Deformação da carepa

Toward Oxide Scale Behavior Management At
High Temperature
Deltombe R. a b , Dubar M. a c , Dubois A. a c and Dubar L. a c
a
Univ Lille Nord de France, F-59000 Lille, France
b
LAMIH, FRE3304,Université de Valenciennes et du Hainaut Cambraisis, Le Mont Houy, 59313
Valenciennes cedex 9-France
c
TEMPO, EA4542,Université de Valenciennes et du Hainaut Cambraisis, Le Mont Houy, 59313
Valenciennes cedex 9-France
Abstract. Oxide scales grow freely on bare metallic surface under environmental conditions
such as high temperature and oxygen. These act as thermal and mechanical shields, especially
during high hot forming processes (>1000°C). But product quality can be impacted by these
oxide scales due to scale remaining on product or sticking on tools. Thus the TEMPO laboratory
has created an original methodology in order to characterize oxide scale under high temperature,
pressure and strain gradients. An experimental device has been developed. The final purpose of
this work is to understand the scale behavior as a function of temperature, reduction ratio and
steel composition.
Keywords: oxide scale, high temperature, metal forming
PACS: 81.40.Pq
INTRODUCTION
In hot metal forming, surface quality is impacted by many parameters. Surface
oxide layers are one of those. This layer appears and evolves during each industrial
process step associated with oxygen gas [3]. It is at the origin of defects on work piece
(roughness, oxide incrustations) and on tools (wear, galling). From an industrial view,
it is necessary to manage the oxide growing or removing from surface [3-4].
Oxide layer structure is granular. During its growing, oxygen gas grips, in a
coherent order, on metallic surface. Iron ions move and insert inside the existing
matrix with equiprobability. Oxide layer development follows a parabolic evolution as
a function of temperature and surrounding gas (oxygen) [5]. Thickness can be reduced
by adding components such as silicon. This element is present as oxide between steel
and iron oxide layer and is sticky and abrasive against tools. For low alloyed steel,
oxides come from Wustite (FeO) for 95%, Magnetite (Fe 3 O 4 ) for 4% and Haematite
(Fe 2 O 3 ) for 1%. Nevertheless, for high alloyed steel, other oxides grow such as
chromium and silicon oxides [6].
Behavior between tool, oxide layer and steel evolves as a function of oxide layer
characteristics (composition, temperature, structure). Indeed, thermal gradient, high
strains and stress (normal and tangential) appear at the interface. These induce
embrittlement, cracks, inserts or delamination of the layer [7]. In order to look at the
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oxide behavior under extreme thermomechanical conditions, an experimental testing
device has been developed at TEMPO laboratory on the basis of last studies [1,2]. Our
study is focused on the comparison between two kinds of steels, a low alloyed steel
and a chromium steel. Specimens are observed and analyzed in order to determine
oxide layer thickness and surface aspect.
OXIDE LAYER BEHAVIOR
Phenomena appearing at the tool, oxide layer and steel interfaces must be
understood. For a better comprehension of these phenomena, a specific device has
been developed to reproduce extreme thermomechanical conditions. Actually the
relative oxide brittle behavior compared to the steel behavior induces heterogeneous
strains (fig. 1) [8].
Parameters acting on oxide layer are numerous and often coupled. From a general
overview, reduction ratio which increases real contact area and thermal gradient at the
interface acts on the thermal shock. Reduction ratio causes the plastic deformation of
layer [9] and cracks initiate when local critical stresses are reached. As a consequence,
technological choices implied in the experimental device development, must be
representative of the industrial processes in order to transpose the results obtained in
laboratory to industry. A peculiar attention is made on oxide trapping and thermal
shock during contact.
FIGURE 1. Oxide strain during hot rolling [8]
Idle rolls have been chosen in order to focus loads on the contact area. This choice
enables to reduce the entry bank which induces oxide cracks by bending before
entering in the work zone (fig. 2 a b). This allows to trap oxide inside the contact area.
Tribochemistry at the interface must be respected in order to be representative of
the hot metal forming processes and the chemical affinities between tool, steel and
oxide. Thus, rolls are obtained by machining industrial tools used for hot metal
forming.
Specimens and tools are analyzed by scanning electron microscopy and are cut to
measure the deformed oxide layer.
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ACKNOWLEDGMENTS
The present research work has been supported by International Campus on Safety and
Intermodality in Transportation the Nord-Pas-de-Calais Region, the European
Community, the Regional Delegation for Research and Technology, the Ministry of
Higher Education and Research, and the National Center for Scientific Research. The
authors gratefully acknowledge the support of these institutions
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