The Impact of Inferior Tube Billets on Seamless Steel Pipes

The quality of tube billets has a crucial impact on the quality of seamless steel pipes (SMLS) and is the root cause of all quality issues. Using inferior billets to manufacture seamless steel pipes is like carving a work of art from rotten wood: no matter how sophisticated the subsequent processing, the finished product will have inherent defects and significant hidden dangers. 

The following are the specific impacts of inferior tube billets on seamless steel pipe quality, which can be categorized into four aspects: surface quality, internal quality, chemical composition and microstructure, dimensional accuracy, and end-use risks:

1. Surface Quality Defects

Common surface defects of inferior tube billets (such as cracks, scars, folds, scratches, and pores) are directly inherited by the seamless steel pipes. These defects result in unsatisfactory surfaces, requiring extensive refining or even rejection, significantly reducing the yield rate. Even after refining, the pipes may still be rejected due to excessive wall thickness.

Common Defect Types:
a. Surface Cracks: These occur during the casting or rolling process of the billet. Impact on Seamless Steel Pipes: During the rolling process, cracks can further expand, becoming stress concentration points, significantly reducing the pipe's fatigue resistance and pressure-bearing capacity, and easily leading to rupture during use.

b. Scratches and warping: Irregular metal scars on the billet surface.
Impact on Seamless Steel Pipes: After rolling, these scars become "interlayers" or "scalp" on the seamless pipe surface, which can easily fall off during subsequent processing, forming pits or leaks, compromising the pipe's integrity and corrosion resistance.

c. Folding: A defect caused by metal being entangled in itself during rolling.
Impact on Seamless Steel Pipes: These cracks are closely integrated with the pipe body, severely disrupting the continuity of the metal and significantly increasing the risk of bursting during hydrostatic testing or pressure-bearing use.

d. Severe scratches: Mechanical damage during production or transportation.
Impact on Seamless Steel Pipes: Deep scratches reduce the effective wall thickness and can also become a source of cracks.

2. Internal Quality Defects
This is the most dangerous and hidden type of problem, typically requiring non-destructive testing to detect. Common internal quality defects include internal cracks, inclusions, central porosity and shrinkage cavities, and air holes. Seamless steel pipes that fail ultrasonic or eddy current testing are considered scrap. If these undetected defects enter the market and are used in pressure-bearing equipment, they become an extremely dangerous time bomb.

3. Chemical Composition and Microstructure Defects
The chemical composition of the billet significantly impacts the quality of seamless steel pipes. If the chemical composition does not meet the requirements, seamless steel pipes may exhibit problems such as insufficient toughness, insufficient strength, and brittleness. Substandard chemical composition control often fails to meet standards, which directly impacts the final performance of the pipes.

Chemical Composition Deviation:
High carbon (C) content results in increased hardness and strength, but decreased plasticity and toughness, poor weldability, and susceptibility to cracking during cold working. Excessive sulfur (S) and phosphorus (P) content: Sulfur forms sulfide inclusions, leading to hot brittleness (cracking during hot working); phosphorus significantly increases cold brittleness (a sharp drop in toughness at low temperatures).

Inhomogeneous alloying elements: This leads to uneven microstructure and properties.

Even after rolling and heat treatment, it is impossible to achieve an ideal, uniformly refined microstructure (such as a normalized microstructure). This can lead to problems such as insufficient strength, poor toughness, and uneven hardness. Ultimately, the mechanical properties (tensile strength, yield strength, elongation, and impact energy) of the steel pipe fail to meet the requirements of product standards (such as ASTM A106 and API 5L).

4. Dimensional Accuracy
The dimensional accuracy of the billet also significantly impacts the quality of seamless steel pipe. If dimensional accuracy does not meet requirements, seamless steel pipes may exhibit problems such as leaks and uneven wall thickness.

Significant Risks in End-Use Applications:

Using seamless steel pipes made from inferior billets may result in the following consequences:

1. Risk of Pressure Equipment Burst: When used in oil wells, gas wells, boilers, and pressure vessels, catastrophic failure can occur at rated operating pressure, leading to explosions, fires, environmental pollution, and other major accidents.
2. Premature Failure: When used in mechanical structural components (such as hydraulic cylinders), premature fracture may occur due to rapid fatigue crack propagation, resulting in equipment downtime and significant economic losses.

3. Processing Scrap: Pipes may crack and become scrapped during subsequent cold bending, flaring, welding, and other processes, wasting all processing costs.
4. Poor Corrosion Resistance: Steel pipes with uneven microstructures and numerous inclusions are more susceptible to pitting and stress corrosion cracking, shortening their service life.

5. Reputation and Legal Risks: For seamless pipe manufacturers and end users, using inferior products can lead to significant loss of quality reputation and even legal liability.

Conclusion:

Eliminating inferior billets at the source is the first and most important step in producing high-quality seamless steel pipes. Cost savings on tube billets are absolutely unacceptable, as the subsequent costs will be tenfold or even a hundredfold higher. Therefore, during the production of seamless steel pipes, tube billets must be rigorously inspected and controlled to ensure they meet quality and process requirements. At the same time, unqualified tube billets must be promptly disposed of and corrected to avoid adversely affecting the quality of the seamless steel pipe.

Read more: China Seamless Pipe Manufacturer & Supplier List or Seamless Steel Pipe Manufacturing Process