Why Do Cold Drawn Seamless Steel Pipes Crack?

Cold drawn seamless steel pipes are widely used in machinery manufacturing, hydraulic systems, automotive parts, boiler equipment, and precision structural components due to their high dimensional accuracy, good surface quality, and stable mechanical properties. However, during production or use, defects such as longitudinal cracks, transverse cracks, or surface cracks sometimes occur. This not only affects product quality but may also lead to equipment failure or even safety accidents.

This article will analyze the causes of cracking in cold drawn seamless steel pipes, providing a reference for preventing and handling similar problems.

Manufacturing Process of Cold Drawn Seamless Steel Pipes

Cold drawn seamless steel pipes are generally produced using a cold drawing process. During manufacturing, hot rolled strip pipes or seamless steel pipes are first pickled to remove rust and then phosphated. They are then cold drawn on a drawing machine, undergoing multiple drawing processes to finally form a seamless steel pipe of a specified diameter and wall thickness. After annealing, they are ready for shipment.

The manufacturing process requires strict control of processing temperature and speed, as well as parameters such as tensile force and cooling water temperature during the drawing process, to ensure the quality of the finished product. During cold drawing, steel pipes undergo 20%–40% plastic deformation. If the process is not properly controlled, work hardening and residual stress accumulation can easily occur, leading to cracks.

Common Cracking Types in Cold Drawn Seamless Steel Pipes

In production practice, steel pipe cracks can generally be classified into the following types:

1. Longitudinal Cracks

These extend along the axial direction of the steel pipe and are often caused by raw material defects or excessive drawing stress.

2. Transverse Cracks

These are perpendicular to the pipe axis and are often caused by excessive cold working or high material brittleness.

3. Surface Cracks

These are fine surface cracks and are often caused by excessive cold deformation.

4. Internal Wall Cracks

These are cracks on the inner surface of the pipe and are often caused by defects on the original inner surface.

5. Stress Cracks

These occur during use and are often caused by residual stress or corrosive environments.

Causes of Cracking in Cold Drawn Seamless Steel Pipes

1. Material Issues

The following problems may exist in the materials used to make cold drawn seamless steel pipes, leading to cracking:

Material Issues: Substandard raw material quality, or the selection of unsuitable materials whose hardness, strength, toughness, and other properties fail to meet usage requirements.

Inclusion Issues: The presence of oxides, sulfides, and silicate inclusions in the steel disrupts its continuity. During cold drawing, these inclusions become stress concentration points, causing crack propagation.

Typical Requirements:

Bubble Issues: Bubbles are formed during the manufacturing process. Their presence reduces the material's strength and stiffness, leading to cracking.

2. Manufacturing Process Issues

The following problems may occur during the manufacturing process of cold drawn seamless steel pipes, leading to cracking:

Over-drawing Issues: Over-drawing can cause cracking of the steel pipe because it creates excessive internal stress, resulting in cracking.

Improper Control During Steel Pipe Stretching: Inadequate control during the steel pipe stretching process can lead to stress concentration, resulting in cracks.

Production Process Issues: Inappropriate production processes, such as using traditional methods like mold curing, can also cause steel pipe cracking. Without proper intermediate annealing, the material's plasticity decreases, making it prone to cracking.

Common annealing temperatures:

3. Environmental Issues

The following problems may lead to cracking during the use of cold drawn seamless steel pipes:

Temperature Issues: Some cold drawn seamless steel pipes need to be used in low-temperature environments (e.g., below -20°C). Exceeding their tolerance temperature range can cause cracking.

Scratches Issues: Scratches on the steel pipe surface can cause surface cracking, accelerating crack propagation and leading to cracking.

Spare Parts Installation Issues: Improper installation of spare parts can cause stress concentration, inducing cracks and leading to steel pipe cracking.

Preventive Measures for Cracking in Cold Drawn Seamless Steel Pipes

1. Use High-Quality Raw Materials

To avoid cracking due to material quality issues, it is recommended to select high-quality, high-toughness, and easily weldable scrap steel and new steel as raw materials during production. This can effectively reduce the incidence of carbonization, oxidation, and carburization problems.

2. Rational Selection of Cold Drawn Seamless Steel Pipe Manufacturing Equipment

Rational equipment selection is key to successful manufacturing. Equipment that improves production efficiency should be selected, and stable production processes should be used to eliminate vibration, wear, and other problems during equipment operation. This will prevent adverse production conditions.

Crucial stages in seamless steel pipe manufacturing require strict control of process parameters to ensure the stability and quality of the production environment. Problems such as excessively low temperatures, excessively high speeds, and excessively high pressures should be avoided during production to prevent defects.

3. Sensitive Monitoring of the Cold Drawn Seamless Steel Pipe Manufacturing Process

During production, timely monitoring is essential to detect problems in the processing promptly. This facilitates preventative and corrective measures. Once problems are detected, they should be addressed immediately, the process adjusted promptly, production parameters rationally controlled, and performance upgrade solutions sought.

4. Improve Workers' Manual Skills

Regardless of the equipment used, excellent manual skills and technical handling capabilities are required from employees. Therefore, enterprises should emphasize employee technical training, improve mechanical skills and handling capabilities, and provide solid process and technical support for steel pipe production.

Conclusion

Cracking is a common problem in the manufacturing process of cold drawn seamless steel pipes. To prevent cracking, high-quality raw materials must be used, equipment must be selected appropriately, the process must be monitored in a timely manner, and workers' manual skills must be improved.