What must be done to heat-treated rivets in the D and DD series that aren't used in time?

Heat-treated rivets, especially in the D and DD series, undergo a specific process to enhance their mechanical properties, such as strength and resistance to fatigue. If these rivets are not utilized within a certain timeframe, they may lose some of their heat-treated properties due to factors such as time or environmental conditions.

It is essential that any heat-treated rivets that have been stored and not used are reheat treated before their next application. This ensures that they retain their intended performance characteristics and structural integrity, which are crucial for safety and reliability in airframe applications. Proper reheat treatment restores the necessary properties that may have diminished during storage, thus maintaining adherence to safety standards and operational requirements.

The other options do not align with proper practices regarding the handling and usage of heat-treated rivets. Reusing untreated rivets or discarding them without assessment fails to consider the importance of maintaining structural integrity, while indefinite storage without addressing the need for reheat treatment could compromise overall safety.