Environmental degradation of fracture resistance in high-temperature water environments of low-alloy reactor pressure vessel steels with high sulphur or phosphorus contents

The fracture behaviour of low-alloy reactor pressure vessel steels with various sulphur and phosphorus contents, different environmentally-assisted cracking (EAC) and temper embrittlement (TE) susceptibilities was evaluated by elastic plastic fracture mechanics tests in air and various simulated light water reactors environments. A moderate but clear reduction of fracture initiation resistance occurred in a) high-sulphur steel with high EAC susceptibility in oxygenated high-temperature water with aggressive occluded crevice environment and preceding EAC growth, and in b) high-phosphorus steel with high TE susceptibility, where the reduction of fracture resistance was most pronounced in hydrogenated high-temperature water.

Investigations and numerical modeling of mechanical properties of tempered martensitic steel Eurofer97 at various loading rates, temperatures and after spallation irradiation

Serafin Knitel

The reduced activation tempered martensitic steel Eurofer97 is a reference steel for a structural applica-tions in fusion reactors. The first-wall and blanket materials will be subjected to high thermal and neutron fluxes and will experience very complex, time-dependent mechanical loading. Owing to impinging neutrons in materials, points defects and gasesous impurities (hydrogen and helium) are created and accummulated in the form of small defect clusters in the microstructure altering the mechanical properties. Notably, fracture toughness decreases as strength increases. Eurofer97, being a body centered cubic alloy, presents a ductile-to-brittle transition temperature T0 that shifts to higher temperatures after irradiation. The main objective was to study the loading rate and irradiation effects on plastic flow and fracture properties of Eurofer97. Tensile and fracture tests were performed at temperatures in the transition region, at low and high loading rates on unirradiated material. Tensile properties of irradiated material were assessed after irradiation in the Swiss neutron spallation source SINQ to a dose of about 11 dpa and helium content of 500 appm at about 250 Â°C. The information gathered from the mechanical tests was used either as input in finite element models of fracture specimens, or as benchmark data to calibrate and verify the predictions of the models. Fractographic observations were also conducted. The tensile tests on unirradiated Eurofer97 showed a strong temperature dependence and strain rate sensitivity of the stress at low temperature, consistent with a mechanism of nucleation and propagation of double kink on screw dislocation. At high loading rate, the strain rate hardening was partially compensated by thermal softening due to quasi-adiabatic heating. The tensile properties after irradiation showed strong irradiation hardening (increase of the yield stress) accompanied by a drastic decrease of uniform elongation. The constitutive behavior at large strains, representative of those at crack tip, was obtained with an inverse method, based on finite element (FE) models of tensile specimens, developed to derive the true stress-strain curves from the load-displacement ones. These curves were used as input for the FE models of fracture specimens. Comparative fractographic observations on unirradiated and irradiated specimens did not reveal any strong effect of irradiation or helium on fracture mechanism, which remains transgranular cleavage. This observation indicates that higher helium concentration than 500 appm is necessary to initiate intergranular cleavage by helium bubble precipitation on grain boundaries. Dynamic fracture tests were performed on unirradiated Eurofer97. A shift of about 10 Â°C of T0 by one order of magnitude in loading rate was determined, which was in good agreement with results obtained on different ferritic steels. Static fracture toughness tests were also performed on disk compact tension specimens that can be accommodated in irradiation tubes of SINQ. Different FE models of fracture specimens were developed to include strain-rate sensitivity of flow stress, heating resulting from the plastic work in the fracture process zone and loading rate. The structure of the stress field at the crack tip was studied and the temperature dependence of a lower bound to toughness data in the transition was reconstructed using a calibrated criterion of local approach to brittle fracture.

EPFL2018

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

Zaiqing Que

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.

EPFL2019

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.