Ameritube operates three tube drawing benches each with a length over 100 feet. The drawbenches are used to draw copper nickel, admiralty brass, monel 400, copper, and other types of tubing for heat transfer, heat exchange, power condenser, petrochemical, HVAC, and artificial lift industries. Ameritube is able to draw a variety of copper alloys.
In the video below, you can see the drawing of 43 foot tube for a condenser. Our drawbenches have the ability to draw 5 tubes at once, creating an opportunity to draw over 100 tubes per hour.
“Draw benches are usually mechanical and have three components: a back bench, die head, and front section. Jaws on a trolley grip the tube and a hook on the back of the trolley engages a moving chain, pulling the tube through a die. Dies are most commonly sintered tungsten carbide inserts with a cobalt binder that have been shrunk-fit into a steel casing.
Tubes are drawn to a finished size using one or more of the following operations:
- Rod or mandrel drawing
- Plug drawing, including fixed, floating, and semifloating (tethered)
Rod Drawing. During rod drawing, a hardened steel mandrel is inserted into the bore of the tube that has been pointed. After the tube has been introduced into the die (see Figure 1), lubricating oil is pumped onto the surface of the tube, the trolley jaws grip the tube or rod tip, the trolley hook engages the chain, and the tube is drawn through the die. The die diameter determines the OD; the rod diameter determines the ID size. Proper die selection minimizes wall thickness changes before the tube contacts the mandrel.
In general, heavy-wall tubes tend to thin before contacting the rod; light walls thicken. High-angle dies tend to thin the wall and low-angle dies tend to thicken the wall. It is critical to remember that the optimum die angle varies with the diameter-to-thickness (D/t) ratio.
After the tube is drawn, it must be expanded for rod removal. A common method is to apply pressure by rotating the tube while passing it through cross rolls. This process generates radial stresses and expands the tube. The process is repeated until the tube is at finished size.
Advantages of rod drawing are that drawing speeds are relatively high and high area reductions (approximately 45 percent for stainless steel) are possible. Disadvantages are that it is a two-person operation and it requires an additional drawing operation, such as a plug draw or sinking, to remove the spiral pattern.
Plug Drawing. Two varieties of plug drawing are fixed and floating. Fixed plug drawing uses a hollow rod anchored at the back of the bench. A lubricant is pumped through the rod to a small hole near the front, allowing lubricant to enter the ID of the tube. A slightly tapered tungsten carbide plug is threaded or brazed onto the end of the rod; the tube is loaded over the rod, lubricant pumped onto the OD surface, and the tube is drawn.
One of the benefits of fixed plug drawing (see Figure 2) is that it produces a smooth ID. Another advantage is that the taper makes it possible to adjust the ID to meet a tight tolerance. While it requires only one operator, the drawing speed is quite slow, and maximum area reductions are low—about 25 percent for stainless steel.
Floating plug drawing (see Figure 3) is well-suited to producing long-length coils economically. This method was used for drawing copper and aluminum for many years. After the lubricant is pumped into the ID of the tube, a tapered plug is inserted, the tube is crimped to hold the plug in place, and the tube is pointed. During drawing the plug is held in position by a combination of forces between the tube ID and the plug. The tooling design is critical to the success of this process. Die angles are generally between 28 and 32 degrees, with plug angles between 20 and 24 degrees. The bearing length should be about 10 to 15 percent of die diameter. Be aware that a plug that is too long can cause scratches on the ID; a plug that is too short will not seat.
Semifloating drawing and tethered plug drawing are floating plug processes adapted for drawing straight lengths. The plug is attached loosely to a back rod and the tube is loaded over the rod and plug for drawing (see Figure 4).
Sinking. Sinking is the term for drawing a tube with no internal support. It is usually performed as a sizing pass after a rod draw. The proper die angle depends on theD/t ratio; a properly chosen die angle minimizes the change in wall thickness. If the wall thickens too much, the ID surface finish will deteriorate.
The bearing length is longer than with other operations, up to 50 percent of the die’s diameter, to ensure the roundness of the finished tube.
Plug drawing and sinking can be used to draw a tube to a finished size.
When designing a drawing schedule, the ratio of wall reduction to diameter reduction is an important quality consideration. Wall reductions tend to iron, or smooth, the ID surface; diameter reductions tend to roughen the surface. A convenient expression for the ratio is the Q value, which is equal to the perce nt wall reduction divided by the percent ID reduction. A Q value of 2 or higher tends to smooth the ID surface. When the schedule does not lend itself to a series of high-Q-value draws, it is better to use a high-Q-value rod draw followed by a hard sink rather than a series of low-Q-value drawing operations. High Q values also result in low residual stress levels for cold-worked tubes. In a recent project, a Q value of 0.91 yielded a residual stress of more than 52,000 pounds per square inch (PSI) as measured by the Sachs and Espy procedure described in ASTM E1928. A draw with a Q value of 2.2 had a residual stress level of only 5,200 PSI. High Q values would result in negative, or compressive, values.
Lubrication. Lubrication is another important consideration, along with tooling and drawing schedule. Most tube mills use chlorinated oils for lubricating stainless steels and nickel alloys. The correct viscosity can be as low as 8,000 SUS (Saybolt Universal Seconds) or more than 100,000 SUS depending on the alloy, tube size, and type of reduction.” (https://www.thefabricator.com/article/tubepipeproduction/cold-drawing-principles, June 2010)
Copper and copper nickel products such as C70600 90/10 Copper Nickel,C70600 Copper Nickel 70/30 are improved through the drawing process, particularly after pilgering and annealing. Ameritube has trained operators who are trained on our internal Standard Operating Procedures (SOP)s for drawing. This involves selecting a die and mandrel to properly achieve the desired reduction in area, the process elongates the grains inside the metal, reduces the outside diameter and wall thickness, and improves the surface resulting in smooth tubes ready for their heat transfer application.
Ameritube has an extensive collection of dies and mandrels to suit your specific application. Ranging in size from 3/8″ to 3″ Ameritube maintains tungsten carbide drawing dies to ensure a smooth surface and repeatable results on the outside diameter and wall thickness. To learn more about the drawing process, you can click here.
If you would like to learn about the copper nickel drawing process and how it affects the performance of copper alloy tubing and other tubing for that matter, please call us at 254.580.9888 x102.