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What Causes the Defects of Hot-rolled Seamless Steel Pipes?


Hot-rolled seamless steel pipe is a round, square, and rectangular steel with a hollow section and no joints on the periphery. When the bending and torsion strength is the same, the weight is lighter. It is economical cross-section steel. What defects are prone to appear in the production process of steel pipes?


1) Cracks (also known as cracks)


Features: The inner and outer surfaces of seamless steel pipes are linear or spiral cracks, some are mesh-shaped and both ends and bottom of the cracks are sharp.


1. Poor steel quality, with subcutaneous pores and subcutaneous inclusions.

2. Improper heating of the tube billet.

3. The deformation pressure is too large.



2) Hairline


Features: continuous or discontinuous hair-like fine lines appear on the outer surface of the seamless steel pipe. Most of the spiral direction is opposite to the direction of rotation of the punch, with a large pitch, and some are similar to a straight line.


1. Poor steel, with subcutaneous pores and subcutaneous inclusions.

2. The surface of the tube blank is incompletely cleaned.



3) Inward fold (commonly known as inner fold)


Features: straight or spiral zigzag defects on the inner surface of the steel pipe. For high-alloy steel pipes, such defects are irregular blocks and distributed at the entrance. If the fold is caused by centering, it is usually half a circle to a circle, and it is on the head.


1. The amount of pressing down the front of the head is too large.

2. Serious headwear.

3. During the piercing process, the metal in the central part of the blank bears strong alternating stress, so the inner wall of the waste tube is prone to inward bending during rolling.

4. The inward folding at the entrance of the high-alloy steel pipe is caused by uneven force on the roll during piercing.



4) Outer folding


Characteristic: the outer surface of the seamless steel pipe is spirally folded, the spiral direction is opposite to the spiral direction of the waste pipe on the punching machine, and the pitch is relatively large.


1. Defects such as cracks or ears remain on the surface of the tube blank.

2. Poor steel quality, inclusions, or serious looseness.



5) Rolling and folding


Characteristics: there are regular folding defects on the outer surface of the seamless steel pipe.


It is mainly caused by damage to the roll of the piercing machine or the roll of the tube mill.



6) Bending


Characteristics: during rolling, the metal enters the gap between the rolls or the tube loses its stability, causing it to form twists and turns. After sizing, the surface of the steel tube shows local convex and concave wrinkles.


1. The outlet pipe diameter of the equalizing machine is larger than the hole type of the fixed and reducing machine.

2. Incorrect adjustment, misalignment of rolls, inconsistent center lines, etc.

3. The sizing frame is installed upside down.


7) Straight inward folding


Features: The inner surface of the seamless steel pipe is symmetrical or a single straight line folding, distributed over the entire length or part of the steel pipe.


1. The head is severely worn or stuck to the metal.

2. The outer diameter of the capillary tube is larger than the width of the grooved hole, the inner diameter is too large or the tube wall is too thin.

3. Unreasonable distribution of reduction of piercing machine and tube rolling machine.

4. Before the second pass of rolling, the 90°overturning was not completed.

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