Identification structurale precise des carbures precipites dans les aciers faiblement allies aux divers stades du revenu, mecanismes de precipitation

Abstract

The crystallographic structure of synthetic iron carbides have been used to predict the X-ray diffraction patterns of ideal lattices. These diffraction patterns were systematically compared with real patterns from carbides extracted from a low alloy silicon-manganese steel at different stages of tempering. This method allowed unambiguous identification of the structural sequence in these steels: α′( L′2 0)→ ϵ( L′3) + α′→ η( L″3 1) + α″→ χ Fe 5 C 2( D 8) + α′→ θ Fe 3 C( DO 11) + α. martensite disordered ordered Hägg carbide cementite A mechanism of the structural sequence is proposed which invokes, to begin with, martensitic shearing (Jack) (α′→ϵ) followed by diffusion of the interstitial atoms (ϵ→η) and finally a “chemical twinning” (Hyde el al.) (η→χ) with simultaneous diffuion of the matrix atoms. The transformation (χ→θ) occurs progressively through variations in the periodicity of the twinning. The crystallographic coherency between the different carbides and the matrix were determined and verified by electron microscope diffraction patterns.