Wire drawing is one of the plastic deformation methods that includes pulling material trough a die and reducing its area which means tensile force applied to the exit side of the die. Materials which are drawable must be having high ductility and good tensile strength.
Wires that have diameter 4-5mm can be produced by hot rolling method. Wires that have smaller in diameter cannot be in required sensitivity because cooling happens fast and undesirable tinder occurs on the surface of wire as a result of heat. In addition, wires can break easily because of huge change on its strength. Because of these reasons sensitive surface and diameter, good strength can be achieved by cold drawing.
Wire drawing process starts with hot rolled wire rod in diameter 5.5-11.5mm. In the beginning of the process slugs are removed from surface with chemical bath or mechanical methods. That operation prevents to harm die during drawing process. In dry rolling grease and soap dust, in wet rolling liquid using as lubricant.
Figure 1.0
A wire can pass trough as many as die untill desired diameter achieved. Every dies continues reducing diameter of wire. In theory, wire drawing process can be done no material left in the end. With this reason length of wire grows and volume remains same. Practically some mechanical differences occurs depens on lubrication kinematics and structure of material. Some cases material requires intermadiate annealing depends on reduction applied to area.
Wire drawing also named cold rolling because no heat applied during drawing process but in smaller diameters, heat occurs while rods properties change with rolling.
Wire drawing process requires basically drawing machines, wire rods, lubricants and dies. Also process named as dry or wet drawing.
Material in Wire Drawing
Tensile test is most common and important test in mechanical examination methods of metals. In generally tensile test is done to determine strength of metals and metal alloys. Controlled tension is applied on cylindrical or any different shaped metal until failure.
Carbon (C): most important material in structure of steel that directly affect strength. Increasing carbon content in steel will increase strength, endurance, water absorption; also will decrease hot shaping ability, welding ability and machinability. But it has no effect corrosive moisture or hot gases resistance. Steels that have 0.2% C content are suitable for drawing process. If carbon content increases it can cause fracture.
Manganese (Mn): it increases tensile strength, yield point, welding ability and hot shaping ability. 3% Mn addition has effect to tensile strength
Silicon (Si): steels always contain silicon in their structure, even very small amounts. It using as de-oxidant in steel production and metalloid of alloys in many case. Also it increase yield point and tensile strength in steels as alloying element. In high temperature it decreases scaling, shows magnetic properties and if steel contains more than 12% it increases acids resistance. With that reason it is using as alloying element in production of spring steels, transformer sheets, acid resistant steels.
Sulfur (S): sulfur is mostly undesirable element in steel. It makes steel brittle and it get into steel structure from raw material. It causes fracture during drawing. Also it may cause cracks with screw wires.
Phosphorus (P): like sulfur, it always found in steel structure and undesirable in drawing. Only it is used above normal amount with free cutting steels for making its shaving brittle and cast irons for increase fluidity.
Cupper (Cu): most characteristic property of cupper is increasing atmospheric resistance of steel even if steel contains low amount of it. It decreases yield point and tensile strength of steel also decreases elongation and makes hard hot forming.
Up to 1.6% C content is suitable for wire production. Steel produced in electric arc furnaces or crucible is enough for these processes. But for some special application, carbon content can be rearranged.
For an example 20MnB4 is suitable for cold deformation which is wire drawing process because of C content is low and Mn content is high. Chemical component is shown as table 2.
Table 2. chemical component of 20MnB4
C%
|
Mn%
|
Si%
|
S%
|
P%
|
0,18/0,23
|
≤ 0,30
|
090/1,20
|
0,025
|
0,025
|
Surface Treatment
Wire rods came from rolling mills has 5.5-8mm diameter and 1500-2000kg weight. There are tinder layers on these wire rods because of hot rolling. These tinder layers can cause corrosion on dies and damage to the surface during drawing. For all these reasons these tinder layers must be removed from surface and this removing operation can be done with two ways.
Chemical Surface Treatment: this operation is called pickling which is done in chemical baths. There is determined amount of
Wire rods that removed from acidic baths may have acid traces on it. Pressured and cold water applied to the wire rods to get rid of these acid traces then they are waited still air until oxide layer occurs on surface. After they are put in phosphate solution (80°C ) to remove acid traces completely and also phosphate makes corrosion resistant layer on surface. As a final step, all wire rods rinse out with water and dry at 100°C in a furnance.
Mechanical Surface Treatment: mechanical surface treatment is using more often because environment consideration. Wire rods are bend bent with rollers and with this way slags are removed from surfaces.
Dies
When wire drawing, wire which has certain strength and hardness passes through a die and reduce its diameter. Therefore these dies must have high stiffness, impact resistance, wear resistance and friction coefficient. For making these properties, sintered carbides and natural or synthetic diamonds should use as die. Usually tungsten carbide using as main component for dies but also component has cobalt as binder. Tungsten carbide and cobalt mixed in dust form then they pressed under high temperatures. Tungsten carbide dies has even harder, denser and more compressive strength than hardened steels and its expansion coefficient is half of hardened steels also. Therefore heat has no effect on tungsten carbide dies during drawing. In addition tungsten carbide has excellent corrosion resistance to prevent scratches on wire during drawing process.
Fig 2. Die Structure
Heat Treatment in Wire Drawing
In wire drawing process, when drawn wire plastically deformed much materials grain size become smaller and many defects occur in crystal structure. This situation does not let wire gets any plastic or elastic deformation and that makes wire brittle. This is undesirable properties in industry. Wire coils heated up to 700°C (below ). Stress relieving, recrystallization and spheroidizing are made. After this process wires gets fine grain size for plastic deformation and drawing process can continue.
If Cq15 (C15E2C) is taken as example, heat treating process can be explained more clearly. Cq15 has ferrite-pearlite microstructure which pearlites observed grain boundaries. Microstructure does not change during annealing but pearlites disperse equally in ferrite structure and increase ductility. Coils heated to 700°C in 4-6h. It passes through recrystallization (500-550°C ), stress relieving (550-650°C ), recrystallization again (650-700°C ) and spheroidizing (starting just below 700°C ). Coils are waited at 700°C for 3+1h in furnaces. Spheroidizing happens in this step and pearlites disperse finely in microstructure. Also nitrogen atmosphere is using for annealing of Cq15 wire coils.
Heat treatment process of wire coil aims to regain
wire rod properties at beginning. Many wire coils are surface treated with acid
baths first time after annealing.
Lubricants
There is always roughness on a surface whatever it
looks flat and shiny. When wires drawing this rougnesses cause corrosion on
surface and could make scratches on surface. Also wire could rapture or die
holes could expand Lubricants are used to prevent that issue. If lubricant
layer is thick enough between wire and die, friction is reduced minimum. A
wires diameter can be reduced 22-30% by passing one die but this rate can go up
to 45% with a good lubrication.
Properties of a lubricant must make a layer between
wire and die to make easier plastic deformation and reduce friction between
them to remove chance of scratch. It must have cooling feature to prevent heat
that occur during drawing. Also it must show detergent feature to clean all
impurities on wires, dices and machine parts.
There are two kind of lubrication in drawing process
named according to they are wet or not and usually saops are using. Dry soap
lubricants are used for thicker wires, wet ones are used for thinner (1.5mm
After drawing lubricant layer can be removed if it is
required. As an example, if wire will be used as tire production or will be
galvanized, lubricants must be removed.
Three kind of soaps are used in wire drawing as
lubricant. They are calcium soaps, sodium based soaps and alluminuim based
soaps.
Calcium Soaps: they are using wire rods that drawed for first time
with slow drawing speed. Calcium soaps also do not disolve in water and their
softening temperature is about 150°C.
Sodium Based Soaps: they are using if clean and fine surface features or
faster drawing speed required. They disolve in water and their softening
temperature is between 140-150°C. They
reacts with lime in certain temperatures to make calcium soaps.
Alluminium Based Soaps: they do not dissolve in water and their softening
temperature is 150°C. They
especially using for wires produced for screw and bolt manufacturing.
In general, oil that using in soaps, amount of that
oil and additives have important role features of soaps.
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