Thermomechanical properties of steel: Viscoplasticity of γ iron and γ Fe–C alloys

Abstract

The available data on minimum strain rate (‘secondary creep’) in creep tests and on maximum stress (first peak stress) in hot deformation tests obtained for γ iron and for γ Fe–C alloys have been reviewed and allocated in the form of an equation ɛ=4·6×10¹⁷ (σ/E)^3·0 exp (α%C) sinh (60·137σ/T) exp (−32 714/T), where ɛ is the strain rate (s⁻¹), σ is the stress (MN m⁻²), E is the elastic (Young's) modulus (MN m⁻²) according to E=230 950−99·04T, %C is the per cent carbon content, and T is the absolute temperature. The range of validity is from approximately Z′=ɛexp (32 714/T) exp (−α%C)=10³ to 10¹⁴ s⁻¹ (where Z′ is a modified Zener–Hollomon parameter), which covers the temperature range from 900 to 1400°C. The factor α describing the influence of carbon content is given by the expression α=2·3−2·1 log (60·137;amp;#x03C3;/T). Hence, the softening of austenite by carbon decreases with increasing stress and decreasing temperature. At very high σ/T ratios, the factor ;amp;#x03B1; may even become negative reversing the effect of carbon to strengthening. A procedure is suggested by which the given equations can be applied also for the description of primary creep.

MST/1386