Physical and Numerical Analysis of Extrusion Process for Production of Bimetallic Tubes Page: 89 of 108
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Figure 4.69 is the EBSD orientation map of the same region, and it shows orientations corresponding
to the ferrite (plain carbon steel) and austenite (stainless steel). It should be noted that there is a
distinct boundary between the two orientations, and that the plain carbon steel side corresponds to
ferrite, further confirming that the dark carbide phase is composed of ferrite plus cementite.
Color Coded Map Type: Inverse Pole Figure 
3.00 pm = 10 steps IPF 
Fig. 4.69. Electron backscatter diffraction orientation map and inverse pole figure of the regions
directly adjacent to the interface. Plain carbon steel (ferrite) is on the left; stainless steel (austenite) is
on the right.
4.3.6 Discussion of Results for Interface Study of Co-extruded Tubes
The interfacial bonding process can be separated into three different stages: primary bonding, bond-
surface extension, and elimination of original joining surface.26 Figure 3.11 is a schematic
representation of the development of the interface and the different stages: (a) faying surfaces are
covered by a layer of surface oxidation/film, (b) primary bonding: initial contact of surfaces causes
break up of surface oxide as asperities deform locally, (c) bond-surface extension: bulk plastic
deformation (metal flow) occurs which generates new film-free surface area, small amounts of
film/oxide are still trapped in the interface, and (d) elimination of original surface: film is dissolved
into solution, microstructural changes occur in the regions near the bond.
During primary bonding, Fig. 3.11 asperities and localized deformation act to break up surface
oxidation and surface layers (surface layers are indicated by the heavy black line in the figure). This
allows for metal-metal contact and the beginnings of the bond to develop. Fresh material is exposed,
allowing for initial diffusion between the materials. Surface oxidation may be present as a result of
room temperature or preheat surface oxidation. Oxides typically reduce diffusion of atomic species,
which prevents a full adherent and tenacious bond between the two alloys. Pressure at the interface
between the materials composing the pre-extruded bimetallic billet may result from either initial
deformation/upsetting in extrusion or residual stresses from "shrink fitting" of billet components.
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Misiolek, W. Z. & Sikka, V. K. Physical and Numerical Analysis of Extrusion Process for Production of Bimetallic Tubes, report, August 10, 2006; United States. (digital.library.unt.edu/ark:/67531/metadc884646/m1/89/: accessed December 15, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.