What are the surface defects of carbon steel pipes?

Surface defects in seamless steel pipes can significantly impact the quality and performance of the final product. Understanding these flaws and their causes is crucial to preventing them and ensuring high-quality production. Below are common surface defects found in seamless steel pipes and their potential causes.

Folding:

This defect appears as a straight or spiral line, either continuous or discontinuous, on the inner or outer surface of the steel pipe. Folding occurs due to poor-quality pipe material, surface inclusions, deep scratches, or cracks that fold during the drawing process. To prevent folding, the quality of the pipe material must be improved, and thorough inspection and grinding should be conducted before drawing.

Cracks:

Cracks are small, linear defects found on the inner and outer surfaces of the steel pipe. These cracks may be continuous or discontinuous and are typically deeper than 1 mm. They often result from subcutaneous bubbles or inclusions in the hot-rolled billet, or from deep pits on the steel pipe before drawing. During the drawing process, these issues can lead to longitudinal scratches. To avoid cracks, it’s essential to improve the quality of the tube blanks and ensure careful inspection and grinding of the material.

Scratches:

Scratches are longitudinal linear defects that appear on the inner and outer surfaces of the pipe. These can be groove-shaped or raised and are primarily caused by internal markings on the pipe material that are not eliminated during drawing, residual iron oxide scale, mold defects, or insufficient hardness of the mold. To prevent scratches, each preparation process before drawing should be optimized, and high-quality, smooth molds with sufficient strength and hardness should be used.

Pits:

Pits are local depressions of varying areas that appear on the surface of the steel pipe. They can be periodic or irregular and are caused by iron oxide scale or other hard dirt being pressed into the pipe surface during drawing or straightening, or by peeling of pre-existing surface layers. Preventing pits involves thoroughly inspecting the pipe material, removing surface defects, maintaining a clean work environment, and preventing dirt from contaminating the pipe surface.

Pitted Surface:

A pitted surface is characterized by small, flaky, or dotted pits on the steel pipe surface. This defect arises from pitting corrosion during pickling, excessive oxide scale formation after annealing, or rusting in a humid environment. Preventing pitted surfaces involves controlling the pickling process, reducing oxide scale formation, and avoiding exposure to moisture during storage.

Spots:

Spots are marks left on the pipe surface due to contact with water or water vapor during processing or storage. If the pipe is not dried properly, water spots may form. Extended contact with water, especially if it is corrosive, can lead to rust spots. While water spots are usually superficial, rust spots can corrode the surface, leaving pitted areas even after removal.

Warped Skin:

Warped skin refers to areas where the inner or outer surface of the pipe separates from the metal substrate, forming discontinuous lumps that may be rooted in the pipe wall. This defect is caused by poor steel quality, subcutaneous bubbles exposed after cold drawing, or residual warping from hot rolling that isn’t removed before further processing. Preventing warped skin requires high-quality steel and proper removal of defects before cold drawing.

Scars:

Scars are thin, flaky defects that form on the pipe surface when earlier defects like folding or warping are not properly removed. These scars are typically lumpy and discontinuous, and while they may be rooted in the pipe wall, they cannot be easily peeled off. To avoid scars, surface defects should be identified and removed promptly during production.

Addressing these defects requires a combination of high-quality materials, careful inspection, and proper processing techniques to ensure that seamless steel pipes meet the required standards for quality and performance.