Wire drawing is a cold metalworking process that reduces the material's diameter and increases its length, creating usable wire. This is done by pointing the material, pulling it through either a single or multiple draw dies using revolving drawing blocks, and employing lubricants to reduce friction and to prevent damage. When multiple draw dies are used, each draw die is incrementally smaller than the previous die until obtaining the final size desired. It is common on multi-hole draw machines to have anywhere from 5 -- 7 draw blocks and draw dies, sometimes more, depending on how small the final diameter is that you are looking to achieve.
This extrusion process also improves the surface finish, mechanical properties, and the diameter tolerance of the original hot roll material. The process is typically performed cold (room temperature) and therefore this process is better known as cold drawing. Cold drawing is done by either drawing coil to coil, coil to straight length bar, or in straight length bar to bar.
The basic process of drawing coil material follows these steps:
The process begins with the steel wire/rod being cleaned to remove the hot roll mill scale. This can be done by either chemically or mechanically descaling the material.
A) Chemical descaling, also known as pickling, is done by immersing in an acid bath containing either sulfuric or hydrochloric acid until the mill scale has been removed. The material is then rinsed, and a lubrication coating is applied. The lubrication coating aids in the cold drawing process by preventing scratching/scoring. Chemical descaling is done on wire/rod by coil only. Various types of coatings may be applied to the material prior to drawing. The most popular being:
- Lime
- inc Phosphate and Lube (reactive or non-reactive lube)
- Zinc Phosphate and Polymer
B) Mechanically descaling can be done on either wire/rod by coil or in straight length bar. There are several ways that this can be achieved:
- Shot-blasting -- cleaning or stripping the material with a high-speed stream of steel particles removes the mill scale and allows the material to be drawn.
- Wire bending - a process in which the material is pulled through a series of pullies that bend the material in different directions to break the mill scale off of the surface.
- Wire brushing - specialized equipment that has a series of wire brush wheels, placed at different angles, spins at increased speeds to remove the scale as the wire/rod runs through the unit.
- Sanding -- very similar to wire brushing, however instead of using wire brushes to remove the hot roll mill scale, the equipment utilizes belt sanding paper.
These various cleaning processes are done either as a separate process or in front of the cold drawing machine. Once the hot roll mill scale has been removed, the material is fed into the drawing machine, and the cold drawing process begins. The end of the wire/rod must be pointed so it will freely slip into the drawing die far enough so that the grips of the machine can be attached and the machine can start pulling the material through the draw die.
The draw die itself sits in what is called a die holder. The die holder is filled with drawing compound which is used to keep the draw die cool and help prevent scratching or scoring of the wire during the cold drawing process. The drawing compound can be either a liquid, a paste, or a powder. Determining which drawing compound to use is a complete topic for discussion. Having a strong partnership with a drawing compound supplier is critical to your success.
A drawing die is a basic tool used to reduce the cross-section of the starting material and can be specially designed based on the grade of the material, the start size and, the desired finish size for each job. The proper approach angle is critical to allow the draw compound to enter the die and carry through onto the wire while drawing.
The die is made up of a carbide nib that is pressed into a steel case and fits snuggly into the die holder. Similar to drawing compounds, having a strong partnership with a good die maker is also critical to your success. For this reason, most draw houses have on staff a die maker or machinist to ensure that the drawing dies are properly maintained.
After the material exits the drawing die, the material is either wrapped around a block to recoil or can go into a straighten and cut machine to make cut-to-length bars.
The cold drawing process can be used to make round to round, round to round corner hex or round corner square, hex to hex, and many other shapes such as double D, single D, and even rounds with keyways to name a few. The final size and shape depend on the design of the drawing die utilized. The possibilities can be considered endless.
Why Do We Draw Steel?
Steel wire is drawn for several important reasons:
- To improve the mechanical properties of the material to meet specific application requirements. These include tensile strength, yield strength, reduction of area, and elongation.
- To improve the diameter tolerance variation and improve the roundness of the product.
- Some customers rely on the cold draw finish of the raw material provided as they do not draw material in front of their header. Other customers will do a light final pass in front of their heading equipment.
Production processes
Material can be drawn in various production processes, sequencing with each providing unique physical properties as well as surface conditions
- Direct Drawn = hot roll material is cold drawn without any heat-treating pre-treatment. This material ships with a drawn surface finish.
- DFAR = drawn from annealed rod, which means that the material was annealed prior to cold drawing. This material ships with a drawn surface finish.
- SAFS = material is spheroidize annealed at finished size, which means the material is cold drawn and then spheroidize annealed. This material receives a final clean and coat process step before shipment. The customer identifies the final coating he/she desires.
- SAIP = spheroidize anneal in process, which means the material is cold drawn, spheroidize annealed, and then cold drawn a final time, which is typically a light reduction (6-9% reduction is common). This material ships with a drawn surface finish.
Different types of materials can be cold drawn, including carbon and alloy steel, stainless steel, copper, aluminum, and others.
Where Is Cold Drawn Material Used?
Cold drawn wire is used in numerous products ranging from piano and guitar strings to tire reinforcement, welding rod, concrete reinforcement, fasteners (nuts, bolts, lug nuts, & rivets), wire racks, wire mesh, and small motor shafts to name a few examples of the uses in the industrial marketplace.
As you can see, drawn wire is vital to a variety of industries. Beta Steel has the capability to draw wire from .064" up to 1.500".
In addition to cold drawing, Beta Steel Group's capabilities for wire/rod by coil and bar stock include:
A ) Annealing both Coils, Slugs (part blanks) and Bars
- Spheroidization
- Lamellar Pearlitic
- Normalizing (Bars only)
- Stress Relief
B) Cleaning and Coating of Coils and Slugs
- Sulfuric Acid Cleaning
- Coatings offered
- i. Lime
- ii. Zinc Phosphate & Lube
- iii. Zinc Phosphate & Polymer
- Saw cutting of Bars
- i. Bundle cut-to-length.
- ii. Slug blanking
- Shot-blasting
- Coils
- Slugs
- Machine Straightening of Bars
- Turn and Polishing of Bars
- Roto-Testing of Bars
We encourage you to contact your Beta Steel representative to discuss your specific requirements.
