The relationship between the transition and upper‐shelf fracture toughness of ferritic steels

Conventionally, the transition fracture toughness and upper-shelf fracture toughness of ferritic steels are viewed as separate properties: they are measured by tests conducted according to different standards, and neither toughness property can be estimated (except in very qualitative terms) based on knowledge of the other. Information presented in this paper demonstrates that quite the opposite is true: transition fracture toughness and upper-shelf fracture toughness are directly related because the microstructural features responsible for both the temperature dependence of fracture toughness and for the magnitude of fracture toughness at any given temperature are the same in both transition and on the upper shelf (the lattice structure controls the temperature dependence, while the size and distribution of second-phase particles control crack initiation and thus the magnitude of fracture toughness). These features bind the transition fracture tough. ness and upper-shelf fracture toughness together in a way that is extremely consistent for all ferritic steels. We present empirical evidence of this relationship based on fracture toughness data from a variety of ferritic steels: various heats of nuclear grade pressure vessel steels and welds (both before and after irradiation), copper precipitation hardened steels used in naval ship construction and mild steels used in ship construction. In total, these data span a broad range of To: from -180 °C to +140 °C. The combination of this theoretically motivated, empirically calibrated relationship between transition and upper-shelf toughness with Wallin's Master Curve (for toughness in fracture mode transition), and EricksonKirk's proposed Master Curve for toughness on the upper shelf, produces a model that predicts the temperature dependence of, and scatter in, fracture toughness of ferritic steels throughout the transition and upper-shelf temperature regimes. As input information, this model needs only the value of To, which can be estimated by performing fracture toughness tests according to the protocols of ASTM test standard E1921-05.