No metalurgist either, but the way chains are made IIRC is they are quenched and tempered steel.
Meaning they are heated really hot. Orange hot. Then quenched in water, that hardens them, but makes them brittle. Then the chain is heated dull red and dunked in water again. That takes the brittleness away but still leaves them strong and tough. Thats the quench and temper process.
By heating and slow cooling (which is what would have happened in the ashes) you have annealed the metal. Taking it back to near its original properties, which aint very tough or strong.
Grade 30 proof coil rating would be about right IMO