The drawing of rebar in a simple form refers to the process of shaping a larger diameter plain or raw rebar into a smaller diameter rebar. In fact, the larger diameter rebar is drawn until it reaches the desired diameter. Additionally, the drawing of rebar is a process that creates stress in the rebar and is performed to increase its resistance and strength against tensile forces.

The drawing of rebar is carried out using modern and highly precise machines. The shape of the drawing die can be round, hexagonal (six-sided), square (four-sided), etc., depending on its application. The raw rebar takes on the desired shape after passing through the die, and after the process, it is commonly referred to as transmission rebar or trans rebar. Its products include various types of trans rebar (oil-drawn, single-polished, double-polished or ground, bearing-drawn, etc.) and six-sided or hexagonal drawn rebar, four-sided or square drawn rebar, and trans strips.

Among the top rebar drawing companies in Tehran, Pars Precision Drawing Company is noted as one of the pioneers in the rebar drawing industry with over 30 years of experience. They are always capable of drawing rebar and producing various types of drawn rebar with very high precision in hundredths and tenths according to user needs and export quality by using the most modern drawing machines.

Rebar drawing has various types, and we will discuss the main ones, namely hot drawing and cold drawing, along with the advantages of each.

Hot Drawing

This is a process in which rebar is subjected to high temperatures and stress using special machines.

Steps of Hot Drawing

1- Initial heating: Initially, the rebars are heated in a special furnace at high temperatures of about 950 to 1050 degrees Celsius, which softens the rebar and speeds up its forming process.

2- Hot rolling: In the second stage, the heated rebars are passed between special rolling rollers. These rollers continuously apply pressure to the rebars, causing them to change shape and size, and this process may be repeated several times until the desired shape and size are achieved.

3- Cooling: After rolling, the rebars are rapidly cooled. This process can occur naturally in open air or be controlled using water and air. Accelerating the cooling of the rebars increases their strength.

Advantages of Hot Drawing

Flexibility: The rebars produced by hot drawing have greater flexibility and are more resistant to breakage.

High strength: This type of drawing increases the hardness and strength of the rebar.

Accelerated production: In hot drawing, due to the use of high temperatures and rolling machines, the production process is carried out quickly, leading to mass production of rebar.

Applications of Hot Drawing

Hot drawing is used in large projects, both construction and industrial, that require high-strength rebar. For example, in the construction of bridges, towers, dams, and other strong and large structures.

Cold Drawing

Cold drawing is a process in which rebar is subjected to tension without the need for heating and at room temperature. This method is mainly used for producing rebar with small diameters and high precision in dimensions and specific mechanical properties.

Steps of Cold Drawing

Cleaning the rebar: Before drawing, the rebar must be cleaned to remove any rust, oil, and surface contaminants. This is done using chemical or mechanical methods.

Drawing the rebar: After cleaning, the rebars are passed through special drawing machines through the die. This process reduces the diameter of the rebar and increases its length. The drawing process may be carried out in one or several stages to achieve the desired results regarding dimensions and diameter.

Stress relief: After drawing, the rebar undergoes heat treatment to eliminate internal stresses and improve its mechanical properties.

Advantages of Cold Drawing

Surface smoothness: The rebars drawn using this method have a smoother and more uniform surface, which is one of its prominent features.

High dimensional accuracy: Producing rebar with high precision in diameter and dimensions is one of the specific and sensitive applications of cold drawing.

Improvement of mechanical properties: Cold-drawn rebars have high strength and hardness and exhibit greater resistance to mechanical and external forces.

Applications of Cold Drawing

This type of rebar has various applications in different industries. The automotive, machinery, and construction industries are some of the sectors that utilize drawn rebars.

Rebar Drawing or Wire Drawing

When metal is subjected to a tensile force at the exit opening or behind the die, the forming process is referred to as drawing or wire drawing.

The force required for drawing or wire drawing is a function of the reduction in cross-section, the angle of the die, and friction. After trimming the rebar, it is prepared for drawing. The rebars are taken to the drawing section, and the trimmed end is easily passed through the drawing die. The drawing clamp, supported by a strong gearbox with a wire rope connection, grips the trimmed end of the rebar and forces the larger diameter rebar to pass through the die hole with great force.

For example, a rolled rebar with an approximate diameter of 14 millimeters is converted to a rebar of 11.75 with a tolerance of 0.01 millimeters. The drawing of rebar or wire drawing reduces the diameter of the rebar and increases its length as it passes through the die. Therefore, the smaller the diameter of the rebar compared to the initial diameter, the more the drawn rebar increases in length.

This change in length can be accurately calculated according to the table below. This table presents the weight of one kilogram of iron per meter for different diameters in three circular, square, and hexagonal cross-sections.

For example, a rebar with a diameter of 10 millimeters has a weight of 0.617 kilograms per meter according to the table, and when converting and drawing to a rebar with a diameter of 8 millimeters, which has a weight of 0.395 kilograms per meter, we calculate as follows: one meter of 8 millimeter rebar weighs 0.395 kilograms. Now, how many meters of 8 millimeter rebar does 0.617 kilograms correspond to? 0.395 1 meter diameter 8 0.617 Kg/m? meter diameter 8 1.56 = ? 0.56 meters change in length.

In cases where drawing of non-circular cross-sections is desired or even in cases where non-geometric and irregular surfaces are intended, the desired profile must be formed to dimensions approximately fifteen percent larger than the final size before drawing through roll forming or hot rolling.

In such cases, calculating the entry angles of the drawing die and considering the uniform drawing force on the profile cross-section is very important. It should be noted that before drawing, there is usually a need for suitable annealing and removal of surface scales before drawing.

For some metals that are high in carbon or contain resistant elements, the annealing process should also be performed after drawing to serve as a stress relief operation and possibly apply grinding or related machining.

In general, under the best conditions and the most optimal drawing method, the reduction in cross-section should not exceed 20 to 22 percent, although in practice, these figures are limited to about 15 percent. In fine drawing of circular sections, diameters of one hundredth of a millimeter are also drawn with extraordinary precision.

Preparation of Metals and Drawing

Longitudinal drawing of metals is one of the main methods of preparing metals for industrial applications. In this method, the angle of the die and its dimensions determine the shape and quality of the final product. Drawing is usually performed at room temperature without the need for heating the metal, with the only exception being the drawing of tungsten, which is used for heating elements (filaments) of thermal lamps.

Metal drawing can be carried out in several stages without the need for annealing, although the number of drawing stages and the extent of reduction in cross-section directly depend on the type of metal and the alloys used in it.

One of the significant advantages of cold drawing that has led to its expansion in the industry is the accuracy and uniformity of the surface after drawing and the speed of production.

In addition to the above advantages, for some metals such as aluminum and copper, surface densification is essential for preparing for subsequent operations, which is done through rolling or drawing, or ultimately rolling and drawing.

In some cases, even for shaping and preparing the surface of specific plastics and even for applying a metal coating on another metal, referred to as the core, rebar drawing is used and has a desirable and significant application. For example, cold drawing is used for copper coating on iron rods for grounding rods.

Rebar Drawing Process | Friction and Proper Lubrication Between the Die and Rebar

In metal forming, the friction conditions between the die and the rebar are significantly influenced by factors such as force, the manner of changing the form, the final properties of the sample, surface roughness, etc.

The material of the drawing die should be made of an alloy called tungsten carbide, and for lubrication and cooling during the passage of the rebar through the die, drawing oil is used.

The friction stress (Ƭ) is measured in terms of force per unit area of contact surface. The contact area is the boundary of the deformed metal. Therefore, the friction or resistance stress is the shear stress at that boundary.

The rebar drawing process | The mechanism of friction is a complex phenomenon and depends on many variables such as the material type, size and diameter of the rebar, rate of deformation, temperature, and atmospheric conditions, etc.

For a specific lubricant, three models are generally used to describe its effect: hydrodynamic, boundary lubrication, and hydrostatic lubrication.

In hydrostatic lubrication, which is common in metal forming, the lubricant is trapped in the form of many small packets at the surface of the sample when the tool contacts it.

In the deformation of the metal, the surface of the rebar twists and takes on the role of the die surface. Therefore, the actual contact area relates to the special adhesion of the contact surface and cannot be disregarded as in elastic deformation.

Thus, the rebar drawing process involves friction in deformation, which is necessarily different from sliding friction in machine parts. In any case, high relative speeds between the workpiece and the tool surface, along with high pressure at the interface or heavy deformation modes, will break the surface film, allowing a new surface to come into contact with the tool surface.

Thus, adhesive contact is facilitated. For such cases, the assumption of no sliding between the workpiece (rebar) and the tool (die) is also incorrect.

For such a composite mechanism of friction that usually occurs in metal forming, friction is a function of sliding and adhesion. Thus, the following shear equation is obtained: [P + AØ]μ = Ƭ

To shape and size rebar and other metal sections, it must pass through a special die, and the necessary conditions must be observed before drawing the rebar. One of the main production processes after hot rolling of metals is cold drawing to prepare the metal for industrial applications. This method, due to its polished and precise surfaces, has been common in the industry for a long time.

In essence, the initial method of reducing the diameter of metals, especially precious metals such as gold and silver, through drawing has been practiced since ancient times, although it was done sporadically and based on personal experiences in specialized goldsmith workshops. It is evident that at this level, production was very minimal and limited to fulfilling individual needs, which had been commonly practiced for years and met the needs of its time.

The Role of Delta Parameter in the Wire Drawing Process

The delta parameter is an important parameter used to express the effect of the geometry of the deformation zone in the wire drawing process.

Δ indicates the ratio of the size of the deformation area perpendicular to the drawing axis (h) to the size of the deformation area parallel to the drawing axis (L) (h/L). This ratio is essentially the average diameter in the conical area divided by the length of this area. h = h0 - h1 and αL = h/2sin in terms of the half-angle of the die (α) and the amount of reduction in cross-section (r), delta can be estimated as the expression /r)][1+(1-r)1/2 ]2α [(where α is in radians and r is the percentage reduction in cross-section divided by 100.

In traditional and common drawing processes, delta values range from 2.5 to 3, while in new drawing systems, this value is less than or equal to 1.5. In other words, the smaller the die angle and the greater the reduction in cross-section, the lower the delta value.

The important role of delta can be explained in the wire drawing process and its effect in preventing and limiting defects such as center-burst and chevron cracks, where delta values equal to or less than 1.5 minimize the growth of chevron cracks.

In general, low delta values in wire drawing processes minimize the additional work required for annealing, die pressure, and die wear. Additionally, when lubrication is performed well, low delta values can reduce the coefficient of friction.

The delta formula allows the effects of die angle and the amount of reduction in cross-section to be effectively combined, providing optimal conditions for wire drawing through changes in the reduction of cross-section and die angle.

One of the disadvantages of low delta values is the need for dies with longer cores, and the amount of friction can increase in situations where lubrication conditions are critical.

Therefore, by selecting an appropriate delta value:

The pressure on the drawing dies can be significantly reduced, thus decreasing the likelihood of die failure.

In a uniform input size of raw materials to the drawing machine and lubrication conditions, a wire with higher strength and flexibility can be produced.

The elastic modulus of the wire output from the drawing machine is increased.

The likelihood of rebar failure during and after drawing is reduced.

Additional work and thus residual stress on the wire are significantly reduced.

The need for annealing in high percentage reductions in cross-section is eliminated.

Die wear is reduced, and the drawing speed is increased to some extent.

This scientific calculation and the articles presented to the factory, with the agreement of the technical department, led to the construction of dies using modern methods and the calculation of die angles, replacing the traditional and inaccurate visual methods. After implementation, it was observed that the lifespan of the dies and the drawing speed increased by up to 3 times, significantly enhancing efficiency.

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