies in the radiation environment of the surveillance capsules. A correlation monitor material for the eastern WWER-1000 (similar as the JRQ for western reactors) is needed in order to determine the reliability of accelerated data for the validation of reactor pressure vessel surveillance data. Reference materials should be well characterised in terms of irradiation behaviour (transition temperature shift, non-destructive signal, etc.). The magnitude of the sensitivity to irradiation for this material should be measurable for the selected exposures. In this subject the IAEA is launching a new co-ordinated research programme. Material is already manufactured, and the JRC-IE has become its custodian. A detailed plan for characterisation of the reference steel is set up, including irradiation conditions, post-irradiation testing techniques and implementation plan. It is expected the participation of several research institutes worldwide in a round robin, which will allow a better comprehension of WWER-1000 steel's behaviour and will be considered as a benchmarking between different laboratories. The JRC-Institute for Energy in collaboration with the Russian Research Centre – Kurchatov Institute is performing the “as received” material characterisation by both destructive methods and non-destructive techniques. The non-destructive techniques used at the JRC-IE premises are novel methods specially developed for non-destructive assessment of the embrittlement state of materials, as the STEAM method and the measurement of magnetic properties. The STEAM technique (Seebeck and Thomson effects on aged material), is based on the measurement of the Seebeck coefficient. The magnetic properties evaluation is done through Barkhausen noise and permeability measurements. This paper presents a preliminary analysis of the results obtained by all involving laboratories.

A preliminary evaluation of irradiation damage in model alloys by electric properties based techniques

The most important effect of the degradation by neutron irradiation is a decrease in the ductility of reactor pressure vessel (RPV) ferritic steels. The main way to determine the mechanical behaviour of the RPV steels is tensile and impact tests, from which the ductile to brittle transition temperature and its increase due to neutron irradiation can be calculated. These tests are destructive and are regularly applied to surveillance specimens to assess the integrity of the RPV. The possibility of applying validated non-destructive aging monitoring techniques would however facilitate the surveillance of the materials that form the reactor vessel. The Institute for Energy of the Joint Research Centre has developed two devices, focussed on the measurement of the electrical properties which prove to give a good non-destructive assessment of the embrittlement state of ferritic steels. The first technique, called Seebeck and Thomson Effects on Aged Material (STEAM), is based on the measurement of the Seebeck coefficient, characteristic of the material and related to the microstructural changes induced by irradiation embrittlement. With the same aim, the second technique, named Resistivity Effects on Aged Material (REAM), measures instead the resistivity of the material. This paper explains (i) preliminary STEAM and REAM results and (ii) results compared with Charpy impact energy temperature shifts due to neutron irradiation. These results will make possible the improvement of such techniques based on the measurement of material electrical properties for their application to non-destructive irradiation embrittlement assessment.

Advanced method for WWER RPV embrittlement assessment

The integrity assessment of reactor pressure vessels (RPV) is a main issue of NPP safe operation. Mechanical properties of the PV materials are degrading progressively under neutron irradiation reducing safety margins. To estimate the current state of embrittlement and to predict actual RPV safe life, it is necessary to develop improved radiation embrittlement understanding models. This is particularly true for the case of re-embrittlement after annealing, typical of several VVER-4000 RPVs. In the present work, on the base of analysis of available information, a semi-mechanistic model is offered. The model is based on the additive contribution of the three main known embrittlement mechanisms: matrix damage, precipitations and segregations which are simply described analytically since the understanding of the role P and Cu in enhancing irradiation embrittlement is rather well known. The model is used to explain and predict embrittlement and re-embrittlement after annealing of high P materials typical of certain VVER-440 RPVs. The proposed model. In this case, proves to have a very practical relevance, since it explains the peculiar re-embrittlement kinetics at low fluence after annealing. This is important for the correct interpretation of early surveillance data avoiding un-correct prediction for longer term embrittlement.

Evaluation of irradiation damage effect by applying electric properties based techniques

The most important effect of the degradation by radiation is the decrease in the ductility of the pressure vessel of the reactor (RPV) ferritic steels. The main way to determine the mechanical behaviour of the RPV steels is tensile and impact tests, from which the ductile to brittle transition temperature (DBTT) and its increase due to neutron irradiation can be calculated. These tests are destructive and regularly applied to surveillance specimens to assess the integrity of RPV. The possibility of applying validated non-destructive ageing monitoring techniques would however facilitate the surveillance of the materials that form the reactor vessel. The JRC-IE has developed two devices, focused on the measurement of the electrical properties to assess non-destructively the embrittlement state of materials. The first technique, called Seebeck and Thomson Effects on Aged Material (STEAM), is based on the measurement of the Seebeck coefficient, characteristic of the material and related to the microstructural changes induced by irradiation embrittlement. With the same aim the second technique, named Resistivity Effects on Aged Material (REAM), measures instead the resistivity of the material. The purpose of this research is to correlate the results of the impact tests, STEAM and REAM measurements with the change in the mechanical properties due to neutron irradiation. These results will make possible the improvement of such techniques based on the measurement of material electrical properties for their application to the irradiation embrittlement assessment.

Cu effects on radiation embrittlement of Ni-containing complex model alloys and the related potentials of the thermoelectric method

One of the major damage contributions to radiation embrittlement of steels is due to copper precipitation. Precipitation mechanism continues until saturation depending on available amount of precipitants, copper concentration in particular. At the JRC-IE measurement of thermoelectric voltage using a specially developed non-destructive method to assess the embrittlement state of materials are regularly performed. The results analysis, from the test of complex sets of nickel containing alloys with different copper content levels, for both fresh and irradiated conditions, are discussed in this paper with the aim of identifying, if copper precipitation to saturation can be followed by non-destructive measurements. The findings are promising showing that the STEAM non-destructive method has very high potential for studying the copper kinetics part in the radiation embrittlement phenomenon.

The effect of ductile crack growth on the temperature dependence of cleavage fracture toughness for a RPV steel with various degrees of embrittlement

The basic properties of cleavage fracture after ductile crack growth are studied on the basis of a probabilistic model for cleavage fracture and a deterministic model for ductile fracture, proposed earlier by the authors. Investigations are applied to a 2Cr–Ni–Mo–V reactor pressure vessel steel in the initial (as-received) and embrittled states. For various temperatures, the dependencies of brittle fracture probability on stress intensity factor and extent of ductile tearing are calculated. For various states of a material, the temperature dependencies of cleavage fracture toughness are predicted with and without regard for ductile crack growth. The basic factors controlling the above dependencies are analysed. The calculated results obtained are compared with test results from CT specimens.