Superplastic behavior of an Al-bearing eutectoid steel

Abstract

Superplasticity is the ability of some polycrystalline materials to achieve extremely large uniform elongations in tension, prior to failure. Most superplastic materials consist of two phases, where the main role of the second phase is stabilizing the grains against grain growth. Early attempts to develop superplasticity in plain carbon steels (carbon content lower than 1%) were unsuccessful [1–3]. This was attributed to the low carbide content (e.g., 12% in an eutectoid steel) necessary to prevent grain growth. The increase of the carbon content between 1.5% to 2.5% lead to the development of steels now designated as ultra high carbon steels (UHCS), which, properly processed, show superplastic behavior. A recent investigation [4] showed, however, that superplastic elongations in steels depends not only on the carbide fraction but also on the carbide size and distribution. The effect of some alloying elements on the superplastic properties of UHCS has been investigated and reported in several papers [5–7]. Best superplastic properties have been found in an UHCS-Al alloy, elongation higher than 1000% at high strain rates have been obtained in this kind of material. The beneficial effect of aluminum in UHCS has been demonstrated in steels with carbon contents higher than 1% [8–10], however, the effect of aluminum in steels with carbon content lower than 1% has not been investigated extensively. In this work the effect of aluminum on the superplastic properties of an eutectoid steel (0.8% C) has been investigated. In order to investigate the effect of aluminum additions on the superplastic behavior of an eutectoid steels, an Al-bearing steel with 0.8% C was melted in an electric tube furnace under a protective argon atmosphere. The chemical composition of the steel is given in Table I. The ingot was homogenized at 1100 ◦C for three hours and then thermomechanically processed. In order to produce the fine microstructure required for superplasticity, the ingot (25 mm thick) was rolled during cooling in the temperature range from 1100 ◦C to 750 ◦C to a final thickness of 5 mm. Tensile test specimens (5 mm width, 3.5 mm thickness and 15 mm length) were machined from the rolled plate in the rolling direction. Strain-rate-change tests at various