Internal Surface Defects
- Inner Fold
Characteristic: A straight or spiral, semi-spiral sawtooth shaped defect on the inner surface of the steel pipe.
Causes:
- Empty tube: center clearance, segregation; serious shrinkage cavity residue; non-metallic inclusions exceed the standard.
- Uneven heating of the empty tube, temperature too high or too low and heating time too long.
- Perforation area: severe upper wear; improper adjustment of drilling machine parameters; aging of punch rolls and so on.
Inspection: Internal bends on the inner surface of the steel tube are not allowed. The internal bend at the end of the pipe must be repaired by grinding or cutting again. The actual value of the wall thickness of the grinding position must not be less than the minimum value required by the standard. The entire length of the inner fold must be rejected.
- Internal Scar
Feature: The inner surface of the steel tube has stains and scars, which generally do not take root and are easy to peel off.
Causes:
- Impurities in graphite lubricant.
- The iron ear at the rear end of the rough tube is pressed against the inner wall of the steel tube.
Inspection: Internal scars on the inner surface of the steel tube are not allowed. The end of the pipe must be ground and cut again. The grinding depth must not exceed the negative deviation from the standard requirements, and the actual wall thickness must not be less than the minimum value required by the standard. The entire extent of the internal scar must be rejected.
- Wavy skin
Characteristic: Small skin with straight or intermittent nail-shaped curls on the inner surface of the steel tube. It often appears on the capillary head and is easy to remove.
Causes:
- Improper adjustment of drilling machine parameters.
- Sticky steel on top.
- Accumulation of oxidized iron film on the rough pipe.
Inspection: Wavy skin that is easy to peel off (or may be burned during heat treatment) is allowed on the inner surface of the steel pipe. For rooted and wavy skin, it must be filed or removed.
- Internal straight scratch
Characteristic: A straight scratch with a certain width and depth on the inner surface of the steel tube.
Causes:
- The laminating temperature is low and the center bar becomes sticky with hard metal objects.
- Impurities in graphite, etc.
Inspection:
- The maximum depth of straight internal scratch allowed for common casings and pipes is 5% (maximum depth for pressure vessels is 0.4 mm).
Careful examination of the Chad's internal straight scratches is necessary and must be ground or removed.
Internal straight scratches with sharp edges must be smoothed by grinding.
- Inner Ridge
Feature: A straight protrusion with a certain width and depth on the inner surface of the steel tube.
Causes: Severe wear of the central bar, irregular or very deep grinding.
Inspection:
- For casing tubes and piping, the maximum allowable internal ridge height is 8% of the wall thickness and the maximum height is 0.8 mm without affecting the diameter. The deviation must be repaired by sanding and cutting again.
- For ordinary tubes and pipes, the maximum allowable height of the inner ridge is 8% of the wall thickness, and the maximum height is 0.8 mm. The deviation must be repaired by sanding and cutting again.
- For steel pipes required to meet L2 (N5) level flaw detection requirements, the maximum height of the internal crest should not exceed 5% (maximum height is 0.5mm). The deviation must be repaired by sanding and cutting again.
- Internal edges with sharp edges must be smoothed by grinding.
- Internal Bulge
Feature: Regular convex protrusions on the inner surface of the steel tube without damage to the outer surface.
Causes: Excessive grinding or loss of meat from the continuous lamination rolls.
Inspection: Inspect according to the requirements for internal protrusions.
- Concave Pull
Feature: Regular or irregular pits on the inner surface of the steel tube without damage to the outer surface.
Causes:
- Incorrect adjustment of continuous lamination and the lamination speeds of each roller in the frame do not match.
- Irregular heating of the empty tube or temperature that is too low.
- Deviation from the rolling axis, collision between the steel tube and the continuous rolling country road, etc. (Note: This reason was proposed in January 2003, and the principle is still being explored.)
Inspection: Tensile concaves with negative deviation not exceeding the wall thickness and actual wall thickness exceeding the minimum wall thickness required are permitted. Excessive traction concaves must be cut out. (Note: Serious development of tensile creases may cause tears, which must be closely inspected.)
- Internal Thread (This defect only occurs on Assel units)
Characteristic: Spiral traces on the inner surface of the steel pipe, which mainly appear on the inner surface of thin-walled pipes and have an obviously uneven feel.
Causes:
- Defects inherent to the inclined rolling process. This defect is most prominent when the Assel tube mill process parameters are incorrectly adjusted.
- The deformation distribution is unreasonable and the wall thickness reduction of the Assel pipe factory is too large.
- Improper configuration of the Assel rolling groove.
Inspection: The depth of the internal thread defect in the steel pipe should not exceed 0.3mm and should be within a certain tolerance range.
External Surface Defects
- External Folding
Features: Spiral-shaped layer bent on the outer surface of the steel tube.
Causes:
- Bends or cracks on the surface of the empty tube.
- Subcutaneous pores or severe subcutaneous inclusions in the empty tube.
- Poor cleaning on the surface of the empty tube, or wrong ear, face, etc.
- Scratches on the surface of the steel tube caused by lifting during rolling, which are then pressed into the steel tube die to form external bends.
Inspection: No presence allowed. Slight bends can be straightened, and after straightening, the actual wall thickness and outer diameter value must not be less than the minimum value required by the standard.
- Delamination
Features: Spiral or block-shaped layers and cracks on the surface of the steel pipe.
Causes: Severe non-metallic inclusions, residual shrinkage holes or severe looseness in the tube blank.
Inspection: No presence allowed.
- External Scar
Characteristics: Stains or scars on the outer surface of the steel pipe.
Causes:
- Stuck roller, aging, severe wear or scuff marks.
- Foreign objects or severe wear on transport rollers.
Inspection:
- The scar area must be filed or removed in pieces.
- For scarred pipe sections, the area with scars greater than 10% must be cut or ground.
- Scars deeper than 5% of the wall thickness must be straightened.
- The actual wall thickness and the value of the external diameter at the grinding site must not be less than the minimum value required by the standard.
- Rough surface
Features: Irregular pits on the surface of the steel tube.
Causes:
- The steel pipe remains in the furnace for a long time or is heated for a long time, resulting in the formation of thick oxide scale on the surface, which is not cleaned properly and curled on the surface of the steel pipe.
- Malfunction of high-pressure water phosphorus removal equipment resulting in incomplete phosphorus removal.
Inspection:
- Local rough surfaces not exceeding the negative wall thickness deviation are permitted.
- The rough surface area should not exceed 20% of the pipe section area with rough surfaces.
- Rough surfaces exceeding the standard may be ground or cut, and the actual wall thickness and outer diameter value at the grinding location must not be less than the minimum value required by the standard.
- Severe rough surfaces are considered waste.
- Green Line
Features: Symmetrical or asymmetrical straight rolling marks on the outer surface of the steel tube.
Causes:
- Misalignment or severe wear of the diameter sizing machine hole shape.
- Irrational design of diameter sizing machine roller hole shape.
- Cold rolling of steel.
- Poor machining of the laminator, with small chamfers on the edges of the rolls.
- Poor assembly of the rollers, with excessive gaps, etc.
Inspection:
- A green line with a height of no more than 0.2 mm is allowed on the outer surface of the casing, and exceeding the standard must be rectified.
- Hand sensitive green lines are not permitted on high pressure container pipes. Hand sensitive green lines should be removed. The grinding location must be smooth, without sharp corners.
- General pipes (structural, fluids, hydraulic support, etc.) allow the existence of a green line with a height of no more than 0.4mm, which must exceed the standard and must be rectified.
- Crisp green lines at the edges should be smooth.
- The actual values of wall thickness and outer diameter at the grinding site must not exceed the minimum value required by the standard.
- Hairline split
Characteristics: Continuous or discontinuous fine cracks on the external surface of the steel tube.
Causes:
- Subcutaneous pores or inclusions in the empty tube.
- Incomplete cleaning of the surface of the tube blank, with the presence of small cracks.
- Worn or aged rollers.
- Poor machining precision of rollers, etc.
Inspection: Visible cracks on the outer surface of the steel pipe are not allowed. If present, they must be completely removed, and the actual values of wall thickness and external diameter after removal must not be less than the minimum value required by the standard.
- Network-type crack
Features: Small striped and helical cracks similar to fish scales on the outer surface of the steel tube.
Causes:
- High content of harmful elements (such as arsenic) in the empty tube.
- Aging and adhesion of drill rolls.
- Gluing guide plates, etc.
Inspection: They must be completely removed. After removal, the actual values of wall thickness and external diameter must not be less than the minimum value required by the standard.
- Scratch
Characteristics: Spiral or straight groove-shaped defects on the outer surface of the steel tube, with the lower part of the groove most visible.
Causes:
- Mechanical scratches mainly occur in roller paths, cold beds, straightening and transportation.
- Poor machining or severe wear of the rolling mill, or foreign objects in the roll seam, etc.
Inspection:
- Local scratches not exceeding 0.5 mm are allowed on the outer surface of the steel pipe, and scratches exceeding 0.5 mm must be rectified. The actual values of wall thickness and external diameter at the grinding site must not be less than the minimum value required by the standard.
- Sharp scratches on the edges should be smooth.
- Tooth
Features: The outer surface of the steel pipe has a concave-convex phenomenon, while the wall thickness of the steel pipe remains intact.
Causes:
- Dents caused by impact during lifting.
- Dents caused by biting during straightening.
- Dents caused by the rear roller path of the diameter sizing machine, etc.
Inspection: There may be local dents that do not exceed the negative deviation of the outer diameter and have a smooth surface. When exceeding the standard, they must be cut.
- Impact damage
Features: Irregular scars on the outer surface of the steel tube due to collision.
Causes: It can occur due to various collisions in cold and hot areas.
Inspection:
- Local impact damage with a depth of no more than 0.4 mm is allowed on the outer surface.
- Impact damage greater than 0.4 mm must be ground smoothly and the actual values of the outer diameter and wall thickness at the grinding location must not be less than the minimum value required by the standard.
- Straightening out Depression
Features: A spiral depression on the outer surface of the steel tube.
Causes:
- Incorrect angle adjustment of the straightener rollers and excessive pressing force.
- Severe wear of the straightening roller, etc.
Inspection: The outer surface of the steel pipe may have straightening depressions without obvious sharp corners and no protrusions on the inner surface, and the outer diameter size must meet the tolerance requirements. Exceeding standard straightening depressions must be cut.
- To bend
Characteristics: Local or longitudinal wrinkles along the steel tube wall with a concave-convex appearance, and the outer surface has a strip-shaped depression.
Causes:
- The hole expansion coefficient selection is very small.
- Improper rolling mill adjustment leads to hole deviation or inconsistent rolling centerline.
- Inadequate distribution of pressing force in each structure of the continuous rolling mill.
For the above reasons, metal enters the space between the rollers during rolling or the tube loses stability, resulting in wrinkles on the tube wall.
Inspection: Not permitted. It must be cut or discarded.
- Tearing
Features: The surface of the steel pipe has the phenomenon of being separated and broken, especially in thin-walled pipes.
Causes:
- Due to the uneven heating of the tube blank, the deformation part is sealed, and the low-temperature part is subjected to tensile rolling. When the pulling force is large, the tube breaks.
- Improper adjustment of the roller speed and seam of each frame of the continuous rolling mill causes the steel to be pulled and torn.
- The wall thickness of the mother pipe supplied by the punching machine to the continuous rolling mill is small, and the deformation of the metal in the continuous rolling mill is smaller than the designed deformation, resulting in rolling and tearing when the pulling force is large.
- The empty tube itself has severe inclusions in some areas.
Inspection: Not permitted. It must be cut or discarded.
The size is very wrong
1. Uneven wall thickness
Features: The wall thickness of the steel pipe is uneven in the same cross section, with a large difference between the maximum and minimum wall thickness.
Causes:
- Uneven heating of the billet.
- Misalignment of the punching machine lamination line and unstable centering rollers.
- Worn or eccentric upper heads.
- Correction of the billet centering hole.
- Excessive curvature or skew of the billet.
Inspection: Measure each pipe individually and cut off any uneven ends from the wall thickness.
2. Deviation in wall thickness
Features: The wall thickness of the steel pipe deviates in one direction, with positive deviation called excessive thickness and negative deviation called insufficient thickness.
Causes:
- Uneven heating of the billet.
- Incorrect adjustment of the drilling machine.
Inspection: Measure each tube individually and cut the offending ends, or re-inspect the entire length and change judgment or discard the tube.
3. Deviation in outer diameter
Characteristics: The outer diameter of the steel pipe is outside specifications, with positive deviation called oversizing and negative deviation called undersizing.
Causes:
- Excessive wear of the gauge hole type or inadequate design of new hole types.
- Unstable lamination end temperature.
Inspection: Measure each tube individually and change the judgment or discard the tube if it is out of specifications.
4. Push-up
Features: A bend in steel pipe that is not straight along its length or that appears like a goose's head at the end is called a “gooseneck bend”.
Causes:
- Caused by localized cooling of water during manual heat inspection.
- Inadequate adjustment during straightening, with severe wear of the straightening rollers.
- Inadequate machining, assembly and adjustment of the sizing machine.
- Bending caused during lifting and transportation.
Inspection: In case of excessive bending, secondary straightening can be carried out, otherwise the tube is discarded. “Gooseneck curves” that cannot be straightened must be cut.
5. Length deviation
Features: The length of the steel pipe exceeds the requirement, with positive deviation called long length and negative deviation called short length.
Causes:
- Billet length exceeds standard.
- Unstable bearing.
- Inadequate control during cutting.
Inspection: Recut long tubes or change the rating, and change the rating or discard short tubes.
