Abstract
Decomposition processes of the body-centered cubic (bcc) δ-ferrite phase in a 25Cr–7Ni–0.14N stainless steel have been investigated at temperatures below 600°C by means of differential scanning calorimetry (DSC), electric resistivity measurement (ERM) and transmission electron microscopy (TEM). Specimens were solution-treated at 1400°C for 15 min to produce δ single-phase structure and then quenched into iced water. The quenched specimens were continuously heated at various constant rates. During the heating up to 600°C, the reactions occurred in the following sequence: concentration fluctuation of nitrogen (N); chromium (Cr) modulated structure; homogeneous precipitation of fine Cr2N platelets; growth of the Cr2N platelets with eliminating the concentration fluctuations; and precipitation of spherical Cu particles. The orientation relationship between the Cr2N and the δ phase matrix can be expressed as [101]δ//[1210]Cr2N and (101)δ//(0001)Cr2N, the habit plane being (100)δ) d //(1212)Cr2N The interface between the Cr2N and the δ phase was coherent, though slight lattice displacements existed. The spherical particles are probably bcc copper (Cu).
