Extruder stakes claim in steels, titanium A look at the process and markets for high-temperature extrusions. Company produces hollow bar stock, standard shapes and near-net profiles in specialty alloys.
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Play the word association game with an extruder, and it isn’t likely that “extrusion” will suggest ferrous metals. Odds on, the most frequent response will be “aluminum.” Nevertheless, there are a few niche firms in North America profitably extruding carbon steels, stainless steels, titanium and other high-temperature metals.
One of them is Plymouth
Engineered Shapes, Hopkinsville, Kentucky, a unit of Plymouth Tube Co. U.S.A., headquartered in Warrenville, Illinois. The company produces hollow bar stock, plus standard and custom profiles, processing 32 grades of carbon and alloy steels; 26 grades of stainless steels; five nickel alloys; and titanium in 6A1-4V and 6A1-6V-2Sn alloys, plus four commercially pure grades. It operates one 2000-ton press extruding shapes that fit within a 6 in. circumscribe circle
Steel and titanium extrusion were Plymouth’s escape from a collapsed market, said Tony Esposito, engineering manager at Hopkinsville. “Plymouth Extruded Shapes was originally intended to supply alloy 304 and 316 stainless steel hollows-starter tube-for seamless tube used in nuclear power plants, as well as the feedwater heater industry.” He related. “The hollows would be shipped to our tube drawing operations in West Monroe, Louisiana and Salisbury, Maryland.”
Changing Direction The Hopkinsville plant was commissioned in late 1980, just before the bottom abruptly dropped out of the nuclear power industry. “It was no longer economically feasible to make a nuclear power tube, so we had to find alternatives. We decided to investigate near-net extruded shapes.”
The first products were stainless steel profiles for valve bodies, and fittings for petroleum and pharmaceutical applications. Plymouth also extruded 1018 low carbon steel hanger bars used to support piping aboard submarines.
Titanium aircraft components came later, the result of a company with a large share of the aircraft market going out of business. Parts must be qualified with each manufacturer, and Plymouth has approvals from all commercial and military aircraft companies.
Direct extrusion of steels and titanium is similar to the process for aluminum, but if differs in details, the most obvious of which is the elevated extrusion temperature; 2000-2050F for titanium, 2200F for carbon steel and 2250-2300F for nickel and stainless steels.
The company operates three induction heaters, each of which achieves extrusion temperature in 9 to 12 minutes. Heating is staged so that the press runs on a 3-4 minute cycle. A conveyor that passes in front of the heaters transfers the billets to a gravity runway that rolls the billet into the press.
Sejournet process Plymouth uses the Sejournet process, in which molten glass
lubricates the high-temperature billets, which also retains heat and _particularly important with titanium-shields against atmospheric contact. The heated billet rolls across a glass frit table on its way to the press, melting the glass and forming, each with the appropriate viscosity, match the various extrusion temperatures.
A pad of compressed frit, called a cookie, is inserted into the container before the billet enters, sandwiching the cookie
between the billet
and the die face.
The cookie melts as
the billet is pushed
through the die,
distributing
additional molten
glass over the
length of the
extrusion. Two mills
of molten glass coat
the billet as it
passes through the
die, preventing
metal-to-metal
contact with the
bearing surfaces.
Steel shot blasting
removes the glass
from the finished
extrusion.
The rule at Plymouth is only one
billet per die. Although there
is no direct contact between the
extrusion billet and the die,
eliminating the possibility of
wear is crucial to maintaining
accuracy. “We used a fresh die
for each billet because we are
concerned about consistency and
repeatability,” Esposito stated
“We operate our own foundry that
casts dies in resin sand molds,
and we recycle the spent
tooling.” Plymouth designs
tooling on a CAD/CAM system and
machines brass master patterns.
The tooling cast from the
patterns us used essentially as
cast. “We remove the mold flash
from the die ports with a
router, Blanchard grind the
contact face
that goes
against the die backer, and we
are ready to run.” Esposito
said. “We do not machine the
baring surfaces, and a typical
cross –sectional tolerance of
our extrusion dies is ± 0.007
in.” One member if the extrusion
line’s four-person crew
exchanges the tooling. A
hydraulic pusher extracts the
used tooling stack: the operator
knocks off the old die and
backer, and inserts new tooling
on a slide and transports the
stack into the press. No puller
grasps the extrusion as it
emerges from the die. Instead,
the extrusion
strand runs
out onto a roller conveyor. Once
the billet is upset, the push
proceeds rapidly, typically
requiring 3 to 5 seconds.
Extrusion ratios run from 10:1
up to 50:1
Extrusions are straightened to 1/8-in. deviation in 5 linear ft in a separate operation. Plymouth has two machines, once 125 ton and one 300 ton, that stretch, straighten, and remove twist in a combined process. The stretch-straighteners have headstocks that apply tension, reducing the cross-section of the extrusion about 1 percent, and also rotate 360 degrees, both clockwise and counterclockwise.
Carbon and stainless steel extrusions are stretch-straightened at ambient temperature. Titanium, however, cannot be cold worked; it is resistance-heated to 1300F.
Heating titanium in air forms a hard oxide layer, called alpha case, on the surface of the metal. Tony Esposito explained how alpha case is controlled.
“When you heat titanium billets, you purge the induction heater with argon. That minimizes oxidation, and the glass coating helps protect against further atmospheric contact during the extrusion process.”
“Heating titanium extrusions for straightening forms alpha case over the entire surface. We remove it with a nitric and hydrofluoric acid etch that also removes a consistent 0.005 in. of metal.”
Steel and titanium can be extruded in hollow shapes, but the opening is limited to circular ID. Unlike aluminum, steel and titanium cannot be separated into strands by spider, porthole or bridge tooling and rejoined downstream of the die to produce irregular hollows.
A hollow is extruded in steel or titanium by drilling the center of the billet and attaching a piercing mandrel to the end of the stem. When the push is made, the mandrel enters the hole to keep the ID open.
Steel and titanium extrusion is generally a short-run business, producing specialized, custom products. Minimum quantities at Plymouth are 2000 lbs. for carbon steel, 1000 lbs. for stainless steel and 250 lbs. for titanium. This does not mean that long runs are impractical; Plymouth has run orders for 200 or more extrusions from the same pattern.
Among the current markets for Plymouth’s extrusions are carbon-alloy blanks for press brake tooling, carbon steel bridge expansion joints, stainless steel elevator and escalator sills, and stainless steel hand rails. Stainless steel, in 400-series alloys, is extruded into the steam turbine blades and vanes, and spray arms for wastewater treatment plants are made of alloy 316 stainless.
Commercial aircraft applications for titanium extrusions are emerging, according to Tony Esposito. “Ten years ago, titanium extrusions were almost exclusively for military aircraft,” he reported. “They were a niche with in a niche. Now they are being used for civilian airliners as well. For example we extrude titanium engine pylons for the Boeing (formerly McDonnell Douglas) MD-90 and MD-95 jetliners, and also produce titanium tracks for Boeing 777 cabin seats. The extrusion process makes the lightweight tracks economically practical, compared to machining them from bar stock, and offers longer lengths that eliminate splicing.”
Plymouth also produces preformed blanks that its customers cold draw to final sizes, usually in one pass. It is more economical than starting with bar stock and running a series of draw passes to produce the finished part. Among customers for performs are makers of firearms, who draw barrels and trigger mechanisms from 400-series stainless steel blanks.
One of Plymouth’s newest products is near-net extrusions for tooling to mold the carbon fiber composite structures found in aircraft, racing cars, and other lightweight high strength applications. The extrusions are made of Invar-36 nickel alloy steel, which is durable and has a low coefficient of thermal expansion, the latter property necessary to withstand the high temperatures at which composite structures are cured.
