Forgeabilité des aciers inoxydables austéno-ferritiques

tel-00672279, version 1 - 21 Feb 2012
Chapter I. Introduction 15
D1
Edge-crack < 5 mm 10 cm
a) b)
D2
Edge-crack > 50 mm 10 cm
Figure I.4. Coils at the end of the hot rolling process showing different levels of edge-cracking depending
on the grade; a) coil of the D1 grade with edge-cracks < 5mm; b) coil of the D2 grade with
edge cracks > 50mm.
I.2.2 Microstructures after continuous casting
The schematic equilibrium phase diagram provides qualitative indications about the origin of the as-
cast microstructures, see Figure I.5. From the liquid, the alloy solidifies into δ-ferrite (Figure I.5.a).
During cooling, the austenite precipitates by a nucleation and growth mechanism. Allotriomorphic aus-
tenite (γ-allotriomorphic) nucleates at existing δ-ferrite grain boundaries (Figure I.5.b). Widmanstätten
austenite laths nucleate from the allotriomorphic austenite with an orientation relationship with the
ferrite close to that of Kurdjumov-Sachs.
γ
L+δ+γ
c)
a)
b)
δ+γ
L
δ
L+δ
a)
δ primary
b)
δ primary
δ primary
δ primary
γ 1-allotriomorphic
c)
δ primary
δ primary
γ 1-allotriomorphic
γ 2-Widmanstätten
Figure I.5. Origin of the as-cast microstructure in duplex stainless steel; a) at high temperature
duplex stainless steel is entirely ferritic; during cooling, the austenite precipitates b) at existing ferrite
grain boundaries and c) inside the ferrite matrix with a lath morphology; thermal history explains
the as-cast microstructure.