Resolving Uneven Ends in Titanium Seamless Tube Rolling

In the rolling process of titanium seamless tubes, the reciprocating (Pilger) cold rolling mill is commonly used. The process involves multiple passes using two-roll (LG) and multi-roll (LD) mills. During deformation, the titanium tubes undergo wall reduction and diameter reduction, requiring 5 to 10 passes to achieve the desired size. While the cold rolling machine is effective for large diameter and wall reduction, it may result in lower dimensional accuracy and issues like cracking and uneven surfaces at the tube ends. This article analyzes the causes of unevenness and proposes effective measures for resolution.

One of the primary reasons for unevenness is identified as the connection between the core rod and the small car, where excessive key clearance causes significant displacement of the core rod. Pure titanium tubes, after rough rolling, typically undergo multiple precision adjustments within one pass. However, in a specific batch, severe undulations, up to 70mm in length, occurred at the tube ends, accounting for 1% of the tube length. This abnormality in wall thickness fluctuation, with a significant difference between protrusions and depressions, was beyond the typical deviation of 0.05-0.10mm. Therefore, equipment or tooling-related issues were suspected.

Wall thickness unevenness could be attributed to factors such as the installation of gears and racks, mold alignment, and mold opening. Measurements indicated that the opening degree difference between upper and lower molds was within the design range. However, discrepancies in the alignment of gears and racks were identified. Despite proper mold installation, deviations in wall thickness may still be due to equipment-related factors, such as feed quantity, rotation angle, and coordination of movements.

Investigations revealed that during the rolling process, the axial movement of the core rod relative to the coaxial tube was excessive, reaching 10mm, violating the requirement of less than 0.5mm. The key clearance at the connection between the core rod and the small car was measured to be 20mm, exceeding the 12mm limit. The excessive key clearance caused the core rod to shift significantly during tube advancement, affecting the accuracy of core rod positioning during rolling.

Adjusting the key clearance at the connection between the core rod and the small car eliminated the unevenness observed at the tube ends. This adjustment, addressing issues related to equipment and tooling, successfully resolved the problem of unevenness at the tube ends.