Iron-Carbon diagram

The iron-carbon phase diagram is important in engineering as it provides the basis for understanding all cast irons and carbon steels and their heat treatment. For structural and mechanical applications, steels and other alloys based on iron (the ferrous alloys) are the dominant engineering alloys. They are intrinsically stiff, strong and tough, and mostly low cost. High density is a drawback for transport applications, allowing competition from light alloys,wood and composites. The wide range of applications reflects the many classes of ferrous alloy – different chemistries combined with different process histories. Here, we focus on ferrous alloys which are predominantly iron and carbon alone.

 Examples of typical applications of iron-carbon alloys are: 

- Cast iron brake discs. Brake discs use sliding friction on the brake pads to decelerate a moving vehicle, 

generating heat in the process.The key material properties are therefore hardness (strength) for wear resistance, good toughness and a high maximum service temperature. Casting is the cheapest way to make the moderately complicated shape of the disc. Cast irons contain typically 2-4 wt % carbon, giving a lot of the hard compound iron carbide, giving excellent precipitation hardening.

- Plain carbon steel: I-beams, cars and cans. If we had to single out one universally dominant material, we might well choose mild steel – iron containing 0.1-0.2 wt % carbon. Almost all structural steels, automotive alloys and steel packaging (beer and food cans) are made of mild steel (or a variant enhanced with a few other alloying additions). All of these applications are wrought – the alloy is deformed extensively to shape. The excellent ductility of plain carbon steel enables this, while the deformation process exploits their work hardening to give the required product strength. Ductility, toughness and decent strength are vital in a structural material.