Influence of nickel and vanadium on superplasticity in ultrahigh-carbon steels

Abstract

The development of cheap structural materials which will deform to large strains under small externally applied stresses during fabrication has been a constant goal for materials scientists, and whilst, in theory, superplasticity can be said to represent the achievement of this goal, in practice problems arise which have limited the range of use of superplastic materials. Either the materials have low strength at room temperature (e.g. Zn-22A1) or are expensive alloys (e.g. In 100, Ti-Al-4V). The present paper explores the influence of small additions of nickel and vanadium upon the formability and the superplastic properties of ultrahigh-carbon (UHC) steels. This new class of superplastic materials (containing 1.0% C to 2.0% C and the other usual steel-making elements, manganese, silicon and sulfur) overcomes the above objections. Nickel is a graphite-forming element whereas vanadium is a carbide stabilizer. This difference is reflected in the microstructures both during the warm working leading to fine structure development and during superplastic deformation. Nickel-containing steels exhibit some graphitization after extensive warm working, whereas vanadium-containing steels do not and very fine structures are developed in this latter steel. Upon superplastic testing, graphitization is enhanced in nickel-containing steels and this is detrimental to properties. Increased grain growth of the ferrite is also observed due to cementite depletion as the graphitization occurs. Good superplastic properties were found in the vanadium-containing steel. An elongation to failure of 817% was achieved in this steel at 650°C. All the steels were found to have better superplstic properties at temperatures just below the A 1 transformation temperature than at temperatures just above it. Room-temperature properties have also been assessed. High strengths were found with good associated ductilities in all the steels. The combination of superplastic formability at warm temperatures with attractive room temperature properties would suggest many applications for these new, inexpensive UHC steels.