Causes of surface defects of cold-drawn seamless steel pipes

Scratches

Features: The surface of the seamless steel pipe exhibits irregular scratches of varying lengths and directions, generally straight, with visible bottoms.

Causes:

Improper handling during annealing.

Scratches occurring during lifting.

Scratches from the sleeve during the straightening process.

Shake Marks (also known as corrugation, bamboo-shaped, and ring-shaped)

Features: The outer surface of the seamless steel pipe has ring marks with uneven full or half circles of waves along the horizontal direction. These marks can be continuous, intermittent, and sometimes only on the surface.

Causes:

Residual iron oxide or dirt on the surface increases friction and causes vibration during drawing.

Lack of a small hole punched at the head.

Poor lubrication of the seamless steel pipe.

Uneven annealing and excessive cooling speed of hot-rolled steel pipes with high carbon content hardening the steel.

Incorrect sizing position of the mold.

Chain vibration during pulling.

Pits (including indentations)

Features: The surface of the seamless steel pipe has localized depressions of varying areas, which can be periodic or irregular. Damaged surfaces exhibit pits, while undamaged surfaces have indentations.

Causes:

Adherence of scrap iron, iron sheet, or other dirt to the surface, which peels off and leaves indentations after being pressed during drawing or rolling.

Foreign matter adhering to the straightening roller, pressing pits into the seamless steel pipe during straightening.

Pockmarks (also known as pitted surface)

Features: The surface of the seamless steel pipe has small, dot-shaped depressions.

Causes:

Severe oxidation of the seamless steel pipe surface.

Over-pickling leading to pitting corrosion on the surface.

Poor preservation causing severe rusting.

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, forming block or fish scale shapes. Some layers remain attached to the pipe wall and are not easily detached, while others are not connected to the metal and are easily detached.

Causes:

Poor steel quality with subcutaneous bubbles.

Issues from hot rolling to cold drawing or cold rolling processes.

Bending at the tail of the seamless steel pipe causing excessive inner diameter reduction and uneven force during drawing, leading to inner warping.

Deep angular transverse pits in the seamless steel pipe formed after drawing.

Delamination

Features: Layered ruptures in a spiral direction within the wall of the seamless steel pipe.

Causes:

Non-metallic inclusions and shrinkage holes in the tube blank.

Severe looseness and other defects.

Cracks

Features: Straight or spiral cracks on the inner and outer surfaces of the seamless steel pipe, with pointed ends and bottoms.

Causes:

Poor steel quality with subcutaneous pores and inclusions.

Cracks in the seamless steel pipe blank.

Improper annealing.

Hairline

Features: Continuous or discontinuous fine hair-like lines on the outer surface of the seamless steel pipe, generally distributed in straight or spiral shapes.

Causes:

Poor steel quality with subcutaneous pores and inclusions.

Incomplete cleaning of the tube surface.

Scratches prone to cause hairline cracks.

Folding (including straight folding)

Features: Linear or spiral folds on the inner and outer surfaces of the seamless steel pipe, which can be continuous or discontinuous.

Causes:

Folds and inclusions on the surface of the tube.

Serious scratches and cracks on the tube surface that extend and fold during drawing and rolling.

Edges and corners produced during drawing and rolling.

Concave Surface (also known as flattening)

Features: The wall of the seamless steel pipe is concave outside and convex inside, but the wall itself is not damaged.

Causes:

Collisions with foreign objects during transportation, especially when taken out of the furnace after annealing.

Excessive stacking of seamless steel pipes, causing the bottom pipes to be crushed.

Excessive swinging of the straightening machine after biting in.

Too tight clamping by the pipe-cutting machine.

Transverse Cracks

Features: Fine transverse cracks, either continuous or discontinuous, appear on the inner and outer surfaces of the seamless steel pipe.

Causes:

Excessive pickling temperature or prolonged exposure, or steel pipe temperature after annealing higher than the acid temperature, causing hydrogen embrittlement.

Insufficient insulation time or uneven temperature during annealing, leading to incomplete elimination of longitudinal stress on the seamless cross-section.

Inadequate drying after acid treatment, resulting in retained hydrogen.

Inconsistent extension of the inner and outer walls of thick-walled pipes.

Tension Cracking/Rolling Cracking

Features: Transverse fractures on the surface of the seamless steel pipe, which may result in the pipe breaking in severe cases.

Causes:

Uneven annealing temperature causing inconsistent longitudinal mechanical properties.

Improper drawing or rolling process with excessive compression rate, leading to tension cracking.

Incorrect mold shape and mold edges scratching the surface.

Serious defects in seamless steel tube billets, such as warping and internal inclusions.

Coarse grains and poor plasticity in high carbon steel and alloy steel.

Cracking

Features: Longitudinal cracks penetrating the tube wall, typically extending along the entire length or occurring at one end.

Causes:

Improper annealing with uneven temperature and elongation.

Excessive reduction causing severe work hardening.

Failure to anneal promptly after drawing, leaving stress uneliminated. High carbon content steel pipes are especially prone to cracking.

Concave Correction (including straightening inclination)

Features: Smooth spiral concave and concave corrections along the longitudinal direction of the seamless steel tube, sometimes presenting a spiral impression with sharp edges and straightening inclination.

Causes:

Incorrect angle of the straightening machine roller, causing the steel tube to rub against the roller's edge shoulder.

Excessive wear on the straightening machine roller, producing sharp edges.

Hammerhead too large or too curved, with excessive bending at the tail of the steel pipe.

Straightening Pattern

Features: Irregular cross-surface uneven patterns on the inner and outer surfaces of the seamless steel pipe during the straightening process, commonly found in thin-walled pipes.

Causes:

Unreasonable pressure distribution across the rollers of the straightening machine, with higher pressure at both ends and lower pressure in the middle, causing mismatched spiral patterns between the first and tail rollers.

Excessive bending of the seamless steel pipe itself, leading to vertical movement during straightening.

Absence of oxide scale on the pipe surface annealed in the sleeve, increasing the likelihood of straightening patterns.