Many structures and parts are fabricated using cold-working techniques (bending, hole-punching, rolling, shearing). Severe cold-working increases the incidence of strain-age embrittlement. The effects of strain-age embrittlement may be accelerated by the galvanizing process, because aging is relatively slow at ambient temperatures but more rapid at the elevated temperatures encountered in the galvanizing process. (See also Bend Diameters).
Any form of cold-working reduces steel’s ductility. Operations such as punching holes, notching, producing fillets of small radii, shearing, or sharp bending may lead to strain-age embrittlement of susceptible steels. Cold-worked steels less than 1/8″ (3 mm) thick subsequently galvanized are unlikely to experience strain-age embrittlement. Since cold-working is the strongest contributing factor to the embrittlement of galvanized steel, these tips are recommended to reduce the incidence of strain-age embrittlement:
Where cold-working cannot be avoided, stress-relieve the part at 1100 F (593 C) for one hour per inch (2.5 cm) of material thickness. ASTM A143, Safeguarding Against Embrittlement of Hot-Dip Galvanized Structural Steel Products and Procedure for Detecting Embrittlement, and CSA Specification G164, Hot-Dip Galvanizing of Irregularly Shaped Articles, provide guidance on cold-working and stress-relieving procedures. However, it is best to avoid severe cold-working of susceptible steels. If there is concern with possible loss of ductility due to strainage embrittlement, advise your galvanizer. A sample quantity of the cold-formed items should be galvanized and tested before further commitment.
Cold-working also must be taken into consideration with respect to another potential mechanical change that may occur during galvanizing’s chemical cleaning process: hydrogen embrittlement. During the chemical reaction between steel, rust, and the chemicals used in the pickling stages of the galvanizing process, hydrogen is produced, which is then absorbed by the steel. Upon immersion into the molten zinc, the hydrogen is expelled as gas. If the steel has been severely cold-worked in a very small region, the hydrogen can be trapped in the steel, causing hydrogen embrittlement when the part is put under stress during use.
Hydrogen embrittlement is of concern only when the ultimate localized tensile strength of the cold-worked steel exceeds 170 ksi (1,200 MPa). Mechanically cleaning of localized high-strength steel areas avoids hydrogen embrittlement.
Design considerations courtesy of American Galvanizers Association. The AGA also has the publication, The Design of Products to be Hot-Dip Galvanized After Fabrication, available for download.
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