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Reasons and Improvement Methods of Internal Threads in Seamless Steel Pipes

Date:2022-06-20    keywords: seamless steel pipe, internal threads
Internal thread is a defect that often occurs in skew rolling mills, and the form is full-length and local. The internal thread defect is the trace left by the deformation of the seamless steel pipe every half-turn. When using fixed mandrels and semi-floating mandrels, sometimes internal thread defects are caused by mandrel wear. The internal thread defects of the skew rolling mill are as difficult to avoid as the straight defects in the automatic tube rolling mill, and it is important to reduce them to a minimum. The reasons for the formation of internal threads of seamless steel pipes are generally considered to be analyzed from the following aspects:

1) The surface state of the mandrel, such as mandrel wear, deformation, sticking to metal and iron oxide scale, etc. Because the mandrel participates in the deformation, the defects on the surface of the mandrel must be reflected on the inner wall of the seamless steel pipe, especially when the fixed mandrel and the semi-floating mandrel are used;

 seamless steel pipe

2) Whether the length of the rolling section in the roll design is sufficient. The role of the rolling section is to equalize the wall. If the internal thread of the seamless steel pipe wall is fully developed in this area, it can be eliminated. However, it is easy to produce internal thread defects when it is subjected to a large amount of deformation in the rolling section;

3) The movement state of the mandrel. When using tension mandrel and fully floating mandrel, the internal thread is less and lighter, especially when using tension mandrel, because the mandrel and the inner surface of seamless steel pipe have relatively large relative sliding, and the axial movement speed of the mandrel is higher than that of metal The axial movement speed is high, so as to eliminate the internal thread, the inner surface of the seamless steel pipe for oil cracking is very smooth, and even no trace of the internal thread can be seen. When a fixed mandrel and a semi-floating mandrel are used, since the mandrel is fixed on the ejector, the beating of the mandrel-ejector system promotes the generation of internal threads. In addition, the mandrel jumps violently in the deformation zone, and it is also prone to internal thread defects;
4) When using a fixed mandrel and a semi-floating mandrel, the secondary iron oxide scale formed on the inner surface of the seamless steel pipe capillary accumulates in the deformation zone of the mandrel and is pressed into the pipe wall, and will leave a spiral trace after falling off ( or Makeng);

5) The process parameters of seamless steel pipe should be selected reasonably. Generally, large elongation coefficient, low rolling temperature, large advance angle, and high roll speed are prone to cause heavier internal thread defects. The actual measurement of the internal thread depth of the two-roll skew rolling machine is generally less than 0.1 mm (except for the fixed mandrel). According to foreign reports, the height of the internal thread of the three-roll skew rolling machine is generally in the range of 0.025 to 0.25 mm.

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