Grain Growth in Duplex Stainless Steels

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Abstract
Grain radii of α and γ phases in duplex stainless steels annealed at 1 000 -1 200°C have been measured, and the growth mode of α and γ grains has been investigated. The results obtained are as follows : (1) The grain size of duplex stainless steels is much finer than those of α and γ single phase stainless steels. This is caused by the facts that α and γ grains restrict the growth of partner's grains by the pinning effect, and the growth mode of both grains becomes similar to the Ostwald ripening. (2) In α-rich duplex stainless steels, the grain growth of both α and γ phases is controlled by volume diffusion in the α phase, and is described by the third power law (R3-R30=k3t). (3) In γ-rich duplex stainless steels, the grain growth is controlled by the diffusion along grain boundary of γ phase, and is expressed by the fourth power law (R4-R40=k4t). If the volume fraction of α phase is smaller than 0.1, however, α grains are mostly located within γ grains, and the grain growth is controlled by volume diffusion, obeying the third power law. (4) The grain radius of major phase, R is related to the grain raius, r and the volume fraction of minor phase, ƒ through the Zener's relation (R=βr/f;). The coefficient β is nearly 4/9, which has been proposed by HELLMAN and HILLERT.

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