In this article we will discuss about the technology, lubrication and machinery of wire drawing.
Technology of Wire Drawing:
Wire Drawing Die:
Modern wire drawing process is similar to the one used centuries ago, however, significant developments have taken place in the machines and the dies. Modern dies are made out of tungsten carbide, ceramic, diamond or hard alloy.
A cross-section of tungsten carbide die is shown in Fig. 9.2. It consists of a tungsten carbide nib imbedded in a casing made out of medium carbon steel. The dimensions of nib and casing are standardized for standard wire diameters. Tungsten carbide dies are used for drawing wires of small, medium and large size, for large diameter wires hard steel alloy dies are also used.
Diamond dies are used for drawing very fine wires of diameters ranging from 0.01 to 1.5 mm. Diamond dies are also preferred for drawing materials like stainless steels which have affinity for tungsten carbide. The nib is made of either single diamond or polycrystalline diamond (PCD). The nib is encased in a casing of tungsten carbide. PCD dies are superior to single diamond dies.
Various methods are used for drilling and polishing the dies. Lasers are now-a-days used extensively for making very small diameter holes. Among other methods, ultrasonic drilling, spark erosion and electrochemical machining may be used. For polishing the die, fine diamond powder or other fine abrasive particles may be used. Dies are, in fact, replacement items just like throw away tool bits in metal cutting. After the die has worn out, it may be re-polished to the next higher size.
At the entry side (Fig. 9.2) the die has a bell shaped or conical approach portion followed by a conical region wherein the wire gets plastic deformation. Towards the small end of the die, the central conical surface merges into a cylindrical region called land. On a new die the diameter of land is equal to the desired wire diameter minus the tolerance so that the die may be used up to the plus side of the tolerance.
The land portion is followed by die relief angle which strengthens the die against chipping of die-end corner. The reduction in wire diameter or plastic deformation takes place in the middle conical region of the die. The cone angle of this region is called die angle.
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The value of this angle should be the optimum i.e., angle at which the process requires the minimum drawing force. The optimum value of die angle varies with the frictional condition and reduction in the wire diameter. Dies with curved middle portion are also used in order to make the metal flow more streamline and thus reduce the drawing force.
Preparation of Steel Wire Rod for Drawing:
For manufacture of steel wires, the raw material is the wire rod which is a hot rolled bar. The wire rod has hard oxide layer or scale on its surface, which must be removed, because otherwise, the abrasive scale will decrease die life as well as damage the surface quality of drawn wire.
The following methods are employed for removal of scale:
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(i) Chemical reaction with dilute hydrochloric or sulfuric acid.
(ii) Mechanical methods which are given below-
(a) Shot blasting.
(b) Loosening the scale by bending over pulleys followed by brushing.
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The first method (chemical reaction) is used by majority of manufacturers. The disadvantage associated with this method is that along with scale the metal is also attacked by acid. The reaction releases nascent hydrogen some of which gets absorbed by the material of wire resulting in making the wire material brittle. And secondly, the wire surface may get pitted by reaction of acid, which spoils the surface quality of drawn wire.
Now-a-days, inhibitors are added to the acid solution which cover up the newly formed metal surface after the scale gets peeled off and act as barrier between the acid and the metal. The temperature of acid solution is raised up to 95°C in order to increase the rate of reaction. For mild steel wires dilute HCl or H2SO4 may be used. Generally the waste acid of other industries is employed.
The oxide layers may consist of FeO which is the inner most layer and adjacent to metal surface, the outer layers may consist of Fe2O3 (black in colour) or Fe3O4 (brown). FeO is the most reactive with acids and in order that acid should reach the inner layers the scale has to be broken.
This is achieved by bending the wire over a small diameter pulleys such that the outer metal layer gets about 8% deformation. The scale being brittle it cracks at places. The bending has to be done on at least 4 pulleys placed at different angles so that cracks in scale are obtained around the circumference.
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The mechanical methods consist of loosening the scale followed by brushing or shot blasting which has to be carried out in different planes so that scale is removed all around the circumference. These methods do not have the disadvantages of pitting and hydrogen embrittlement. But in spite of all the care in mechanical methods, some specks of scale are left on the wire surface, which get into the die and increase die wear. After scale removal the wire is prepared for lubrication.
Lubrication in Wire Drawing:
Both solid and liquid lubricants are used for wire drawing. Drawing with solid lubricants is called dry drawing and that with liquid lubricants is called wet drawing.
i. Dry Drawing:
In dry drawing, the lubricant is in the form of solid powder which is generally a mixture of soap powder, graphite and lime or a mixture of solid fat and lime (CaO) and Ca(OH)2. The soaps generally used are calcium stearate and stearates of other metals like sodium, zinc and aluminium.
Sodium soap is particularly useful after the phosphate coating process which is described below. The typical compositions contain 10-15% aluminium stearate, 15-20% calcium stearate, hydrated lime 60-70% and graphite 2 to 5%.
Dry drawing is generally used for heavy reductions, though the surface produced is dull. The drawn wire also carries a thin film of lubricant which helps the subsequent processing such as upset forging of headed components like rivets, screws, bolts, etc.
In order that the wire surface may be able to carry the lubricant into die, it is coated with lime, for which, the wire is dipped in hot solution of lime at about 70°C, then taken out and dried. It may require several dippings before the desired film thickness is achieved. It is finally dried and baked at 200°C. The lime particles get firmly attached to the wire surface and can entrap the lubricant particles when the wire passes through the lubricant box which is placed just before the die.
During the drawing operation the wire becomes hot and the soap tends to melt. The lime helps in keeping the lubricant thick and viscous and in maintaining lubricant film on the interface so that metal to metal contact between die and wire does not take place. However, for small diameter wires (d < 0.5 mm) lime is not used.
ii. Wet Drawing:
Wet drawing is used for obtaining bright surface finish and for using hydrodynamic lubrication devices. Also it is used for small diameter wires and for wires which have to be coated after the drawing process because wet lubricant is easier to remove.
In wet drawing, liquid lubricants are mixtures of soap solutions with up to 3% oil. Soap emulsions with water and fatty oils may be used for copper, silver, gold and platinum wires. For aluminium wires petro-oils with fatty oils may be used.
Phosphate Coating:
It is beneficial to use phosphate coating for drawing wires of high strength steels. Phosphate coating is very adherent and spongy and can absorb the lubricant and hence acts as a very good lubricant carrier. The coating prevents metal to metal contact even under extreme pressures.
For phosphate coating, the wire surface is thoroughly cleaned of rust and grease. It is rinsed before dipping in a bath containing phosphoric acid and zinc phosphate.
The following reactions may take place:
As the process proceeds the chemical reaction gradually slows down due to following two reasons:
(i) Hydrogen which evolves during the reaction, makes a gaseous shield between metal and the solution. An oxidizing agent is added to the solution to convert hydrogen into water. Sodium nitrate or sodium chlorate may be used. Addition of these chemicals accelerates the process.
(ii) The second problem arises due to dissolving of iron in the solution. This changes the composition of solution from Zn phosphate to iron-zinc phosphate resulting in slowing down the process. Addition of sodium chlorate helps in converting ferrous phosphate to ferric phosphate which precipitates out of the solution, thus it helps in maintaining the original composition of the solution.
After phosphate coating the wire is dipped in a liquid lubricant generally containing soap and MoS2. The spongy and adherent phosphate coating absorbs the lubricant. If the coated wire is dipped in a solution of sodium stearate, a chemical reaction takes place as a result of which zinc- stearate is formed at the top layer of coating.
This makes a highly adherent lubricant film because zinc stearate is chemically bonded to zinc phosphate which in turn is chemically bonded to wire metal. Experience with this coating shows that wire may be drawn through 10 to 12 dies without an additional lubricant box.
Hydrodynamic Lubrication in Wire Drawing:
Now-a-days, wires are drawn at pretty high speeds, and therefore, hydrodynamic effect may be utilized to enhance the lubrication at the die-wire interface. Hydrodynamic lubrication decreases the die wear as well as the drawing force. In conventional wire drawing the motion of lubricant is illustrated in Fig. 9.3.
As the wire passes through the liquid lubricant box, its surface picks up a layer of lubricant, which moves along with the wire. Because of high interface pressure between the wire and the die, only a small fraction of the lubricant layer can go through the die, the remaining lubricant coming at high speed with the wire comes to a stop and possibly flows back (Fig. 9.3).
The change of momentum of high speed fluid (Newton’s second law of motion) gives rise to very high pressures near entrance to the die. This is equivalent to a pressurized lubrication. The hydrodynamic lubrication may be enhanced by fixing a short tube length at the entrance to the die (Fig. 9.4). The clearance between the tube and the wire is very small, just sufficient for the fluid film to get into the tube along with the wire.
A second device shown in Fig. 9.5 employs two dies fixed at either end of a high pressure chamber. A low reduction is given in the first die while a high reduction is given in the second die. The low reduction allows more fluid lubricant to get into the chamber while the high reduction in the second die allows less quantity of fluid to get out of the chamber. Thus, a high pressure is built up inside the chamber. The second die gets high pressure lubrication.
Die Rotation and Use of Ultrasonic Vibrations:
Rotation of die during wire drawing process helps in making the wear of die more uniform and hence increases die life.
Application of ultrasonic vibrations perpendicular to the axis of wire helps in reducing friction. However the effect is rather small and the cost is high. Therefore, at present it is not economical due to high initial cost of the equipment and its maintenance.
Wire Drawing Machinery:
Large diameter wires and straight bars are generally drawn on draw benches (Fig. 9.6). Small diameter wires which can be wound are drawn on block or drum type machinery. The rotation of circular drum or block on which the wire is wound pulls the wire through the die (Fig. 9.7).
The draw blocks are slightly tapered toward top so that the wire loops may slip along as the new loop is wound at the bottom of the drum (Fig. 9.7). The drawn wire is taken off from the top end of the drum. It may be fed to another die and drum placed in succession.
A wire drawing machine may have one or more draw blocks. In high speed wire drawing, the wire gets quite hot and contact of wire with the metal drum helps in cooling the wire. Double drums are sometimes used to increase contact time in order to cool the wire. Besides, other cooling devices may also be used.