1) uneven wall thickness of the hollow

There are seven main reasons causing uneven wall thickness of the hollow pipe (uneven heating of billet, deviation of centering hole, asymmetry of L&R guide disk, bending of plug bar, improper rolling line, small opening of centering Roll and improper adjustment etc.).

a) uneven heating of billet

The non-uniform thickness of hollow wall caused by uneven heating of billet is easier and intuitive be found. The effective method of prevention is to perform different heating methods according to different materials.

b) the small open of three-roll centering device

This value indicates the distance between the centering roller and the center of the plug bar when the hollow pass through the centering roller. This value is also different according to different pass and pass specifications so the value needs constant adjustment. The theoretical requirement is the diameter of the inner circle is equal to the outer diameter of the hollow +10 ~ 12MM. However, only measure distance between the plug bar and three rollers during production. Due to mechanical assembly accuracy and adjustment reasons, this value is also changing, especially the small open values of the 1/2/3-roll centering must be measured frequently to prevent changes.

C) skewed rolling line

The standard of line skewed is to extend the straight plug bar between the upper and lower rollers, the centering roller is locked, measure the distance between the two rollers, if distance is different indicates the line irregularity, first consider whether the roll position is equal. If equal, it is necessary to consider whether the rolling line of three-roll centering is in a line. Sometimes, when the three roll centering is small open, the values of the three rolls are equal, maybe the rolling line is skewed, need to adjust the Central Line of stand one or two. 

2) scarring on the inner surface

For prevent air enter the hollow tube and oxidation occurs, a lot of units set up the deoxidization system to blow borax into the hollow tube. High-temperature borax can make the oxidized iron sheet fall off, and then use nitrogen to blow away the residue together with the iron sheet.

3) folds

The hole cavity is formed in advance during the perforation process, and the metal in the hole cavity is oxidized which will form internal folding because of cannot be welded during the deformation process. There is mainly centering internal folding at the front end of the tube due to centering. The reason is that centering punches are cracked and concave edges are formed on the inner surface of centering holes. Some include internal fold formed by rupture and oxidation of the pore cavity; Large tawny scale-like folds formed due to high heating temperature (even overheating) or excessive perforation temperature rise; There is a small scale inner folding due to low heating temperature, difficulty in biting, and increasing the amount of pressure in front of the plug. An internal spiral wound by the use of a broken plug or binding metal.