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Fusion In-vessel components, assembled and maintained using laser welding, one of the most promising techniques, exhibit complex distributions of residual stress, microstructures, and material properties. These residual stresses can compromise structural integrity and lifespan of critical components. Although using advanced experimental measurements can evaluate the residual stress for individual case, extending the measurements to massive number of components are costly and time-consuming. Traditional machine learning (ML) models struggle to account for the heterogeneity and anisotropy of these stress distributions. Here, we develop a novel ML framework based on the Eurofer97 steel, the structural material for in-vessel components. The ML framework is trained on high-resolution residual stress data derived from recently-developed evaluation techniques. Combining with microstructures, the model enables prediction of heterogenous and anisotropic residual stress distribution. It successfully predicts the compressive residual stress in fusion zone (∼−200 MPa) balanced by tensile residual stress in heat affected zone (∼300 MPa), aligning closely with experimental results with the R-squared value of 0.989 and the mean square error of 10−4. Unlike experiments that take hours, the ML model provides predictions within seconds. It offers valuable insights into residual stress prediction for various joints, enhancing the reliability and lifetime prediction of in-vessel components.

EUROFER97 steel samples were exposed to deuterium plasma in the linear plasma device PISCES-A with the ion energies of 90 and 140 eV at exposure temperatures between 360 and 773 K to deuterium ion fluences between 3.8 × 1023 and 6.3 × 1025 m−2. RBS analysis of these plasma-exposed samples showed that the uppermost surface layers were enriched with tungsten and tantalum. Deuterium depth profiles in the plasma-exposed samples were examined using the energy-scanning NRA technique allowing measurements of the deuterium depth profiles at depths of up to about 9 μm. With an increase in the exposure temperature from 360 to 773 K, the deuterium retention in the 1 µm thick sub-surface layer decreased almost monotonically from about 1 × 1019 D/m2 to 5 × 1017 D/m2. The deuterium concentration at a depth of 8 μm demonstrated a different temperature dependence - with an increase in the exposure temperature from 360 to 500 K, the deuterium concentration significantly decreased from about 4 × 10−3 to (4 ± 3) × 10−5 at.%. With a further increase in the temperature up to 773 K, the deuterium concentration at a depth of 8 μm became equal to or less than 5 × 10−5 at.%.

• The novel process improves the mechanical properties without compromising fatigue. • Fatigue cycling entails grain size increase due to the instability of grain boundaries. • As an effect of grain growth, the material softens with a hardness decrease up to 8 %. • The growth of micrometric grains driven by stress below the homologous temperature is a novelty. • It is necessary to assess whether the same phenomenon occurs in standard EUROFER-97 steel. The present paper has investigated the high cycle fatigue behavior of EUROFER97 steel submitted to a novel process consisting of cold rolling with reduction ratio of 80 % followed by a heat treatment at 650 °C for 1 h. This process had already proved to be highly effective under static loads leading to a significant improvement in yield stress. The fatigue tests have been performed with 0.2 load ratio at room temperature. Results indicate that the fatigue strength of the steel submitted to the novel treatment is comparable to that of the standard EUROFER97. Moreover, the fatigued samples underwent a microstructural evolution consisting of grain size increase and texture change due to the stress-driven instability of grain boundaries, especially LAGBs. The collapse of some boundaries involves partial annihilation and re-arrangement of dislocations, and grain coalescence. As a consequence of such microstructural change the material softens with hardness variations up to 8 %. [ABSTRACT FROM AUTHOR]

The present paper demonstrates the overview of ten years NRG effort in investigating the Eurofer97 steel mechanical properties response after neutron irradiation in the High Flux Reactor in Petten, and highlights the main trends found during analyses of the extensive mechanical data sets. [ABSTRACT FROM AUTHOR]

Highlights: [•] The surface heat flux load of 3.5MW/m2 produced substantial stresses and inelastic strains in the heat-loaded surface region, especially at the notch root. [•] The notch root exhibited a typical notch effect such as stress concentration and localized inelastic yield leading to a preferred damage development. [•] The predicted damage evolution feature agrees well with the experimental observation. [•] The smooth surface also experiences considerable stresses and inelastic strains. However, the stress intensity and the amount of inelastic deformation are not high enough to cause any serious damage. [•] The level of maximum inelastic strain is higher at the notch root than at the smooth surface. On the other hand, the amplitude of inelastic strain variation is comparable at both positions. [•] The amount of inelastic deformation is significantly affected by the length of pulse duration time indicating the important role of creep. [Copyright &y& Elsevier]

Highlights: [•] Clear evidence of microscopic damage and crack formation at the notch root in the early stage of the fatigue loading (50–100 load cycles). [•] Propagation of fatigue crack at the notch root in the course of subsequent cyclic heat-flux loading followed by saturation after roughly 600 load cycles. [•] No sign of damage on the notch-free surface up to 800 load cycles. [•] No obvious effect of the pulse time duration on the crack extension. [•] Slight change in the grain microstructure due to the formation of sub-grain boundaries by plastic deformation. [Copyright &y& Elsevier]

• Neutron irradiations cause hardening, ductility loss and strain hardening capacity reduction in Eurofer97 steel variants (∼3 dpa, 300 °C). • Post-necking ductility mainly contributes to the irradiated tensile elongation, with mostly ductile failures. • RA decreases after irradiation, and the irradiation-induced yield stress increase and loss in RA appear to be inversely correlated. • RAFM steel designing should target a large separation between plastic-instability stress and yield stress. Ten Eurofer97 steel variants, produced by non-standard fabrication-processing routes and modified alloying chemistries, were studied by neutron irradiations in the high flux isotope reactor. The irradiations were performed to ITER-TBM relevant conditions of ∼255–350 °C, 2.94–3.24 dpa. We quantified the irradiation-induced degradation of the steels using mechanical property tests. All the steels suffered from irradiation hardening, where a significant increase in Vickers microhardness and yield stress (σ YS) occurred, accompanied with severe loss of tensile elongation. The extent of hardening was material dependent. For T irr = 300±30 °C, most steels showed σ YS increase in the range of ∼30% to as high as ∼66%, except for a low temperature tempered steel with σ YS increase below 15%. Despite large losses in elongation, most failures were ductile. Significant post-necking ductility was retained with reduction in area (RA) between 65–75%, but <50% for low temperature tempered steels. The ultimate tensile stress to yield stress (σ UTS /σ YS) ratios decreased significantly after irradiation, highlighting irradiation-induced strain hardening capacity reduction. No major effect of irradiation on the plastic instability stress (σ PIS) and true fracture stress of the steels was observed. By comparing the tensile stresses in true stress units and with literature, the results suggest that RAFM steel designing should target materials with a large separation between σ PIS and σ YS , to ensure the materials can maintain large work hardening and uniform deformation capability after irradiation. The tensile data of the steels additionally revealed a compelling evidence of an inverse trend between the change in RA and increase in σ YS of the neutron irradiated Eurofer97 type steels. [ABSTRACT FROM AUTHOR]