Plastic Torsional Deformation and Fracture of Low Carbon Steel at Low Temperatures
1965 Volume 51 Issue 1 Pages 59-64
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1965 Volume 51 Issue 1 Pages 59-64
The present investigation was carried out for the effects of testing temperatures and the ferrite grain diameters on the low temperature torsional deformation and fracture behaviour of O'08 percent carbon steel, using a newly developed dynamic torsion machine.Dynamic torque-twist curves were obtained photographically at various temperatures.
Variation of lower yield stress ry with grain size 2d in torsional deformation satisfied the Petch formula τy=Ti+kyd-1/2, which was obtained from tension test.Value of τi increased rapidly with a decreasing of the testing temperature and also ky increased with a lowering of the testing temperature.It is shown experimentally that the flow stress τƒ at constant strain is related to the grain diameter by τƒ=τiƒ+kƒd-1/2 where τƒ and k are constants. Value of kf at various testing temperatures is lower than those of ky and kƒ has a small sensitivity to temperature.
At the slow rate of straining, a transition from ductile to completely brittle behaviour occurs at -183°C, when the grain diameter is d-1/2≤5.23. Substituting values of τy, d-1/2and Ky obtained in this experiment into Cottrell's formula of ductile to brittle fracture, the effective surface energy γ is found to be γ≥7.45×103 erg/cm2. This value of the surface energy for fracture is expected to be greater than the true surface energy, but it is the same value as that of γ obtained by tensile test.
Large plastic deformation to fracture took place in the torsion test, even in the specimen broken by cleavage fracture.
