Tube Making


Topics:


Introduction

The primary shape product from which a seamless tube can be made may be a can or an extruded heavy wall tube. A tube with a welded seam may be produced by Roll-Forming from a strip. The primary process entitled "Tube Extrusion" for the fabrication of heavy wall large diameter tube is handled separately. Cans, also called cups, as a primary shape can be made by each of the following three processes, or by combinations thereof:

  • Impact Extrusion, as described in Chapter (6)
  • Deep Drawing, as described in Chapter (10)
  • Ironing, as described in Chapter (11)

of the "Handbook of Metalforming Processes", by Avitzur. Published by John Wiley, 1983. Tube making is described in Chapter (9).

For example, a shallow can of relatively thick wall can be produced by either deep drawing or impact extrusion. The wall thickness can be reduced by ironing.

The cans made by the processes described above may on occasions be used as the raw material for the production of tube by removing the bottom of the can. This procedure then may compete with other tube making procedures, such as tube extrusion. The different routes are then defined as The "Pass Reduction Schedules" or simply "Reduction Schedules." The reduction schedule contains the number of passes by each process, i.e., deep-drawing, redraw, sinking, plug drawing, etc. The pass reduction schedule contains also the details for each pass, including, die angle, reduction, friction value, etc. Intermittent treatments such as intermediate anneals are also part of the pass reduction schedule.

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Pass Reduction Schedules

The alternate Reduction Schedules for the subsequent finishing sequence of the tube are described in this section. For a schematic diagram of possible pass reduction schedules see Fig. <1> below. To simplify the diagram, intermittent anneals are omitted from Fig. <1>. Many alternate routes may lead to the same final product. Circumstances in different facilities may lead to different optimal paths.

For any selected Alloy a pass reduction schedule, similar to Fig. <1> could be constructed, with details of the process, the tooling, the number of passes and the reduction, die angles, lubrication, intermittent anneals, and speeds for each pass. These pass reduction schedules should be constructed by the team from the plant, together with Metalforming personnel. It is a trial and error process. While goals may be set, the diagram will always remain an evolving document. Reduction schedules always change, hopefully for the better.

PASS REDUCTION SCHEDULE FIGURE <1> :  GIF [ ]      [58 ]

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Background on Tube Drawing, Sinking and Ironing

Foreword

This page focuses on selected problem areas in the secondary processing of tube, reducing its wall thickness and/or diameter. Mainly we deal with tubes that began as hot extruded tubes, deep drawn tubes, or roll formed and welded tubes. These secondary processes of tube sinking, drawing, and ironing are described in Chapter (9) of the Handbook. It also deals with surface defects such as scratches, and lubricant bonding on the surface of the extruded billet. We focus on the pass reduction scheduling, i.e., selection of reductions per pass, appropriate die angle, and the sequencing of sinking, drawing, with mandrel, and floating plugs, and ironing. We deal with the design of, and with the introduction of more frequent use of, the floating plug, instead of sinking. The proper use of the ironing process to achieve thinner wall on finer tube is also treated.

First we classify and describe the processes briefly. For a deeper analysis see Chapter 9 of the Handbook, and access the software on tube sinking which is based on the analysis. The software for the analysis of tube sinking is instrumental in providing graphs for the design of the die and plug for tube drawing with floating plugs.

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Tube Sinking

When a tube is pulled through a (conical) converging die to decrease its outer diameter, without internal tooling (see Fig. <7>) the process is called free tube drawing or tube sinking. It can be performed on a draw bench to form straight tubes of finite length, or it can be performed on bull block, to make a continuous coiled tube.

In Fig. <7> a tube of original outer radius Ro and inner radius Ri is pulled through a conical die of semi-cone angle a with an exit hole of radius Rof. The original thickness of the tube, to, is given by the difference Ro - Ri. The tube, while passing through the die, shrinks in outer radius from the original radius Ro to a final nominal radius Rof. In wire- and tube-drawing practice it may sometimes be observed that the exit diameter of the product is slightly smaller than the exit of the die. The parameters Ro, Ri, Rof, a, the length of the land of the die L (also called the bearing), and the back tension sxb (if applied) are all independent parameters of the process.

The inner radius of the emerging tube, Rif, is not directly controlled by the tooling of the process. The final thickness of the tube (tf = Rof - Rif) is a dependent parameter, dependent on Ro, Ri, Rof, a, sxb, and the friction between the tube and the die. The drawing stress sxf required to perform tube sinking is another dependent process parameter. In the treatment of tube sinking as performed in the Handbook the dependence of the dependent process parameters - the drawing stress sxf and the exit thickness tf - on the independent process parameters Ro, Ri, Rof, a, L, sxb, and the friction between the tube and the die - is determined. The optimal die angle that minimizes the drawing force and at the same time maximizes the maximum possible reduction are also studied. Software, based on the analysis is available for the determination of the following dependent process parameters:

  • drawing stress
  • emerging wall thickness
  • optimal die angle
  • maximum possible Reduction

on the independent parameters. The understanding of the process of tube sinking, through the analysis and through experimental work, is essential for the development of criteria for design and tooling for mandrel and plug drawing, as discussed next. User friendly software for the analysis is available through Metalforming Inc. The software is presented and provided for an on hand experience in the appropriate Seminar on tube making.

TUBE SINKING FIGURE <7> [Cannot perform fsize(): Win32 Error Code = 2
] GIF

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Mandrel and Plug Drawing

A better control on the inner surface of the tube is provided by placing a cylindrical mandrel inside the tube at the throat of the die. The mandrel exert a size determination of the wall thickness and inner diameter, as well as affecting a smooth mirror like surface finish inside the tube. In order to be functional the diameter of the mandrel must be larger than the diameter expected from identical processing by tube sinking. If the mandrel is smaller it will have no control of the process at all. The mandrel have only to be slightly larger because otherwise it will cause a large increase in the drawing force and may lead to tube tearing. The recommended die angle for drawing with a mandrel is nominally the same as that found to be the optimal angle for tube sinking.

The two popular designs of mandrels are the stationary and the moving mandrel (Figs. 9.51 and 9.52a of the Handbook). A stationary mandrel is clamped to the frame of the draw bench, while a moving mandrel adheres to the drawn tube through friction and pressure and moves with it. In either case the shaping of the inner surface of the tube commences only at the exit of the tube from the die. While converging through the die the inner surface of the tube deforms freely.

The conical plug (Fig. <8>) mounted at the tip of a stationary mandrel was first introduced to affect the inner surface earlier than possible with the cylindrical mandrel. The diameter of the tip of the plug is that recommended for the mandrel, i.e., slightly larger than that of the emerging tube when tube sinking is practiced. The cone on the plug is of a slightly smaller angle than that observed under identical tube sinking conditions.

It is noticeable that the tension on the mandrel depends on friction between the plug and the tube, and on the cone angle of the plug. The larger the plug angle the smaller is the tension on the mandrel. Under the proper conditions the tension on the plug may be eliminated. The plug then places itself at the throat of the die even when there is no mandrel to hold it there. Such a plug is called floating plug. The design of the floating plug, i.e., diameter of the tip, and plug cone angle are determined through experimental tube sinking, or through the use of the software for the analysis of tube sinking. When a floating plug is used the tube can be drawn on a bull block, from coil to coil.

MANDREL AND TUBE DRAWING FIGURE <8> [Cannot perform fsize(): Win32 Error Code = 2
] GIF

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Ironing

The process of ironing of cans is already described earlier. When a tube is ironed we can't rely on ram force acting on the bottom of the tube. Ironing of tubes is done with a draw bench, pulling at the exit side of the die. Usually the clamp or tong may grab a hold of the tube forcing it onto the mandrel and pulling the two together. However one can pull on the mandrel itself and if the die angle is small enough the mandrel will drag the tube with it through friction. (see chapter 11 of the Handbook).

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Monday April 26 2010

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