In hardening gray iron, the casting is heated to a temperature high enough to promote the formation of austenitic, held at that temperature until the desired amount of carbon has been dissolved, and then quenching at a suitable rate.

The temperature to which a casting must be heated is determined by the transformation range of the particular gray iron of which it is made. The transformation range can extend more than 55°C above the At transformation-start temperature. A formula for determining the approximate A, transformation temperature of unalloyed gray iron is:

A (°C) = 730 + 28.0 (% Si) - 25.0 (% Mn) Chromium raises the transformation range of gray iron. In high-nickel, high-silicon irons, for example, each percent of chromium raises the transformation range by about 10 to 15°C. Nickel, on the other hand, lowers the critical range. In a gray iron containing from 4 to 5% Ni, the upper limit of the transformation range is about 710°C.

Castings should be treated through the lower temperature range slowly, in order to avoid cracking. Above a range of 595 to 650°C, which is above the stress-relieving range, heating may be as rapid as desired. In fact, time may be saved by heating the casting slowly to about 650°C in one furnace and then transferring it to a second furnace and bringing it rapidly up to the austenitizing temperature.