 Steel Bars Combine Forging and Rolling
Scot Forge's Tartan Bars feature a fully consolidated
forged center, and a surface that is finished by multiple-pass rolling.
Combining forging and rolling cuts press time in half; produces short runs in
short order.
By J. NEILAND PENNINGTON
Senior Editor - Modern Metals
Until now, the cross section of forged round bars from Scot Forge have not been
truly circular. They were actually polygons with as many as 50 sides, produced
by multiple planishing strokes in the forging press.
Now the Spring Grove, Illinois, company has introduced a forged bar that is
round to one-half of the AISI tolerance for hot rolled bar. Trade named Tartan
Bar, the material is produced in diameters of 6 to 16 1/4 in. and lengths from
8.8 to 30 ft. by a new automated bar mill that began production runs this
February at Scot Forge's Clinton, Wisconsin, works.
The bar mill completes a $ 14 million expansion begun three years ago. The
project also includes two new plant bays totaling 50,000 sq. ft., nine tip-up
heat treating furnaces, a 50,000-gallon tank for both water and polymer
quenching, a 50,000-lb. capacity stacking crane, and a centerless bar peeler
with a 4 in. through 16 1/4 in. diameter capacity.
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Sixteen-sided preforms are forged in this 1250-ton
open-die forging press.Roll planishing operation that follows forging refines
surface finish, cuts forging time in half.
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Manufacturing Tartan Bars combines conventional open-die forging with a roll
planishing mill that Scot Forge says is unique in the bar industry. "This is
not the same as hot-rolled bar," stated Dick Statton, Clinton vice president
and plant manager. "It is forged bar with a fully consolidated center and a
hot-rolled surface. Metallurgically, it is identical to a bar formed entirely
by forging."
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| Each of four parabolic openings on the single-stand reversing
planishing mill accommodates a range of diameters. Bar lengths range from 8.8
to 40 ft.; are limited by length of infeed and outfeed conveyors. |
Guaranteed sound center
"Most conventional hot rolling mills won't guarantee a sound center in
diameters above 10 to 12 in. We guarantee a sound center through 16 1/4 in."
The technology is the result of efforts to reduce the forging press time
required to planish a bar, and minimize press wear from the repeated pounding
of planishing strokes.
"Jim McKinley, our president, knew there must be a better surface finishing
technique than forging 50 flats on a bar," said Sharon Haverstock, vice
president of marketing. He had been at work developing a roll system of our
own when he heard a presentation by GFM, GmbH, in Steyr, Austria, which
developed the roll planishing mill. He was very excited about its capability,
and brought the idea to our management group. Another intent of the roll
planishing line is to free up forging press time. We have been working at full
capacity, and our lead times have extended. We were looking for ways to improve
both the product and the process.
Forged preforms
Producing Tartan Bars begins conventionally. Cast ingots are cogged into
four-sided billets on one of two open die forging presses, then reheated and
forged into 16-sided preforms. The process requires about half the press time
of forging plus planishing on the forging press.
A further shortening of the forging cycle is contemplated. "We may forge
preforms into true octagons, rather than 16-sided double octagons, if we can
maintain quality." Dick Statton said. "Our goal is to spend less and less time
on the forging press."
The preforms are transferred to the new bar line, which combines the roll
planishing mill with a Braun abrasive saw and an Info-Sight programmable end
marker. The runout table and multiple-stage cooling bed were built by Voest
Alpine, which also integrated the line controls.
The 1250-hp single-stand reversing planishing mill reduces the diameter of the
bars by up to 12 percent. The rolling rate is 3 ft./second, and the rolls are
automatically positioned for the required diameter, controlled by a computer
that operates the entire line. The bars are typically rolled in 13 to 21 passes
(always an odd number so the bar will exit downstream), depending on surface
requirements. Rolling time for each bar averages about three minutes, and the
production goal is 12 bars per hour.
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Bars are 1850 F at the center and 1500 F on the
surface when they are trimmed by the abrasive saw. Accuracy is comparable to a
toothed carbide on a cold saw. |
Automatic reversing cycle
The automatic reversing cycle is initiated by entering the bar dimensions and
alloy. The program selects the required number of passes, and closes the roll
gap to progress from the starting to the finished diameter.
Bar ends are trimmed by the automated abrasive saw, which spins a 5 ft. diameter
glass fiber wheel coated with silicon carbide. A 16 1/4 in. diameter bar is
trimmed in less than 30 seconds, and squareness of the end is comparable to bar
stock cold-cut with a circular saw.
Ends of the bars are imprinted with order and heat numbers by the automated dot
matrix end stamper. Coding is currently entered manually, but will eventually
be downloaded from the mill operations computer.
The imprinted bars are transferred to a five-stage cooling table that rotates
the bars to maintain straightness. The sixth station is an accumulator rack,
and bars are removed by a 40,000-lb. capacity GCC (Gerlinger Carrier Co.)
Straddle-Loader.
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Separate from the automated planishing line is this Hetran centerless bar peeler
for diameters up to 16 1/4 in. Peeler produces a 125-250 RMS finish.
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"The reversing mill gives us the ability to do with one stand
at a lower cost what would normally require several stands, and the line
automation allows us to make custom sizes."
--Chris Scheiblhofer
Planishing mill manager |
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"[Tartan Bar] is a forged bar with a fully consolidated
center and a hot-rolled surface. Metallurgically, it is identical
to a bar formed entirely by forging."
-- Dick Statton,
V.P. and plant manager |
Small quantities quickly
Tartan Bars can be produced in smaller quantities with shorter lead times than
hot-rolled bar, said Chris Scheiblhofer, planishing mill manager. "Hot-rolled
bars are made in large runs and standard sizes by a series of roll stands that
reduces the diameter more than our single-stand mill. Orders are necessarily
large, and lead times can run two to four months."
"The reversing mill gives us the ability to do with one stand at a lower cost
what would normally require several stands, and the line automation allows us
to make custom sizes. It is economically practical to purchase the production
of only one ingot. Lead times are now from two to four weeks, and we are
working on a guaranteed delivery program for Tartan Bars. Our goal is one week
for untreated bars, and two weeks for heat-treated metal."
Half-spec straightness
Roundness is not the only Tartan Bar tolerance that is half the AISI
specification for hot rolled bars. The standard straightness is also one-half
the rolled bar specifications. One-eighth inch in any 5 ft. is standard, and
1/16 in. is available on special order. For hot rolled bar, the limits are 1/4
in. and 1/8 in. in any 5 ft.
Tartan Bars are produced in all standard carbon and low-alloy steel grades, plus
forgeable tool steels,and 300 and 400 series stainless steels. Both OEMs and
service centers are seen as large potential markets. Sharon Haverstock believes
that Tartan Bars will be of particular interest to service centers, given the
combination of large size range, small minimum orders and rapid delivery.
Rolling the surface of Tartan Bars generally lowers their cost, compared to
conventionally forged steel. "If you are purchasing a bar made entirely in a
forging press, you typically buy at least 1/2 in. of additional diameter to
allow for machining," Haverstock advised. "Because the surface of Tartan Bars
is smoother, you need less stock allowance, usually no more than 3/8 in. The
net cost of the Tartan Bar will be less because you are buying fewer pounds of
metal."
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Reprinted from Modern Metals — May 1998 Copyright Trend Publishing Inc.
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