Industrial steel pipe finishing defects and their prevention

The finishing process of steel pipes is critical for removing defects, improving quality, meeting special product requirements, and defining the "identity" of the products. Key finishing processes include straightening, cutting ends (chamfering, sizing), inspection and testing (surface quality, geometric dimensions, non-destructive testing, hydraulic tests), grinding, length measurement, weighing, painting, printing, and packaging. Steel pipes for special purposes may also undergo surface shot peening, mechanical processing, and anti-corrosion treatments.

1. Steel Pipe Straightening Quality Defects

Steel pipe straightening machines can be categorized into pressure straightening machines, inclined roller straightening machines, and tension straightening machines. The straightening process involves repeated elastic-plastic bending and flattening to reduce the curvature and ovality of the pipes. Common defects during straightening include:

- Unstraightened pipes (goose-head bends at the ends)

- Concave or square corrections

- Crack corrections

- Surface scratches and indentations

Factors affecting straightening quality include the type and adjustment of the straightening machine, hole shape, steel pipe characteristics (original curvature, size, material), and the size and quality of the inlet and outlet guide cylinders. The guide cylinder should be 30-50mm larger than the pipe's outer diameter, and its center line should align with the straightening center line to prevent surface scratches and misalignments.

The D/S value (outer diameter/wall thickness) significantly influences straightening quality. Optimal straightening occurs when the D/S value is between 9 and 10. High-strength steel pipes with greater original curvature require larger straightening forces, increasing the risk of surface defects. For high alloy content or significantly curved pipes, a combination of primary and fine straightening processes is recommended.

2. Steel Pipe Grinding and Cutting Defects and Their Prevention

Surface grinding aims to remove defects that must be cleaned per standards, improving surface quality. Common grinding defects include excessive depth or irregular shapes, causing negative deviations in outer diameter or wall thickness. Grinding should meet the following requirements:

- Wall thickness at grinding points must not fall below nominal wall thickness deviations.

- Outer diameter at grinding points must meet specified requirements.

- Grinding should leave a smooth, curved surface, with a depth-to-width-to-length ratio of 1:6:8.

- No overburning or polygonal marks should remain after grinding.

- The number of grinding points should not exceed standard specifications.

3. Surface Processing Defects of Steel Pipes and Their Prevention

Surface processing includes shot peening, overall grinding, and mechanical processing, aimed at improving surface quality or dimensional accuracy.

- Shot Peening:

Sprays iron or quartz sand shots at high speed to remove iron oxide scales and reveal hidden defects.

Factors like sand shot size, hardness, and spraying speed affect the quality. Overly large, hard, or fast shots may cause surface pits, while too small or slow shots may not remove all scales.

The thickness and density of iron oxide scales also influence the cleaning effect.

- Overall Grinding:

Uses sanding belts, grinding wheels, or machines for outer surface grinding; grinding wheels or internal mesh machines for inner surfaces.

Removes iron oxide scales, improves surface finish, and eliminates small defects like cracks, pits, and scratches.

Common defects from grinding include black skin (from insufficient grinding), excessive wall thickness, flat or polygonal surfaces, pits, burns, and wear marks.

Grinding with abrasive belts generally results in better surface quality but lower efficiency.

In summary, understanding and addressing the factors affecting each finishing process can significantly enhance the quality and reliability of steel pipes, meeting stringent industrial standards and special use requirements.