Properties and fracture of structural steels with yield stress of 373–737 MPa in ambient to nil ductility temperature range

By lowering the testing temperature down to below the nil ductility temperature (NDT −20°C) the yield stress, tensile strength and the uniform elongation are increased, while the reduction of area is slightly decreased and the Charpy notch toughness strongly decreased. After strain ageing, the tensile properties are increased, the uniform elongation strongly decreased and the reduction of area slightly decreased. The effect of low temperature on the tensile properties is similar for as-delivered and strain-aged steels. The tensile fracture of as-delivered and strain-aged steels at the NDT −20°C is ductile, while the Charpy-V notch toughness (CVN) fracture is brittle at a significantly higher temperature. The fracture of CVN specimens in the transition temperature range is basically similar to the fracture of notched tensile specimens tested with loading rates in the range 0.1–1500 m/s at the NDT. The mechanism of CVN fracture is discussed.