Forgeabilité des aciers inoxydables austéno-ferritiques

tel-00672279, version 1 - 21 Feb 2012
Chapter III. HOT CRACKING RESISTANCE 69
1380°C / 15 min
As-received material
1050°C
WQ WQ
eγ = 29 µm eγ = 60 µm
a) b)
1050°C
δ
γ -Widmanstätten
Figure III.25. Influence of the cooling rate on the austenite lath thickness in the D2 Widmanstätten microstructures.
III.4.3 γ-Equiaxed microstructure
In order to generate from the as-received material an equiaxed microstructure (E), that is thermally
stable at 1050°C, an annealing at 1250°C (temperature required to dissolve quickly the fine secondary
austenite, chromium nitrides, sigma or chi phase which could be present initially in the as-received
condition) was followed by water quenching (WQ). An isotropic microstructure with a volume fraction
of austenite close to the equilibrium fraction (%γeq = 35%) was obtained (Figure III.26.a).
As-received
material
1250°C / 4h
WQ
% γ = 36 ± 3 %
a)
1h
1050°C 24h 42h
WQ WQ WQ
γ 2 γ -Equiaxed δ
% γ = 51 ± 3 % % γ = 49 ± 3 % % γ = 49 ± 3 %
b) c) d)
Figure III.26. Schematics of the HTE.1 applied to D2 and resulting microstructures; white = γ, grey = δ;
a) 1250°C/4h + WQ; b) 1250°C/4h + WQ + 1050°C/1h + WQ; c) 1250°C/4h + WQ + 1050°C/24h +WQ;
d) 1250°C/4h + WQ + 1050°C/42h + WQ.