Microstructures of Carbon Steels
Steels are iron-based alloys. The most common are carbon steels, which may contain up to 1.5% carbon. Cast irons typically contain between 2.5 and 4% carbon. Figure 4.1 is the phase diagram showing the metastable equilibrium between iron and iron carbide. Below 912°C, pure iron has a body-centered cubic (bcc) crystal structure and is called ferrite, which is designated by the symbol α. Between 912 and 1400°C, the crystal structure is face-centered cubic (fcc). This phase, called austenite, is designated by the symbol γ. Between 1400°C and the melting point, iron is again bcc. This phase is called δ-ferrite, but it is really no different from α-ferrite. The maximum solubility of carbon in α (bcc iron) iron is 0.02% C and in γ (fcc iron) is about 2%. Iron carbide, Fe3C, is called cementite and has a composition of 6.67% C. The structure developed by the eutectoid reaction, γ → α + Fe3C at 727°C, consists of alternating platelets of ferrite and carbide (Figure 4.2) and is called pearlite.
Steels containing less than 0.77% C are called hypoeutectoid, and those with more than 0.77% C are called hypereutectoid. The microstructures of medium-carbon steels (0.2 to 0.7% C) depend on how rapidly they are cooled from the austenitic temperature. If the cooling is very slow (furnace cooling), the proeutectoid ferrite will form in the austenite grain boundaries, surrounding regions of austenite that subsequently transforms to pearlite, as shown in Figure 4.3.