Considerations for Steel Cold-worked Prior to Galvanizing
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).
In order to minimize the effects of galvanizing on cold-worked fabrications, consider these guidelines:
- Select steel with carbon contents below 0.25%,
- Specify aluminum-killed steels, which show less susceptibility to embrittlement than do silicon-killed steels,
- Avoid designs with notches, which increase stress; flame-cutting or sawing is preferred, particularly for heavy sections,
- Drill, rather than punch, holes in material thicker than 3/4” (19 mm). If holes must be punched, they should be punched undersize and then reamed an additional 1/8” (3 mm) overall, or drilled to size. Material between 1/4”- 3/4” (6.5-19 mm) thick is not seriously affected by cold-punching if the punching is done under good shop practice. Materials up to 1/4” (6.5 mm) thick that have been cold-punched do not need stress-relieving operations before galvanizing,
- Choose steels with low transition temperatures, since cold-working raises the ductile-brittle transition temperature and the temperature cycle of galvanizing may raise it even further.
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.
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 150 ksi (1100 MPa). Mechanically cleaning of localized high-strength steel areas avoids hydrogen embrittlement.