Identification of precipitate phases in an 11 pct Cr ferritic and martensitic steel after thermomechanical treatment

Abstract

An 11 mass% Cr ferritic/martensitic steel was subjected to a standard heat treatment consisting of normalizing and tempering and subsequently, a thermomechanical treatment (TMT) process involving austenization at 1100 °C for 1 hour, warm-rolling with 93% deformation at 650 °C and tempering at 650 °C for 1 hour. The precipitate phases of the TMT-processed steel were qualitatively analyzed using transmission electron microscopes and energy dispersive X-ray spectrometers in combination of lattice parameter calculation. Nb-rich MC carbides and Nb-rich M(C,N) carbonitrides pre-existing in the normalized-and-tempered steel were also observed in the TMT-processed steel. Eight types of precipitate phases introduced by the TMT were identified in the TMT-processed steel. They are Nd-rich MC carbide with a face-centered cubic crystal structure and lattice parameter a = 1.1408 nm, Cr-rich M₂C carbides/Cr-rich M₂X (Cr₂C type) carbonitrides/Cr-rich M₂X (Cr₂N type) carbonitride with a hexagonal crystal structure, Cr-rich M₇C₃ carbides/Cr-rich M₃C₂ carbide/V-rich M₂(C,N) carbonitride with a simple orthorhombic crystal structure, and Mn-rich M₅C₂ carbide with a base-centered monoclinic crystal structure. Among these identified precipitate phases, Nb-rich MC carbides, Nb-rich M(C,N) carbonitrides, Cr-rich M₂C carbides and Cr-rich M₂X (Cr₂C type) carbonitrides are dominant phases, while the other six precipitate phases are minor phases, in the TMT-processed steel. The identified precipitate phases are also discussed.