THE STRAIN INDUCED CEMENTITE DISSOLUTION IN CARBON STEELS - EXPERIMENTAL
FACTS AND THEORETICAL APPROACH.
Yu. V. Ivanisenko, Dept. Of Mat. Sci., Ulm University, 89081 Ulm, Germany
W. Lojkowski, UNIPRESS PAN, 01-142 Warsaw, Poland
R. Z. Valiev, Inst. Of Physics Of Adv. Mat., USATU, 450000 Ufa, Russia
H.-J. Fecht, Dept. Of Mat. Sci., Ulm University, 89081 Ulm, Germany
Strain induced cementite dissolution at the room temperature takes
place at various modes of severe deformation - rolling, wire drawing,
High Pressure Torsion (HPT) and Ball Milling. Recently it was shown,
that cementite dissolution occurs on the surface of the railroad
treads as well, so this process is very important from the practical
point of view. Cementite dissolution accompanies by the gradual grain
refinement up to nanoscale. In spite of the progress in the
investigation of this phenomena, some problems remain unclear still.
For example, peculiarities and mechanisms of both the nanostructure
formation and dissolution of second phases, as well as a question
about the distribution of carbon atoms in the structure of the
severely deformed steel.
In the present research we studied the microstructure and phase
composition of the carbon steel UIC 860 subjected to the HPT in the
wide strain range and tried to correlate it with the dissolution
mechanism. It was shown, that decomposition of cementite proceeds not
monotonously with strain degree increase, and depends on the existing
type of the structure. At the early stages of deformation a very fast
cementite dissolution up to 40 % occures. Then, in relatively wide
strain range the degree of cementite dissolution remains almost the
same and only slowly increases up to 60 % . Within this range the cell
structure exist. The total cementite dissolution occurs at the N=5
turns, at the same strain degree the nanocrystalline structure with a
mean grain size of 10 nm forms. The increasing of the strain degree
doesn't lead to further grain refinement. On the base of the obtained
experimental results a model of strain induced cementite dissolution
is developed.
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