It is a common experience that metals expand when they are heated. Like any metal, steel will also expand when heated; how­ever, in the region between the critical temperatures, Acb and Ac3, a marked contraction occurs upon heating. This is shown schemati­cally in Fig. 11-14 for a.2 percent carbon steel (AISI 1020).

When the steel cools slowly from an elevated temperature, like other metals, it will contract. However, here again, something unex­pected happens: the steel expands in the region between the critical temperatures, as shown in Fig. 11-14.

The reason for this behavior is that a phase change occurs in the region between the critical temperatures. Austenite is a denser structure than ferrite and pearlite, so that upon heating to austenite the metal must contract. Upon cooling from austenite to pearlite and ferrite, the metal must expand.

When.8 percent carbon steel is rapidly cooled or quenched from a temperature at which it is austenite, ferrite and pearlite do not form. Instead, the hard martensite structure forms. Martensite is less dense than pearlite, ferrite, or austenite. Thus, when the martensite is formed, the metal must expand. As shown in Fig. 11-14, this always occurs at a lower temperature.