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How to use Cerrobend to Bend Tubes and Sections?

Service News Monday, August 6, 2012: CS Alloys

Many fields require the usage of thin wall tubing, making it important to deal with manipulation problems and acquire perfect bends. However, this easy said, than done.

Different materials were used for tube bending, with different success rates. Pitch, resin, lead, sand, spiral springs and even internal mandrels were used, though they avert flattening, buckling or rupture of tube walls to some extent, they fail often when the bend required is thin or light. Sand seemed difficult to pack tightly and hence, did not offer adequate support for the tube wall. On the other hand, lead shrinks with setting and the results are even worse, and also required high melting points leaving it impossible for use with light alloys. As for resin and pitch, results were better though, they fail when it is about sharper bends and are dangerous to handle, leave alone being messy. Last but not the least, spiral springs and internal mandrels offer limited usage and only suitable for gentle bends.

Above all, besides spring and mandrel, all other fillers are extremely difficult to remove completely and can possibly leave traces after bending. When such small particles lurk behind, it can become dislodged or cause problems that can be serious, or even trigger areas of marked weakness when they are put through heat treatment.

Cerrobend, which is an alloy made of tin, bismuth, cadmium and lead. It has unique properties that make it ideal filler during tube bending. The melting point of Cerrobend is 158º F, which means, it is lesser than the temperature of boiling water. So it is possible to deploy Cerrobend effectively for bending of tubes, even when the wall is only 0.007” thin, to smaller radii. Cerrobend perfectly snuggles to the inner wall of the tube and then, it can be bent much like bending a solid bar. There’s another unique quality of Cerrobend, which is its expanding ability with solidification, unlike other fillers than contract.

Molten Cerrobend will cool and crystallize slowly, making it coarse, crystalline and brittle. Even so, if it is chilled down at a faster pace, it gets a fine, granular texture and also, it is very ductile in this condition. When Cerrobend is utilized for bending metal tube, the alloy should be kept in a stainless steel pot and then, kept in boiling water. For smaller quantity, a regular kitchen double- boiler should do. When water begins to boil, Cerrobend melts fully, though it should not be overheated. Another alternative would be using natural gas, or electric pots that come with controlled thermostats. Heating the alloy over direct flame is not safe, though with constant monitoring and usage of thermometer, there shouldn’t be any problems. When heating the alloy, one end of the tube should be sealed with a cork or stopped. Also, the tube should be coated with olive oil or any other cooking oil that does not contain detergent. The oil is to be poured into the tube and then, excess oil can be poured out, leaving some at the bottom. If the tubes are oiled prior to leading, the alloy will not come in contact with the tube wall, since the oil film cannot be broken at 212º F.

Molten Cerrobend, can now be poured into the tube and during this process, the tube should be held at an angle, to allow the metal flow along the inside wall of the tube. This way, there would be no air bubbles. One the molten alloy fills the tube, it is quickly dipped into a tank of cold water (about 50º F), with cork end going down first. Now, the molten alloy becomes a fine crystalline structure that’s flexible and easier to bend.

With low thermal conductivity, Cerrobend cools down really slow, even with cold water. SO, right after chilling, the tube and the filling should be allowed to reach room temperature. At this temperature, any bending equipment is suitable for bending, though it should be done slowly and with uniform pressure. When too much pressure or speed is deployed, it can cause failure. For thin wall aluminum or duralumin tubing that’s bent to small radius should be done in multiple stages and not in one go.

When bending is done, Cerrobend can be removed from the tube with ease, by heating with boiling water, steam, or even using hot air that matches the temperature of boiling water (212º F.). Since open flame can damage the tube, or cause overheating, it is not advisable. When in molten state, the alloy can be drained out and used over and over again. Besides, after draining the alloy, the tubes should be cleaned with steam, or using a pull-through to remove any small granules of Cerrobend, which can remain stuck to the oil film and interfere during heat treatment that will follow.

If the diameter is too small, the molten metal can be sucked into it, using suction. This is typically ideal for tubes that are ¼” in diameter or lesser. Copper, duralumin, stainless steel, brass and even steel, can be bent using Cerrobend. Tubes that are plated with nickel or chrome can be bent without plate flaking off.

Cerrobend is widely used in airplane manufacturing units, pipe lines, hydraulic tubes, etc. Tubes used in constructing airplanes frames also use this alloy, since irregular tubes can also be bent readily like round tubes.

Yet another usage of Cerrobend is in the manufacturing of extruded or rolled sections. With this alloy, the need for cumbersome rolling machines is nullified. The alloy is cast into a suitable mold to embed the section and then, it is bent to the required dimension around the section. In such conditions (in particular, when the former is grooved to perfectly accommodate the cross-section of this alloy block, without any cross-sectional distortion), it is not possible for the section to spread, or ripple in any direction and also, perfect bending is possible.