Impact strength and fracture features of 12 % chromium ferritic-martensitic steel EP-823 in temperature range from –196 to 100 °С

The authors investigated the patterns of fracture during impact bending tests and determined the values of impact strength and temperature of the ductile-brittle transition in temperature range from –196 to 100 °С of heat-resistant 12 % chromium ferritic-martensitic steel EP-823 in structural states after traditional heat (THT) and high-temperature thermomechanical (TMT) treatments. After THT, temperature of the ductile-brittle transition T dbt is approximately –45 °С, after HTMT – approximately –40 °С. At these temperatures, the impact energy ( KCV ) after THT is approximately 36 J/cm 2 , after HTMT – 32 J/cm 2 . Fractographic studies conducted by scanning electron microscopy of the fracture features of impact steel samples after two treatments (THT and HTMT) in the low-temperature test area (at cryogenic temperatures) showed a predominantly brittle nature of fracture, while fracture occurs by the mechanism of a transcrystalline quasi-cleavage. In the temperature range of the ductile-brittle transition, a mixed nature of fracture is observed, which passes through the mechanism of a transcrystalline quasi-cleavage with elements of ductile dimple fracture. In the temperature range from 50 to 100 °С, the extremely ductile nature of the fracture was detected, realized by the transcrystalline dimple fracture mechanism. After HTMT, there is a slight decrease (relative to THT) in the steel impact strength in almost the entire temperature range under consideration and, accordingly, an increase in the temperature of its ductile-brittle transition. This is due to the tests’ geometry, in which the direction of impact occurs in the plane of the layered structure, and it facilitates the formation of delamination cracks.