Forgeabilité des aciers inoxydables austéno-ferritiques

tel-00672279, version 1 - 21 Feb 2012
Chapter III. HOT CRACKING RESISTANCE 45
Chapter III. Hot cracking resistance
The characterization of the high temperature fracture resistance of metallic materials and the under-
standing of the influence of microstructural parameters remain an experimental and theoretical chal-
lenge. The main difficulty is the definition of a relevant fracture parameter that properly quantifies the
controlling fracture phenomenon, for example during high temperature roughing operations. In some
applications, the resistance to damage initiation and growth is the main issue. In that case, the high
temperature fracture strain measured on tensile specimens usually provides valuable information. In
some other forming operations, the resistance to cracking initiation from a pre-existing defect is the
main issue. Finally, the tearing resistance, i.e. the resistance to the propagation of a crack, is probably
the key property in edge cracks issues during hot rolling.
The outline of this chapter is as follows. The main concepts of the Essential Work of Fracture are
summarized in section III.1. The experimental procedures are described in section III.2. Then, the
following materials will be discussed in this chapter:
� D1 versus D2 in the as-cast conditions (section III.3); these preliminary results obtained on as-
cast materials will reveal the necessity of generating model microstructures in which the mor-
phology, the volume fraction and the size of both phases will be well-controlled (section III.4).
� D1_W versus D2_W (section III.5), where ‘W’ means Widmanstätten and refers to the mor-
phology of the austenite. This comparison will allow investigating the effect of the major alloy-
ing elements.
� D2_W versus D2_E (section III.6), where ‘E’ means equiaxed that corresponds to a different
austenite morphology compared to the Widmanstätten one.
� D1_W versus D1bis_W (section III.7), where D1bis_W is the same material compared to
D1_W, i.e. with the same chemical composition but arising from a different industrial casting.
This will allow investigating the influence of the impurities.
III.1 Literature review about the Essential Work of Fracture
concept
The essential work of fracture (EWF) concept was introduced by Cotterell and Reddel [70] as a mean
of quantifying the fracture resistance of thin ductile metal sheets. The basic idea is simple. The EWF
concept was introduced to handle ductile fracture from the point of view of complete fracture of a spe-
cimen and not from cracking initiation measurement such as in the traditional fracture mechanics ap-
proach. The purpose of the method consists in separating, based on dimensional considerations, the