What must be done to heat-treated rivets in the D and DD series if they are not used within the prescribed time frame?

Heat-treated rivets, specifically in the D and DD series, have a predetermined shelf life once they are manufactured. This shelf life is designed to ensure that the rivets maintain their desired mechanical properties and structural integrity. Over time, if these rivets are not utilized, they can lose their strength due to changes in the material structure that might occur during storage.

If the rivets have exceeded their prescribed usage time, reheat treating them before use is necessary to restore their mechanical properties and ensure they perform as expected in structural applications. Reheat treating involves subjecting the rivets to specific temperature controls that re-align the material grain structure, thus reinstating the hardness and strength characteristics that are critical for their performance in an application.

Discarding the rivets would not be the best choice as it does not allow for possible reuse after proper re-treatment. Polishing and painting do not address the underlying material properties that may have deteriorated, so they would also not be viable solutions to ensure the rivets meet performance standards. Hence, reheat treating is the proper approach to ensure these rivets are safe and effective for use in sheet metal structures.