Knowing how to determine wall reduction is important; however it is equally important to know the characteristics of the popular tubing materials. A simple rule of thumb is the harder the material, the less wall reduction required to obtain a tube joint.
The following are guidelines that have been used in the industry throughout the years. These are in no way recommendations for all heat transfer equipment but are offered as general suggestions. Always consult the manufacturer of the heat transfer vessel for specific information before undertaking any maintenance procedures.
When expanding tubes, to increase tool life and expansion quality, it is important to lubricate each tube end and make certain that the inside of the tubes and the tube expanders are kept clean.
Steel, Carbon Steel, & Admiralty Brass 7-8%
Admiralty Brass is widely used in condensers. The tube wall should be reduced by approximately 7% – 8% for optimum tube joints. In general, only a 4% to 10% reduction in wall thickness is necessary to produce a tight tube in a serrated hole. On the other hand, a reduction in excess of 15% may cause leaking, splits or flaked tubes.
Carbon Steel is used in almost every type of pressure vessel built today. Tube wall reduction should be approximately 7% – 8%. If the tube is cracking or if the tooling shows excessive wear, tube hardness should be checked. Carbon Steel tubes should be 90 to 120 Brinnel hardness for rolling. It is possible to roll tubes up to 150 Brinnel, however, flaking and cracking is more likely to occur as the tube hardness increases.
Copper & Cupro Nickel 8-10%
When rolling Copper and Cupro Nickel, consider approximately 8 – 10% wall reduction to be a proper tube joint. Since copper is one of the softer tubes used in pressure vessels, it can be easily rolled.
Stainless Steel & Titanium 4-5%
When rolling Stainless Steel and Titanium tubes, approximately 4% – 5% wall reduction is sufficient to produce a secure tube joint. When rolling these alloys, the entire wall reduction should be done quickly, since they have a greater tendency to work harden.
When rolling Titanium, it is often recommended that you use a four or five roll expander. This will decrease the diaphragm of a thin wall and help eliminate tube end cracking.
When rolling 3003 or 4004 Aluminum you should not reduce the walls over 5%. When rolling 6061-T Minimum, which is one of the most popular materials used in aircraft fittings, you can reduce the wall 10% – 12% for a mechanical joint.
For information on calculating a wall-reduction percentage, check out our article on the Basic Principles of Tube Expanding.