Cold drawing of wire and tube

Introduction
As pointed out earlier, hot processing normally produces low-strength materials and uneven dimensional properties of
the product. If high-quality fabricates are required, cold processing must follow the preliminary hot method of shape
acquisition. In axisymmetric components, such as wire (rod) and seamless tubing, this is achieved by cold drawing the
hot-finished product through a die (or a series of dies). This treatment imparts good mechanical properties and effectively
regulates dimensional tolerances.

Basically, a nozzled (in the case of a tube) specimen is inserted in the die and is then gripped by a suitable device
which can pull it forward on a mechanical or hydraulic bench. A reduction in the diameter (solid specimens) and wall
thickness (tubular specimens) results. Multi-die drawing is necessary in wire production, whereas single-die processing is
more usual in the case of a tube although, for special reasons, tandem drawing is sometimes employed.

Coiled rod, obtained in a hot-rolling or extruding process, forms the starting stage of wire drawing, whereas a hotfinished
tube is used for a cold-drawing operation.

There are a number of variants of wire-drawing processes.

Processes are usually based on the method of lubrication adopted, but the four major routes are:

1. conventional dry or wet drawing,
2. hydrodynamic lubrication system,
3. hydrostatic lubrication system, and
4. ultrasonic vibration system.

The main operational tube drawing processes are:

1. sinking,
2. floating plug,
3. stationary plug,
4. mandrel, and
5. ultrasonic vibration.

The main characteristics of some of these operational systems are shown diagrammatically in Figure 16.52. In a
sinking operation, the tube is drawn without any internal support of the bore and, therefore, the change in wall thickness
cannot be controlled. Because of the bending and unbending that the wall undergoes in its passage through the die,
thinning or thickening of up to 411% can take place. The process is generally used only as a preliminary operation to
further manipulation.

The normally adopted route is that of drawing over a cylindrical plug. Either a stationary or a floating tool can be
used. In the former case the tool is positioned in the die throat and is held there by a rigid plug bar. The tube, of a bore
slightly larger than the plug diameter, is pulled over the plug and the plug bar. Initially the tube is ‘sunk’ onto the plug and
is then drawn in what is known as ‘close pass’ or ‘pure draft’. Both the diameter and the wall thickness of the tube are
reduced in a controlled manner.

For the drawing of long, small bore, small diameter, thinwalled tubes, floating plug or semi-floating ‘captive’ processes
are used. Here, an initially hot-processed or cold-annealed tube is inserted into a die which contains, in one case, an
unsupported conical free plug. The tube is drawn over the plug and is then coiled on a drum for ease of storage and transportation.

In the other variant of the operation, a conical plug that is free to float is prevented from moving in one direction
by a bar.

The older method of drawing over a long mandrel, which supports the bore, is now seldom used, mainly because the
tube drawn in this way clamps tightly onto the mandrel. Thus, to remove the tube after drawing, the tube and mandrel may
have to be reeled in a cross-rolling reeler which expands the tube and thus frees it from the mandrel. The reeling operation
can impart helical markings to the outer surface of the tube, thus necessitating another drawing operation, and is also likely
to affect the uniformity of dimensions along the whole length of the tube. In some cases, where highly polished and dimensionally
accurate, but relatively short tubing is needed, mandrel drawing can be used if withdrawal of the tool is possible
without reeling. This is the case for tubing with sufficient wall thickness that buckling does not occur when the tube is freed
by pulling the mandrevtube assembly through a ‘gate’ of a diameter only fractionally larger than that of the mandrel.
This results in a stripping operation.

In ultrasonic vibratory systems, used either in wire or tube drawing, the die is vibrated at an appropriate frequency to
increase the efficiency of the process by affecting the rate of feed of lubricant and the mechanics of the drawing.
The techniques described above are equally applicable to the manufacture of ferrous and non-ferrous tubing, but conventionall cola and warm drawing of stainless steels and cold drawing ‘of square sections are of particular interest.
In the case of stainless steel, an increase in temperature to about 300°C reduces drawing forces by up to 35% and
increases the attainable deformation by about 55%. Warm drawing thus constitutes an important development in this
type of processing.

The development of the floating-plug technique, with its saving in tool material, better carry-through of lubricant, and
saving of space when coil drawing, has been rapid and, in some cases, has overtaken the use of more conventional
fixed-plug operations. Again of particular interest is the ever-increasing use of this process in the coil drawing of
stainless and carbon steels. Irrespective of the material being drawn, the rate of production is increased; some 40% saving in
time is possible when, for example, coil drawing on 0.9 m diameter drums.

Although the spring back or the final diameter of the drawn coil depends on the material processed, it is easily assessable
for aluminium, brass, copper and alloy steels.

Wire drawing, often thought of only in terms of steel and copper, is in fact used to produce satisfactory lengths of rods
and wires in a number of more exotic metals and alloys. On the single-metal side, brittle materials such as molybdenum
and beryllium can be drawn, whilst tungsten wires must be manufactured at high temperature in order to counteract the
unfavourable mechanical properties of this material. Bimetallic wires are manufactured by coaxial drawing of solid cores
surrounded by hollow tubular sheaths, including ultrafine composites of niobium and copper, and copper and alumialuminium.
Transformation-induced-plasticity steel has been processed, as has ausformed silicon-chromium steel. Clearly. the range of application of cold drawing is large and it continues to increase bringing in non-circular section rods and wires.

Basic concepts of wire drawing

Although it is customary when considering theoretical aspects of wire drawing to refer to a single pass, it must be made clear that drawing from a coiled rod to the finished product must be carried out in a multistage, normally automated. machine or draw bench. Successive passes call for the correct design of dies and the provision of suitable lubricating conditions. Die design requires minimization of the degree of redundancy and size of the drawing load, leading to the choice of the optimum (for the given conditions) effective die angle, while at the same time allowing a protective lubricating film to develop and be maintained constantly throughout the operation. Failure in lubrication results in impaired quality of the surface finish and an increase in the rate of wear of the die. The cost of
remachining or replacement of the tool is then added to the cost due to the loss of dimensional accuracy of the product. Since successive drawing passes are not able to rectify original surface faults or the presence of scale created by annealing the rod, the preparation of the rod is all important. Here, either ‘dry’ or ‘wet’ processing is adopted. In a typical ‘dry’ in-line system for rod drawing, the specimen undergoes the following sequence of operations:

1.grit blasting by three or four sets of guns at, say, 90” to
each other;
2.grit extraction through filters in a chamber;
3.air blasting to remove the dust;
4.lubricating in an enclosed chamber; and 

5. drawing.

A dry blasting operation results in pitting of the surface or, at least, a matt finish, either of which assists in trapping the lubricant and creating local conditions of hydrostatic lubrication. This, in turn, promotes the possibility of applying single heavy passes. Various alloys, including stainless steels, can be treated in this way without any additional surface preparation. The ‘wet’ descaling processes include the well-established acid pickling, improved, and sometimes accelerated by, the passage of an electric current of some 7 A cm-’ and ultrasonic vibration, both of which help to dislodge and precipitate the scale.

However, environmental considerations weigh against the atmospheric and effluent pollution associated with the use of acids and, therefore, attention has been focused on reducing the original amount of scale and on using molten salt bath heating in place of acid pickling.

A controlled carbon oxide atmosphere is suitable for highspeed, low-scale-forming annealing, and is sometimes preferable to vacuum annealing which may cause strand welding. Depending on the alloy processed, resistance heating can be used for, say, brass annealing, or fluidized beds can be used to reduce the time of operation and the amount of scale. Electrochemical lubrication, resulting from the use of suitable molten salts of, for instance, potassium and lithium chlorides is the direct result of descaling in a non-acidic environment. Descaled and cleaned rods are then coiled and fed into drawing machines. Drawing of the wire itself is carried out at speeds ranging from 30 to 2500 m min-’, depending on the material, with resulting reductions in cross-sectional area of 15-25’?!! in the case of narrow diameter wires and 2045% for coarser wires.

The two basic techniques employed are, again, the ‘wet’ and ‘dry’ processes. In the former, the entire production line is usually immersed in the lubricating liquid, whereas in the latter the wire ‘picks up’ the lubricant on passing through a container.

Copper and copper alloy, some aluminium, and very fine diameter wires are normally processed in the wet condition,
while ferrous alloys and all other materials tend to be drawn dry. In consequence, a wide range of lubricants is employed. The lubricant used must account for not only the specific material requirements, but also for the effects of cooling that are now recognized as being of major importance. For most ferrous materials, the preferred method of lubrication is that of precoating the wire. Ordinary carbon steels are phosphate coated, but stainless steels require either oxalate compounds or borax as a lubricant. Although oxides protect the surface from die damage during drawing, they do not necessarily act as lubricants and so two-stage lubrication is still, reluctantly, used. In addition to oxide films, crystalline lubricants are required, and chlorine and sulphur type additives are used.

Electrochemical deposition of lubricants is still in the development stage but, if proven sufficiently economical, it will
provide a relatively easy answer to standard processing lubrication problems in that the deposition and removal (combined with temperature control) of substances such as molybdenum sulphide is relatively simple by this means.
The removal, and possibly re-use (for reasons of economy), of lubricant is of importance. Cleanliness of the lubricant as achieved by, say, filtration of debris, can be high and will thus affect the ‘brightness’ of the surface when the wire undergoes the final series of passes. When drawing very fine wires, ultrasonic cleaning is necessary.

Non-standard wire-drawing techniques

These techniques include principally hydrodynamic lubrication systems and ultrasonic drawing. The basic idea behind
hydrodynamic lubrication is to provide a continuous, but sufficiently thick, layer of pressurized lubricant that will
separate the tool from the wire. In this way tool wear is considerably reduced.

An effective technique of achieving these conditions is that of drawing through a sealed tube containing oil as lubricant (although occasionally soap can be employed) and terminating in a constriction or nozzle and, eventually, in a die. As thespeed of drawing increases (and high speeds are necessary) so does the oil pressure until, theoretically, it reaches the value of the yield stress of the processed material. The difficulties experienced in operating the system are related to the problems of sealing at high pressures and velocities, and to those of precision nozzle design which is ‘adjustable’ to materials and conditions.

Copper and aluminium wires can be drawn, but the pressures required for ‘harder materials’ such as, for example, steels, are too high to maintain successfully over a period of time. Nevertheless, since very good lubrication is generated with, consequently, low friction and tool wear, the technique should be considered where economy of operation is important.

In an ultrasonic multidie system, one or more dies when vibrated in the direction of drawing create conditions of
back-pull which, in turn, alter the force requirement and directly affect its magnitude. As a result of die vibration, an
oscillatory force is induced in the wires, between the consecutive dies and, eventually, the coiler drum. In consequence, greater reductions in cross-sectional area are achievable without either an increase in the degree of plasticity of the material or any decrease in interface friction. The level of this reduction depends on the value of the back-pull exerted and increases with decreasing back-pull factor. The surface finish and mechanical properties of the drawn wire remain unaffected by the oscillatory nature of the force system.

Special tube-drawing operations

Two completely diverse drawing systems now in operation are: ultrasonic drawing of ferrous and non-ferrous alloys; and fixed-plug drawing of explosively prewelded bi- or tri-metallic tubing.

Ultrasonic drawing is based on either the volume or surface effects produced by the oscillatory vibration of tools. The
volume effect is subdivided into:

1. the superposition mechanism,
2. metallurgical effects, and
3. the swaging effect

The surface effects comprise:

1.the change in friction between the tool and the work piece, and
2. the friction vector.

If only the plug is vibrated at about 20 kHz, friction is reduced by surface effects, i.e. by a thickening of the film of lubricant between the tools and the work piece. This reduces the drawing load and/or eliminates pick-up and chatter. However, practical difficulties are experienced in designing the plug and plug bar that will give the required resonance.

If only the plug is vibrated at about 20 kHz, friction is reduced by surface effects, i.e. by a thickening of the film of lubricant
between the tools and the work piece. This reduces the drawing load and/or eliminates pick-up and chatter. However,
practical difficulties are experienced in designing the plug and plug bar that will give the required resonance.

If, however, the die is vibrated ultrasonically in the radial direction, rather than remain stationary as in a conventional
drawing operation, a swaging effect is produced in the tube which, although conforming to the subdivision listed above,
indicates that, industrially, the division between volume and surface effects is not all that important. Again, the details of the design of suitable equipment  cannot be reproduced here, but information is available in the publications listed in the Further Reading at the end of this chapter

The process has the following basic characteristics:

1 the reduction in the drawing stress increases with increasing ultrasonic energy density,
2 a reduction in area of more than 54% is achievable,
3 the ultrasonic energy density can be increased by reducing
the speed of drawing rather than by increasing the energy
input,
4.surface finish is improved when radial die oscillations are
used, and
5.pick-up and chatter are low or absent.

The high degree of sophistication required in the rapidly developing technology of the manufacture of petrochemicals,
electronic devices (particularly cybernetics, cryogenic systems, and of atomic pile and toxic metals remote control systems)
calls for the development of a new range of engineering, tubular composites. Some of these cannot be produced by
conventional techniques alone or can only be manufactured at high coljt. A typical example of the first group is a multilayer,
multimetallic cylindrical pressure vessel, whereas an example of the second is a semiconductor system enclosed permanently
in a protective tubular metal sheath. In between these extremes there is a wide range of components such as special bior
multi-metallic heat exchangers, that combine structural strength with anticorrosive properties and ensure a rate of
heat flow as good as that shown by the individual metals of the composite. Conventional codrawing or coextruding of such assemblies gives less satisfactory results, because even with a high degree of process control it is practically impossible to ensure that no lubricant or debris is trapped on the surfaces. Furthermore, lack of cohesion between the original components of the
assembly is likely to lead to very high differential deformation and the associated in-built shearing stress. Many of these limitations can be eradicated if the integrity of the component is assured a priori by, say, explosive welding. In this respect, the manufacture of duplex or triplex,

bimetallic or multimetallic cylindrical pressure containers is of practical importance and this has been accomplished successfully.
Because of the technical problems involved when long composite cylinders are explosively welded, the usual technique
is to weld short, large diameter combinations, and then to obtain the required dimensions by means of cold-plug
drawing.

The characteristic features of drawing are the changes effected in the distribution of hardness across the tube section,
and in the quality of the weld. While the former simply reflects the effect of strain hardening of the composite associated with
the imparted deformation, the latter represents the effect of shearing at interfaces. Unlike hydrostatic extrusion, discussed
earlier, where the adhesion of implosively welded elements is improved, in drawing weakening and even failure of the weld
can occur. Of course, the failure, often only local, of the weld does not reduce the strength or tightness of the cylinder, since
the function of the weld is only to promote more ‘homogeneous’ drawing conditions.

The high degree of sophistication required in the rapidly developing technology of the manufacture of petrochemicals,
electronic devices (particularly cybernetics, cryogenic systems, and of atomic pile and toxic metals remote control systems)
calls for the development of a new range of engineering, tubular composites. Some of these cannot be produced by
conventional techniques alone or can only be manufactured at high coljt. A typical example of the first group is a multilayer,
multimetallic cylindrical pressure vessel, whereas an example of the second is a semiconductor system enclosed permanently
in a protective tubular metal sheath. In between these extremes there is a wide range of components such as special bior
multi-metallic heat exchangers, that combine structural strength with anticorrosive properties and ensure a rate of
heat flow as good as that shown by the individual metals of the composite. Conventional codrawing or coextruding of such assemblies gives less satisfactory results, because even with a high degree of process control it is practically impossible to ensure that no lubricant or debris is trapped on the surfaces. Furthermore, lack of cohesion between the original components of the
assembly is likely to lead to very high differential deformation and the associated in-built shearing stress.Many of these limitations can be eradicated if the integrity of the component is assured a priori by, say, explosive welding. In this respect, the manufacture of duplex or triplex, bimetallic or multimetallic cylindrical pressure containers is of practical importance and this has been accomplished successfully. Because of the technical problems involved when long composite cylinders are explosively welded, the usual technique is to weld short, large diameter combinations, and then to obtain the required dimensions by means of cold-plug
drawing. The characteristic features of drawing are the changes effected in the distribution of hardness across the tube section,
and in the quality of the weld. While the former simply reflects the effect of strain hardening of the composite associated with
the imparted deformation, the latter represents the effect of shearing at interfaces. Unlike hydrostatic extrusion, discussed
earlier, where the adhesion of implosively welded elements is improved, in drawing weakening and even failure of the weld
can occur. Of course, the failure, often only local, of the weld does not reduce the strength or tightness of the cylinder, since
the function of the weld is only to promote more ‘homogeneous’ drawing conditions.