CHALEUR Project

Four batches of microstructurally homogeneous, low-alloy RPV materials, with the same chemical composition, were prepared by EDF R&D using a GLEEBLE device, which is a state-of-the-art instrument for heat treatment. Two reference materials from the foot and head of a large forging vessel were compared in general homogeneity studies. Additionally, the reference state from the foot underwent heat treatment to simulate long tempering and high austenitization processes. All batches were post-HT characterized using instrumented Charpy impact and tensile tests. The reference foot batch was also investigated using mini CT fracture toughness to compare the obtained reference temperature from the Master Curve T0 with the T41J of the Charpy impact test. A larger-than-expected shift was found between the two DBTT indications. Various benchmarks were performed between SCK CEN and EDF R&D, including round-robin tests and the effect of analyses and calculations from the micro-cleavage fracture stress temperature-stress diagram approach. The results are presented with respect to dispersion due to various analysis methods and the inherent variability of large forged components.

The objectives outlined in the technical proposal have been met.

Objective 1, which aimed to obtain “microstructurally homogeneous” low-alloyed steel forgings across different forging locations and heat treatments, have been achieved. This is supported by test repeatability, limited data scatter in Charpy results, and preliminary fracture surface analyses. However, full confirmation requires further microstructural and compositional characterization at the mesoscale using techniques such as electron microscopy, EBSD, and OES/EDS, which were out of the scope of the present project proposal.

Objective 2 was successfully completed by the user lab (SCK CEN). Four material batches were characterized through Charpy impact, tensile, and fracture toughness tests. Detailed analyses included DBTT curves, load diagram construction, micro-cleavage fracture stress calculations, flow curve assessment, and a Master Curve evaluation for one reference batch.

Objective 3, which focused on benchmarking the results and analyses between the two laboratories, was successfully accomplished. Benchmarks were conducted on several aspects, including dimensional measurements, instrumented Charpy impact curves, Charpy impact curve fitting (e.g., T56J, USE), stress-temperature diagrams, single fracture toughness tests, reference T₀ temperature obtained from Master Curve analysis, and comparative analyses of Charpy and fracture toughness data using different software. The benchmarking results demonstrated a high level of agreement between the two labs. In summary, all three objectives were met with minor deviations.

The material processing and testing were conducted systematically. Four batches of low-alloyed steel were prepared and characterized according to the proposal (Objective 1). Testing and data analysis (Objective 2) were handled thoroughly by SCK CEN. All batches underwent detailed mechanical testing, with comprehensive analysis of DBTT behavior, fracture mechanics, and flow characteristics. These tasks were completed on schedule and in line with the technical scope. For benchmarking (Objective 3), coordination between the infrastructure and user labs was effective. Dimensional benchmarks were completed successfully. Comparisons of Charpy, stress-temperature diagram and fracture toughness results show strong consistency between the two labs. Some minor issues were encountered during the course of the project. These issues, related to dimensional control, specimens’ engraving, and data acquisition and holding time during Charpy impact tests, were appropriately documented through internal non-conformity reports. In each case, corrective actions were identified and implemented, either through internal discussion or in collaboration with the external infrastructure partner (EDF) when necessary. As a result, all issues have been resolved without impact on the project timeline or outcomes. The project was very successful and delivered important input for the lifetime evaluation of light water reactors, contributing to a better and thorough understanding of the influence of heat-treatment and the associated microstructure on fast fracture properties of pressure vessel materials.

1. Mechanical properties of low-alloyed steels obtained by precise heat treatments and innovative testing procedures CHALEUR To be published in journal article (JNM, IJPVP or EngFractMech: expected in 2025).
2. The influence of heat treatment on the fast fracture properties of low alloyed steels Journal article (planned) N.A. 2026-06-01 (estimate, depends on submission, review process) Scientific community of interest N.A.
3. Mechanical properties of low-alloyed steels obtained by precise heat treatments and innovative testing procedures CHALEUR Conference proceedings IGRDM 2025 2025-10-18 Scientific community, NPP utilities, other research institutions N.A.
4. Fracture behavior of low-alloy RPV steels in the ductile–brittle transition region: influence of processing and test conditions Conference presentation (planned) Fontevraud 11 (2026) 2026-09-14 to 16 Scientific community, NPP utilities, other research institutions N.A.
5. Influence of microstructural features on Charpy impact toughness properties of low alloy steels Conference proceedings Euromat 2025 2025-09-14 Scientific community of interest N.A.

UCLouvain, SCK CEN, EDF R&D, CEA, VTT, JAEA, NRA/CRIEPI, IGRDM community, … (non-nuclear scientific community, steel producers, nuclear scientific community, …)