Also known as Cold Finished and Cold Drawn Seamless (CDS), this process is sometimes considered an extension of the cold rolling method. The reason for this is because of similarities like
- The pipes are usually processed at room temperature in cold rolling and drawing.
- Both methods use hot-rolled pipes as the base stock for production.
- Cold-rolled and cold-drawn pipes take longer and cost more to manufacture.
However, several differences like the following also exist between these two processes.
How is Cold Drawing Different from Cold Rolling?
|Manufacturing Process||Application of Pressure||Difference in Temperature|
|Cold Rolling||The tubing is rolled through mills for elongation and thinning.||The rolling of the steel mills applies pressure on the steel tube.||The cold rolling is usually conducted at room temperature.|
|Cold Drawing||The tubing gets drawn through a die, with or without a mandrel.||The pressure is applied locally as the tubing gets drawn through the die.||The drawing may take place under high temperatures to hot work the tubing.|
What is Cold Drawing
A cold-drawn pipe is tubing drawn cold through steel or carbide die to elongate and improve its dimensional accuracy. The process may or may not have a mandrel or a floating plug inserted inside the tube to help enhance the wall thickness.
Cold drawing costs more than hot rolling as the manufacturing process involves more labor, time, and energy. However, the result of this process is also superior, with the products revealing a better surface finish and more even concentricity.
The exposure to extreme temperatures results in forming iron oxide scales on the surface of the hot-finished pipes.
- To get rid of these marks and scales, the hot rolled steel gets passed through an acid bath. This process is also known as pickling.
- Hot rolled pipes also go through pickling before they can be cold-drawn.
- This scale removal helps the pipe retain its performance and improves product longevity.
Before the hot-rolled pipe can be inserted into the drawing die, it gets smeared with lubricants like oil and phosphate for easy insertion. This process is known as lubing.
- Once lubed, the tubing is ready to be cold-drawn.
- Once the tubing enters the opening, a drawing machine begins slowly pulling the pipe forward while the die exerts pressure from all sides.
- Room temp is the typical temperature for this process, and the steel tube is extruded at one end before the insertion because the die diameter is smaller than the hot-finished pipe.
This pressure is responsible for giving cold-drawn tubes their signature polished finish. It also helps improve tensile strength while altering the pipe dimensions to fit uniform tolerances. This drawing process may get repeated many times to change the shape and tolerance while enhancing elasticity.
Heat may also be applied to the finished product during cold drawing to eliminate internal stresses by improving the microstructural strength.
Important Characteristics Of Cold Drawn/Cold Finished Pipes
- Cold Drawn Pipes are usually in small diameter sizes and are commonly quoted to fit lower than average specifications.
- These pipes often get used where dimensional tolerances are tighter than average.
- Cold drawing is sometimes considered to be part of cold rolling.
- The starting stock in the cold drawing is usually a hot rolled tube
- Cold-drawn pipes get pulled over a die, with or without a mandrel.
ASTM A179/A178M Specifications For Seamless Cold-Drawn Low-Carbon Steel Heat-Exchanger And Condenser Tubes
|Yield Strength (Minimum)||325 MPa|
|Tensile Strength (Minimum)||180 MPa|
|Longitudinal elongation in 2 in (Minimum %)||35|
- The tube hardness should not exceed 72 HRB
- After the final cold draw, the tubes are to be heated at a temperature of 1200 Degrees Fahrenheit
- This specification covers tubes of ⅛ to 3 inches in outside diameter
Standard Specification For Seamless Carbon And Alloy Steel Mechanical Tubing: ASTM A519/ASTM A519M-17
Chemical Requirements For Low-Carbon Steels
|MT X 1015||0.40||0.50||0.90||0.20|
|MT X 1020||0.40||0.50||1.0||0.25|
Advantages Of Cold Drawn Seamless Pipes
- When it comes to cold drawing, the stock tube is drawn at room temperature, which leads to a high degree of microstructural consistency and unity.
- Cold-drawn pipes are exposed to high pressure as they get drawn through dies for elongation. This pressure leads to higher tensile strength in these products.
- Unlike hot rolling, the tubing must get drawn multiple times to make a cold finished product. This process allows producers to make pipes with close dimensional tolerances and even thickness and diameter.
- Drawing the stock at lower temperatures gives the tubes a high strength-to-weight ratio improving their safety and application potential.
- Using the die and the mandrel in cold rolling improves the outside and the inside diameter surface quality.
- Cold drawing improves the machinability of the hot rolled steel tube, widening its range of applications.
- Hot rolled pipes get pickled to remove the surface contaminants, mill scales, and iron oxides. This process enhances the pipe's aesthetic appeal, giving it a glossier, even surface finish.
Applications Of Cold Drawn Pipes
Cold drawn seamless pipes have a wider variety of applications than hot-rolled tubes. Manufacturers devote more labor and time to produce a more accurate and durable pipe, which gets used in:
- Industrial machinery; in tool extensions, machine components, frames, hydraulics, and more.
- Automobile industry; as fenders, axels, car seating frames, chassis components, roll cages, shaft tubes, and more.
- Furniture making; as frames for benches, beds, desks, chairs, displays, fixtures, and more.
- Oil and energy: as bearings, turbine parts, pumps, columns, rolls, and more.
- In artwork, installations, and more.
Disadvantages Of Cold Drawn Seamless Pipes
- For cold-drawn steel to be tougher and have a higher tolerance than hot-rolled steel, it must be drawn through the dye several times.
- Cold-drawn steel requires multiple drawings to achieve the intended shape and thickness, adding to the labor and cost.
- Unlike hot rolling, which involves a minimal number of steps, in cold rolling, the tube has to be lubricated, drawn through a die or a mandrel, and annealed before the product can get finished.
- Since the tube gets drawn at room temperature, it takes longer to see a reduction in diameter than it would if it was exposed to temperatures above the recrystallization point.