Difference between the tough-to-brittle transition temperature and the plasticity-free transition temperature

In the context of mechanics and temperature, there are two main concepts related to metals. One is that at high temperatures, metals soften and transition from a solid state to a liquid state; they change from being plastic bodies to rheological bodies. Rheological bodies represent an intermediate state between solid and liquid states, and such deformation is not a brittle one. As the temperature of a metal decreases, it transforms from a plastic material to a brittle one; this temperature is known as the material’s brittle transition temperature. Because in the vast majority of materials, at certain temperatures, atoms move with difficulty, and dislocations—the basis of plastic deformation—cannot move. The material changes from plastic to brittle. The national standard GB/T6803-1986 \"Method for determining the temperature of plasticity loss in ferritic steels – Drop hammer test\" (NDT), and the American standard ASTM E208-81 \"Standard method for determining the temperature of plasticity loss in ferritic steels – Drop hammer test\"”

Toughness-fragility transition temperature: This refers to the change in the internal crystal structure of steel as temperature changes, which in turn leads to corresponding changes in the steel’s toughness and fragility. 1. Low-temperature condition: When the temperature drops to low levels (depending on the type of steel), steel that is normally tough loses its toughness and becomes as brittle as a glass rod, easily breaking. Therefore, steel used in cold regions (such as Siberia in winter and the Arctic and Antarctic) must be of a type suitable for such cold conditions. Low-temperature brittleness is influenced by the dislocation migration force, known as the Peierls force. At low temperatures, it is difficult for the Peierls force to cause dislocations to move, which leads to a sharp increase in the material’s yield strength; at a certain temperature, this yield strength equals the fracture strength. This temperature is the ductile-brittle transition temperature. As the temperature continues to drop, the yield strength increases further, exceeding the fracture strength; therefore, at low temperatures the material undergoes brittle fracture without any plastic deformation. The fracture strength of the material is little affected by temperature. 2. Hot steel: Steel has a basically crystalline structure. When the temperature rises to 200–300°C, the increase in internal energy causes the crystal bonds to break. At this point, the steel is still in a relatively hard solid state, so it becomes brittle and prone to breaking. 3. Toughness-to-fracture transition temperature: For body-centered cubic crystal metals and alloys, or certain hexagonal close-packed crystal metals and alloys, when the temperature drops below a certain value tk, the material transitions from a tough state to a brittle state; this temperature is known as the toughness-to-fracture transition temperature.

The toughness of ferritic structural steel decreases as the temperature drops. When the temperature reaches a certain value, a critical point is reached at which the fracture pattern is entirely that of crystalline brittle fracture; this temperature is known as the non-ductile transition temperature (NDTT)