By J. Neiland Pennington, Senior Editor - Modern Metals
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.
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.
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."
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.
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.
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.
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."
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."