Strain Rate Effect on Strength and Orientation Effect on Toughness of Modern High-Strength Pipe Steels

Tensile properties (stress-strain curve) including effects of strain rate and temperature as well as fracture resistance curves are required for advanced engineering critical assessment (ECA) of imperfections discovered during construction or service, and for modelling dynamic fracture. However, these fundamental mechanical properties do not appear to be widely available for modern high-strength pipe steels. In this experimental investigation, seven modern high-strength pipe steels including X70, X80, X100 and X120 grades were tested. Tensile tests were performed using cylindrical specimens over a range of strain rate (0.00075 to 1 s-1) and temperature (23 to –150°C). For ferritic steels, the thermal component of flow strength Δσ = σ – σ0 where σ0 is the flow stress at room temperature and quasi-static rate follows a “master curve” constitutive equation, where T is the temperature in Kelvin andε  is the strain rate in s-1. Standard Charpy absorbed energies, and J-resistance and crack-tip opening displacement (CTOD) resistance curves using single-edge bend, SE(B), specimens, were determined with different specimen orientations. The initiation fracture toughness values (J0.2 mm and CTOD0.2 mm) were also determined. Generally, toughness was higher for longitudinal than for transverse specimens for the same notch configuration, and for surface-notched than for through-thickness-notched specimens for the same specimen orientation. These results provide historical perspective on trends in the evolution of strength and toughness.