Seamless pipe internal thread

Posted by zora li on November 22nd, 2022

Seamless pipe internal thread

Internal thread is a common defect of skew rolling mill in the form of full-length and partial. The internal thread defect is the trace left by the deformation of each half circle of the seamless steel pipe. When using fixed and semi-floating mandrels, sometimes female thread defects are caused by mandrel wear. The internal thread defects of skew rolling mills are as unavoidable as the straight line defects of automatic rolling mills. It is important to minimize them. 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, metal adhesion, scaling, etc. Since 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 the use of fixed mandrels and semi-floating mandrels;

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

3) The movement state of the mandrel. When using tension mandrel and fully floating mandrel, the internal thread is small and light, especially when using tension mandrel, due to the relatively large relative sliding between the inner rod and the inner surface of the seamless steel pipe, the axial movement speed of the mandrel is higher than that of the metal rod. . Axial movement speed to eliminate internal threads, the inner surface of the seamless steel pipe that rolls out cracks is so smooth that even the internal threads cannot be seen. When using fixed mandrels and semi-floating mandrels, the bouncing of the mandrel and ram system results in internal threads as the mandrel is fixed to the ram. In addition, the mandrel jumps strongly 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 is piled into the pipe wall during rolling in the deformation zone of the mandrel, leaving a spiral mark (or as a pit) after falling off.

5) Reasonable selection of seamless steel pipe process parameters, generally large elongation coefficient, low rolling temperature, large advance angle, and high rolling speed are likely to cause heavier internal thread defects. In fact, the depth of the internal thread of the two-roll cross-rolling stretching machine is generally less than 0.1mm (except for the fixed mandrel). According to foreign reports, the height of the internal thread of the three-roll cross rolling mill is generally in the range of 0.025 to 0.25 mm.

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