Defects and prevention of steel pipes

 
1. Fold

Defect name	Fold
Defect characteristics	The inner and outer surfaces of the steel pipe are folded in a straight or spiral direction. Partial or full length appears on the steel pipe.
Cause	(1) There are folds or inclusions on the surface of the pipe material. (2) There are serious scratches or cracks on the surface of the pipe material. (3) There are corners or depth-to-width ratios in the pipe grinding area
Prevention and elimination methods	Strictly implement the concentrate policy. Unqualified tubes are not put into production.
Check and judge	Check with the naked eye. Folding of the outer surface allows grinding, after the defects are completely removed. Those whose wall thickness and diameter do not exceed the negative deviation shall be judged as qualified products. The inner surface is partially folded and removed. The elders of Tongtong are totally abolished.

 

2. Straight inward

Defect name	Straight turn
Defect characteristics	The inner surface of the steel pipe presents a straight zigzag fold, with partial or full length, single or multiple.
Cause	Brought by raw materials. Unable to eliminate after drawing.
Check and judge	Check with the naked eye. Local straight incision and resection. A long straight road is judged invalid.

 
 
 
3. Scarring

Defect name	Scarring
Defect characteristic	There are local metal separation slices on the surface of the steel pipe. Lumpy or scaly. The metal connected to the pipe wall is not easy to fall off, and the unconnected metal is easy to fall off.
Cause	The hot-rolled tube material is defective and has not been eliminated after drawing.
Check and judge	If there are scars on the steel pipe, they should be removed. After the removal, the wall thickness and diameter of the steel pipe do not exceed the negative deviation. It is allowed to exist if the local slight scarring depth does not exceed the negative deviation of the wall thickness.

 

 
4. Cross crack

Defect  name	Cross crack
Defect characteristic	There is continuous or intermittent lateral cracking on the surface of the steel pipe.
Cause	(1) The temperature of the acid solution is too high, the pickling time is too long, and the temperature of the steel pipe is higher than the temperature of the acid solution, causing hydrogen embrittlement. (2) Insufficient heat treatment time or insufficient heating temperature. On the cross section of the steel pipe, the processing stress could not be fully eliminated, causing transverse cracking. (3) When the tube is pulled out, the reduction is too large, exceeding the tensile strength of the metal. (4) The speed of heating the thick-walled steel pipe is too fast, resulting in excessive thermal stress. (5) The thick-walled tube with empty drawn causes inconsistent elongation of the inner and outer walls. (6) Thick-walled tubes are subject to rapid cooling (especially in winter) when heated, causing transverse cracking.
Prevention and elimination methods	(1) Properly execute the operating instructions for pickling and heat treatment technology. (2) Choose the reduction amount reasonably. (3) Heat treatment of thick-walled pipes should be heated evenly.
Check and judge	Check with the naked eye. Transverse crack defects are not allowed. This kind of defect is not allowed to be repaired, because it is not easy to check after repair, and it is impossible to judge whether it is completely removed, so it should be directly scrapped.

 


5.Rowing
 

Defect  name	Rowing
Defect characteristic	The surface of the steel pipe presents a longitudinal linear scratch called a scratch. The length of the track is not the same, the width is not the same, most of them are groove-shaped, and the bottom of the groove can be seen. A scratch is called a scratch on the surface, and a green line is on the surface without damage.
Cause	(1) The inner surface of the draft is not smooth, with cracks or bonded metal. (2) The transition part of the hammer head has corners and wear tools. (3) Under-pickling or residual iron oxide on the capillary. (4) Poor saponification, drawing speed is too fast, the mold is easy to adhere to the metal. (5) The surface of the inner mold is not smooth or bonded with metal. (6) The pipe material has internal lanes, green lines, etc., but it is not cleared after pulling. (7) The mold is damaged, and it is easy to cause blue lines on the surface of the steel pipe. (8) The amount of deformation is too large, the hardness of the mold is not enough, resulting in serious wear of the mold, and scratches are produced during drawing. (9) Uneven annealing and insufficient deformation
Prevention and elimination methods	(1) Improve the surface quality of the extubation mold. (2) The iron oxide skin should be rinsed clean and the coating should be uniform and firm. (3) The transition part of the hammer head should be smooth and without edges and corners. (4) Inspect the surface of the mold and steel pipe frequently, and find the problem and deal with it in time.
Check and judge	Check with the naked eye. General structural pipes and steel pipes used for processing mechanical parts are allowed to exist when the depth of the defect does not exceed the negative deviation of the wall thickness. The steel pipes used for pressure pipes and similar purposes are judged according to the corresponding technical standards.

 


6. Steel pipe shaking pattern

Defect  name	Steel pipe shaking pattern
Defect characteristic	The surface of the steel pipe presents uneven waves or wave marks along the length, arranged one after another. There are partial or full length on the surface of the steel pipe (sometimes only on the inner surface).
Cause	The steel pipe is shaken during the drawing process. The causes of jitter are as follows: (1) Uneven heat treatment performance, excessive tensile strength, work hardening caused by rolling low temperature steel in hot rolling workshop. (2) Dirty washing after pickling, poor lubrication, uneven coating, and jitter during drawing. (3) The tube is too long, and the saponification of the hammerhead part is not sufficient. (4) The core rod is thin, and the core rod is elastically deformed to cause jitter during drawing. (5) The chain vibrates when the starting speed is fast or when it is pulled open.
Prevention and elimination methods	(1) The steel pipe should be fully heat-treated to achieve softening and uniform performance. (2) The surface of the steel pipe should be washed, the coating is uniform and firm, and it is fully saponified. (3) The starting speed should be appropriately slowed down during drawing. (4) Drawing according to the prescribed deformation. (5) Correctly select the core rod size.
Check and judge	Check with the naked eye or touch with hands. The essence of the chattering defects is the uneven longitudinal diameter and uneven wall thickness. A slight trembling pattern that can see traces without feeling can be judged as a qualified product; those that are clearly visible but have a feeling can be judged as a waste product.

 


7. Pull out concave

Defect  name	Pull out concave
Defect characteristic	In the longitudinal direction of the steel pipe, the wall of the pipe is recessed from the outer surface to the inner surface (sometimes there is a protrusion on the inner surface), and its length is irregular.
Cause	(1) When the thin-walled steel pipe is drawn without a head, the diameter reduction is too large. (2) The end of the steel pipe hammer head has edges and corners, and the transition part is wrinkled. (3) Thin wall of the tube material (such as repair point)
Prevention and elimination methods	(1) When drawing thin-walled pipes without a plug, the diameter reduction should be reasonably distributed. (2) The end of the hammer head should be free of edges and wrinkles. (3) For the local defects on the surface of the pipe material, the aspect ratio of the cleaning should be qualified.
Check and judge	Check with the naked eye. Pulling out the concave is a defect that is not allowed, and cannot be repaired. Partial ones are removed, and the entire root is scrapped.

 
 
 
8.Air pull

Defect name	Air pull
Defect characteristic	When there is a plug drawing, because the plug is not given, the wall thickness is over-positive, the outer diameter is slightly smaller, and the inner surface has a ring-shaped step. Mostly at the end of the hammer head
Cause	(1) The mandrel head of the pipe drawing machine is not adjusted to the required position, or the mandrel cannot be pushed to the drawing position due to the bending of the mandrel or the bending of the steel pipe. (2) The chamfering of the head is unreasonable and the head cannot be given. (3) The gap between the pin and tongue of the core rod of the pipe drawing machine is too large. (4) The longitudinal thickness of the pipe material is uneven, and the plug cannot be given. (5) Insufficient power of jack cylinder or failure of plug. (6) There is a large burr in the steel pipe, which is pushed to the end of the hammer head when the head is worn. So you can't give it a head.
Prevention and elimination methods	(1) Correct selection and preparation of extubation tools. (2) Before extubation, the position of each part of the extubation machine should be adjusted correctly. (3) Check the pipe puller frequently to eliminate the back-and-forth movement of the core rod.
Check and judge	Air-drawing defects are not allowed, and the air-drawing parts should be removed.

 

 
9. Folding
 

Defect name	Folding
Defect characteristic	The outer surface of the steel tube is folded in a zigzag shape along the corner or the direction of feeding and rolling.
Cause	(1) The width of the pass is not enough, the roll seam is too small, and ears appear during the rolling process, and then rolled to form a fold. (2) Excessive roll feed or equipment imbalance, causing cone penetration. (3) The upper and lower hole patterns are misaligned, the hole patterns drop meat, and the corners are not suitable. (4) The outer diameter of the pipe material is excessively poor, or the hole pattern is seriously worn. (5) There are serious scratches, cracks and other defects on the surface of the pipe material. (6) The tube material is too deep and the aspect ratio is out of specification.
Prevention and elimination methods	(1) Reasonably design the hole pattern. (2) Correct selection of process parameters (3) Carefully adjust the rolling mill. (6) Carry out the fine material policy, and the unqualified materials will not be put into production.
Check and judge	Folding defects are not allowed to exist, and must be ground. After the ground thickness and diameter do not exceed the negative deviation, it is judged as qualified.

 


10. Cracking

Defect name	Cracking
Defect characteristic	The steel pipe exhibits longitudinal cracking through the pipe wall. There are full-length cracks and local cracks.
Cause	(1) The amount of reduction is too large, and the pipe material is hardened and hardened, which is easy to produce when there is no plug drawing. (2) Improper heat treatment results in inconsistent plasticity of the pipe material, or work hardening is not completely eliminated. (3) The steel pipe was not heat treated in time after drawing. (4) Too many consecutive drawing passes and severe work hardening. (5) Steel types with low plasticity (high carbon content and many alloying elements) are more prone to cracking.
Prevention and elimination methods	(1) Reasonable layout and drawing. (2) Alloy steel pipes and continuous drawn steel pipes should be heat treated in time after drawing. (3) Perform heat treatment according to the process requirements, and the cooling rate should be reasonably controlled to make its performance evenly eliminate work hardening.
Check and judge	Steel pipe cracking is a waste product, and local cracking can be removed.

 
 

10. Over pickled

Defect name	Over pickled
Defect characteristic	The surface of the steel pipe showed small longitudinal cracks or needle-like holes and sesame-shaped cracks. The degree of severity is different, the inner surface is more serious than the outer surface, some are in a band shape, and some are only on the end or inner surface.
Cause	(1) The thickness of the scale on the surface of the steel pipe is uneven, and the degree of looseness and firmness are different. (2) The pickling time is too long. (3) The concentration of acid solution is low, and the content of ferrous sulfate is high. When the temperature of the acid solution is high, it is easy to over pickle. (4) The concentration of acid solution is uneven, the solution does not flow, and the corrosion is uneven. (5) The steel pipe is not cleaned after pickling. (6) The steel pipes are parked for too long in the pickling room.
Prevention and elimination methods	(1) Properly execute the operation rules of pickling technology. (2) Master the heat treatment operation to prevent uneven thickness of the scale on the surface of the steel pipe. (3) Strictly implement diligent lifting, diligent viewing and diligent checking to prevent over pickling.
Check and judge	Checking with the naked eye, over-pickling defects have large-scale pinhole-like corrosion on the surface of the steel pipe and can produce hydrogen embrittlement, so they are not allowed to exist.

 


12. Scratches

Defect name	Scratches
Defect characteristic	There is mechanical damage on the surface of the steel pipe. Generally, the depth of the defect is shallow, and the bottom is visible to the naked eye.
Cause	(1) During the transportation process, the steel pipe was scratched by steel wire racks and other machinery. (2) In the process of straightening, the steel pipe produces scratches in the catheter. (3) There are burrs on the end of the pipe or the heads collide with each other to produce scratches. (4) When the steel pipe is straightened, it is scratched by the roller edge due to the excessive bending of the steel pipe or improper hole pattern. (5) The roller of the pipe cutter is bad, which may cause scratches.
Prevention and elimination methods	(1) To prevent bumps when transporting steel pipes, keep the storage places such as material racks and inclined grate bars clean. (2) Remove the burr from the tube end. (3) Correctly adjust the straightening machine to eliminate the hidden danger of abrasions in the straightening process (4) Improve the spreader to prevent the steel rope from hanging the pipe directly.
Check and judge	Check with the naked eye, the edges of the scratches are more rounded, and those with a smaller depth and width are allowed to exist. If the edge of the defect has sharp corners or a relatively large depth and width, it should be reground. Those who do not exceed the negative deviation of the diameter and wall thickness after refining shall be qualified.

 


13. Pit
 

Defect name	Pit
Defect characteristic	There are local depressions of different sizes on the surface of the steel pipe. They are arranged in a spiral shape or periodically, and some are distributed locally or in clusters.
Cause	(1) Oxide scale or other contaminants adhere to the surface of the steel pipe, and then peel off after pressing into the surface during drawing. (2) Before hot-rolling and reducing diameter, when the steel pipe is reheated, it is pitted by the dirt at the bottom of the furnace, which is not eliminated after cold drawing. (3) Straightening rollers bond metal. (4) Soap liquid, lubricating oil, emulsion, etc. are not clean. (5) The steel pipe burr is not clean. (6) Tool bonding metal. Tools such as mandrels and receivers are not cleaned properly. (7) There are plugs and split heads at the end of the pipe (the pipe size is too thin, the heat treatment is not good, the hole gap is not adjusted properly, the core head is unreasonable, etc.) (8) The place where the pipe material is parked is not clean, and the inner surface enters the dirt. (9) The coating is not strong, the copper layer is dropped, and the oil is not cleaned after rolling.
Prevention and elimination methods	(1) Wash and flush the scale and dirt on the surface of the steel pipe. (2) Regularly sharpen the surface of the straightening roller to prevent metal adhesion. (3) Wipe the core head, core rod, and take over during rolling. (4) Grind the end burrs of the steel pipe before rolling.
Check and judge	(1) Visual inspection. Local pits with a depth not exceeding the negative deviation of the wall thickness are allowed to exist. Large pits should be rejected. (2) When the depth of the defect cannot be determined by the naked eye, the deeper pit can be partially repaired, and then the depth can be measured by a ruler. If the diameter and wall thickness do not exceed the negative deviation, it is judged as qualified.

 


14. Concave

Defect name	Concave
Defect characteristic	The steel pipe presents a relatively smooth spiral convex and concave. Spiral marks sometimes with sharp edges and corners. Continuous or discontinuous, local or full length appears on the surface of the steel pipe.
Cause	(1) The angle of the straightening roller is small, or the amount of reduction is not adjusted correctly. (2) Straightening roller is seriously worn. (3) Before the straightening, the steel pipe is bent too much, or the head and tail are bent too much. (4) The design of the straightening roller is unreasonable. (5) Out-of-tolerance of pipe material is easy to cause concave correction.
Prevention and elimination methods	(1) The straightening machine should be overhauled frequently, and the straightening roller should be replaced in time. (2) Adjust the straightener correctly. (3) Reasonably design the straightening roller shape.
Check and judge	Check with the naked eye. The outer surface is smooth without edges and corners, and the inner surface has no obvious protrusions, which is allowed to exist. Those with obvious protrusions on the inner surface or sharp corners on the outer surface shall be rejected.

 
 
 
 
15. Pockmarks (pocket noodles, pockmarks)

Defect name	Pockmarks (pocket noodles, pockmarks)
Defect characteristic	Thin spot-shaped pits on the surface of the steel pipe. Appears locally or continuously on the surface of the steel pipe.
Cause	(1) The iron oxide skin is too thick due to heat treatment, and the iron oxide skin is pressed in during straightening, and pitting occurs after peeling. (2) After pickling, iron oxide skin or saponification (lubricating) agent remains dirty, and pitting occurs after extraction. (3) After pickling the steel pipe, the parking time is too long to produce rust, causing pitting. There is oil stain on the surface of the pipe, which coalesces into oil spots during heat treatment and burns into numb surface when heated.
Prevention and elimination methods	(1) Before heat treatment, remove the oil stains on the surface of the steel pipe, and correctly execute the heat treatment system when installing the furnace. (2) Do a good job in civilized production, insist on rapid turnover, and ensure the cleanliness of each process. (3) Pickling can not be too acidic, not underwashing, and drawn out in time after pickling.
Check and judge	For general structural steel pipes and steel pipes used for processing mechanical parts, slight pitting is allowed. For the steel pipes in the boiler industry, fluid pipelines and other similar purposes, due to the defects of the pitted surface that affect the process performance and easily cause pitting corrosion, they should be checked and accepted according to the corresponding technical conditions.

 
 

16. Bending

Defect name	Bending
Defect characteristic	The steel pipe is not straight along the length.
Cause	(1) During the heat treatment of steel pipes, the heating temperature is uneven. (2) The straightener adjustment is abnormal. (3) Uneven performance of steel pipes. (4) The tube bend is too large, and there is residual bend after straightening.
Prevention and elimination methods	(1) Reasonable distribution of diameter reduction during drawing. (2) Heat evenly. (3) Correctly adjust the straightener.
Check and judge	Bending defects are common defects. Use a one-meter square ruler to lean against the maximum bending point of the steel pipe. Use a feeler gauge to measure the maximum radius. If the bending degree does not exceed the standard, it is judged as qualified; if it exceeds the standard, it can be re-straightened; Should be removed.