クロムの動ひずみ時効

抄録

Tensile test for sintered and cast chromiums was carried out in vacuum at temperatures up to 1273 K to exartme the dynamic strain aging behavior. Three strain rates 2.4×10⁻², 2.4×10⁻¹ and 2.4 s⁻¹ were applied.
With increasing temperature, tensile strength decreased gradually at first and showed a minimum at a certain temperature. Above this temperature, the strength continued to increase to a maximum value. Serrated flow appeared between the temperatures of the minimum and maximum tensile strengths. A minimum tensile ductility evaluated by the fracture elongation was observed around the temperature at which the tensile strength became a maximum.
Relation between the strain rate \dotε and the temperature T of the maximum tensile strength could be well described by Arrhenius’ equation, \dotε=\dotε₀exp(−Q⁄RT), where \dotε₀ is a constant, Q is the activation energy for dynamic strain aging, and R is a gas constant. Activation energy of 101.3 kJ/mol was obtained, which was in good agreement with the activation energy for diffusion of the nitrogen atom in chromium.
It was found previously that chromium lost its rolling workability at temperatures above 1173 K owing to the large tensile stress in the rolling direction of the workpiece-surface in contact with rolls. The decreased rolling workability of chromium was related to the dynamic strain aging which brought about less ductility.