Embrittlement Trend Curves for Notch and Fracture Toughness of 15Kh2MFAA Type Steel

Reactor pressure vessel integrity and lifetime evaluation is based on the use of fracture mechanics apparatus but most of the material vessel material data and their degradation during operation are based on results from Charpy V-notch impact tests. Then, empirical correlations between transition shift of temperature dependence of notch toughness and fracture toughness are applied. Elaboration of „Master Curve“ approach for fracture toughness experimental data analysis allows to use fracture toughness data directly to the reactor pressure vessel integrity evaluation. Wider use of this approach is limited by the lack of appropriate database from surveillance specimen test data, as mostly only Charpy impact specimen are included into the Surveillance specimen programs. Fortunately, all WWER Surveillance programs contain also fracture toughness specimens, either pre-cracked Charpy size or CT-0.5. Thus, database of fracture toughness data from Surveillance programs of WWER-440/V-213C type reactor pressure vessels, operated in the Czech Republic, Slovakia and Hungary and manufactured only by one manufacturer - SKODA JS, was collated and analyzed. These vessels were manufactured from 15Kh2MFAA type steel and appropriate weld metal, both of Cr-Mo-V type with low content of detrimental impurities — P and Cu. Analysis of the data in fluence interval up to 6×1024 m−2 (with neutron energies En larger than 0.5 MeV) show that transition temperature shifts in fracture toughness temperature dependence are higher than for Charpy impact tests. Several formulae have been applied for fitting these shift dependencies with chemical composition of materials and finally new Embrittlement Trend Curves for Charpy shifts have been corrected. Additionally, new Embrittlement Trend Curves for fracture toughness shifts based on “Master Curve” approach have been also proposed. Both trends are using simple power law on fluence with exponents around 0.6 and depend on phosphorus and copper contents even though effect of other elements has been also checked.