Wire and Rod Drawing Process for Steel
Wire and Rod Drawing Process for Steel
Drawing of wire from steel rod is a metal working process used for the reduction of the cross-section of the rod. Similarly rods are drawn from steel rounds of larger diameters. During drawing the volume remains the same and hence there is increased in the length of the drawn wire or rod. It is carried out by pulling the wire/rod through a single or a series of the drawing dies. In the case of series of drawing dies, the subsequent drawing die is to have smaller bore diameter than the previous drawing die. Drawing is usually performed in round sections at room temperature, thus it is classified as a cold working process. However, it can be performed at higher temperatures for large wires to reduce forces.
Drawing process normally is most frequently used to produce round cross sections, but squares and other shapes can also be drawn. Wire/rod drawing is an important industrial process, providing commercial products. Rod and wire products cover a very wide range of applications which include shafts for power transmission, machine and structural components, blanks for bolts and rivets, electrical wiring, cables, wire stock for fences, rod stock to produce nails, screws, rivets, springs and many others. Drawing of rods from steel rounds is used to produce rods for machining, forging, and other processes etc.
Advantages of drawing in the above applications include (i) close dimensional control, (ii) good surface finish, (iii) improved mechanical properties such as strength and hardness, and (iv) adaptability to economical batch or mass production.
In the process of drawing, the cross section of a long rod or wire is reduced or changed by pulling (hence the term drawing) it through a die called a draw die. Pulling of rod through the die is done by means of a tensile force applied to the exit side of the die. The plastic flow is caused by compression force, arising from the reaction of the metal with the die.
Thus, the difference between drawing and extrusion is that in extrusion the material is pushed through a die, whereas in drawing it is pulled through it. Although the presence of tensile stresses is obvious in drawing, compression also plays a significant role since the steel material is squeezed down as it passes through the die opening. For this reason, the deformation which occurs in drawing is sometimes stated to as indirect compression.
The major processing variables in drawing are reduction in cross-sectional area, die angle, friction along the die-work piece interface, and drawing speed. The die angle influences the drawing force and the quality of the drawn product.
The basic difference between drawing of rod and wire drawing is the size of the starting material which is processed. Bar drawing is the term used for drawing of rods from the steel rounds, while wire drawing applies to drawing of wires from steel wire rods. Wire sizes down to 0.03 mm are possible in wire drawing.
Drawing speeds for steels can be usually as high as 10 meters per second for very fine wire. In drawing, reductions in the cross-sectional area per pass range up to about 45 %. Usually, the smaller the initial cross section, the smaller the reduction per pass. Fine wires usually are drawn at 15 % to 25 % reduction per pass and larger sizes at 20 % to 45 % per pass. A light reduction (sizing pass) can also be given on rods to improve their surface finish and dimensional accuracy.
Drawing of rods from rounds is generally accomplished as a single draft operation which means that the round is pulled through one die opening. Because the starting round has a large diameter, it is in the form of a straight cylindrical piece rather than coiled. This limits the length of the rod that can be drawn. By contrast, wire is drawn from wire rods in coils consisting of several hundred of meters and is passed through a series of draw dies. The number of dies varies typically between 4 and 12.
The process characteristics of wire/rod drawing consists of (i) pulling of the wire rod/round through the die to reduce its diameter, (ii) drawing increases the length of the wire/rod as its diameter decreases, (iii) several dies are used in succession (tandem) for small diameter wire, (iv) drawn wire/rod properties gets improved due to cold working, and (v) wire temper can be controlled by swaging, drawing, and annealing treatments.
Since the drawing process consists of pulling a rod or wire through a die, it results in a stretching or elongation of the material along with a reduction in cross sectional area. The pulling force is limited by the strength of the steel material. In case the wire/rod is pulled too hard then the material breaks. The force needed to pull the wire/rod through the die is determined by the extent of the reduction in cross-sectional area. The larger is the reduction, the greater is the force needed. Thus it can be seen that the maximum achievable reduction in diameter is limited by the yield strength of the steel being drawn.
Yield strength depends on the steel composition and typically the reduction in area through a die is in the region of 15 % to 45 %. If a greater reduction is needed then this is done by drawing the wire through a series of dies, each one smaller than the one before. However, the plastic deformation experienced by the steel rod/wire as it is pulled through the die tends to increase hardness and reduce ductility. Here ductility refers to the ease with which steel can be deformed. As the process of drawing in series makes it harder to reduce the cross-section, it is often necessary to perform an annealing process between successive draws to assist the process of steel deformation. On the other hand, the increased tensile strength resulting from drawing is often seen as a very desirable material property.
Very small diameters can be obtained by successive drawing operations through dies of progressively smaller diameters. Annealing before each set of reductions allows large reduction percentages. In steel drawing process the annealing process is also called patenting.
Wire drawing is usually performed cold, although there are some cases where steel rod/wire is drawn hot to improve ductility. Die lubrication is essential in cold drawing to achieve a good surface finish as well to maximize the life of the die.
Wire drawing involves stretching metal to the required shape, and as such is considered deformation rather than removal processes. A significant advantage of drawing is that there is very little material waste. However, this benefit has to be set against the high cost of the dies and the possible need to carry out annealing to counteract work hardening.
The process of drawing is shown in Fig 1. In the figure ‘Do’ is the initial diameter of the rod and ‘Df’ is the diameter after drawing. ‘F’ is the applied force. ‘Alpha’ is the die angle and ‘Lc’ is the line of contact.
Fig 1 Process of drawing
By pulling a rod or wire through a die the cross section is reduced. The percentage reduction of area (% r) is given by the following equation “% r = 100 x (Ao – Af)/Ao” Where ‘Ao’ is the initial area and ‘Af’ is the final area of the wire/rod after drawing.
Before the actual drawing, the material to be drawn is properly prepared. This involves three steps namely (i) annealing, (ii) cleaning, and (iii) pointing. The purpose of annealing is to increase the ductility of the starting material to accept deformation during drawing. Annealing is also sometimes needed between steps in continuous drawing. Cleaning of the wire rods/rounds is required to prevent damage of the work surface and draw die. It involves removal of surface contaminants (e.g., scale and rust) by means of chemical pickling or shot blasting. In some cases, pre-lubrication of the work surface is accomplished subsequent to cleaning. Pointing involves the reduction in diameter of the starting end of the wire rods/rounds so that they can be inserted through the draw die to start the process. This is usually accomplished by swaging, rolling, or turning. The pointed end of the wire rods/rounds is then gripped by the carriage jaws or other device to initiate the drawing process.
In case of drawing of stainless steel wire/rod, the surface of the wire rod/round is examined first. This is done by tensile and hardness testing, and measuring of the diameter. Surface preparation is done by pickling in acid (ferritic and martensitic steels) or basic solutions (austenitic steels). The prepared skin is then coated with lubricant. Cold drawing is carried out through diamond dies or tungsten carbide dies till the desired diameter is achieved. Cleaning off oil/lubricant is then carried out and the wire is heat-treated (annealing at around 1100 deg C or plus skin pass).
Drawing of rods of larger diameter is carried out on draw benches which consists an entry table, die stand (which contains the draw die), carriage, and exit rack. The carriage is used to pull the stock through the draw die. It is powered by hydraulic cylinders or motor-driven chains. The die stand is often designed to hold more than one dies, so that several bars can be pulled simultaneously through their respective dies.
Wire drawing is done on continuous drawing machines which consist of multiple draw dies, separated by accumulating drums between the dies. Each drum, called a capstan or block, is motor driven to provide the proper pull force to draw the wire stock through the upstream die. It also maintains a modest tension on the wire as it proceeds to the next draw die in the series. Each die provides a certain amount of reduction in the wire, so that the desired total reduction is achieved by the series. Depending on the steel to be processed and the total reduction, annealing of the wire is sometimes required between groups of dies in the series.
Draw bench for rod drawing and continuous wire drawing is shown in Fig 2.
Fig 2 Draw bench for rod drawing and continuous wire drawing
Terminology of a typical die used for drawing rod/wire is shown in Fig 3. A typical draw die has four distinguishing regions. These regions are (i) entry, (ii) approach angle, (iii) bearing surface (land), and (iv) back relief. The entry region is usually a bell-shaped mouth that does not contact the work. Shape of the bell causes hydrostatic pressure to increase and promotes the flow of lubricant into the die and prevents scoring of wire rod/round being drawn and die surfaces. The approach is where the drawing process occurs. It is cone-shaped with an angle (half angle) normally ranging from around 6 degrees to 20 degrees. The proper angle varies according to wire rod/round material. The bearing surface, or land, determines the size of the final drawn wire/rod. It produces a frictional drag on the wire/rod and also removes surface damage due to die wear, without changing dimensions. Finally, the back relief is the exit zone. It is provided with a back relief angle (half-angle) of around 30 degrees. The back relief allows the steel material to expand slightly as the wire leaves the die and also minimizes abrasion if the drawing stops or the die is out of alignment.
Fig 3 Terminology of a typical die
Draw dies are made of tool steels or cemented carbides. For hot drawing, cast-steel dies are used because of their high resistance to wear at elevated temperatures. Dies for high-speed wire drawing operations frequently use inserts made of diamond (both synthetic and natural) for the wear surfaces. Cemented carbide is composed of carbides of titanium, tungsten, nickel, molybdenum, and tantalum. Cemented carbides are the most widely used for drawing dies due to their superior strength, toughness, and wear resistance. Diamond dies are used for drawing fine wire with diameters ranging from 2 micrometers to 1.5 mm. They are either made from a single-crystal diamond or in polycrystalline form with diamond particles in a metal matrix (compacts). Polycrystalline diamond is used for wire drawing dies have longer die life, high resistance to wear, cracking or bearing. Both carbide and diamond dies are typically used as inserts or nibs, which are supported in a steel casing.
Mechanics of wire drawing
Deformation during wire/rod drawing is influenced by a number of factors which include steel chemistry, approach angle, lubrication, drawing speed, and reduction as the most significant.
Although the fact that volume is not lost during deformation is obvious, it is, in fact, a highly useful concept which forms the basis for analyzing a number of drawing problems. One of the most common applications involves the determination of wire speed at different stands and the necessary capstan speeds which is to be used. Simply stated, constancy of volume states that the volumetric rate of wire entering a die must be the same as that exiting. Because the cross-sectional area is reduced during drawing, it is necessary that a wire must increase in speed for the same volumetric rate of material to enter and exit the die. Volumetric rate is defined as the cross sectional area of the wire multiplied by the wire velocity.
In multi-pass drawing, wire speed exiting each die is to increase so that the volumetric rate of metal flow is equal at all dies. Hence, capstans, having an angular velocity equal to the exiting wire speed, are used to pull the wire through the die after each reduction. If this is not done, the wire breaks due to unequal wire tension between dies.
Wire diameter increases as drawing die wears during the process of drawing. Hence, based on constancy of volume, wire speed decreases as the die increases in size. If the linear speed of the pulling capstan is matched to the wire size of a new die, capstan speed becomes faster than the wire speed as the wire diameter increases. This increased capstan speed applies high tensile stress on the wire, frequently breaking the wire. Hence, capstans in multi-pass drawing units are designed so that the wire slips on the capstan as the die wears and the wire speed decreases. Slip is facilitated by limiting the number of wraps around the pulling capstan and wetting the wire and capstan surfaces with drawing lubricant.
Although it seems that the forces and power in wiredrawing can be analyzed by using simple tension, deformation conditions in wire are, in fact, far more complex due to compressive and drag forces generated by the die surface. Draw force represents the total force which is required to be applied at the die block to overcome friction at the die surface and resistance of the deforming steel material. Since the draw force is being transmitted by unsupported material, the draw force is to be limited to prevent any plastic deformation from occurring outside of the die. Thus, yield stress of the drawn wire represents an upper limit to the allowable draw stress. Accepted drawing practice normally limits draw stress to 60 % of the yield strength of the drawn wire. Draw stress is found by dividing the draw force by the cross-sectional area of the drawn wire.
While it generally appears that the work or energy consumed at a given draw stand is dictated by the material and reduction taken, the actual amount needed is considerably higher in practice. This is the due to the inefficiencies that exist during deformation, which are primarily governed by the approach angle. Such inefficiencies do not make any useful contributions in reducing the cross-sectional area and generally serve only to increase energy requirements and adversely influence wire/rod quality. The total work consumed at a draw stand can be partitioned into three components namely (i) useful homogeneous work required to reduce the cross section, (ii) work required to overcome frictional resistance, and (iii) redundant (inhomogeneous) work required to change the flow direction.
Homogeneous work is determined by drafting (reduction), and is essentially independent of the approach angle. Friction and redundant work, on the other hand, are closely coupled to die geometry and have an opposite effect as the approach angle is changed. Under normal drawing conditions, typical losses are on the order of 20 % for frictional work and around 12 % for redundant work.
Redundant work and frictional work have adverse effects on wire properties in addition to increasing the energy needed for drawing. One consequence is that mechanical properties are not homogeneous across the wire cross section. Because redundant and frictional deformations are concentrated near the wire surface, higher levels of strain hardening results in the surface and near-surface layers (analogous to temper rolling) and is greater than the strain which results from cross section reduction. Also, redundant deformation has an adverse effect on ductility.
Ductility is inversely related to strain and hence, redundant deformation also acts to limit the number of passes and maximum reduction which can be taken prior to annealing. Even if this does not lead to problems in drawing, the resultant loss in ductility can lead to fracturing in subsequent forming processes such as bending and cold heading.
Layers at the wire/rod surface usually not only undergo a change in cross section, but they also deform in shear because of drag presented by the die surface. Even for highly polished die surfaces and hydrodynamic lubrication, a certain amount of frictional work is always present. Frictional work dominates at low die angles where surface drag is increased as a result of higher contact length in the approach zone for a given reduction. Frictional work can be decreased by using a larger approach angle and, to a lesser extent, by improving lubrication or die surface condition. Although friction forces are also related to die load, normally little effort is made to control friction by limiting reduction since this requires additional stands. Instead, normal practice is to optimize approach angle and lubrication effectiveness.
As wire enters the approach zone of a drawing die, material layers near the surface undergo deformation due to the reduction in area and change direction of flow, i.e., bending to conform to the direction change going from the approach zone into the bearing zone of the die. Redundant deformation, like frictional deformation, is not evenly distributed over the wire and is normally at maximum at the surface with a corresponding increase in hardness. Redundant deformation is promoted by larger die angles since material further away from the centerline undergo a sharper change in direction than the material near the centerline and hence experience higher levels of distortion. Redundant deformation influences the level of residual stress in drawn wire. As the approach angle is increased, the deformation gradient between the surface and centre line also increases. This leads to progressively higher tensile stresses at the surface and compression stresses at the core. The reverse effect occurs during drawing, and centre bursts can develop due to the high levels of tensile stresses generated in the core of the wire.
Selection of the proper die angle is crucial for the success of any wiredrawing operation. Based on the fact that frictional work increases with decreasing die angle and redundant work increases with increasing die angle, an optimum approach angle is to exist. The optimum approach angle minimizes both frictional and redundant work and, as a consequence, the drawing force. In addition to minimizing force requirements, the optimum die angle also provides improved surface quality and finish.
The geometry of the working part (approach zone) of a die is a key factor in wiredrawing. This geometry can be defined by the delta factor, which is the ratio of the circular arc spanning the midpoints of the die face to the length of contact between wire/rod and die. Low delta values (small semi-angle or higher reduction in area) indicate larger friction effects and surface heating due to longer wire/rod contact in the approach zone. Higher values of delta (large semi-angle or lower reduction in area) are indicative of increased levels of redundant deformation and surface hardening due to excessive direction change during flow through the die. Large delta often results in a greater tendency toward void formation and centre bursting. Delta values of 1.50 perform well in many commercial drawing operations, while delta factors in excess of 3.0 are to be avoided in general.
Drawing dies can extract only a small amount of heat, so proper attention is required to be paid to inter-pass cooling, particularly at the later stages of reductions. While some of the heat is transferred to the die, most stays in the wire and attempts to use die cooling to reduce wire temperature have proved largely unsuccessful. Various studies on the effectiveness of die cooling found that a die typically removes less than 5 % to 20 % of the heat generated in the wire. This is due to the fact that a given area of wire is in contact with the die surface for only thousandths of a second. Even though the die is expected to remove only minimal heat from the wire, die temperatures cannot be overlooked, and cooling of the die case is often necessary. This is particularly true when carbide inserts are being used in a steel casing due to the large difference in coefficients of thermal expansion.
A good rule of thumb for temperature increase per pass in dry drawing (other than the first die) is 60 deg C to 80 deg C for mild steels and 100 deg C to 160 deg C for high carbon steels. These values are halved for wet drawing. The three modes of wire cooling used normally are (i) direct cooling where water or coolant is sprayed onto wire exiting the die or on the take-up capstan, (ii) indirect cooling where water or coolant is sprayed onto the die casing or is circulated on the inside on the die casing or take-up block, and (iii) air blast where forced air impinges on wire on the block or capstan.
Inter-pass cooling often employs direct water cooling on the wire exiting a drawing die, and using the residual heat in the wire to remove the last of the water by evaporation. Direct cooling combined with internal block cooling can bring the wire temperature to below 120 deg C, which is a reasonable starting temperature for the next reduction. It is important to prevent oxidation and fouling of internal surfaces of the blocks to maintain good heat transfer between the hot wire and cooling water. The effective means of cooling drawn wire (i) ensure that wire enters die as cold as practical, (ii) avoid heavy reduction, (iii) employ the best possible lubrication, (iv) consider using back pull, (v) increase time intervals between reductions, (vi) increase number of wraps on the block, and (vii) increase block diameter.
Two primary variables which control die life are pressure and temperature. Pressure acting on the die in wiredrawing is much lower than found in other cold forming operations. Hence, temperature is often a far more critical factor in controlling die life. Although it seems logical that wear occurs uniformly along the approach zone, this is not the case in practice. Maximum wear (measured in volume loss) normally occurs at the point at which the wire/rod initially contacts the die. There, a deep annular crater is formed, which is normally known as a ‘wear ring’. Ringing is due to the plane of impingement of wire/rod on the die oscillates about a mean position because of irregularities of size and vibration of the wire. As a result, a narrow zone of the die bore is subjected to a cyclic load with eventual subcutaneous failure by fatigue.
Once a wear ring develops, deformation can occur prior to the contact point in the drawing die. This is called ‘bulging’ and results from backup or upsetting of near-surface regions of the wire as contact is made at the wear ring location in the die. Bulging occurring at the initial point of contact in the die throat limits lubricant entry into the die and accelerates die wear. Lesser amounts of wear occur along the contact length of the approach zone, although here too wear is not uniform and often results in an oval rather than a circular wear surface.
Wires sliding against the working area of a drawing die cause die wear so that wear depends on the surface area of wire, and consequently the length of wire, passing through a drawing die. Often, die life is measured in terms of weight of wire drawn or time of drawing. However, such measures are to be converted to length of wire drawn to get a fundamental indication of die wear. Hence, a practical measure of die life is the mean length of wire drawn per unit increase in die diameter. As a general rule, steels having a high yield strength are more resistant to wear. However, recent studies have shown that die hardness does not control die wear, i.e., increasing hardness of die material does not lead to a substantial increase in die life.
Defects in the drawn wire/rod can be either due to the defects in the starting material (seams, slivers and pipe) or can be introduced by the deformation process.
Typical defects in a drawn wire/rod are centre cracking. The defect centre burst or cracking (cupping) occurs for low die angles at low reductions. Centre cracks can occur in drawn products due to larger die angle, lower reduction per pass, and friction etc. Another major type of defect in drawing is seams, which are longitudinal scratches or folds in the material. Seams can open up during subsequent forming operations (such as upsetting, heading, thread rolling, or bending of the rod or wire), and they can cause serious quality-control problems. Various other surface defects (such as scratches and die marks) also can result from improper selection of the process parameters, poor lubrication, or poor die condition.
Because the materials being drawn undergo non-uniform deformation during drawing, cold-drawn products usually have residual stresses. For light reductions, such as only a few percent, the longitudinal surface residual stresses are compressive (while the bulk is in tension) and fatigue life is thus improved. Conversely, heavier reductions induce tensile surface stresses (while the bulk is in compression). Residual stresses can be significant in causing stress-corrosion cracking of the part over time. Moreover, they cause the component to warp if a layer of material subsequently is removed such as by slitting, machining, or grinding.
Rods which are not sufficiently straight (or are supplied as coil) can be straightened by passing them through an arrangement of rolls placed at different axes.