Steel Manufacturing Blog: Keeping it Steel

Hot Dip Galvanized Threaded Rod Grade 55 and B-7

Posted on Wed, Nov 14, 2018 @ 04:10 PM

The Steel Supply Company has added several new items to the selection of All Threaded Rod. These include;

Threaded Rod Grade 55 HDG
  • Grade 55 Hot Dip Galvanized
    • 3/4 – 10
    • 1 – 8
    • 1-1/4 – 7
  • Grade B-7 Hot Dip Galvanized
    • 3/4 - 10
    • 1 – 8
    • 1-1/4 – 7
    • 1-1/2 – 6
    • 1-3/4 – 5
    • 2 – 4.5

Galvanized long threaded are difficult products to produce for several reasons. First, A-307 threaded rod is required to hold only a 1A thread diameter tolerance. This is the widest tolerance permitted by ASTM specifications and therefor the easiest to produce. It is noticeable when threading a nut and manually checking the amount of free play. When the Hot Dip Galvanized coating is applied it introduces a diameter variable that is difficult to control but the wide tolerance allowed with A-307 makes it much easier to adhere to.

When elevating the tensile and yield strength to Grade 55 or B-7, ASTM also requires the thread tolerance conform to 2A. These are tighter minimum and maximum thread diameters. Again, the difficulty increases when the threaded rod requires a hot dip galvanized coating.

Unlike I-Beams, Plate and Steel Shapes, which can be galvanized and will drip clear before the zinc is solidified threaded products require an extra step to clear the threads. Smaller items are galvanized in steel baskets approximately the size of 55-gallon drums. As soon as the basket is removed from the molten zinc bath it is inserted in a centrifuge and spun at approximately 500 rpm. The centrifugal force generated evacuates the liquid zinc from the threads and produces a uniform coating thickness. This process is employed for smaller bolts, up to about 24” in length.

At 12’ long the All Threaded Rod requires its own special treatment. With an anchor bolt or turnbuckle rod when the length exceeds the spinner basket size the threads are typically hand brushed. This is effective because the threaded area is only at the ends. With a 12’ all threaded rod it is impossible to brush the entire length before the zinc cools and solidifies in the threads.

This all goes back to tighter thread tolerances. To Hot Dip Galvanized 12’ threaded rods special racks are employed and function similar to the centrifugal basket. The rods travel in the rack through the entire process and when removed from the molten zinc entire rack is spun. While most galvanizing is done in open air factories, this process is in closed areas where the environment can be kept at elevated temperatures. This allows the zinc to remain in liquid form for an extended period which assure the spinning process will be effective and the thread diameter will be within specifications.

This is one step in a series that allows The Steel Supply Company to provided Grade 55 and Grade B-7 All Threaded Rod, Hot Dip Galvanized, that is fully Melted and Made in America.

Tags: Threaded Rod