Abstract
In this paper the abnormal behavior of stress relaxation during the rapid heating transformation of commercial pure iron, hypoeutectoid steel (S 45 C) and eutectoid steel (SK 5) has been examined using the apparatus originally designed for stress relaxation measurements. The stress relaxation rate-time curve during the rapid heating transformation was measured and analysed with respect to the transformation superplastic deformation. The main results obtained are as follows:(1) During the thermal cycle across the A1 or A3 transformation temperature, the stress relaxed perfectly independant of initial stress.(2) In pure iron two maximum rates of stress relaxation rate appeared at 630°C and near the starting temperature of A3 transformation, but only one maximum rate appeared in S 45 C and SK 5 at the starting temperature of A1 transformation. All the rates of stress relaxation depend on the initial stress and the heating rate.(3) By the stress-relaxation method, the mechanical behavior of transformation superplasticity can be described by the equation in the form(This article is not displayable. Please see full text pdf.) \ oindentwhere \dotγ is the strain rate, K is a constant, τ is the applied stress, and n is the stress exponent.
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