ASM Metals HandBook Vol. 14 - Forming and Forging

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With high-strength materials such as steels, stainless steels, and high-temperature alloys, the surface of the rod or wire can

be coated either with a softer metal or with a conversion coating. Copper or tin can be chemically deposited on the surface

of the metal. This thin layer of softer metal acts as a solid lubricant during drawing. Conversion coatings may consist of

sulfate or oxalate coatings on the rod; these are then typically coated with soap, as a lubricant. Polymers are also used as

solid lubricants, such as in the drawing of titanium.

In the case of steels, the rod to be drawn is first surface treated by pickling. This removes the surface scale that could lead

to surface defects and therefore increases die life.

In wet drawing, the lubricant is chosen both for its tribological attributes and for its cooling power, and it can be either

oil-base or aqueous. It can be applied to the die inlet, the wire, and often also to the capstan, or the entire machine can be

submerged in a bath. When the machine operates with slip, the lubricant must reduce wear of the capstan while

maintaining some minimum friction. This wet-drawing practice is typical of all nonferrous metals and of steel wires less

than 0.5 to 1 mm (0.02 to 0.04 in.) in diameter.

A transition between the two techniques is sometimes used, particularly in the low-speed drawing of bar and tube. A high-

viscosity liquid or semisolid is applied to the workpiece and/or die. Reference 8 provides additional details on the

lubrication of ferrous wire.

References cited in this section

S. Kalpakjian, Manufacturing Processes for Engineering Materials, Addison-Wesley, 1984

A.B. Dove, Ed., Steel Wire Handbook, Vol 4, The Wire Association International, Inc., 1980

Wire, Rod, and Tube Drawing

The Manufacture of Commercial Superconductors

The design requirements of commercial superconductors have challenged metal extrusion and composite metal-drawing

technology such that superconductors with 10,000 to 40,000 filaments, several microns in diameter, are available in wire

form. On an experimental basis, wires have been produced with 1 million filaments less than 1 m (40 in.) in diameter.

Understanding the reasons for this challenge to metal-forming technology requires a brief introduction to engineering

requirements for commercial superconductors.

The design and application of superconductors is mainly controlled by the critical temperature, T

. Niobium-base

superconductors are usually used at liquid helium temperatures (4.2 K). At these temperatures, specific heats of materials

are typically in the 10

-3

J range. Small mechanical or electromagnetic disturbances can provide sufficient heat to raise the

temperature of the superconductor above T

; this increase in temperature causes the normally high resistance to return.

Commercial superconductors are designed to prevent and/or control the change to the nonsuperconducting state.

Copper and aluminum are usually used as the matrix; copper is the preferred matrix because of its mechanical

compatibility with the niobium-base superconductors. If an event occurs that is sufficient to return the superconductor to

the normal state, the copper temporarily conducts the current until the superconductor is cooled below the critical

temperature.

The sizes of the filaments of the superconductor are chosen to be in the 100 μm (4000 μin.) range--small enough to

prevent an electromagnetic instability called a flux jump. For applications requiring precise magnetic fields, as in dipole

magnets, filaments must be in the 1 μm (40 μin.) range. Power frequency applications require filaments of less than 1 μm

(40 μin.).

Commercial superconductors for power applications are manufactured by a coextrusion and composite-drawing process.

The resulting wire consists of one to tens of thousands of filaments of the superconductor, each individually surrounded

by a normal metal matrix. The superconductor itself is usually a ductile alloy of niobium and titanium (Fig. 14) or a brittle

intermetallic of niobium and tin (Nb

Sn) (Fig. 15 and 16).

Fig. 14 Cross section of 500 niobium-titanium filaments separated by a copper substrate and enclosed within a

copper-nickel tube. Courtesy of Oxford Superconducting Technology.

Fig. 15 Cross section of niobium filaments reacted with tin in the bronze substrate to form Nb

Sn.

Courtesy of

Oxford Superconducting Technology.

Fig. 16 Cross section of 3000 filaments of Nb

Sn in final conductor.

Courtesy of Oxford Superconducting

Technology.

Superconducting multifilamentary conductors are manufactured using a combination of extrusion and wire-drawing

techniques (up to 40 to 50 such separate sequences may be necessary) to make up to 1 million individual wire filaments of

microscopic size enclosed within a wire having outside diameter of fractions of an inch. The two primary techniques used

are billet stacking and the modified jelly-roll method.

Billet Stacking Method. The manufacture of a typical niobium-titanium superconductor with filaments in the 10 to

100 μm (400 to 4000 μin.) range begins with the assembly of a billet (Fig. 17). The billet is assembled by inserting rods of

the superconductor into an array of tubes of CDA 101 copper with a hexagonal outer shape and a round inner diameter.

The array approximates a circle having a diameter slightly less than the copper extrusion can placed over it. A typical

billet is 305 mm (12 in.) in diameter and 762 mm 30 in. long.

Fig. 17 Fabrication steps and process parameters required to manufacture multicore niobium-

titanium filament

conductors. Courtesy of Intermagnetics General Corporation.

The billet is evacuated to remove the air and electron beam welded to form a vacuum-tight seal. The elements in the billet

are metallurgically bonded and uniformly reduced in area by a hot direct extrusion. Reduction ratios of 16 to 1 are

generally used, requiring extrusion forces typically in the 31 to 44 MN (3500 to 5000 tonf) range. The resulting extrudate

is normally 10 m (33 ft) long by 85 mm (3.3 in.) in diameter. This rod is then cold drawn using a proprietary die and

reduction schedules designed to ensure uniform coreduction of the superconducting filaments. The initial draw process

requires benches as long as 60 m (200 ft), with 590 kN (60,000 kgf) of draw force to be used to ensure that the rod does

not have to be cut before it is coiled for further drawing. The remaining process involves performing heat treatment and

draw cycles to develop the current capacity of the superconductor, annealing the matrix, and uniformly coreducing the

filaments. Specially modified wire-drawing machinery is generally used. The final step is an anneal to restore the ductility

and resistivity of the copper matrix. Preceding this step is a twisting operation, which twists the wire upon itself. This

twists the filaments inside the composite, ensuring that the filaments act individually under electromagnetic fields.

Superconductors requiring more than 5000 or 6000 filaments or, correspondingly, filaments of sizes less than 10 m (400

in.) are made by coextruding a single filament, which, in the case of niobium-titanium, usually has a diffusion barrier.

This ensures that no damaging intermetallics form during the extrusion or heat treatment process. This extrudate is drawn

to the appropriate size and assembled into a second extrusion billet; the process is then repeated.

The extrusion and draw process can be repeated a number of times to produce wires with 20,000 to 1 million or more

filaments. As shown in Fig. 18, a first-generation billet can yield 19 individual filaments in the wire configuration. Sixty

one of these 19-filament wires were then stacked, extruded, and drawn to yield a 19 × 61 array of 1159 filaments in the

second-generation billet. The third-generation billet, consisting of 61 of the 1159 filament wires obtained in the second-

generation billet, was stacked, extruded, and drawn to yield a 70,699 filament superconducting wire. However, the

problem of "sausaging," or filament nonuniformity, especially in the outer diameter of the conductor, becomes more

evident as more extrusions and drawings of the wire are attempted.

Fig. 18 Cross section of billet assembly used to produce a 70 699-filament wire using up to third-

generation

billets. (a) First-generation billet. (b) Second- and third-generation billets.

For these developmental conductors, the matrix is often composed of several metals or alloys of metals, such as copper

nickel, or is alloyed slightly with magnetic impurities, such as manganese. This ensures that the electromagnetic and

physical sizes of the micron-size filaments are the same.

The manufacture of Nb

Sn in filamentary form has presented some unusual problems because of the brittle nature of the

superconductor. The accepted processes are based on forming the brittle phase at the final stage. Early technology carried

the tin in a 13 wt% bronze matrix. The manufacturing process was a multiple coextrusion and codrawing process similar

to that for niobium-titanium. The work-hardening rates of bronze require many anneals, and this can lead to premature

formation of the brittle phase. More recently, a process based on maintaining the tin in its pure phase has gained

acceptance. This is called the internal tin process.

The standard process begins with a billet of copper and niobium filaments assembled into a tubular array (Fig. 19). The

billet follows the usual procedures for niobium-titanium, but is extruded with a mandrel to maintain the hole. The

resulting extrudate typically has several hundred filaments of niobium in a copper matrix with a hole of about 10% of the

diameter running throughout the length at the center. Tin is inserted into the hole, and the resulting composite is drawn to

a size suitable for assembly into the stabilizer tube. This tube is also formed from a hollow extrusion but is composed of

copper and a diffusion barrier such as niobium or vanadium. The diffusion barrier keeps the tin from alloying with the

copper in the stabilizer. Quantities of 19 to 37 composite elements are assembled and inserted into the stabilizer tube. The

resulting rod is repeatedly drawn without any annealing to yield a wire having a diameter ranging from 1 mm to fractions

of a millimeter. The resulting conductor frequently has several to tens of thousands of filaments in the several micron size

range with multiple cores of tin. The wire is then heat treated to diffuse and then react the tin with the niobium to form the

Sn.

Fig. 19 Sequence of manufacturing operations involved in the formation of Nb

Sn multifilamentary wire using

the internal tin process. Courtesy of Intermagnetics General Corporation.

Modified Jelly-Roll Method. Instead of using rods, the modified jelly-roll method (Ref 9) utilizes thin (0.05 to 0.50

mm, or 0.0020 to 0.020 in., thick) niobium foil sheets that are slit with a number of discontinuous and staggered parallel

slits having controlled interconnection distances, d, of 5 to 150 mm (0.197 to 5.9 in.), as shown in Fig. 20. This niobium

sheet is subsequently stretched at right angles to the original slits to produce a diamond-shaped array of continuously

connected filaments--a hexagonal matrix that is 45 to 85% open. The expanded niobium sheet is then rolled up with the

designated matrix material (bronze, copper, and aluminum), inserted into a copper container, sealed, extruded, and

processed as conventional wire. As area reduction of the cross section progresses, the horizontal dimension d is stretched

and elongated, and the vertical dimension w is compressed by a factor of 100,000 to 1,000,000 to form the individual

filaments (Fig. 21(a) and 21(b)).

Fig. 20 Schematic of the modified jelly-roll process for producing superconducting multi-filamentary wire.

Fig. 21(a)

Detail of niobium sheet mesh rolled into a jelly roll using a matrix material to show how the

hexagonal mesh is transformed into an individual filament through elongation in the horizontal direction a

compression in the vertical direction when subjected to extrusion and drawing.

Fig. 21(b) Cross section of a 0.78-mm (0.0307-in.) diam unreacted niobium-

tin multifilamentary composite

wire consisting of 18 subelements that were produced using the modified jelly-

roll method. The wire was cold

worked to a 160,000 to 1 reduction in area. Left: 65×. Right: Close-up of one of the

18 subelements showing

the individual niobium filaments surrounding a tin/copper alloy core and a vanadium barrier. 465× (differential

interference contrast). Courtesy of P. E. Danielson, Teledyne Wah Chang Albany.

Numerous billets 75 mm (3 in.) in diameter have been produced with a final niobium filament size of 1 to 2 m (40 to 80

in.) in diameter. It should be possible to achieve submicron filaments. At the commercial level, the process can be used

for Va

Ga, Nb

Al, Nb

Sn, NbTi, and other composites.

Reference cited in this section

S. Foner and B.B. Swartz, Ed.,

Superconductor Materials Science: Metallurgy, Fabrication, and

Applications, Plenum Press, 1981

Note cited in this section

* This section was written by B.A. Zeitlin, Intermagnetics General Corporation

Wire, Rod, and Tube Drawing

References

1. G.E. Dieter, Mechanical Metallurgy, 2nd ed., McGraw-Hill, 1976, p 658

2. K. Lange, Ed., Massiveforming, in Textbook of Forming Technology, Vol II, Springer-

Verlag, 1974, p 227

(in German)

3. "Rathbone Cold-Drawn Profile Shapes and Pinion Rods," Technical Brochure, Rathbone Corporation

4. M.B. Bever, Ed., Encyclopedia of Material Sciences and Engineering,

Vol 2, Pergamon Press and The MIT

Press, 1986

5. W. Wick, Ed., Forming, Vol II, 4th ed., Tool and Manufacturing Engineers Handbook,

Society of

Manufacturing Engineers, 1984

6. Designer's Handbook: Steel Wire, American Iron and Steel Institute, 1974

7. S. Kalpakjian, Manufacturing Processes for Engineering Materials, Addison-Wesley, 1984

8. A.B. Dove, Ed., Steel Wire Handbook, Vol 4, The Wire Association International, Inc., 1980

9. S. Foner and B.B. Swartz, Ed.,

Superconductor Materials Science: Metallurgy, Fabrication, and

Applications, Plenum Press, 1981

Flat, Bar, and Shape Rolling

G. D. Lahoti, The Timken Company; S.L. Semiatin, Battelle Columbus Division

Introduction

ROLLING OF METALS is perhaps the most important metalworking process. More than 90% of all the steel, aluminum,

and copper produced--in 1985, some 800 million tons of material worldwide--go through the rolling process at least one

time. Thus, rolled products represent a significant portion of the manufacturing economy and can be found in many

sectors. Beams and columns used in buildings are rolled from steel. Railroad tracks and cars are made from rolled steel,

and airplane bodies are made from rolled aluminum and titanium alloys. The wire used in fences, elevator ropes, electrical

conductors, and cables are drawn from rolled rods. Many consumer items, including automobiles, home appliances,

kitchen utensils, and beverage cans, use rolled sheet materials.

In rolling, a squeezing type of deformation is accomplished by using two work rolls (Fig. 1) rotating in opposite

directions. The principal advantage of rolling lies in its ability to produce desired shapes from relatively large pieces of

metals at very high speeds in a somewhat continuous manner. Because other methods of metalworking, such as forging,

are relatively slow, most ingots and large blooms are rolled into billets, bars, structural shapes, rods (for drawing into

wire), and rounds for making seamless tubing. Steel slabs are rolled into plate and sheet.

Fig. 1 Typical rolling mill stand

Although the rolling of metals has been done for some time and has been a very productive means of working large

quantities of metals to a variety of shapes and sizes, the state of the technology had been somewhat stagnant until

recently, when major innovations started to appear. With the advent of computer-assisted controls, highly automated, very

high-speed rolling mills were installed beginning in the 1970s. One rod mill commissioned in 1980, for example, is

reported to roll steel wire rod at the rate of 335 kph (210 mph). This mill has a rated output of 545,000 Mg (600,000 tons)

per year, and the entire mill is operated from three climate-controlled pulpits equipped with computerized controls and

closed-circuit video monitors. Another modern mill came on stream in the early 1980s. It is a 200 cm (80 in.) hot strip

mill capable of producing steel coils up to 188 cm (74 in.) wide and weighing up to 33.6 Mg (37 tons). The mill features

computer controls that automatically adjust water flow rates, roll speeds, and strip temperatures to meet metallurgical

requirements. In addition to these developments, computer-aided modeling of the rolling process is now routinely used at

several locations for design of rolls and optimization of the process parameters (see the section "Mechanics of Plate

Rolling" in this article). Understanding of the materials also has improved considerably, thereby permitting development

of new products such as high-strength low-alloy (HSLA) steels, which require controlled rolling. In short, significant

developments are happening in this field, which was largely neglected for decades.

Acknowledgements