A Study on the Blue-brittle Behaviour of a Mild Steel in Tortional Deformation

Abstract

The blue-brittle behaviour of a mild steel in torsional deformation was examined at strain rates of 1.1×10⁻³, 3.4×10⁻², 3.1×10, 9.0×10 and 1.8×10² sec⁻¹ between room temperature and 700°C.
The temperature of the maximum flow stress and the temperature of the minimum strain to failure increased with increasing strain rate.
The former was lower than the latter at the lower strain rates of 1.1×10⁻³ and 3.4×10⁻² sec⁻¹, while at the higher strain rates both of the temperatures were nearly eaqual. The relationship between the strain rates and the testing temperatures of the blue-brittleness depends not only on the diffusion coefficients of nitrogen and carbon in α iron, but also on the concentrations of nitrogen and carbon in it. At the higher strain rates, the specimen was heated by the dissipated mechanical energy and the blue-brittleness appeared at a rather low temperature.
The blue-brittle behaviour at the lower strain rates, which was observed below about 350°C, may be mainly due to the strain aging by nitrogen atoms.