Precipitation of M₂₃C₆ at austenite/ferrite interfaces in duplex stainless steel

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.