High-Temperature Water Effects on the Fracture Behaviour of Low-Alloy Reactor Pressure Vessel Steels

The structural integrity of the reactor pressure vessel (RPV) of light water reactors (LWR) is of utmost importance regarding operation safety and lifetime. High-temperature water (HTW) and hydrogen absorbed from environment, in synergy with irradiation embrittlement, dynamic strain aging (DSA), environmentally-assisted cracking (EAC) or temper embrittlement (TE) may reduce the fracture resistance of RPV steels. The fracture behaviour in the upper shelf region of low-alloy RPV steels with different microstructures and DSA, EAC and TE susceptibilities in various simulated LWR environments was evaluated by elastic-plastic fracture mechanics tests.

In the reference RPV steels with low sulphur and phosphorus contents and low DSA, EAC and TE susceptibilities, environmental effects on fracture resistance are absent or marginal in both oxygenated and hydrogenated HTW. However, moderate but clear reduction of fracture initiation resistance occurred in: a) Simulated coarse grain heat-affected zone material with high yield stress, since a higher yield stress facilitates the hydrogen enrichment in the fracture process zone; b) Low-sulphur RPV steels with high DSA susceptibility, where the reduction of fracture initiation resistance increased with decreasing loading rate at 288 °C and was most pronounced in hydrogenated HTW due to the localization of plastic deformation by the interaction between DSA and hydrogen; c) High-sulphur RPV steels with high EAC susceptibility in aggressive occluded crevice environment (oxygenated HTW with addition of impurities) with preceding EAC crack growth, resulting from the enhancement of hydrogen availability and uptake; and d) High-phosphorus RPV steel with high TE susceptibility, where the reduction of fracture initiation resistance was most pronounced in hydrogenated HTW, indicating that TE effect dominates over effects of occluded crevice chemistry.

In hydrogenated HTW, the reduction of fracture resistance correlated fairly well with the DSA susceptibility of different steels and sulphur content played no or a minor role. In oxygenated HTW, the reduction of fracture resistance increased with steel sulphur content for steels with low DSA susceptibility. Stable ductile transgranular tearing by micro-void coalescence dominates in both air and HTW environments. The fracture surface of specimens tested in air was mainly ductile. In contrast, specimens tested in HTW environments, varying amounts (a few %) of secondary cracking, macro-voids, quasi-cleavage and intergranular cracking were observed on fracture surface. The observed fracture modes and morphology suggest a combination of hydrogen-enhanced local plasticity and hydrogen-enhanced strain-induced vacancies mechanisms with minor contributions of hydrogen-enhanced decohesion embrittlement mechanism.

The main reason for the moderate degradation effects is the low hydrogen availability in HTW together with a high density of (fine-dispersed and strong) hydrogen traps in RPV steels. In addition, the environmental reduction and softening by hydrogen and HTW environments was partially compensated by the toughness increase due to DSA. Although the environmental effects are moderate, effects can be more critical for RPV materials with low initial upper shelf toughness under unfavourable combinations (high sulphur content, increased strength, high EAC, TE and DSA susceptibilities) or old plants with small margins regarding irradiation embrittlement.

Constitutive behavior and fracture properties of tempered martensitic steels for nuclear applications

Raul Alejandro Bonadé

In this study, the tensile and fracture properties and the microstructure of the reduced-activation tempered martensitic steel Eurofer97 have been investigated. This technical alloy is a 9%Cr steel developed within the European fusion material research program. To a lesser extend, the plastic flow properties of a equiaxed ferritic Fe9%Cr model alloy were also studied for comparison with those of the tempered martensitic structure. The main objectives of this work are described hereafter: to correlate the microstructural features with the plastic flow properties measured by the tensile tests for both the Eurofer97 steel and the model alloy. The correlation established should be reflected in a physically-based model of plastic flow. to study the fracture properties of the Eurofer97 steel in details in the lower ductile-to-brittle transition region. to calculate with finite element modeling the stress fields at the crack tip. This information is further used in conjunction with a local approach model for quasi-cleavage to reconstruct the fracture toughness-temperature curve. Tensile tests were carried out at different imposed nominal strain rate at several temperatures from 77K up to 473K, on both the Eurofer97 steel and the Fe9%Cr model alloy. The temperature dependence of the yield stress was precisely determined. As expected for body-centered cubic (BCC) materials, a strong increase of the yield stress by decreasing temperature, below 200 K, was observed. At higher temperatures, the temperature dependence of the yield stress was found much weaker, being associated with the temperature dependence of the shear modulus. Efforts were made to analyze in details the post-yield behavior (strain-hardening) as a function of temperature. The post-yield behavior was modeled using the Kocks phenomenological model based on the competition of storage and annihilation of dislocations. While this model was originally developed for face-centered cubic (FCC) metals where the rate-controlling mechanism of dislocation motion is the dislocation-dislocation interaction, we used is model in the high temperature domain (T>200K) of BCC materials, to model the strain-hardening evolution of the Eurofer97 steel and the model alloy. The values obtained for the key parameters of the model, namely the dislocation mean free path and annihilation coefficient, were found consistent with the microstructural features. The parameters temperature-dependence observed was also consistent with the physics of two basic mechanisms of dislocation storage and annihilation (dynamical recovery). For the low temperature domain, the strain-hardening model was modified to account for the strong Peierls lattice friction, based on an original idea of Rauch. Transmission electron microscopy observations were done to characterize the undeformed microstructures and their evolution with strain. A clear correlation was established between the stress-dependence of the strain-hardening and the microstructures. Fracture toughness tests on the Eurofer97 steel were performed with 0.2T C(T) and 0.4T C(T) specimens in the lower transition. Five temperatures were selected at which many tests were repeated to determine the amplitude of the inherent scatter of quasi-cleavage. The temperature were chosen in such a way that the measure fracture toughness remain below 150 MPa m1/2, which correspond for the 0.4T C(T) at a M value larger than 70 at the highest temperature. Such a M value for 0.4T C(T) specimens is known to ensure enough constraint. An attempt to analyze the experimental data in the framework of the master-curve approach by following the ASTM E-1921 standard was done. It was clearly demonstrated that the assumed shape of the toughness-temperature curve as described in the ASTM E-1921 standard for the reactor pressure vessel steels is not adequate for the tempered martensitic Eurofer97 steel, which present a particularly steep transition. Our Eurofer97 fracture database was then compared to the existing one on another similar steel, the F82H. Differences in the amplitude of the scatter of both steels was found while the lower bound of the toughness-temperature curve, describing the 1% failure probability was shown to be the same. 2D finite element simulations of the compact tension specimens were performed at various temperatures using the constitutive laws determined previously. The stress field around the crack tip were calculated and used to determine a local criterion of quasi-cleavage. The criterion is defined by the attainment of a critical stress encompassing a critical area. The lower bound of the toughness-temperature curve was then successfully reconstructed by using this local criterion. Finally, the relationship between the critical area and the applied stress intensity factor for the C(T) specimen was shown to follow a power law whose coefficients are dependent on the real dimensions of the specimen. Such a relationship allows scaling the C(T) toughness data from one size to another in case of in-plane constraint loss.

EPFL2006

Liquid lead-bismuth embrittlement effects on unirradiated and irradiated ferritic/martensitic steels for nuclear applications

Bin Long

In liquid metal spallation targets such as the MEGAPIE (the megawatt pilot experiment) target and the future ADS (accelerator driven system) spallation targets, which utilize liquid lead-bismuth eutectic (LBE) as the target material, liquid metal embrittlement (LME) effects on the target structural materials are considered as one of the critical issues that determine the lifetime of the targets. However, nowadays, the LBE embrittlement effects are not yet well understood, particularly under irradiation conditions. The aim of this work was: (a) to investigate the effects of LBE embrittlement on the mechanical properties of ferritic-martensitic (FM) steels (candidate materials for applications in spallation targets) in various states, and (b) to study the mechanisms of LBE embrittlement effects on the mechanical properties of FM steels. These two goals have been reached by studying LBE embrittlement effects on the mechanical properties of the T91 steel in various conditions: (i) the standard normalized and tempered condition, (ii) hardened by tempering at lower temperatures, and (iii) after irradiation under high energy proton and spallation neutron mixed spectrum. Mechanical tests such as slow-strain-rate tensile (SSRT) tests and 3-point bending tests have been performed to characterize the mechanical properties of the T91 steel and to determine the effects of LBE embrittlement on these mechanical properties. Microstructural analyses such as transition electron microscopy (TEM) and scanning electron mirocopy (SEM) observations have been conducted to obtain the microstructural information needed for understanding the embrittlement mechanisms. SSRT tests on the T91 steel in the standard metallurgical (SM) state, tempered at 760°C and denoted as HT760, revealed that it may encounter LBE embrittlement in the temperature range of 300 to 425°C. For the first time, a well defined "ductility trough" for a FM steel – LBE system was evidenced from the reduction of the fracture strain (or total elongation) of the T91 FM steel in this temperature range. SEM observations of the fracture surfaces showed that the fracture mode of the specimens suffering embrittlement effects is brittle transgranular cleavage. SSRT tests on the hardened T91 steel, tempered at 500 or 600°C and denoted as HT500 and HT600, respectively, showed more pronounced LBE embrittlement effects. In comparison to that of HT760, the "ductility troughs" of HT600 and HT500 cover a wider temperature range. The LME onset temperature TE is not exactly known but most probably lower than 150°C, which means close to the melting point temperature (125°C) of LBE. The ductility recovery temperature TR of HT500 is higher than that of HT760 and HT600. The results of SSRT tests on the HT600 and HT500 materials demonstrate clearly that the LBE embrittlement effects on the tensile properties of FM steels are strongly enhanced by hardening. Both the influenced temperature range and the degradation level increase with the hardening amount. The fracture mode of HT600 and HT500 materials tested in liquid LBE at T ≤ 450°C showed mainly brittle transgranular cleavage. SSRT tests on the T91 and F82H FM steels irradiated at temperatures in the range of 140-500°C to doses in the range of 0.048 to 20 dpa revealed significant irradiation-induced hardening and embrittlement effects (loss of ductility) as compared to the unirradiated ones. The SSRT tests performed on irradiated specimens in liquid LBE revealed that the specimens undergo a further reduction in ductility. The ductility of some irradiated specimens is reduced to a very low level of about 2-3%. 3-point bending tests on the T91 steel showed that the load-displacement curves of specimens tested in liquid LBE are different from those of specimens tested in Ar. The evaluated J-integral values of specimens tested in liquid LBE are always lower that of the specimens tested in Ar. It means that the crack propagation in a specimen tested in liquid LBE needs less energy as compared to a specimen tested in Ar. These results suggest that the LBE embrittlement is so-called "crack propagation controlled" rather than "crack nucleation controlled". SEM observations revealed that the crack propagation mode undergoes a ductile-to-brittle transition. In liquid LBE the fracture toughness of FM steels can be reduced by about 20% as compared to that in inert Ar environment. 3-point bending tests on the hardened T91 materials HT500 and HT600 demonstrated that the LBE embrittlement effects are much more drastic for hardened materials, similarly to what was observed as a result from SSRT tests. The JQ value is reduced by as much as 95% for both HT600 and HT500 materials tested in liquid LBE. The hardened materials exhibit unstable crack propagation behavior in liquid LBE in a wide temperature range of 150 to 500°C. 3-point bending tests on the irradiated T91 steel revealed not only irradiation-induced embrittlement effects but also LBE embrittlement effects. The fracture toughness of the irradiated specimens was found to decrease further as a result from testing in liquid LBE. SEM observations of the fracture surface showed a mixture of intergranular and transgranular fracture. The LME phenomena evidenced for the T91 steel – LBE couple could be interpreted using the model of adsorption-induced reduction in cohesion of atomic bonds combined with the Kelly-Tyson-Cottrell (KTC) criterion (σ/τ ≥ σmax/τmax). According to the KTC criterion, the tendency to fail by cleavage increases as (σmax/τmax) decreases. The σmax value can be greatly reduced by adsorption of liquid Pb or Bi atoms at crack tips, while the τmax value can be increased by hardening the steel by means of either deformation, or tempering, or irradiation. Practically, the present results indicate that a great attention should be paid to application of FM steels in liquid LBE, since the mechanical properties of FM steels, such as the ductility and the fracture toughness, can be substantially degraded by LBE embrittlement effects, particularly under irradiation conditions as the LBE embrittlement are enhanced by irradiation-induced hardening. Fundamentally, the observations made in this work suggest that the mechanism responsible for liquid LBE-induced embrittlement effects on FM steels can be essentially interpreted by using the adsorption-induced reduction in cohesion of atomic bonds model combined with the KTC criterion for brittle cleavage fracture.

EPFL2009

Effect of dynamic strain ageing on environmental degradation of fracture resistance of low-alloy RPV steels in high-temperature water environments

Zaiqing Que, Hans-Peter Seifert, Philippe Spätig

Elastic plastic fracture mechanics tests on different low-alloy reactor pressure vessel (RPV) steels in air and various simulated light water reactor environments revealed a clear, but moderate reduction of the fracture initiation resistance of RPV steels with high susceptibility to dynamic strain ageing (DSA) in high-temperature water (HTW). In HTW, the reduction of fracture initiation resistance increased with decreasing loading rate and was most pronounced in hydrogenated HTW. The stronger reduction of fracture resistance in hydrogenated HTW was mainly due to the localization of plastic deformation by interaction between DSA and hydrogen.