Common Defects of Hot-rolled Mild Steel Tubes

Surface defects

1. Cracks

Cause: During the heating stage of the billet, if the heating speed is too fast or the temperature is uneven, thermal stress will be generated inside the billet, which may cause cracks on the surface of the mild steel tube during the rolling process. During the rolling process, if the rolling force is too large or the surface condition of the roller is not good, such as wear and dents on the roller, cracks will also appear on the surface of the mild steel tube. In addition, during the cooling process of the mild steel tube, if the cooling speed is too fast, especially in the parts where stress concentration exists, cracks may also appear.

Impact: Surface cracks will seriously affect the strength and sealing of the mild steel tube. For mild steel tubes used to transport fluids, cracks may cause liquid leakage and pose a safety hazard. Moreover, cracks will become the starting point of corrosion, accelerate the corrosion of the mild steel tube, and shorten its service life.

2. Folding

Cause: It is mainly generated during the rolling process. If the metal flow is uneven during the perforation or continuous rolling stage, part of the metal will be rolled into the surface of the mild steel tube to form folds.

Impact: The discontinuous metal structure at the folding point will reduce the strength of the mild steel tube. At the same time, impurities and corrosive media are easily accumulated at the folding point, causing local corrosion, and in the subsequent processing (such as welding, bending, etc.), the folding may further expand, affecting the quality of the steel pipe.

3. Scratches

Cause: During the production process of mild steel tubes, friction with equipment components (such as guides, conveyor rollers, etc.) may cause scratches. For example, when the surface of the guide is not smooth enough or has sharp edges, the mild steel tube is easily scratched during the passage. In addition, in the process of lifting and stacking, if the operation is improper, the collision between steel pipes will also cause scratches.

Impact: Minor scratches will affect the appearance of the steel pipe, while deeper scratches will destroy the surface integrity of the steel pipe and reduce its corrosion resistance. For some steel pipes used in high-precision equipment or in occasions with high requirements for appearance, scratches may make them not meet the use requirements.

Dimensional deviation

1. Uneven wall thickness

Cause: In the process of perforation and rolling, improper control of process parameters is the main cause of uneven wall thickness.

Impact: Uneven wall thickness will affect the bearing capacity of the steel pipe. When subjected to internal pressure, the thinner wall parts are prone to local deformation or even rupture. This is a serious quality problem for mild steel tubes used to transport high-pressure fluids or withstand large external forces.

2. Outer diameter deviation

Cause: The sizing process is a key link in controlling the outer diameter of the steel pipe. If the roller size of the sizing machine is inaccurate, the roller is worn, or the rolling force during the sizing process is unstable, it will cause outer diameter deviation. In addition, during the rolling process, if the temperature of the mild steel tube changes greatly, its thermal expansion will also cause fluctuations in the outer diameter.

Impact: The outer diameter deviation will affect the matching accuracy of the steel pipe with other pipe fittings (such as flanges, joints, etc.). If the outer diameter is too large, it may not be properly installed with the pipe fittings; if the outer diameter is too small, the sealing of the connection cannot be guaranteed, and leakage is likely to occur during fluid transportation.

Internal defects

1. Inclusions

Cause: The inclusions in the billet itself are one of the main reasons. In the steelmaking process, if the refining links such as deoxidation and desulfurization are not thorough, inclusions such as oxides and sulfides will remain in the molten steel. These inclusions will remain inside the mild steel tube after the billet is rolled.

Impact: Inclusions will destroy the continuity of the metal inside the mild steel tube and reduce its strength and toughness. When the steel pipe is subjected to pressure or external force, stress concentration is likely to occur around the inclusions, causing cracks, which seriously affects the quality and service life of the steel pipe.

2. Delamination

Cause: It may be caused by the fact that the shrinkage holes and looseness inside the billet are not fully welded during the rolling process. In addition, if the metal deformation inside the mild steel tube is uneven during the rolling process, delamination may also occur.

Impact: Delamination will greatly reduce the strength of the mild steel tube, especially when it is subjected to axial tension or bending force, the delamination part is prone to fracture. For some application scenarios with high requirements for mechanical properties, such as mechanical structure support, delamination is a quality defect that is not allowed to occur.

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