Abstract
A study has been made of the precipitation of M23C6 in a duplex stainless steel (26Cr–5Ni–1·3Mo–0·03C) heated in the range 650–850°C. In the fully ferritic condition M23C6 precipitates at α/α boundaries and on dislocations within the grains. When austenite crystals grow in the ferrite matrix M23C6 nucleates at the γ/α interfaces which subsequently move away from the precipitates. These carbide particles have a cube–cube relation with the austenite and a Kurdjumov–Sachs (K–S) relationship with the ferrite resulting in a three-phase crystallographic relationship, which confirms that the carbide forms on the interphase boundary. High-resolution electron microscopy using Moiré fringes and lattice-plane imaging allowed a detailed examination both of the orientation relations and of the interfacial structures. Interfacial dislocation arrays were observed which corresponded to those expected from the lattice mismatch. Precipitation of M23C6 occurred both on interfaces without dislocation arrays, but containing extrinsic dislocations, and also on interfaces with arrays where the carbides grew more rapidly. Ledges were observed on coherent γ/α interfaces, the density increasing with decreasing aging temperature, and because of their relatively slow movement M23C6 particles nucleated on these steps.