As the most common form of embrittlement encountered during the hot-dip galvanizing process, strain-age embrittlement is often misnamed "galvanizing embrittlement." Strain-age embrittlement is defined by the change of properties of cold worked steel over time. These changes occur rapidly at higher temperatures and only in steel that has been cold worked. Therefore, concerns of strain-age embrittlement will stem from galvanizing cold worked steel.
For strain-ageing to occur, two components are necessary. The first component is stresses must be induced into the steel prior to the galvanizing process. This normally occurs through cold working the steel. Cold working can include bending, punching, or shearing the steel. If the stresses from these cold working practices are not relieved prior to galvanizing, they become points of high residual stress during the galvanizing process and can lead to strain-age embrittlement. Strain-ageing can also be caused by impurities in the steel, such as those found in lower quality steels used for reinforcing bar.
If a part cracks due to strain-age embrittlement, the cracking occurs immediately after galvanizing, but is also often seen at the job site, as in the case of reinforcing bar. Often times simple handling is enough stress to cause a strain-age embrittled member to crack.
There are several ways to reduce the occurrence of strain-age embrittlement, but all methods focus on one aspect reducing the stresses induced into the steel prior to galvanizing. Instead of cold working, which induces stresses into steel, the steel can be hot worked at temperatures between 1100F and 1300F. If it is necessary to cold work the steel, relieving the stresses induced from cold working can be accomplished by heating the steel to 1200 1300F for heavy cold working and up to no more than 1200F for less severe cold working. When bending steel, it is best to allow for a bend diameter at least three times the section thickness. When punching is necessary on thicker steels (3/4 or greater in thickness), the holes should be reamed at least 1/16 around the edge of the hole. When flame cut coping is necessary, such as on structural beams, the minimum radius of the copes should be one inch or greater. Grinding the areas around the cope is recommended to remove small micro-cracks in the steel from the flame cutting process.
© 2019 American Galvanizers Association. The material provided herein has been developed to provide accurate and authoritative information about after-fabrication hot-dip galvanized steel. This material provides general information only and is not intended as a substitute for competent professional examination and verification as to suitability and applicability. The information provided herein is not intended as a representation or warranty on the part of the AGA. Anyone making use of this information assumes all liability arising from such use.