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Classification and Causes of Quality Defects of Seamless Steel Pipes

Date:2022-02-17    keywords:quality defects, seamless steel pipes

1. Surface quality of carbon seamless steel pipe: requirements for surface smoothness


a. Severe defects of seamless steel pipes: cracks, internal folding, external folding, rolling, separation, scarring, concave pull, convex hull, etc.

b. General defects of seamless steel pipes: pits, blue lines, scratches, bumps, slight inner and outer straights, roll marks, etc.


seamless steel pipe


2. The reasons for the defects of seamless steel pipes


① Due to surface defects or internal defects of the tube blank.
② In the production process, such as incorrect design of rolling process parameters, unsmooth die surface, poor lubrication conditions, and unreasonable pass design and adjustment.
③ In the process of heating rolling, heat treatment and straightening of the billet (seamless steel pipe), if the heating temperature is not properly controlled, the deformation is uneven, the heating and cooling rate is unreasonable or the straightening deformation is too large, resulting in excessive residual stress , then it may also cause surface cracks in the seamless steel pipe.

3. Steel pipe geometric accuracy and shape

a. The accuracy of the outer diameter of the steel pipe: depends on the method of determining (reducing) the diameter, the operation of the equipment, and the process system.
   Outer diameter tolerance δ=(D-Di)/Di ×100%
   D: maximum or minimum outer diameter mm, Di: nominal outer diameter mm

b. Steel tube wall thickness accuracy: It is related to the heating quality of the tube blank, the process design parameters and adjustment parameters of each deformation process, the quality of the tool and its lubrication quality.
   Wall thickness tolerance: ρ=(S-Si)/Si×100%
   S: maximum or minimum wall thickness in cross section, Si: nominal wall thickness in mm

c. Steel pipe ovality: It indicates the degree of out-of-roundness of the steel pipe.
d. Steel pipe length: normal length, fixed (multiple) ruler length, length allowable deviation
e. Steel pipe bending degree: indicates the radius of the steel tube: the bending degree of the steel pipe length per meter, the bending degree of the whole length of the steel pipe
f. The inclination of the end face of the steel pipe: indicates the inclination of the end face of the steel pipe and the cross section of the steel pipe
g. Bevel angle and blunt edge of steel pipe end face

4. Production process

①The main production process of hot-rolled seamless steel pipe (△main inspection process):
Tube blank preparation and inspection△→tube blank heating→piercing→tube rolling→barren tube reheating→diameter determination (reduction)→heat treatment△→finished pipe straightening→finishing→inspection△(non-destructive, physical and chemical, Taiwan inspection) →in library

②The main production process of cold-rolled (drawn) seamless steel pipe:

Billet preparation→pickling lubrication→cold rolling (drawing)→heat treatment→straightening→finishing→inspection


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