Physical and Numerical Analysis of Extrusion Process for Production of Bimetallic Tubes Page: 29 of 108
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14+ L L
1200 -- --
Nkel content. wt%
25 20 15 10 S 0
5 10 15 20 25 30
Chromium c rrini, wt%
Fig. 3.4. Vertical section of Fe-Ni-Cr ternary phase
diagram at 70% Fe.
The Cr content of the 304 stainless steel is approximately 18% (Table 3.2), which at the extrusion
temperatures of 1100-12000C is fully austenitic is located in the phase field that is continuous to
Table 3.2. Chemistry in weight percent of the steels obtained via arc-spark spectroscopy
Alloy C Cr Mn Ni P S Si Fe
1020 0.19 - 0.71 - 0.009 0.050 - Balance
304 0.02 18.27 1.13 9.32 0.028 0.025 0.41 Balance
Because of the strong influence of temperature on microstructure and thus on the extrudability of
ferrous materials, great care must be taken in temperature management during extrusion. If the
preheat temperature is too high, temperature increases due to frictional and deformation heating may
result in a heterogeneous microstructure consisting of both y-austenite and 6-ferrite that may lead to
hot shortness or localized material tearing. Low preheat temperatures or excessive heat loss to tooling
prior to extrusion, on the other hand, will lead to the formation of a-ferrite and the associated
decrease in ductility and increase in flow stress leading to nonuniform material flow as well as
extrusion defect formation. As flow stress increases, tooling life decreases rapidly; however,
excessive tooling temperature also reduces tooling life.
Lubricants commonly employed in non-ferrous material extrusion for the most part are unsuitable for
ferrous material extrusion. The high temperatures and extrusion pressures associated with steel
extrusion cause most conventional extrusion lubricants to break down or fail. Two common methods
exist for steel lubrication. The first is a thin film of highly viscous molten glass. The glass softens at
temperatures below the steel extrusion temperature and is often applied in a powdered form that
rapidly melts to become a surface film that acts to insulate the billet. Another suitable lubricant for
high-temperature ferrous extrusion is a suspension of graphite in liquid that evaporates when applied
to the hot billet. This type of lubricant was used in this research (Necrolene). The paste is applied to
the billet and begins to vaporize, leaving behind a graphite film that acts as the lubricant during
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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/m1/29/: accessed December 10, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.