Common defects of cold drawn seamless steel pipe
Common defects of cold drawn seamless steel pipe
Causes of common defect features of cold drawn seamless steel tubes and methods for their prevention and elimination
1. Fold
After the seamless steel pipe is drawn, the inner and outer surfaces of the steel pipe are folded in a straight line or a spiral direction, and partially or entirely appear on the steel pipe.
Causes: There are folds or inclusions on the surface of the pipe material, there are serious scratches and cracks, and the corner or aspect ratio (H/b) of the pipe material is not sufficient.
Prevention and elimination methods: strictly control the quality of piercing and hot rolling.
2. Out of size (including out of wall thickness, uneven wall thickness, out of diameter, and eccentric ellipse)
The diameter of the seamless steel pipe exceeds the standard deviation. On the same section, the pipe wall is thin on one side and thick on the other, with different diameters. The difference between the long and short axes exceeds the standard.
Causes:
1. Improper selection of drawing mold, or improper adjustment of mandrel (inner mold).
2. The design and manufacture of the inner and outer molds are unreasonable or severely worn, or the hardness is not enough to cause deformation and wear.
3. Long heat treatment time, high temperature, or uneven heat treatment performance.
4. Improper control of the law of increasing and decreasing walls.
5. Unsuccessful preparation of drawing table.
6. The steel pipe is flattened during straightening, resulting in a large error.
Prevention and elimination methods:
1. Correctly design and select the pipe pulling mold.
2. Perform heat treatment system correctly and heat evenly.
3. Correctly adjust the straightener, and often calibrate the components and measuring tools of the pipe drawing machine.
4. Master the rule of increasing and decreasing wall of different steel types and different specifications of steel pipes.
5. Correctly and reasonably prepare the drawing table.
6. The ovality can be re-straightened, and the local ovality can be removed.
Three, rowing
The longitudinal straight scratches on the surface of seamless steel tubes are called scribe lanes. The length of the scribe lanes is different, and the width is not the same.
Causes:
1. The draft surface is not smooth, with cracks or bonded metal.
2. The excessive part of the hammer head has corners and the tools are worn.
3. Under-pickling or residual iron oxide on the capillary.
4. Improper operation of phosphating and saponification processes.
5. The inner and outer molds are damaged or worn seriously.
6. Uneven annealing in the middle and insufficient deformation.
Prevention and elimination methods:
1. Improve the surface quality of the extubation mold.
2. After pickling the steel pipe, the iron oxide skin should be rinsed clean.
3. The excessive part of the hammer head should be smooth and without edges.
4. Inspect the surface of the mold and steel pipe frequently, and deal with any problems found in time.
Four, bucket pattern
The surface of the seamless steel pipe is unevenly arranged along the length of the ring-shaped waves or the waves are arranged one by one, and the local or full length appears on the inner and outer surfaces of the steel pipe.
Causes:
1. Uneven performance after heat treatment, low temperature steel caused uneven performance during hot rolling.
2. Rinsing after pickling is not clean, and poor phosphating leads to uneven saponification.
3. The core rod is thin, and the core rod is elastically deformed to cause jitter during drawing.
4. The shape of the draft is unreasonable, the entrance cone angle is too large, so that the contact area of the steel pipe and the die hole is too small, so that the drawing deformation is unstable and jitters.
Prevention and elimination methods:
1. Perform heat treatment according to the requirements of the operation regulations to achieve uniform softening performance
2. The quality of pickling, phosphating and saponification should be well controlled.
3. Drawing according to the prescribed deformation.
4. Choose the core rod size correctly.
Five, pull out concave
In the longitudinal direction of the seamless steel pipe, the wall of the pipe is indented in strips, and its length is irregular.
Causes:
1. When drawing thin-walled steel pipes without mandrel (empty drawing), the diameter reduction is too large.
2. The end of the steel pipe hammer head has corners or excessive parts are wrinkled and the amount of deformation is too large.
3. The pipe material is partially thin (such as grinding points).
Prevention and elimination methods:
1. When pulling out thin-walled tubes, the diameter reduction should be reasonably distributed.
2. There should be no edges and wrinkles at the end of the hammer head.
3. Clean the local defects on the surface of the pipe material.
Six. Cracking (longitudinal splitting)
The seamless steel pipe exhibits a longitudinal crack that penetrates the wall of the pipe, and has a long partial crack.
Causes:
1. The amount of reduction is too large, the number of consecutive drawing passes is too much, the pipe material is hardened and hardened, especially the empty drawing.
2. Improper heat treatment, resulting 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. Steel types with low plasticity are more prone to cracking.
Prevention and elimination methods:
1. Reasonably compile and draw tables.
2. Alloy steel pipes and continuous drawn steel pipes should be heat-treated in time.
3. Perform heat treatment according to the process requirements, and the cooling rate should be reasonably controlled to make its performance uniform and eliminate work hardening.
Seven, over pickling (sulfuric acid)
The surface of the seamless steel tube shows holes and sesame-shaped cracks with different degrees of severity. Generally, the outer surface is more serious than the inner surface.
Causes:
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 acid concentration is low and the content of ferrous sulfate is high. It is easy to over pickle when the pickling temperature is too high.
4. The concentration of acid solution is uneven, the solution does not flow, and the corrosion is uneven.
Prevention and elimination methods:
1. Properly execute the operating instructions for 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 inspections to prevent over pickling.
Eight, pits (pit noodles, pits)
The surface of the steel pipe is formed with thin dot-shaped pits, which partially or continuously appear on the surface of the steel pipe.
Causes:
1. The oxide scale is too thick (overburned) during heat treatment. When the iron oxide scale is pressed or peeled off during straightening, pitting occurs.
2. There are dirt in the residual iron oxide skin or saponification liquid after pickling, and pitting occurs after pulling.
3. After pickling the pipe, it will be parked for too long and will rust and cause numbness.
4. There is oil stain on the surface of the steel pipe, coalescing into oil spots during heat treatment and burning into numb surface when heating.
Prevention and elimination methods:
1. The heat treatment system is correctly implemented according to the requirements of the steel type during furnace installation.
2. Adhere to rapid turnover.
3. Pickling can not be washed, not underwashed, drawn out in time after pickling.
Nine, drawing fracture
During drawing, there is a sharp groan or intermittent "hoop..." sound, the internal and external molds are brushed, and the internal and external surface of the steel pipe is scratched. The lack of lubrication effect and the increased load during the drawing of the machine cause the steel pipe to break laterally.
Causes:
1. Pickling and carbonization (hydrogen embrittlement), causing the steel pipe to be drawn and broken.
2. Phosphating is not up to standard. Excessive phosphating (too old) will cause the phosphating film to fall off and lose the purpose of lubrication.
3. If the phosphating is too light, the lubricating can not form adhesion on the surface of the steel pipe, that is, the intermittent regular sound of "t...
4. The lubricating temperature is too low or too high, the concentration is insufficient, the lubricant is insufficiently adsorbed or the lubricant is acidified to lose the lubricating effect.
Prevention and elimination methods:
1. Strictly control the concentration, temperature and time of pickling, wash and change water frequently.
2. Always check the total acidity and free acidity of the phosphating solution to ensure that the phosphating acid ratio is normal.
3. Control the lubrication temperature and concentration, keep the working fluid clean and avoid acidification.
4. Check the deformation and whether the internal and external molds meet the standard.
