ASM Metals HandBook Vol. 14 - Forming and Forging

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In the simplest method of hydrostatic extrusion, the metal is extruded through a conical die into the atmosphere in much

the same way as in conventional extrusion (Fig. 1). The container with the pressure-transmitting fluid is sealed with high-

pressure seals at the ram and the die. Extrusion begins as soon as the hydrostatic pressure has reached a sufficiently high

value, depending on the flow stress of the material and the extrusion ratio.

Fig. 1 Comparison of the conventional extrusion (a) and hydrostatic extrusion (b) processes

Extrusion Pressure. A pressure peak is found at the beginning of extrusion in the hydrostatic process. It is needed to

initiate metal flow, which is hindered by friction at the die until a lubricating film and steady-state conditions have

developed. This pressure peak can be very high, particularly if breakdown of the lubricant film occurs. The rapid decrease

in pressure after extrusion begins can sometimes lead to the development of periodic oscillations in pressure; this is

known as the stick-slip effect. It can be eliminated through the use of viscous dampers (Ref 1, 2) or by minimizing the

amount of hydrostatic fluid used.

After the initial pressure peak, the steady-state pressure remains constant because there is no friction at the wall of the

container (Fig. 2). The steady-state extrusion pressure required depends on the work material and is linearly related to the

natural logarithm of the extrusion ratio R according to the empirical equation:

p = k

ln R + k

(Eq 1)

where k

and k

are constants. Figure 3 illustrates this relationship for the cold extrusion of round billets to rod for several

alloys. The gradient of the lines would be slightly steeper for more complex sections.

Fig. 2 Pressure variation versus ram displacement during hydrostatic extrusion

Fig. 3 Extrusion pressure versus reduction of area in cold hydrostatic extrusion of various materials

References cited in this section

A.H. Low and C.J.H. Donaldson, Report 289, The National Engineering Laboratory, 1967

H.L.D. Pugh, Hydrostatic Extrusion of Steel, Iron Steel, Vol 45, 1972, p 29-44, 49-51

Hydrostatic Extrusion

Simple Hydrostatic Extrusion of Brittle Materials

Most brittle materials are subject to circumferential (transverse) and longitudinal surface cracking during hydrostatic

extrusion. This cracking can be avoided through the use of either fluid-to-fluid extrusion or double-reduction dies. In

fluid-to-fluid extrusion, the billet is hydrostatically extruded into a fluid at a lower pressure. This method has several

disadvantages, including high tooling and operating costs, extrusion lengths that are limited to the length of the secondary

chamber, and increased fluid pressure required for extrusion. For these reasons, the fluid-to-fluid process may not be

suitable for many industrial applications.

The problem of extruding low-ductility metals was approached in a different way by researchers at Battelle Columbus

Division (Ref 3), who established that the cracks or fracture first developed in the rear section of the die land,

immediately before the exit plane, and that the surface cracking resulted from residual tensile stresses as the product left

the die. The cracks observed were either longitudinal or transverse across the extruded product, depending on whether the

predominating residual stresses were longitudinal or circumferential. This phenomenon was noted much earlier in rod and

tube drawing (Ref 4). It was discovered that it is possible to reverse the residual stresses at the surface to compressive

stresses by a subsequent draw with a low reduction in area (<2%).

The work of these investigators led to the development of the double-reduction die (Ref 4). Figure 4 compares a standard

die with a double-reduction die. The double-reduction die used for the experiments was designed to give a 2% reduction

in the second step. This method has been successfully applied to the extrusion of brittle materials, including beryllium and

TZM molybdenum alloy, without any cracking. The lubricant used was polytetrafluoroethylene (PTFE), and the

pressurizing fluid was castor oil. The results may be applicable to conventional cold extrusion through a lubricated

conical die (Ref 3).

Fig. 4 Standard die (a) and double reduction die (b) for the hydrostatic extrusion of brittle materials. H,

distance between the start of each bearing; θ, included angle at second reduction.

Dimensions given in inches.

Source: Ref 5

It is believed that the small second reduction prevents cracking by imposing an annular counterpressure on the extrusion

as it exits the first portion of the die. This counters the axial tensile stresses arising from residual stresses, elastic bending,

and friction. Prevention of circumferential cracks upon exit from the second portion of the die is believed to be associated

with the favorable permanent change in residual stresses in the workpiece caused by the small second reduction (Ref 5).

References cited in this section

R.J. Fiorentino, B.D. Richardson, and A.M. Sabroff, Hydrostatic Extrusion of B

rittle Materials: Role of

Design and Residual Stress Formation, Met. Form., 1969, p 107-110

H. Buhler, Austrian patent 139,790, 1934; British patent 423,868, 1935

R.J. Fiorentino, Selected Hydrostatic Extrusion Methods and Extruded Materials, in Hy

drostatic Extrusion:

Theory and Applications, N. Inoue and M. Nishihara, Ed., Elsevier Applied Science Publishers, 1985, p 284-

322

Wire, Rod, and Tube Drawing

Introduction

IN THE DRAWING PROCESS, the cross-sectional area and/or the shape of a rod, bar, tube, or wire is reduced by pulling

through a die. One of the oldest metal-forming operations, drawing allows excellent surface finishes and closely

controlled dimensions to be obtained in long products that have constant cross sections. In drawing, a previously rolled,

extruded, or fabricated product with a solid or hollow cross section is pulled through a die at exit speeds as high as several

thousand feet per minute (Ref 1, 2). The die geometry determines the final dimensions, the cross-sectional area of the

drawn product, and the reduction in area. Drawing is usually conducted at room temperature using a number of passes or

reductions through consecutively located dies. An important exception is the warm drawing of tungsten to make

incandescent lamp filaments. Annealing may occasionally be necessary after a number of drawing passes before the

drawing operation is continued. The deformation is accomplished by a combination of tensile and compressive stresses

that are created by the pulling force at the exit from the die and by the die configuration.

In wire or rod drawing (Fig. 1 and 2), the section is usually round but could also be a shape. In the cold drawing of

shapes, the basic contour of the incoming shape is established by cold-rolling passes that are usually preceded by

annealing. After rolling, the section shape is refined and reduced to close tolerances by cold drawing (Ref 3). Again, a

number of annealing steps may be necessary to eliminate the effects of strain hardening, that is, to reduce the flow stress

and to increase the ductility.

Fig. 1 Drawing of rod or wire (a) and tube (b).

Fig. 2 Drawing of a bar through undriven rolls.

In tube drawing without a mandrel (Fig. 3), also called tube sinking, the tube is initially pointed to facilitate feeding

through the die; it is then reduced in outside diameter while the wall thickness and the tube length are increased. The

magnitudes of thickness increase and tube elongation depend on the flow stress of the drawn part, die geometry, and

interface friction.

Fig. 3 Tube drawing without a mandrel (tube sinking).

Drawing with a fixed plug (Fig. 4) is widely known and used for drawing large-to-medium diameter straight tubes. The

plug, when pushed into the deformation zone, is pulled forward by the frictional force created by the sliding movement of

the deforming tube. Therefore, the plug must be held in the correct position with a plug bar. In drawing long and small-

diameter tubes, the plug bar may stretch and even break. In such cases, it is advantageous to use a floating plug (Fig. 5).

This process can be used to draw any length of tubing by coiling the drawn tube at speeds as high as 10 m/s (2000 ft/min).

In drawing with a moving mandrel (Fig. 6), the mandrel travels at the speed at which the section exits the die. This

process, also called ironing, is widely used for thinning the walls of drawn cups or shells in, for example, the production

of beverage cans or artillery shells.

Fig. 4 Drawing with a fixed plug.

Fig. 5 Drawing with a floating plug.

Fig. 6 Drawing with a moving mandrel.

References

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

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

Verlag, 1974, p 227

(in German)

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

Wire, Rod, and Tube Drawing

Basic Mechanics of Drawing (Ref 4)

It is fundamental that the pulling force, or drawing stress, cannot exceed the strength of the wire or rod being drawn

(otherwise, fracture or unstable deformation would occur). In fact, practical considerations often limit the drawing stress

to about 60% of the as-drawn flow stress. Therefore, the area reduction per drawing pass is rarely greater than 30 to 35%.

A particularly common reduction is that of an American Wire Gauge of 1, or about 20.7%. Thus, many reductions or

drawing passes are needed to achieve a large overall reduction. Much larger reductions can be achieved in a single

operation with extrusion. Alternatively, drawing can be used to generate larger quantities of small-diameter product (for

example, 0.01 mm, or 0.0004 in.) with excellent dimensional control (assuming proper die maintenance).

Approach Angle. A typical carbide drawing die is illustrated in Fig. 7. The wire or rod makes contact in the drawing

cone along the approach angle and is reduced to the dimensions of the drawing cone exit. The bearing region involves no

further reduction and allows the die to be refinished without a change in the exit dimensions of the drawing cone. The

back relief reduces the amount of abrasion that takes place if the drawing stops or if the die is out of alignment. A

lubricant is introduced at the bell portion of the die and is pulled into the die/wire interface by the moving wire.

Fig. 7 Cross section

of a typical wire die for drawing 5.5 mm (0.218 in.) diam rod to 4.6 mm (0.180 in.) diam

wire (17% reduction per pass).

The approach angle is perhaps the most important feature of the die for most applications. The effect of the approach

angle on metal flow cannot easily be considered independent of the drawing reduction, and modern drawing theory

incorporates both into the Δ parameter:

where is the approach semiangle (one-half the included angle) in radians and r is the fractional drawing reduction,

given by:

r = 1 - A

where A

and A

are the starting and finishing cross-sectional areas, respectively. Commercial die design often involves

approach semiangles in the range of 6 to 10° and drawing reductions of about 20%. The corresponding Δ values typically

range from 2 to 3, with higher values corresponding to lower reductions and higher die angles, and lower values

corresponding to higher reductions and lower die angles.

Effect of Friction. Basically, low Δ values may involve excessive frictional work between the wire and the drawing

cone, and high Δ values involve redundant work or plastic strain beyond that calculable from the reduction in area of the

pass. Some degree of redundant work exists for Δ> 1, with redundant work increasing as Δ increases, much as frictional

work can increase as Δ decreases. The net effect is that some intermediate value of Δ involves the minimum work, and

therefore the minimum drawing force, because the drawing force multiplied by the drawing velocity is the work

consumed per unit time. Similarly, the drawing stress equals the work per unit volume of wire drawn. The Δ for minimum

drawing stress can be approximated by:

where μ is the coefficient of friction between the wire and the drawing cone. The drawing stress σ

can be usefully

approximated as:

where is the average strength or flow stress of the wire during the drawing pass.

Redundant Work of Deformation. Redundant work is expressed in terms of the redundant work factor or the ratio

of total plastic deformation work to the work implied by dimensional change. Experimental studies suggest that the

redundant work factor can be estimated to be:

/6 + 1

Heat Generation During Drawing. The management of heat is of great concern in drawing; practical cold-drawing

operations can involve wire temperature increases of a few hundred degrees Kelvin. Much heat is generated directly by

the plastic deformation, and this heat is only partially removed by interpass cooling. The dies extract little heat under

commercial conditions and become very hot. Under adiabatic conditions, the temperature increase T

associated with

plastic deformation in a single pass is approximately:

= ln (1/1 - r)/C

where C and ρ are the heat capacity and density of the wire, respectively. Additional heat generation is associated with

frictional work. This heat is concentrated at the die/wire interface and can lead to diminished lubrication, further heating,

and catastrophic lubricant breakdown. Accompanying problems include poor wire surface quality and metallurgical

changes near the wire surface. If the coefficient of friction is not influenced by Δ, frictional heating is aggravated by low

Δ processing. Fortunately, there is a tendency for low approach angles (and thus low Δ) to foster hydrodynamic

lubrication and a reduced coefficient of friction.

Reference cited in this section

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

Vol 2, Pergamon Press and The MIT

Press, 1986

Wire, Rod, and Tube Drawing

Preparation for Drawing (Ref 5)

One or more of three basic preparation steps--heat treatment, surface preparation, and pointing--are usually required prior

to successful cold drawing. These three steps are naturally dependent on the state of the part before drawing and on the

desired drawing results.

Heat treatment usually involves annealing or softening so that the material is ductile enough for the intended

percentage of reduction. This is particularly necessary for certain metals that are hard or brittle in the hot-worked state or

for previously cold-drawn parts that have already been work hardened too much to allow further reduction.

Annealing. In the wire industry, a wide variety of in-process annealing operations are available for rendering coiled

material suitable for further processing that may require formability, drawability, machinability, or a combination of these

characteristics. One large wire mill reported using 42 separate and distinct annealing cycles, most of which represented

compromises between practical considerations and optimal properties. For example, annealing temperatures below those

that might yield optimal softness sometimes must be used in order to avoid the scaling of wire coils, which can often

occur even in controlled-atmosphere furnaces. Even slight scaling can cause the coil wraps to stick together, and this can

impede coil payoff in subsequent operations.

Patenting is a special form of annealing that is peculiar to the rod and wire industry. In this process, which is usually

applied to medium- and higher-carbon grades of steel, the rod or wire products are uncoiled, and the strands are delivered

to an austenitizing station. The strands are then cooled rapidly from above the full annealing temperature (A

) in a molten

medium (usually lead at about 540 °C, or 1000 °F) for a period of time sufficient to allow complete transformation to a

fine pearlitic structure. Salt baths and fluidized beds have also been used for this purpose. This treatment increases

considerably the amount of subsequent wire-drawing reduction that the product can withstand and permits the production

of high-strength wire. Successive drawing and patenting steps can be used to obtain the desired size and strength level.

Surface Preparation. To prevent damage to the workpiece surface or the draw die during cold drawing, the starting

stock must first be cleaned of surface contaminants, such as scale, glass, and heavy rust. This cleaning usually involves

the use of various pickling or shotblasting methods. In many cases, especially when tubes are being drawn, the surface

can also be coated or prelubricated by phosphatizing, plating, soaping, or liming methods. If no intermediate annealing is

required, some of the prelubricating methods permit several cold-drawing passes without repeated treatment. Solid bars or

rods are generally lubricated by oil during the drawing process.

To provide a wire of good surface quality, it is necessary to have clean wire rod with a smooth oxide-free surface.

Conventional hot-rolled rod must be cleaned in a separate operation, but with the advent of continuous casting, which

provides better surface quality, a separate cleaning operation is not required. Instead, the rod passes through a cleaning

station as it exits the rolling mill.

Pointing, sometimes called chamfering, involves the preparation of a short length of one end of the starting part to a size

slightly smaller than the draw die. The prepared end, called the point, is thus ready for insertion through the draw die for

gripping. The actual pointing operation is usually performed at room temperature by swaging, rolling, or turning.

However, it can be performed after preheating and can also be done by hammering, acid etching, or grinding.

In some cases, these pointing operations can be avoided through the use of push pointing, which involves pushing the end

a short distance through the die. Pushing forces, however, are much higher than pulling forces. As a result, starting parts

having small diameters and slender sections may buckle during the push-pointing process. This buckling action can be

minimized by proper support, but parts having a diameter of about 9.5 mm (0.37 in.) or less generally must be pointed by

one of the methods previously described.

Reference cited in this section

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

Society of

Manufacturing Engineers, 1984

Wire, Rod, and Tube Drawing

Drawing of Rod and Wire (Ref 5)

An overall view of the process by which steel wire is drawn from rods is shown in Fig. 8. Methods and equipment used

for the cold drawing of rod and wire, as well as small-diameter tubing, are generally designed so that the products can be