Phase Transformations and Heat Treatment Lecture Note

Heat treating of steel involves phase transformations. These can be diffusion-dependent transformations such as the formation of pearlite and diffusion less transformations such as when martensite forms. Let’s begin by reviewing the phases identified in Figure 1 – namely ferrite, austenite, cementite and pearlite.

At room temperature, the most stable form of iron is ferrite, also known asa-iron (“alpha” iron), which has a body-centered cubic (bcc) crystal structure. Ferrite is a fairly soft material that can dissolve only a very small amount of carbon – no more than 0.021% at 1670°F (910 °C) and only 0.008% at room temperature. Ferrite is the phase that exists below the upper critical temperature of a steel with less than 0.80% carbon.

The formation of austenite or g-iron (“gamma” iron) begins when steel is heated above its lower critical temperature, the Ac1 (A1) line shown on the diagram. The structure is fully austenitic above the Ac3 (A3) or Accm line. Austenite has a face-centered cubic (fcc) crystal structure and can contain up to 2.03% carbon at 2110°F (1154°C) or approximately 100 times greater than the maximum limit for ferrite. Carbon strengthens steel and gives it the ability to be hardened by heat treatment.