High-temperature Laves precipitation and its effects on recrystallisation behaviour and Lüders deformation in super ferritic stainless steels

Abstract

The interaction of microstructural evolution, precipitation and mechanical properties of cold rolled 27Cr–4Mo–2Ni steels stabilised by Nb/Ti during annealing are investigated. Prior deformations provide an acceleration effect on bulk sigma precipitation and increase the starting precipitation temperature to ~1000 °C, which can induce a brittle fracture mode for annealed specimens. The precipitation temperature of Fe2Nb-type Laves phase is observed to be as high as 1050 °C, and these nanoscale Laves phases are intertwined with Nb2C particles, which are located at sub-grain boundaries. The abnormal relationship between the grain size and the annealing temperature, which is also related to the formation of Nb-containing particles, is determined. The transformation from Nb2C to Laves phase below 1050 °C release C atoms into the matrix around the Nb2C particles. This further leads to Lüders deformation during tensile test because of Cottrell atmosphere formation. The γ-fibre texture development during the annealing process is explained based on orientated nucleation and orientation growth theory. Super ferritic stainless steels exhibit a good combination of strength and ductility via Laves phase precipitation control during annealing at approximately 1050 °C for 5–60 min; these properties are advantageous for their application with strong γ-fibre texture. Keywords: Super ferritic stainless steel, Lüders deformation, Annealing, Recrystallization texture, Laves phase, Embrittlement