Why do some fabricated assemblies distort during galvanizing?
Stresses induced during steel production and in subsequent fabricating operations, are sometimes relieved when exposed to the high temperature of the molten zinc in the galvanizing kettle (generally around 840 deg. F). Also, parts of dissimilar thicknesses that are welded together can expand and contract at differing rates, based upon their thickness. When one of these materials tries to expand or contract and the other material is stationary, distortion can occur. Galvanizing these types of materials separately can be of great benefit to the manufacturer, from a distortion control standpoint. For example, with a channel frame and thin plate assembly, the frame and plate should be galvanized separately and bolted or welded together later, rather than welded together before galvanizing.
Guidelines for minimizing distortion and warpage are provided in ASTM A384,Safeguarding Against Warpage and Distortion During Hot-Dip Galvanizing of Steel Assemblies, and CSA Specification G 164, Hot Dip Galvanizing of Irregularly Shaped Articles.
Tips for Minimizing Distortion
To minimize changes to shape and/or alignment, design engineers should observe the following recommendations:
-Where possible, use symmetrically rolled sections in preference to angle or channel frames.
I-beams are preferred to angles or channels.
-Use parts in assemblies that are of equal or near equal thickness, especially at joints (Figure 5).
-Use temporary bracing or reinforcing on thin-walled and asymmetrical designs (Figure 6).
-Bend members to the largest acceptable radii to minimize local stress concentration.
-Accurately preform members of an assembly so it is not necessary to force, spring, or bend them into position during joining.
-Continuously weld joints using balanced welding techniques to reduce uneven thermal stresses. Pinholes from welding are very dangerous in items to be galvanized and must be avoided. Staggered welding techniques to produce a structural weld are acceptable. For staggered welding of 1/8-inch(4 mm) or lighter material, weld centers should be closer than 4 inches (10 cm).
-Avoid designs that require progressive-dip galvanizing. It is preferable to build assemblies and sub-assemblies in suitable modules, so they can be immersed quickly and galvanized in a single dip. In this way, the entire fabrication can expand and contract uniformly. Where progressive-dip galvanizing is required, consult your galvanizer.
The last tip brings about a great point - if you have any questions regarding proper fabrication techniques to facilitate galvanizing, ALWAYS contact your galvanizer! At Hubbell Galvanizing, we are happy to assist you with any questions you may have. Feel free to contact us at the following:
(800) 244-4258 - Toll-free
(315) 736-8311 - Office
(315) 736-0381 - Fax