Corso di Studi di Fabbricazione

Transcription

1 Corso di Studi di Fabbricazione 3b Richiami dei processi tecnologici di trasformazione TUBE ROLLING 1 2 TUBE ROLLING Rolling allows to obtain seamless tubes that are strong and reliable. Altenatively, tubes can be produced as follows: from a stock strip, bending it along the axis and welding it lenghtwise; through extrusion (also of steel). 2

2 3 SEAMLESS TUBES AND WELDED TUBES 3 4 APPLICATIONS FOR SEAMLESS TUBES Seamless tubes are used in fields requiring a high integrity of the tube: for example in civil construction, in order to reduce the instability due to combined bending and compressive stress reduce the onset of localized corrosion satisfy aesthetic needs... 4

3 5 TUBE ROLLING Or in plants with pressure fluids, in order to match high mechanical characteristics reduce the onset of localized corrosion TUBE ROLLING The process of tube rolling can be divided into three phases: production of the hollow piece: the billet is transformed in a hollow shape with limited length and high wall thickness. This is a hot process. stretching: the tube is made directly from the hollow piece, by stretching it and reducing its wall thickness. Stretching also is a hot process. 6

4 7 TUBE ROLLING The third phase is: finishing: the tube gets to its final dimensions through rolling (hot or cold process) or drawing (hot process). If necessary, it is also cold-straightened. 7 8 The production of hollow pieces takes place generally through a MANNESMANN Mill (or oblique rolling). Generally, the raw piece is a billet but it can be also a continuous casting. However, the production process of hollow pieces is discontinuous (billets up to about 5 meters). The billet is heated up to C and taken to the mill. 8

5 9 The rolls are shaped like two cones joined along the longer base and have skew axes. The two rolls are set into rotation, the billet is put between them and kept in place by opportune rolls or lateral guides (not visible here). Also a central mandrel is present. 9 0 The action of friction T between the rolls and the billet can be resolved into an axial load (A) and a tangential load (R). The first generates the translation of the billet, while the second generates rotation: the billet moves forward with a helicoidal motion. A T sen R T cos Generally: 10

6 1 While moving forward, the billet meets the mandrel that is between the rolls and tears the central material to generate the tube. The tearing action is actually supported by the particular stress state generated by the two rolls Consider the stress state in a generic section subject to the radial load P generated by the contact with the rolls: dq A D T O B' C' P P T A 1 D A' B 2 D' C B' C' Consider now some material elements as a circular ring section viewed within an angle dq. You see as the radial stress on the outer element is a tensile stress on the inner surface (centripetal) which balances the circumferencial load generated by P. 12

7 3 The adjacent element nearest to the center is stressed on the outer surface by a radial tensile load (centrifugal). To reach a balance, the stress on the inner surface must be greater since the surface is smaller and the horizontal component of the circumferencial load is added. Going on towards the center, the central element is subject to a tensile stress in horizontal direction and to a compressive stress in vertical direction. Therefore, the material is subject to a tensile-compressive stress cycle every quarter turn. The material is quickly torn and the P A B madrel shapes T the hole. dq A D O B' C' P T 1 D A' 2 D' C B' C' 13 4 To summarize Because of the central mandrel, the process is discontinuous: hollow shapes are produced, from which the central mandrel must be extracted. 14

8 5 HOLLOW PIECE PRODUCTION: Billet Piercing Sometimes, before getting to the Mannesmann mill, a blind hole is done on one end of the billet to facilitate the start of the processing and the piercing of the billet The hollow piece you get must be stretched and eventually reduced. The diameter is reduced and certainly the wall thickness is also reduced. The hollow piece is then taken to the next station. 16

9 7 STRETCHING: Pilgrim Mill At this point, the hollow piece is generally processed through a stretching mill, named Pilgrim Mill, which shapes the hollow piece as an actual tube. The mill consists of two rolls and a feed system. A mandrel is put inside the hollow piece and is continuously fed (from right to left, as shown in the figure). The rolls counter-rotate and have a particular geometry. A B N 17 8 STRETCHING: Pilgrim Mill For a section of the rotation arc, the two rolls aren't in contact with the tube which moves forward to the left (1). In figure (2), the rolls come into contact with the material and stretch the hollow piece, moving it back. A tube with reduced thickness (3, 4 e 5) is generated, due to a strong axial stretching. In these phases, the hollow piece and the mandrel may move back. In figure (6), the rolls release the material which continues to move to the left. (1) (2) (3) (4) (5) (6) 18

10 9 STRETCHING: Pilgrim Mill Therefore, the hollow piece moves: to the left during the feed phase to the right during the stretching phase This movement reminds the way of walking of the medieval pilgrims (that s why it is called pilgrim mill). As the material comes out of the pilgrim mill, the shape is already that of a tube, although the wall thickness and the diameter are still rather big and geometric tolerances are rough. Now you have to: further stretch the tube to reduce diameter and thickness improve dimensional accuracy 19 0 HEATING Before continuing with stretching, hollow shapes are often cut and heated again (singularly or in lots) to take them back to a temperature which allows hot forming. 20

11 1 STRETCHING: Continuous Mill Stretching and final dimensions are obtained through a continuous mill or a reducing and stretching mill. The continuous mill consists of a group of sizing rolls, generally two or three, shaped in such a way that can reduce the tube thickness. The tube always contains a sizing mandrel, which is generally idle. p Dg Dt L M 21 2 STRETCHING: Continuous Mill The continuous mill is composed of several stands (even more than 10); each stand is composed of a group of sizing rolls which can reduce the thickness only by 5-10%. 22

12 3 STRETCHING: Reducing and Stretching Mill The reducing and stretching mill is similar to the continuous mill described before, but it does not use a central mandrel. Therefore, a reduction in diameter is always present. The stretching action is obtained by imposing a pull action between the stands and controlling appropriately the roll speed in the different stands. Pull action generated by the next stand 23 4 STRETCHING In this phase, you can also give the tube a section other than a circular section: At this point, the tubes are cooled or get cooler. 24

13 5 STRAIGHTENING When the product is cold, you can impose a permanent plastic strain to improve the straightness of the tube and its circularity. The processing is generally carried out through double bending: 25 6 THE CYCLE To summarize, the production cycle of a seamless tube is the following: Ingot Rotary Furnace Ingot Piercing Mannesmann Mill 26

14 7 THE CYCLE followed by... Pilgrim Mill Cut Heating 27 8 THE CYCLE followed by... Stretching and Finishing Straightening 28

15 9 FINISHED PRODUCTS 29 0 PRODUCTION PLANT A typical production plant has the following layout: Reducing and Stretching Mill Rotary Furnace Pilgrim Mill Mannesmann Mill Heating Furnace 30