You don’t define breakdown so I will assume a permanent change to the original specified mechanical properties.
For ASTM A 514 Grade H steel (high yield, Q&T), above 400 deg F to 1100 deg F you will see a reduction in mechanical properties while at elevated temperature; cool to RT and properties are back to near original.
Exposure to temperatures above 1150 deg F (original minimum tempering temperature by specification) will have a step change to lower mechanical properties and upon cooling to RT will be a permanent change (lowered) to these properties unless this material is reheat treated.