Design of a superplastic alloy steel

Abstract

An iron alloy, containing a relatively low concentration of alloying elements, which shows good superplastic behaviour in the temperature range 900°–960°C, has been designed from first principles. The alloy, of composition Fe–4%Ni–3%Mo–1·6%Ti, was developed so that at high temperatures it would have a duplex α-γ structure containing approximately equal proportions of the two phases. Grain refinement was achieved by hot rolling in the two-phase field or by cold working and recrystallizing in the two-phase field. At elevated temperatures values of strain-rate sensitivity for the microduplex structure lay in the range 0·5–0·6, and under optimum conditions tensile elongations of >BOO % were obtained. Cavitation occurred during superplastic straining and failure was associated with the growth and interlinkage of cavities. In the air-cooled condition the alloy was fully ferritic and showed good room-temperature tensile properties. A similar alloy which had a lower titanium content and, as a consequence, a larger volume fraction of austenite, did not exhibit such marked superplastic behaviour. Tensile failure was again associated with cavitation but the addition of a trace amount of boron to the alloy led to a marked reduction in the extent of cavitation during deformation.