Temperature-dependent deformation and fracture properties of low-carbon martensitic steel in different stress states

The temperature-dependent ductile-brittle transition (DBT) in low-carbon martensitic steel is investigated in different stress states. To capture the effects of the stress state, tensile tests have been performed with flat specimens and three-point bending tests have been conducted with Charpy specimens at dynamic (impact) and quasi-static loading rates, covering a broad temperature range of 77 K–423 K. The results reveal that ductile-brittle transition temperature (DBTT) is significantly affected by stress triaxiality, wherein the transition temperature in uniaxial tension is ∼142 K lower than that in plane strain condition of Charpy specimens. The energy absorption capacity in the ductile resistance domain during three-point bending tests is sensitive to the loading rate. Moreover, dynamic strain aging poses a decrease in ductile resistance upon quasi-static loading when the temperature exceeds 323 K and thus without a specific upper shelf plateau. Tensile strength and ductility tend to increase with decreasing temperature, and the associated failure mode changes from shear-dominated to nearly pure tension due to the specific discrepancy of temperature influence on the critical normal and shear strengths, wherein the formation of shear bands that dominate the tensile deformation could be strictly inhibited by the formation of nanoscale dislocation cells based on dislocation analysis. The failure mechanisms in tensile tests correlate with the tensile specimens' macroscopic fracture angle. In the end, a new local parameter is proposed for temperature-dependent toughness estimation in tensile tests.