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

The proper handling of heat-treated rivets in the D and DD series that are not used within the prescribed time is critical to ensure their performance and integrity when used in applications. Heat-treated rivets undergo a specific heat treatment process designed to enhance their mechanical properties, such as strength and ductility. However, if these rivets are not used within the designated time frame after treatment, they may lose their effectiveness.

Reheat treating the rivets before use restores their intended properties, ensuring they meet the required strength and durability standards. This process reestablishes the rivet's hardness and performance characteristics that might be degraded over time due to exposure to various environmental factors or simply due to the aging of the material.

The other options do not address the necessary precautions for maintaining the structural integrity of the rivets. Storing at room temperature, discarding, or polishing does not remediate the critical loss of mechanical properties that can occur if the rivets are not utilized within the specified timeframe. Reheat treating is the appropriate action to ensure that the rivets perform as intended in their applications.