Abstract

Direct-quenched structural steels are a cost-effective ultra-high-strength solution for demanding applications. These untempered, mainly S900 and S960 grade steels can possess good impact toughness and weldability when they contain low carbon contents and have low carbon equivalents. However, it is reported that as regards brittle fracture toughness these steels do not follow the commonly used correlation between the Charpy-V impact toughness transition temperature T_28J and the fracture toughness reference temperature T₀, i.e. T₀ = T_28J − 18 °C. These T₀ estimates are on the unconservative side, so there is a risk of overestimating the brittle fracture toughness of these steels in structural design when relying solely on impact toughness transition temperature values. In this study, the correlation between T₀ and T_28J temperatures of low-carbon ultra-high-strength martensitic and martensitic-bainitic steels in the quenched state is analyzed. In total, 78 new and re-analyzed data sets are reported i.e. data for 39 steels tested in both longitudinal and transverse orientations. These data sets are then evaluated using the procedures found in the literature. A recently updated T₀ − T_28J correlation is tested and it is shown that it gives less unconservative estimates of T0 by including the effects of yield strength and upper shelf energy. Finally a new correlation between T₀ and T_28J for as-quenched low-carbon steels is proposed, i.e. T₀ = 0.8*T_28J + 14 °C.