Formation Process of Microduplex Structure in (α+γ) Two Phase Stainless Steel

The effect of the amount of cold work (from 0% to 90%) of ferrite (α)) and the aging temperature on the precipitation behavior of austenite (γ) from α during the aging in (α+γ) two phase region have been investigated using Fe-25%Cr-7%Ni-3%Mo and Fe-28%Cr-4%Ni-4%Mo (α+γ) two phase stainless steels.The starting structure of the alloys before the aging is fully ferritic. It is found that the resulting structures are remarkably affected by the amount of cold work of α, the aging temperature and the aging time. When the amount of cold work is small (<25%), the precipitation of γ preferentially occurs at α-grain boundaries and deformation bands at an early stage of aging, and then γ particles precipitate inside α-grains at a later stage of aging. When the amount of cold work of α is large (>50%), the subgrain structure is rapidly formed by recovery in α-matrix during heating up to the aging temperature or during the earliest stage of aging, and then γ particles precipitate at these α subgrain boundaries. As the amount of cold work of α increases, size of α subgrains decreases, so that the precipitated γ particles become to disperse finely and homogeneously. In particular, when the amount of cold work of α exceeds about 70%, a microduplex structure, which consists of very fine α subgrains and γ particles, is obtained. However, it is found that a recrystallization of α matrix occurs during aging when the volume fraction of precipitated γ or the size of γ particles is small, because the pinning effect of γ particles on the migration of α subgrain boundary is deduced. Therefore, it is necessary to hold a large amount of cold work of α and to precipitate the large volume fraction of γ to obtain the microduplex structure.