Iron–Carbon Equilibrium Diagram

Since most steels contain carbon, the basic principles of microstructural development can be explained by the iron–carbon equilibrium diagram. This diagram, shown in Fig. 1, is essentially a map of the phases that exist in iron at various carbon contents and temperatures under equilibrium conditions. Iron is an interesting chemical element in that it undergoes three phase changes when heated from room temperature to liquid. For example, from room temperature to 912 C, pure iron exists as ferrite (also called alpha iron), from 912 to 1394 C, it exists as austenite (gamma iron), from 1394 to 1538 C it exists as ferrite again (delta iron), and above 1538 C it is liquid.

In other words, upon heating, iron undergoes allotropic phase transformations from ferrite to austenite at 912 C, austenite to ferrite at 1394 C, and ferrite to liquid at 1538 C. Each transformation undergoes a change in crystal structure or arrangement of the iron atoms in the crystal lattice.

It must be remembered that all chemical elements in their solid form have specific arrangements of atoms that are essentially the basic building blocks in producing the element in the form that we physically observe. These atomic arrangements form a latticework containing billions of atoms all aligned in a systematic way. Some of these lattices have a cubic arrangement, with an atom at each corner of the cube and another atom at the cube center.

This arrangement is called body-centered-cubic (bcc). Others have an atom at each corner of the cube and atoms at the center of each face of the cube. This is called face-centeredcubic (fcc). Other arrangements are hexagonal, some are tetragonal, etc. As an example, pure iron as ferrite has a bcc arrangement. Austenite has a fcc arrangement. Upon heating, bcc ferrite will transform to fcc austenite at 912 C. These arrangements or crystal structures impart different properties to steel. For example, a bcc ferritic stainless steel will have properties much different from a fcc austenitic stainless steel, as described later in this chapter.

Since pure iron is very soft and of low strength, it is of little interest commercially.

Therefore, carbon and other alloying elements are added to enhance properties. Adding carbon to pure iron has a profound effect on ferrite and austenite, discussed above. One way to understand the effect of carbon is to examine the iron–carbon diagram (Fig. 1). This is a binary (two-element) diagram of temperature and composition (carbon content) constructed under near-equilibrium conditions. In this diagram, as carbon is added to iron, the ferrite and austenite phase fields expand and contract depending upon the carbon level and temperature.

Also, there are fields consisting of two phases, e.g., ferrite plus austenite. Since carbon has a small atomic diameter when compared with iron, it is called an interstitial element because it can fill the interstices between the iron atoms in the cubic lattice. Nitrogen is another interstitial element. On the other hand, elements such as manganese, silicon, nickel, chromium, and molybdenum have atomic diameters similar to iron and are called substitutional alloying elements. These substitutional elements can thus replace iron atoms at the cube corners, faces, or center positions. There are many binary phase  diagrams (Fe–Mn, Fe–Cr, Fe–Mo, etc.) and tertiary-phase diagrams (Fe–C–Mn, Fe–C–Cr,etc.) showing the effect of interstitial and substitutional elements on the phase fields of ferrite and austenite. These diagrams are found in the handbooks listed at the end of the chapter. Being an interstitial or a substitutional element is important in the development of steel
properties. Interstitial elements such as carbon can move easily about the crystal lattice whereas a substitutional element such as manganese is much more difficult to move. The movement of elements in a crystal lattice is called diffusion.


Mechanical Engineers’
Handbook: Materials and Mechanical Design,
Volume 1, Third Edition.
Edited by Myer Kutz

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