Mills for Reducing Tube Thickness and Sizing | Metallurgy

The following points highlight the six main mills for reducing tube thickness and sizing. The mills are:- 1. Pilger Mill 2. Mandrel Mill 3. Plug Mill 4. Reeling Process 5. Assel Mill 6. Stretch Reducing Mill.

1. Pilger Mill:

The tubular shells produced in piercing process are further processed. Pilger mill is commonly used after rotary piercing. This process consists of rolling in two contoured rolls. The shape of pilgering rolls is shown in Fig. 12.19.

Figure 12.20(a-d) shows the sequence of events in the process. After the thick tube is fed forward in Fig. 12.20(a), the pilgering rolls take bite in Fig. 12.20 (b), the rotation of the roll reduces the thickness of the small length of tube by pushing a wave of material ahead of it. During rolling the rotation of rolls pushes back the tube along with the mandrel.

The back motion is utilized to compress an air cylinder and thus store the energy which is utilized for pushing back the tube when the rolls clear the stock. After the tube is cleared by pilgering rolls it is also rotated by 90° and is fed forward by same length as previously. The process is repeated. Thus it is rolled again.

The shell gets an elongation of 5 to 10 and cross-section is reduced by 80 to 90 percent. After pilgering, the tube is rolled in sizing mill. The sizing mill consists of three or more stands each having two or three rolls. After sizing, tube is transferred to cooling bed and then to finishing workshops.

2. Mandrel Mill:

Figure 12.21 shows another sequence of processes which follow the piercing. In this figure the second process is rolling in a mandrel mill which is suitable for diameters from 20 to 150 mm. The process consists of heating the billet which is then put through the piercing mill. The shell so formed is rolled in a mandrel mill. The process is used for mass production runs of small size tubes up to 125 mm diameter.

The process is also suitable for continuous runs of tube sizes from 60 mm to a 180 mm outside diameter. A continuous mandrel mill consists of 6 to 10 stands which are placed in tandem, each stand is inclined at 45 degrees with the horizontal plane and is offset by 90 degrees with respect to its immediate neighbor so that the tube is reduced all along the circumference.

After the piercing operation the mandrel bar is inserted into the shell to a certain length and the assembly is inserted into mandrel rolling mill. After rolling the mandrel is pulled out of the shell which is reheated and fed to stretch reducing mill or sizing mill. Modern mandrel mills are capable of manufacturing tube lengths up to 30 meter.

3. Plug Mill:

Plug mill (Fig. 12.22) is still the most commonly used in seamless tube manufacturing plants, because plug mill allows the complete range and thickness to be rolled. The sequence of various operations carried on a plug mill are illustrated in Fig. 12.22. The plug mill constitutes a two high main stand and a two high stripper stand. To start the process the shell obtained from piercing mill is pushed with the help of air cylinder into the gap between the rolls.

On the opposite side there is a mandrel on which a plug is mounted from the tube entry side. Once the tube is gripped by rolls there is no need of further push. The tube is rolled over the plug. At the end of the process the plug is removed and tube is returned with the help of stripper rolls.

It is turned through 90 degrees and a second pass is given with a new plug which is 1-3mm larger in diameter. In the old plug mills the rolls carry a number of grooves and the tube can be given a second or third pass after changing the groove and the plug. Besides a number of sizes can be rolled.

Most of the modern plug mills are single groove type because with better rolling-stand design it is easier to change the rolls. On automated roll changer it takes less than 10 minutes to change to a new set of rolls. One of the advantages of single groove is simple feed table with low maintenance. Besides the bearings are equally loaded which is not the case in multi-groove stands.

The billets after passing through plug mills are passed through reelers in the same heat. The major benefit of faster plug mill is saving on heat energy because more processes may be carried out in the same heat.

4. Reeling Process:

After rolling the tube to desired thickness it is transferred to reeling mill. The reeling stand (Fig. 12.23) which consists of two barrel shaped rolls, also roll the tube on a mandrel. The modern reelers are more powerful and the take part of load of plug mills. The thickness decreases a bit while diameter may slightly increase. The tube gets more rounded and smoothened. After this the tube goes through sizing mill followed by finishing operations.

5. Assel Mill:

Assel mill represent the cross rolling technology. Three shaped rolls place at equal angles with respect the work piece, roll the pierced tube between them. The rolls may be forced on to the job with the help of hydraulic cylinders.

Shafts may also be rolled by similar technology. Assel tube rolling mill consists of rotary furnace, piercing mill followed by Assel mill (Fig. 12.24). The blanks are heated again and rolled through stretch reducing mill. A rotary sizer (Fig. 12.25) may be used in place of stretch reducer. The process is used to manufacture medium and heavy wall tubes.

6. Stretch Reducing Mill:

In stretch reducing no internal mandrel is used. The mill may consist of 24 to 30 stands placed in tandem and tube is rolled continuously through all the stands as shown in Fig. 12.26(a). Each stand has separate speed control system. The modern stretch reducers have three rolls placed symmetrically around the circumference. Figure 12.26(b) shows the arrangement of rolls in different stands. By adjusting the inter-stand tension it is possible to reduce the thickness as well.