Analysis of the causes of surface defects of cold-drawn (rolled) seamless steel pipes

1. Scratches.

Features:

The surface of seamless steel pipes 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 and scratches of varying lengths, mostly in the shape of grooves. Surface damage is scratches, and no surface damage is blue lines.

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 mold heating and welding 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, mold wear, and scratches.

3. Shake marks (also known as corrugation, bamboo-shaped, 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 scale 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, and there are no indentations on the surface.

Causes:

1) Scrap iron, iron sheets, or other dirt adhere to the surface of the steel pipe, peel off, and leave indentations after being pressed into the surface during drawing or rolling.

2) The straightening roller adheres to foreign matter, and the surface of the seamless steel pipe is pressed into pits during the straightening process.

5. Pockmarks (also known as pitted surface),

Features:

The surface of the seamless steel pipe is a patch of small depressions in the form of dots.

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 severely rusted.

6. Scarring (including scales, peeling, and rolling),

Features:

The thin layer on the inner and outer surfaces of the seamless steel pipe is 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 tube.

3) The bending of the tail of the seamless steel tube causes excessive inner diameter reduction, uneven force during drawing, and the formation of inner warping.

4) The seamless steel tube has deep angular transverse pits, which are formed after drawing.

7. Delamination,

Characteristics:

Laminar rupture in the wall of the seamless steel tube in a spiral direction.

Causes:

1) There are non-metallic inclusions and shrinkage holes in the tube blank.

2) Severe looseness, etc.

8. Cracks,

Characteristics:

On the inner and outer surfaces of the seamless steel tube, there are straight or spiral cracks, and the two ends and bottom of the cracks are pointed.

Causes:

1) Poor steel quality, with subcutaneous pores and subcutaneous inclusions.

2) There are cracks in the seamless steel tube blank.

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) The surface of the tube billet is not cleaned thoroughly.

3) Hairline lines are also easily caused after abrasion.

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 billet has folds and inclusions.

2) The surface of the tube billet has serious scratches and cracks, which are extended and folded after drawing and rolling.

3) The grinding part of the tube billet has edges and corners, which 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) Seamless steel pipes collide with foreign objects during transportation, especially when they are taken out of the furnace after annealing.

2) There are too many seamless steel pipes stacked, and the bottom steel pipes are easily crushed.

3) The straightening machine swings too much after biting in.

4) The pipe-cutting machine is clamped too tightly.

12. Transverse cracks

Characteristics:

There are continuous or discontinuous small transverse cracks on the inner and outer surfaces of seamless steel pipes.

Causes:

1) The pickling temperature of seamless steel pipes is too high for too long or the temperature of the steel pipes after annealing is higher than the acid temperature, causing hydrogen embrittlement of seamless steel pipes.

2) During annealing, the insulation time is not enough the temperature is uneven, 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 thick-walled pipes is inconsistent.

13. Tensile cracking/rolling cracking.

Features:

The seamless steel pipe has transverse fractures on its surface, 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 tensile cracking.

3) The mold shape is wrong, and the edges and corners of the mold scratch the surface of the seamless steel pipe.

4) The seamless steel pipe billet has 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 pipe shows longitudinal cracks that penetrate the pipe wall, generally occurring along the entire length and sometimes at one end.

Causes:

1) Improper annealing, uneven temperature, and different elongation.

2) Excessive reduction and severe work hardening.

3) Failure to anneal in time after drawing and stress is not eliminated. Steel pipes with high carbon content are prone to cracking.

15. Straightening concave (including straightening inclination).

Features:

The surface of the seamless steel pipe presents a relatively smooth spiral concave and straightening concave along the longitudinal direction, and sometimes presents a spiral mark with sharp edges and straightening inclination.

Causes:

1) The angle of the straightening machine roller is incorrect, and the seamless steel pipe rubs against the edge shoulder of the roller.

2) The straightening machine roller is excessively worn and produces sharp edges.

3) The hammerhead end is too large or bent and the tail of the steel pipe is bent too much.

16. Straightening pattern.

Features:

During the straightening process, irregular cross-surface uneven patterns appear on the inner and outer surfaces of the seamless steel pipe, which are generally produced in thin-walled pipes.

Causes:

1) The pressure distribution of each roller of the straightening machine is unreasonable, with large ends and a small 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 tube annealed in the sleeve has no oxide scale and is most likely to form a straightening pattern.