Physical and Numerical Analysis of Extrusion Process for Production of Bimetallic Tubes Page: 63 of 108
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extrusion, the effect on the tube eccentricity must be addressed. This section deals with measuring the
wall thickness of co-extruded tubes along the tube length and comparing the observed product with
the values of eccentricity.
The extruded tubes used in this project were composed of a stainless steel (UNS 530400, referred to
as 304) core and a plain carbon steel sleeve (UNSG10211, referred to as 1020). Testing was
performed on multiple billet geometries in reference to initial core and sleeve thicknesses. The
extrusion ratio, R, was equal to 10.6 for the results presented here (a subset of a larger experimental
matrix). Table 4.4 outlines the geometry of the billets tested before extrusion.
4.2.1 Eccentricity Measurements
The eccentricity measurements were made on the bimetallic tube produced from a billet design given
in Table 4.4. For each billet set-identified by the thickness of the stainless steel core material (i.e.,
Low or High)-there were three billets: one with a full-length core (designated by the numeral "1"
after the core description), one with the core shortened 10% of the overall billet length (designated by
the numeral "2" after the core description), and one with the core shortened 20% (designated by the
numeral "3" after the core description). The center-hole diameter was 38.1 mm, overall billet length
was 127 mm, and the overall billet diameter was 137.2 mm.
Table 4.4. Billet geometry identification for eccentricity measurements
ID Core thickness Sleeve thickness Core length Core length
Sample (mm) (mm) (mm) (% of billet)
HighCorel 8.9 40.6 127.0 100
HighCore2 8.9 40.6 114.3 90
HighCore3 8.9 40.6 101.6 80
LowCorel 6.4 43.2 127.0 100
LowCore2 6.4 43.2 114.3 90
LowCore3 6.4 43.2 101.6 80
Each billet set in Table 4.4 was preheated at 12000C for approximately 2.5 h in a box furnace with a
nitrogen atmosphere. The billets were lubricated with a graphite-based lubricant and extruded with a
mandrel attached to the dummy block on a horizontal hydraulic press of 1200-ton capacity. A
graphite pad (preheated with the billet) was placed between the dummy block and the billet so that the
entire billet could be extruded. This graphite pad deforms during extrusion and forces the entire billet
through the die so that no billet material is left in the container at the end of extrusion. The die orifice
diameter was 55.6 mm.
After extrusion, the tubes were cut into 50.8-mm-thick slices, with one side prepared for optical
measurements. Core and sleeve thicknesses were measured at four positions for each slice (12, 3, 6,
and 9 o'clock also refered to as North, East, South, and West, respectively) as shown in Fig. 4.32.
Prior to sectioning the tube, a reference groove was cut into the tube along the length so that the four
measurement positions were constant throughout the tube. Measurements were made using a macro
camera interfaced with a Leco 3001A Image Analysis software package.
Two assumptions were made so that the eccentricity could be calculated: the ID was constant and
equal to mandrel diameter (35.6 mm) and the outer diameter (OD) was constant, which was
determined by die orifice (55.6 mm).
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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/m1/63/: accessed November 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.