Isothermal decomposition of δ-ferrite in a 25Cr–7Ni–0.14N stainless steel Original Research Article

Isothermal decomposition processes of δ-ferrite in a 25Cr–7Ni–0.14N stainless steel have been investigated by means of transmission electron microscopy and Vickers hardness measurement. The specimens solution-treated at 1400°C were isothermally transformed at various temperatures between 450 and 600°C. Fine Cr2N platelets precipitate homogeneously in all the temperature range investigated. The orientation relationship between the Cr2N and the ferrite matrix can be expressed as; [101]δ//⌊12̄10⌋Cr2N and (1̄01)δ//(0001)Cr2N, the habit plane being (1̄00)δ//(1̄21̄2)Cr2N. Spherical Cu particles precipitate in the temperature range between 550 and 600°C. These Cu particles transform from bcc to 9R structure when their diameter reaches a critical size. Austenite formation is observed at temperatures above 550°C. Occasionally, the austenite particles exhibit twinned structure, and often nucleate on Cr2N platelets. R-phase precipitates after prolonged aging at 600°C.