Cold Drawn vs. Cold Rolled Pipe

Cold-drawn and cold rolled pipes, as two common steel pipe processing methods, play important roles in industrial production. Although their names are similar and both fall under the category of cold working, their production processes, surface quality, dimensional accuracy, quality control, mechanical properties, processing temperatures, and application areas differ significantly. These differences will be analyzed in detail below.

Commonalities between Cold Drawn and Cold Rolled Pipes

Both processes significantly improve material properties through room-temperature deformation:

1. Increased Strength

Cold deformation refines the grain size, increasing tensile strength by 20%-30% compared to the raw material.

2. Improved Surface Quality

Both processes achieve a smooth surface with Ra≤1.6μm, superior to the Ra≥12.5μm of hot rolled pipes.

3. Dimensional Accuracy

Outer diameter tolerances are generally 3-5 grades better than hot rolled pipes, making them particularly suitable for precision fitting applications such as hydraulic cylinders.

4. Common Limitations of Cold Working

Due to work hardening, the plasticity indicators (such as elongation) of finished pipes are typically 15%-25% lower than those of hot-rolled pipes. For complex forming, intermediate annealing is required to restore processing performance.

Cold Drawn Pipe vs. Cold Rolled Pipe

1. Manufacturing Process

Cold drawn pipe manufacturing is carried out on a cold-drawing machine. The billet steel pipe is heated to a certain temperature and then pressed through a die, subjecting the steel pipe to tensile stress on both the inner and outer sides simultaneously. Under tension, the diameter and wall thickness of the steel pipe both decrease until the desired size is achieved.

Cold drawn process: Round billet → Pickling → Lubrication → Multi-pass drawing → Annealing (stress relief) → Straightening;

Cold rolled pipe manufacturing involves heating the billet steel pipe to a certain temperature and then rolling it. Unlike cold-drawn pipes, cold rolled pipes undergo only unidirectional extrusion during rolling, and the radial dimensions and wall thickness of the steel pipe do not change significantly. Therefore, the manufacturing cost of cold-rolled pipes is relatively lower than that of cold-drawn pipes.

Cold rolled process: Hot-rolled pipe → Pickling → Continuous rolling on a cold rolling mill → Online inspection → Finishing.

2. Dimensional Accuracy

Cold drawn steel pipes typically achieve an outer diameter accuracy of ±0.1mm and a wall thickness tolerance of around ±5%, making them suitable for producing pipes with smaller diameters (usually ≤150mm) and larger length-to-diameter ratios.

Cold rolled steel pipes offer even greater advantages in dimensional control, with an outer diameter accuracy of ±0.05mm and a wall thickness tolerance controlled within ±3%, making them particularly suitable for producing precision hydraulic pipes, bearing pipes, and other dimensionally sensitive products.

3. Surface Quality

Cold drawn pipes often have longitudinal striations on their surface, with a roughness Ra value typically ranging from 1.6 to 3.2μm. Cold rolled pipes, on the other hand, have a smoother surface, with an Ra value reaching 0.4-0.8μm when using high-precision rolls.

This difference gives cold rolled pipes an advantage in applications such as surface coating and precision fitting. However, it is worth noting that the cold rolling process may produce unique "rolling marks," which need to be controlled by optimizing the mill rigidity and process parameters.

4. Mechanical Properties

Cold drawn pipes possess higher axial strength and hardness, making them suitable for manufacturing components subjected to axial loads.

Cold rolled pipes, due to uniform work hardening, combine good strength and plasticity, and their fatigue performance is generally superior to that of cold-drawn pipes.

5. Key Quality Control Points

Cold drawing steel production requires close attention to die wear, lubrication, and drawing force fluctuations; cold rolling steel production, on the other hand, requires more precise control over the matching of roll gap, feed rate, and rotation speed.

Statistical Process Control (SPC) data shows that the critical dimension CPK value of cold rolling is typically 0.2-0.3 higher than that of cold drawing, indicating better process stability.

6. Processing of Special Materials

Cold rolling is more advantageous for difficult-to-deform materials such as high-strength alloy steel and titanium alloys. Its multi-directional stress state effectively prevents crack formation.

Cold drawing often requires more intermediate annealing cycles when processing these materials.

7. Residual Stress Distribution

Residual stress distribution is a key factor affecting product stability.

The residual stress in cold drawn steel pipes exhibits a distinct layered distribution, with the surface in a tensile stress state. Cold rolled steel pipes, due to more uniform deformation, show lower peak residual stress and a more gradual distribution.

This difference makes cold-drawn pipes more prone to deformation during subsequent processing, requiring stress-relief annealing to improve this. A precision hydraulic system manufacturer's tests found that the straightness stability of cold-rolled pipe fittings was more than 35% higher than that of cold-drawn products.

8. Different Processing Temperatures

The processing temperatures for cold rolling and cold drawing also differ. Cold rolling is performed below the crystallization temperature, generally at room temperature, while cold drawing is performed under heating conditions at approximately 800°C to 950°C.

Therefore, cold drawing requires heating the steel pipe in a furnace before sending it to a drawing machine for processing, ensuring that defects such as cracks do not appear on the steel pipe surface.

9. Application Areas

Cold-drawn pipes are used in the manufacture of precision mechanical parts, automotive components, pipes, hydraulic cylinder housings, diesel engine fuel injection pumps, and other high-precision steel products.

Cold rolled pipes are used to manufacture general steel products such as low- and medium-pressure boiler pipes, hydraulic supports, automobile axle sleeves, automobile bearing housings, and bicycle frames.

Pros and Cons of Cold Drawn Seamless Pipe

1. Cold Drawn Seamless Pipe Pros

Improved Mechanical Properties: The cold drawing process allows for greater plastic deformation of the steel pipe, thereby improving its toughness and tensile strength, resulting in better mechanical properties.

High Dimensional Accuracy: The cold drawing process allows for precise control of the seamless steel pipe sizes, meeting the requirements of high-precision applications.

Good Surface Quality: The surface of the seamless pipe is less prone to oxide scale formation during cold drawing, resulting in better surface quality, which is beneficial for subsequent processing and use.

2. Cold Drawn Seamless Pipe Cons

Large Residual Stress: During cold drawing, significant residual stress is generated inside the seamless pipe, which may affect its overall and local buckling characteristics.

Limited Drawing Length: Due to the presence of residual stress, the length of cold-drawn seamless steel pipes is limited and cannot be drawn too long.

Higher Cost: The cold drawing process is relatively complex and requires advanced equipment and technology, resulting in relatively high production costs.

Pros and Cons of Cold Rolled Seamless Pipes

1. Cold Rolled Seamless Pipe Pros

High Forming Speed and Output: Cold rolling allows for rapid forming at room temperature, resulting in high production efficiency and no damage to the coating.

Diverse Cross-Sectional Shapes: Cold rolling can produce a variety of cross-sectional shapes to adapt to different application conditions.

Increased Yield Point: The cold rolling process induces plastic deformation in the seamless pipe, thereby increasing its yield point and enhancing the material's load-bearing capacity.

2. Cold Rolled Seamless Pipe Cons

Residual Stress: Although there is no hot plastic compression during cold rolling, residual stress still exists within the cross-section, which may affect the performance of the steel pipe.

Poor Torsional Resistance: Cold rolled steel sections are generally open, resulting in lower free torsional stiffness and relatively poor torsional resistance.

Smaller Wall Thickness: Cold rolled steel has a thinner wall thickness, and there is no thickening at the corners where plates meet, making it less able to withstand localized concentrated loads.

How to Choose?

1. Diameter Range

Cold-drawn pipes are better suited for diameters of Φ6-127mm, while cold rolled pipes excel in the Φ4-80mm thin-walled pipe segment.

2. Batch Size Requirements

Cold drawing offers high single-batch forming efficiency, suitable for small to medium batches; cold rolling is suitable for large-volume precision orders.

3. Cost Control

Cold drawn pipes are 8%-12% cheaper for the same specifications, but cold rolling is more economical for ultra-thin walls (<1mm).

4. Subsequent Processing

Cold rolled pipes are preferred for deep drawing due to their lower anisotropy.