Chapter 6 – (Rod, Wire and Tube Drawing)

Introduction Drawing is an operation in which the cross-sectional area of a bar or tube is reduced or changed in shape by pulling it through a converging die (dies are usually tapered). The drawing process is somewhat similar to extrusion, except that in drawing, the bar is under tension, whereas in extrusion it is under compression.

Introduction Although drawing applies tensile stress, compression also plays a significant role since metal is squeezed as it passes through die opening. Rod and wire drawing are generally finishing process, and the product is either used as produced or is further processed into other shapes, such as by bending or machining. Rods are used for various applications, such as small pistons, structural members, shafts, spindles, and as raw material for making fasteners such as bolt and screws. Wire and wire product have a wide range of applications, such as electrical wiring, electrical equipment's, cables, springs, fencing, welding electrodes and shopping carts. Wire diameter may be as small as 0.025 mm

Rod products

Wire products

Drawing The concept of drawing involves pulling wire, rod, or bar though a die, or converging channel to decrease cross-sectional area in increase length. In the majority of the cases the cross section is circular, although none-circular cross sections may be drawn/ or created by drawing. In a comparison to rolling, drawing offers Much better dimensional control. Lower capital equipment cost. And extension to small cross sections. In comparison to extrusion, drawing offers Continuous processing. Lower equipment cost.

Drawing Large quantities of wires, rods, tubes and other sections are produced by drawing process which is basically a cold working process (Drawing is usually performed at room temperature, thus classified a cold working process, but it may be performed at elevated temperatures for large wires to reduce forces). In this process the material is pulled through a die in order to reduce it to the desired shape and size. In a typical wire drawing operation, once the wire is gripped and pulled to pass through the opening of the die, its diameter reduced to the desired one.

Cold drawing properties Improved Size and Section Tighter size & section tolerances Dimensional consistency within each bar Dimensional consistency from bar to bar Improved Surface Finish / Reduces surface machining Improved Straightness Improved Cost Effectiveness / Production of Precision Shapes to Precision Tolerances Increased Mechanical Properties / Can reduce the need for hardening. Yield strength Tensile strength Hardness

Wire Drawing vs. Bar Drawing Difference between bar drawing and wire drawing is stock size Bar drawing - Large diameter bar and rod stock (bar drawing usually involves stock that is too large in cross section to be coiled. Round bar stock may be 1 to 10 cm in diameter or even larger) Wire drawing - Small diameter stock - wire sizes down to 0.025 mm (0.001 in.) are possible Although the mechanics are the same, the methods, equipment.

Drawing Practice and Products – Usually performed as cold working – Most frequently used for round cross-sections • Products: – Wire: electrical wire; wire stock for fences, coat hangers, and shopping carts – Rod stock : for nails, screws, rivets, and springs – Bar stock: metal bars for machining, forging, and other processes

How dose drawing work Why not simply stretch wire, rod or bar? It can be argued, at least in principle, that some of the objectives of drawing could be achieved by simply stretching the wire with a pulling force. The cross section could be reduced and elongation accomplished, but dies would not be needed and the friction and metal flow issues presented by die could be a voided. The principle problem with just stretching the wire with a pulling force is the necking phenomena. Basically, after a certain amount of uniform stretching, all further elongation will be concentrated at a single location ( a neck), which will rapidly thin and break. This occurs because the decrease in cross-sectional area eventually weakens wire more than any strengthening that occurs by work hardening.

How dose drawing work Simple explanation of drawing process. In the drawing process, a pulling force and a pressure force from the die, combine to cause the wire to extend and reduce in cross-sectional area, while passing through the die as shown in the figure :

How dose drawing work Because of this combined effect , the pulling force or drawing force is less than the force that would cause the wire to stretch, or neck and break downstream from the die. On the other hand, if a reduction too large in cross-sectional area is attempted at the die, the drawing force may break the wire. In commercial practice, engineering pulling loads are rarely above 60% of the as-drawn strength, and the area reduction in a single drawing pass is rarely above 30% or 35%, and is often much lower. A particular common reduction in none ferrous drawing is the American Wire Gage (AWG) number, or about 20.7%. So many drawing passes are needed to achieve large overall reduction

Wire Drawing Wire drawing involves stock that can be easily coiled and subjected to sequential or tandem drawing operations with as many as a dozen or more draws occurring with a given drawing machine. Each drawing operation or “pass” will involve delivery of the wire to the die from a coil on capstan OR drum, passage through the die, and take-up on a capstan that pulls the wire through the die Continuous drawing machines consisting of multiple draw dies (typically 4 to 12) separated by accumulating drums Each drum (capstan) provides proper force to draw wire stock through upstream die Each die provides a small reduction, so desired total reduction is achieved by the series of dies.

Wire Drawing Fine wire drawing typically refers to round wire with a diameter of less than 0.1 mm. Ultra fine wire drawing typically refers to round wire as fine as 0.0025 mm in diameter.

Wire Drawing Annealing before each drawing operation permits large area reduction (Annealing sometimes required between dies to relieve work hardening). Because of the strain hardening, intermediate annealing between passes may be necessary in cold drawing in order to maintain sufficient ductility to the material and avoid failure. Tungsten Carbide dies are used to for drawing hard wires, and diamond dies is the choice for fine wires.

Bar Drawing Hydraulically operated draw bench for drawing metal bars Bar drawing usually involves stock that is too large in cross section, and hence must be drawn straight. Round bar stock may be 1 to 10 cm in diameter and even larger. Accomplished as a single-draft operation - the stock is pulled through one die opening Beginning stock has large diameter and is a straight cylinder Hydraulically operated draw bench for drawing metal bars

Bar Drawing The reduction in area is usually restricted to 20 to 50%, because greater reductions would exceed the tensile strength of the material, depending on its ductility. To achieve a certain size or shape multiple passes through progressively smaller dies or intermediate anneals may be required

Tub Drawing Tubes produced by extrusion or other process (such as shape rolling) Also tubes can be reduced in thickness or diameter by tube drawing. The shape of tubes can be changed by using dies and mandrels with various profiles

Tube drawing Tube drawing is also similar to wire drawing, except that a mandrel of appropriate diameter is required to form the internal hole. Here three arrangements are shown in figure (a) with a floating plug and (b) fixe plug drawing(c) with a moving mandrel

Fixed plug drawing This is the oldest tube drawing method. Fixed plug drawing, also known as stationary mandrel drawing, uses a mandrel at the end of the die to shape the ID of the tube. This process is slow and the area reductions are limited (lengths of tubes are limited), but it gives the best inner surface finish of any of the processes.

Floating plug drawing Floating plug drawing, also known as floating mandrel drawing, uses a mandrel that is not anchored whatsoever to shape the ID of the tube. The mandrel is held in by the friction forces between the mandrel and the tube. The greatest advantage of this that it can be used on extremely long lengths, sometimes up to 1,000 feet (300 m). The disadvantage is it requires a precise design otherwise it will give inadequate results. This process is often used for oil- tubing

Floating plug drawing

Moving mandrel drawing Is the process that draws the tube with a mandrel inside the tube; the mandrel is drawn with the tube (moving mandrel).  Tubes as large as (30 m)in diameter can be drawn. The advantage to this process is that the mandrel defines the ID and the surface finish. The disadvantages are that lengths are limited by the length of the mandrel, usually no more than 100 feet (30 m), and that a second operation is required to remove the mandrel, called reeling. This type of process is usually used on heavy walled or small ID tubes. Common applications include super-high pressure tubing and hydraulic tubing This process is also use for precision manufacturing of trombone hand slides

Moving mandrel drawing Also known as semi-floating mandrel drawing, is a mix between floating plug drawing and fixed plug drawing. The mandrel is allowed to float at the end of tube, but it still anchored. This process gives similar results to the floating plug process, except that it is designed for multiple diameter of straight tubes. It gives a better inner surface finish than rod drawing.

Preparation of the Work for Wire or Bar Drawing Annealing – to increase ductility of stock Annealing: This is a thermal treatment generally used to soften the material being drawn, to modify the microstructure, the mechanical properties and the machining characteristics of the steel and/or to remove internal stresses in the product. Depending on the desired characteristics of the finished product, annealing may be used before, during (between passes) or after the cold drawing operation, depending on material requirements.

Preparation of the Work for Wire or Bar Drawing • Raw Stock: Hot rolled steel bar or rod coils are used as raw material. Because the hot rolled products are produced at elevated temperatures (1700 – 2200 i.e. hot rolling), they generally have a rough and scaled surface and may also exhibit variations in section and size. Cleaning - To prevent damage to work surface and draw die (Abrasive scale (iron oxide) on the surface of the hot rolled rough stock is removed) it is done by pickling process. Pointing – to reduce diameter of starting end to allow insertion through draw die (Several inches of the lead ends of the bar or coil are reduced in size by swaging ,so that it can pass freely through the drawing die. Note: This is done because the die opening is always smaller than the original bar or coil section size) and also to prevent die from wearing. Coating: The surface of the bar or coil is coated with a drawing lubricant to aid cold drawing.

Lubrication Proper lubrication is important in drawing operations. The basic methods of lubrication used in wire drawing are: Dry drawing : in dry drawing, the surface of the wire is coated with various lubricants, depending on the strength and frictional characteristics of the material. The rod to be drawn is first surface treated by pickling, which removes the surface scale that could lead to surface defects and considerably reduce die life (because of it’s abrasiveness). The bar then goes through a box (stuffing box) filled with soap powder to coat it Pickling is a metal surface treatment used to remove impurities, contaminants, rust or scale from ferrous metals, copper, and aluminum alloys. A solution called pickle liquor, which contains strong acids, is used to remove the surface impurities. It is commonly used to descale or clean steel in various steelmaking processes

Lubrication Wet drawing : in wet drawing, the dies and rod are completely immersed in a lubricant. Typical lubricants include oils and emulsion (containing fatty or chlorinated additives) and various chemical compounds. Metal coating : for high - strength materials, such as steels, stainless steels, and high-temperature alloys, the surface of the rod may be coated with a softer metal or with conversion coating. Conversion coatings may consists of sulfate oxalate coatings on the rod, which typically are then coated with soap, as a lubricant. Copper or tin can be chemically deposited as a thin layer on surface of the metal, whereby it acts as a solid lubricant Polymers may also be used as a solid lubricants, such as in drawing titanium

Features of a Draw Die Entry region - funnels lubricant into the die to prevent scoring of work and die Approach - cone-shaped region where drawing occurs Bearing surface (land) - determines final stock size. The purpose of the land is to size, that is, to set the final diameter of the product Back relief - exit zone – if the exit of die bearing has a sharp edge, this can shave the wire. This will produce metal particles, dust or fines, which will Pollute drawing lubricant Block the entrance of the next die

Features of a Draw Die Back relief The friction against sharp edge also imparts vibration to the wire, which can be transmitted through the machine. The negative results of this are: Abnormal wear of the die Poor wire quality Numerous wire breaks To avoid this sharp edge, it is necessary to produce A well smooth transition zone between the bearing and the exit zone

Die materials Die materials for drawing are generally alloy tool steels, carbides, or diamond. For drawing fine wires the die may be diamond, either a single crystal or a polycrystalline diamond. Carbide and diamond dies are made as inserts or nibs, which are then supported in a steel casting as shown:

Die Wear A typical wear pattern on a drawing die is shown below. The die wear is highest at the entry. Although the pressure is highest in this region and may be partially reasonable for wear of die.

Die Wear Other factors that are involved in wearing of die include: Variations in the diameter of the entering wire. Vibration, which subjects the die-entry contact zone to fluctuating stresses The presence of abrasive scale on the surface of entering wire.

Equipment Drawing equipment can be of several designs. These designs can be classified into two basic types; Draw bench, and Bull block: A draw bench : A draw bench uses a single die and the pulling force is supplied by a chain drive or by hydraulic means. Draw bench is used for single length drawing of rod or tube with diameter greater than 20mm (may be 1 to 10 cm in diameter and even larger). Length can be as much as 30 m. similar to a long horizontal tensile testing machine but with hydraulic or chain-drive mechanism, is used for single draws of straight rods with large cross sections and for tubes with length up to 30 m. Bull block : smaller cross sections are usually drawn by a bull block, which is basically a rotating drum around which wire is wrapped. The tension in the setup provides the force required to draw the wire

Equipment  Drawing equipment can be of several designs. These designs can be classified into two basic types Draw bench for rod and tube and Bull block for wire A draw bench uses a single die and the pulling force is supplied by a chain drive or by hydraulic means. Draw bench is used for single length drawing of rod or tube with diameter greater than 20mm. Length can be as much as 30 m

Draw bench

Bull block

Drawing Defects Defects in drawing are similar to those observed in extrusion especially center cracking. The factors influencing center cracking are: The tendency for cracking increase with increasing die angle. Decreasing reduction per pass. Friction The presence of inclusions in the material. A type of surface defect in drawing is the formation of seams. These are longitudinal scratches or folds in the material which can open up during subsequent forming operation, such as by upsetting, heading, thread rolling, or by bending of the wire or rod.

Residual stress in drawing operation Drawing Defects Residual stress in drawing operation Because of inhomogeneous deformation that the material undergoes, a cold drawn rod, wire, or tube usually contains residual stresses. Typically a wide range of residual stresses can be present within the rod in three principles directions : Transverse direction Longitudinal direction. Radial direction.

Residual stress in drawing operation For very light reductions, the surface residual stresses are compressive. Note that light reductions are equivalent to shot peening or surface rolling which induce compressive residual stresses on surface, thus improving fatigue life. Residual stress can be significant in stress application causing cracking or in warping of the component when a layer is subsequently removed, as by machining or grinding

shot peening Shot peening is a cold working process used to produce a compressive residual stress layer and modify mechanical properties of metals. It entails impacting a surface with shot (round metallic, glass, or ceramic particles) with force sufficient to create plastic deformation

Mechanics of rod and wire drawing The major variables in drawing process are: The reduction in cross-sectional area Die angle Friction also plays a major role

Idea drawing force-with no friction The drawing stress, σd, for the simplest case of ideal deformation-no strain hardening ( that is no friction or redundant work ) is obtained by : Where Y is the yield stress of the material For the material that exhibits strain hardening with the true-stress-strain behavior, Y is replaced by an average flow stress Ȳ: The quantity, Ȳ, is obtained from the expression: Where Ȳ is the average flow stress

Idea drawing force-with no friction The ideal drawing force ( that is no friction or redundant work ) is obtained by : Where iis the average flow stress and can be obtained by : A0: is the initial cross sectional area. Af: is the final cross sectional area. K is the stress (strength) coefficient, n is the strain hardening (work hardening) exponent

Idea drawing force-with friction Friction at the die-workpiece interface increases the drawing force because work has to be supplied externally to overcome friction. The drawing stress with friction is obtained by following expression: .

Die pressure Noting that the compressive stresses in the two directions are equal to p, the die pressure along the die contact length can be obtained: Yf :is the flow stress , σ :is the tensile stress in the deformation zone. Yf

Example A round rod of annealed 302 stainless steel is being drawn from a diameter of 10 mm to 8 mm at s speed of 0.5m/s. assume that frictional and redundant work together constitute 40% of the ideal work of the deformation. Calculate the power required for this operation? Calculate the die pressure at the exit of the die?

Comparison of bulk deformation processes

Comparison of bulk deformation processes

Comparison of bulk deformation processes