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Analysis of the causes of surface defects of cold-drawn (rolled) seamless steel pipes
Date:2024-05-31      View(s):166      Tag:seamless steel pipe, cold drawn seamless steel pipe, cold rolled seamless steel pipe
1. Scratches.
Features: The surface of the seamless steel pipe presents irregular scratches of varying lengths and directions, which are generally straight and can be seen at the bottom.
Causes: 1) Improper operation during annealing. 2) Scratches during lifting. 3) During the straightening process, the seamless steel pipe is scratched at the sleeve.

2. Scratches (also known as wire drawing, roughening, straight lines, including blue lines).
Features: On the inner and outer surfaces of the steel pipe, there are longitudinal straight lines of varying lengths, mostly in the shape of grooves. Scratches are caused when the surface is damaged, and blue lines are caused when the surface is not damaged.
Causes: 1) Scratches on the rough pipe. 2) There is iron oxide remaining on the rough pipe. 3) Poor lubricant coating causes increased friction, resulting in heat and welding of the mold metal. 4) The hardness of the inner and outer molds is insufficient or uneven, and the finish is poor. 5) The poor transition part of the hammerhead produces sharp edges and corners, and the mold wear causes scratches.

3. Shake marks (also known as corrugation, bamboo-shaped, and ring-shaped).
Features: On the outer surface of the seamless steel pipe, there are ring marks with uneven full or half circles of waves along the horizontal direction, which are continuous, intermittent, and sometimes only on the surface.
Causes: 1) There is residual iron oxide or dirt on the surface of the seamless steel pipe, which increases friction and generates vibration during drawing. 2) There is no small hole punched at the head. 3) The seamless steel pipe is poorly lubricated. 4) Uneven annealing and excessive cooling speed of hot-rolled steel pipes with high carbon content cause the steel to be harder. 5) The sizing position of the mold is incorrect. 6) The chain vibrates when pulling.

4. Pits (including indentations).
Features: The surface of the seamless steel pipe is a local depression of varying areas, some of which are periodic and some are irregular. The defective surface has pits with damage, and there are indentations without damage on the surface.
Causes: 1) Scrap iron, iron sheet, or other dirt adheres to the surface of the steel pipe, and peels off and leaves indentations after being pressed into the surface during drawing or rolling. 2) Foreign matter adheres to the straightening roller, and the surface of the seamless steel pipe is pressed into pits during the straightening process.

5. Pockmarks (also known as pitted surface).
Features: Small, dot-shaped depressions on the surface of the seamless steel pipe.
Causes: 1) The surface of the seamless steel pipe is seriously oxidized. 2) The seamless steel pipe is over-pickled, and pitting corrosion occurs on the surface. 3) The seamless steel pipe is not well preserved and is seriously rusted.

6. Scarring (including scales, warping, and rolling).
Features: Thin layers on the inner and outer surfaces of the seamless steel pipe are partially separated from the metal, in block or fish scale shape, some are connected to the metal of the pipe wall and are not easy to fall off, and some are not connected to the metal and are easy to fall off.
Causes: 1) Poor steel quality, with subcutaneous bubbles. 2) From hot rolling to cold drawing or cold rolling pipe. 3) The bending of the tail of the seamless steel pipe causes the inner diameter reduction to be too large, and the force is uneven during drawing, forming an inner warp. 4) The seamless steel pipe has a deep angular transverse pit, which is formed after drawing.

7. Delamination.
Features: Layered rupture in a spiral direction in the wall of the seamless steel pipe.
Causes: 1) There are non-metallic inclusions and shrinkage holes in the tube blank. 2) Severe looseness, etc.

8. Cracks.
Features: On the inner and outer surfaces of the seamless steel pipe, there are straight or spiral cracks, and the two ends and bottom of the cracks are pointed.
Causes: 1) Poor steel quality, subcutaneous pores, and subcutaneous inclusions. 2) The seamless steel pipe blank has cracks. 3) Improper annealing.

9. Hairline.
Features: On the outer surface of the seamless steel pipe, there are continuous or discontinuous hair-like fine lines, generally distributed in a straight line or spiral shape.
Causes: 1) Poor steel quality, with subcutaneous pores and subcutaneous inclusions. 2) Incomplete cleaning of the tube surface. 3) Scratches are also prone to cause hairline cracks.

10. Folding (including straight folding).
Features: On the inner and outer surfaces of the seamless steel pipe, there are linear or spiral folds, which are continuous or discontinuous.
Causes: 1) The surface of the tube has folds and inclusions. 2) The surface of the tube has serious scratches and cracks that extend and fold with each other after drawing and rolling. 3) The edges and corners of the tube are produced during drawing and rolling.

11. Concave surface (also known as flattening).
Features: The wall of the seamless steel pipe is concave outside and convex inside, but the wall is not damaged.
Causes: 1) The seamless steel pipe collides with foreign objects during transportation, especially when it is taken out of the furnace after annealing. 2) Too many seamless steel pipes are stacked, and the bottom steel pipe is easy to be crushed. 3) The straightening machine swings too much after biting in. 4) The pipe-cutting machine clamps too tightly.

12. Transverse cracks.
Features: There are continuous or discontinuous fine transverse cracks on the inner and outer surfaces of the seamless steel pipe.
Causes: 1) The pickling temperature of the seamless steel pipe is too high for too long or the steel pipe temperature after annealing is higher than the acid temperature, which causes hydrogen embrittlement of the seamless steel pipe. 2) The insulation time is not enough or the temperature is uneven during annealing, and the longitudinal stress on the seamless cross-section cannot be fully eliminated. 3) The seamless steel pipe is not dried or not dried thoroughly after acid treatment, and the hydrogen cannot be removed. 4) The extension of the inner and outer walls of the thick-walled pipe is inconsistent.

13. Tension cracking/rolling cracking.
Features: There are transverse fractures on the surface of the seamless steel pipe, and in severe cases, it will break.
Causes: 1) Uneven annealing temperature causes inconsistent longitudinal mechanical properties. 2) Improper drawing or rolling process, excessive compression rate, easy to cause tension cracking. 3) The mold shape is wrong and the mold edges scratch the surface of the seamless steel pipe. 4) Seamless steel tube billets have serious defects such as warping and internal inclusions. 5) High carbon steel and alloy steel have coarse grains and poor plasticity.

14. Cracking.
Features: The seamless steel tube shows longitudinal cracks that penetrate the tube wall, which generally occur along the entire length and sometimes at one end.
Causes: 1) Improper annealing, uneven temperature, and uneven elongation. 2) Excessive reduction and severe work hardening. 3) Failure to anneal in time after drawing and stress not eliminated. Steel pipes with high carbon content are prone to cracking.

15. Concave correction (including straightening inclination).
Features: The surface of the seamless steel tube presents a relatively smooth spiral concave and concave correction along the longitudinal direction, and sometimes presents a spiral impression with sharp edges and straightening inclination.
Causes: 1) The angle of the straightening machine roller is incorrect, and the seamless steel tube rubs against the edge shoulder of the roller. 2) The straightening machine roller is excessively worn and produces sharp edges. 3) The hammerhead is too large or too curved, and the tail of the steel pipe is bent too much.

16. Straightening pattern.
Features: During the straightening process, the inner and outer surfaces of the seamless steel pipe show irregular cross-surface uneven patterns, which are generally produced in thin-walled pipes.
Causes: 1) The pressure distribution of each roller of the straightening machine is unreasonable, with large pressure at both ends and small pressure in the middle. Therefore, the spiral pattern of the first roller and the spiral pattern of the tail roller do not match. 2) The seamless steel pipe itself is bent too much and moves up and down during the straightening process. 3) The surface of the pipe annealed in the sleeve has no oxide scale, which is most likely to form straightening patterns.
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