Tempering — The plates shall be reheated to a
temperature within the range from 1150 to 1225°F [620
to 665°C], held within that temperature range for a minimum
of 1 h/in. [2.4 min/mm] of thickness, but in no case
less than 15 min, and then water-quenched or air cooled
to below 300°F [150°C]