A Model for Predicting Fracture Toughness and Scatter in Thermally Embrittled Steels

Temper embrittlement in a thick plate of a Mn-Ni-Mo pressure vessel steel was investigated using fracture toughness tests on Charpy and CT type specimens. A shift of the ductile-to-brittle transition temperature (~30–40 °C) was measured when the material was aged at 450 °C for 5000 h. Moreover an unusual scatter in fracture toughness tests was determined on aged material, such as \(K_{IC} (P_{R} = 90\,\% )/K_{IC} (P_{R} = 10\,\% ) \sim 5\), where P _R is the probability of failure. Scanning electron micrographs (SEM) indicated that the fracture surface was partly intergranular along micro-segregated zones (MSZ). This observation was made both on the initial and the aged conditions. Intergranular facets were largely covered by phosphorus segregation. A fully predictive model involving a combination of a local approach to fracture based on Beremin theory and accounting for MSZ distribution, and on the modelling of segregation kinetics in ternary (Fe-C-P) systems is developed to analyze these results. This model predicts the scatter in fracture toughness measurements and the shift in DBT. Moreover the statistical distribution of MSZ leads to a size effect in fracture toughness measurements which is different from the \(K^{4} B\) law inferred from the Beremin model applied to a homogeneous material.