The production of hot-rolled seamless steel pipes can result in various defects due to factors such as steel quality, rolling processes, and equipment conditions. Below is a detailed analysis of common defects, their characteristics, and underlying causes.
- Cracks
Characteristics:
Appear as straight or spiral cracks on the inner and outer surfaces.
Some cracks form a mesh pattern, with sharp ends and bottoms.
Causes:
Poor steel quality with subcutaneous pores or inclusions.
Improper heating of the tube blank.
Excessive deformation pressure.
- Hair Lines
Characteristics:
Fine, hair-like lines, usually spiral, on the outer surface.
The spiral direction is opposite to the rotation of the punching machine.
Causes:
Subsurface defects in steel, including pores and inclusions.
Incomplete cleaning of the tube blank surface before processing.
- Inner Fold
Characteristics:
Serrated defects on the inner surface, appearing as straight or spiral folds.
In high-alloy steel pipes, defects appear as irregular blocks near the inlet.
Causes:
Excessive pressure during piercing.
Worn-out piercing heads.
Uneven stress distribution during rolling.
- Outer Fold
Characteristics:
Spiral folds on the outer surface, opposite in direction to the rough pipe’s spiral on the piercing machine.
Causes:
Pre-existing defects such as cracks or ears on the tube blank.
Poor steel quality, with inclusions or severe looseness.
- Rolling Folding
Characteristics:
Regular folding defects on the outer surface.
Causes:
Damaged rolls in the piercing machine or tube rolling mill.
- Tie Folding
Characteristics:
Twists and turns formed during rolling.
After sizing, local convex and concave wrinkles appear on the surface.
Causes:
Improper alignment of the sizing machine.
Incorrect roll adjustment, causing instability.
- Straight Folding
Characteristics:
Straight-line folds, either symmetrical or single, affecting the entire length or part of the pipe.
Causes:
Worn-out or improperly adjusted piercing heads.
Excessively large or small tube diameters leading to deformation.
Poor distribution of pressure in the piercing and rolling process.
- Straight Track
Characteristics:
Linear scratches of consistent width and depth along the pipe.
Causes:
Bent top rods causing misalignment.
Worn-out heads leading to inner surface damage.
- Delamination
Characteristics:
Spiral or block-shaped layered fractures inside the pipe wall.
Causes:
Non-metallic inclusions and shrinkage holes in the tube blank.
Looseness in the tube blank structure.
Improper heating of high-alloy steel tube blanks.
- Scarring
Characteristics:
Surface scars on the inner and outer steel pipe.
Causes:
Hard block formation from lubricant inclusions.
Detached rough tube ears being pressed onto the surface.
Oxidation of plug surfaces during piercing.
- Pits (Depressions)
Characteristics:
Irregular surface depressions on the inner wall.
Causes:
External injuries, such as collision damage.
Metal debris pressing into the tube surface.
- Concave Surface (Bumps)
Characteristics:
The wall is concave on the outside and convex on the inside but remains intact.
Causes:
Impact during handling or excessive vibration in the straightener.
- Pockmarks (Pitted Surface)
Characteristics:
Small, localized pits on the inner surface, sometimes periodic.
Causes:
Rough grooves in rolling or sizing mill rolls.
Severe rusting or over-pickling leading to corrosion.
- Convex Surface (Bulges)
Characteristics:
Uneven bulges on the outer surface.
Causes:
Pitting or depressions on the automatic rolling mill rollers.
- Concave Correction
Characteristics:
Smooth spiral depressions on the pipe surface.
Causes:
Excessive pressure or incorrect roller angle in the straightening machine.
- Scratches
Characteristics:
Spiral or linear groove defects, often exposing the bottom of the groove.
Causes:
Guide plate wear, nozzle misalignment, or iron oxide adhesion.
Mechanical damage during rolling, straightening, or transport.
- Internal Spiral
Characteristics:
Spiral concave and convex patterns on the inner surface.
Causes:
Over-expansion during leveling.
Uneven tool wear or poor machine adjustments.
- Blue Line
Characteristics:
Longitudinal marks corresponding to the hole opening of the sizing machine.
Causes:
Misaligned or severely worn rolls.
Poor groove turning.
- Burrs
Characteristics:
Circular or partial cutting residue at the pipe end.
Causes:
Improper pipe-cutting blade angle.
Summary
Various defects can occur during the production of hot-rolled seamless steel pipes, including cracks, folds, pitting, scratches, and more. These defects often result from steel quality issues, rolling process errors, and equipment misalignment. Proper control of materials, heating processes, and equipment settings can help minimize such defects and improve product quality.
English
Español
