Common defects and causes of hot-rolled seamless steel pipe

Hot-rolled seamless steel pipe defects are mainly caused by raw material quality, billet heating, piercing and rolling parameters, equipment condition, and improper process control. Common defects include surface scratches, pits, cracks, folds, seams, hairline defects, lamination, dimensional deviations, and straightness problems. Effective billet inspection, accurate temperature control, optimized rolling parameters, regular equipment maintenance, and final inspection can significantly reduce these defects and improve the quality and reliability of hot-rolled seamless steel pipes.

This article explains the most common defects in hot-rolled seamless steel pipes, their causes, and the main measures used to prevent them.

Common Defects in Hot-Rolled Seamless Steel Pipes

The following table summarizes the major defects, their typical appearance, main causes, and recommended prevention measures.

Defect	Typical Appearance	Main Causes	Prevention
Surface scratches	Linear marks or scratches on the inner or outer surface	Billet surface defects, oxide scale, friction with equipment	Improve billet surface quality and maintain rolling equipment
Pits	Small or localized depressions on the pipe surface	Oxide scale, inclusions, surface contamination, or equipment damage	Clean billets and inspect equipment regularly
Cracks	Straight, longitudinal, transverse, or irregular cracks	Poor billet quality, improper heating, excessive deformation, or stress concentration	Inspect billets and optimize heating and rolling parameters
Hairline defects	Fine continuous or discontinuous lines on the surface	Subsurface inclusions, cracks, scale, or improper piercing/rolling	Improve billet quality and control piercing and rolling conditions
Inner folds	Folded or irregular defects on the inner surface	Billet defects, improper piercing, uneven deformation, or incorrect rolling parameters	Optimize piercing and rolling parameters
Outer folds / seams	Longitudinal or spiral surface defects	Billet surface defects, excessive deformation, or improper mill adjustment	Improve billet inspection and mill adjustment
Lamination	Layer-like or planar separation on the surface or inside the pipe	Inclusions, porosity, or other billet-related defects	Strengthen billet quality control and use appropriate NDT
Dimensional deviation	OD, ID, or wall thickness outside the specified tolerance	Incorrect sizing parameters, worn rolls, or unstable rolling conditions	Calibrate equipment and monitor dimensions during production
Poor straightness	Pipe bends or deviates from the specified straightness	Uneven cooling, improper straightening, or dimensional variation	Control cooling and optimize straightening
Uneven wall thickness	Wall thickness varies around or along the pipe	Off-center piercing, uneven deformation, or improper sizing	Control piercing alignment and optimize rolling parameters

Common defect

1. Surface scratches or pits
The main performance is that there is a certain width and depth of pits or scratches on the inner and outer surfaces of the steel pipe. Such defects mainly come from the residual oxide skin on the surface of the raw material or the impurities mixed in the raw material, which makes the billet and the equipment friction, thus forming scratches, pits or black spots.

2. Folding defect
In the process of hot rolling, due to the cracks and pitting in the subskin bubbles, inclusions or surface of the tube billet, the folding phenomenon is easy to occur in the process of hot rolling and subsequent drawing, which is manifested as the folding, depression or local uneven thickness of the inner wall of the tube.

3. Row lengthwise
Due to the improper control of mold parameters, rolling tension and heating temperature in the rolling process, scratches or lines along the length direction may be produced on the surface of the pipe, which not only affects the appearance, but also may become a hidden danger of subsequent crack growth.

4. Folding defects
In the continuous rolling process, if the billet temperature is uneven or the mill parameters are not set properly, it is easy to have complex defects such as double seam folding. Such defects are usually shown as abnormal section shape in local area, which affects the overall mechanical properties of the pipe.

The main cause of defects

1. Raw material problem

Billet defects: there are subcutaneous bubbles, inclusions, cracks and pitting defects in the raw material, if it is not completely eliminated before hot rolling, it will be amplified during the rolling process, resulting in internal folding, scratches and other problems.
Surface pollution: impurities such as residual oxide and rust on the billet surface are easy to form local defects during high temperature rolling.

2. Improper control of process parameters

Temperature control: In the hot rolling process, uneven heating or insufficient temperature of the billet will lead to insufficient local plasticity, resulting in cracks, internal folding or scratches.
Rolling tension and speed: the mill transfer tension is too large or improper speed, easy to cause uneven material deformation or surface stress concentration, resulting in longitudinal tracing and folding defects.

3. Equipment adjustment and process operation problems

Mold status: Mold wear or inaccurate correction will lead to steel pipe size deviation and surface defects.
Process connection problems: improper connection of hot rolling, sizing, straightening, cutting and other processes may form stress concentration areas between different processes, resulting in surface cracks and defects.

Improvement measure

Improve raw material quality: strengthen billet inspection to ensure that raw materials have no obvious subcutaneous bubbles, inclusions or cracks.
Optimize process parameters: strictly control key parameters such as heating temperature, rolling tension and rotational speed to ensure uniform temperature throughout the hot rolling process and reduce stress concentration.
Equipment maintenance and mold management: regular inspection and correction of rolling mill equipment and mold to ensure that they are in good condition to reduce defects caused by equipment problems.
Improve the process connection: strengthen the connection management between each production process, ensure that hot rolling, sizing, straightening and other processes seamless docking, reduce the risk of stress concentration between processes.

Frequently Asked Questions About Hot-Rolled Seamless Steel Pipe Defects

1. What are the most common defects in hot-rolled seamless steel pipes?

The most common defects include surface scratches, pits, cracks, hairline defects, folds, seams, lamination, uneven wall thickness, dimensional deviations, and poor straightness. Most defects are related to billet quality, heating, piercing, rolling, equipment condition, or process control.

2. What causes cracks in hot-rolled seamless steel pipes?

Cracks can be caused by poor billet quality, subsurface cracks or inclusions, improper heating, excessive deformation, incorrect piercing conditions, or local stress concentration during rolling.

3. What causes folding defects in seamless steel pipes?

Folding defects are commonly associated with billet surface or subsurface defects, inclusions, uneven heating, improper piercing, excessive deformation, and incorrect rolling or equipment parameters.

4. What causes surface scratches on hot-rolled seamless steel pipes?

Surface scratches can result from billet surface defects, residual oxide scale, foreign particles, or friction between the pipe and damaged or contaminated production equipment.

In summary, by improving raw material quality, optimizing process parameters and strengthening equipment management, these defects can be effectively prevented and reduced, thereby improving product quality and safety.

Read more : What is a seamless steel pipe?