Methods for Identifying The Quality of Spiral Steel Pipes

Methods for Identifying the Quality of Spiral Steel Pipes

1. The inner diameter of substandard thick-walled steel pipes fluctuates significantly. The reasons are: 1. Unstable steel temperature and uneven heating. 2. Uneven steel composition. 3. Due to rudimentary equipment and low foundation strength, the rolling mill experiences significant bouncing. This results in large variations in the inner diameter within the same circumference, leading to uneven stress distribution and easy fracture.

2. High-quality pipes have standardized trademarks and markings. For large threaded steel pipes with a diameter of 16 mm or more, the distance between the two trademarks is usually more than 1 meter. The longitudinal ribs of substandard rebar often appear wavy. Substandard thick-walled steel pipe manufacturers often lack overhead cranes, resulting in loose packaging. The side profile is often elliptical.

3. Substandard thick-walled steel pipes are prone to folding. Folding refers to various fold lines formed on the surface of the thick-walled steel pipe. This defect often runs along the entire length of the product. The reason for folding is that substandard manufacturers pursue high efficiency, resulting in excessive reduction during rolling, creating "ears" that then fold during subsequent rolling. Folded products will crack after bending, significantly reducing the strength of the steel.

4. Substandard thick-walled steel pipes often have a rough surface. This rough surface is a defect caused by severe wear of the rolling grooves, resulting in irregular bumps and depressions on the steel surface. Because substandard thick-walled steel pipe manufacturers prioritize profit, they often exceed the rolling groove wear limits.

5. Substandard thick-walled steel pipes are prone to surface scaling. There are two reasons: (1) The material of substandard steel pipes is uneven and contains many impurities. (2) Substandard manufacturers have rudimentary guiding equipment, making it easy for steel to stick to the rollers. These impurities then cause scaling.

6. Substandard materials are prone to surface cracks. This is because their billets are made from low-quality materials with many pores. During the cooling process, these billets develop cracks due to thermal stress, which then become visible after rolling.

7. Substandard thick-walled steel pipes are easily scratched. This is because substandard manufacturers have rudimentary equipment, which easily produces burrs that scratch the steel surface. Deep scratches reduce the strength of the steel. 8. Counterfeit thick-walled steel pipes lack metallic luster and appear light red or similar to cast iron. There are two reasons for this: 1. The raw material is low-quality billet. 2. The rolling temperature of the counterfeit material is not standardized; the steel temperature is determined by visual inspection, making it impossible to roll within the specified austenitic region, and thus the steel's performance naturally fails to meet standards.

9. The transverse ribs of counterfeit thick-walled steel pipes are thin and low, often showing incomplete filling. This is because the manufacturer uses excessive reduction in the initial rolling passes to achieve a large negative tolerance, resulting in a smaller die size and incomplete filling of the mold.

10. The cross-section of counterfeit thick-walled steel pipes is elliptical. This is because the manufacturer uses excessive reduction in the first two passes of the finishing rolls to save material. This significantly reduces the strength of the rebar and does not meet the standard dimensions of rebar.

11. High-quality steel has a uniform composition, and the cold shearing machine has a high tonnage, resulting in a smooth and neat cut end face. However, due to the poor quality of counterfeit materials, the cut end face often shows chipping, i.e., unevenness, and lacks metallic luster. Furthermore, because counterfeit manufacturers cut fewer ends, large burrs appear at the ends.

12. Counterfeit thick-walled spiral pipes contain many impurities, have a lower steel density, and have serious dimensional deviations. Therefore, in the absence of a caliper, weighing can be used for verification. For example, for 20mm rebar, the national standard specifies a maximum negative tolerance of 5%. For a 9M length, the theoretical weight of a single piece is 120 kg. The minimum weight should be: 120 x (1 - 5%) = 114 kg. If the actual weight of a single piece is less than 114 kg, it is counterfeit steel because its negative tolerance exceeds 5%. Generally, weighing the entire batch will yield better results, mainly considering cumulative errors and probability theory.