Typical Iron-Iron Carbide Applications

The purpose of adding carbon to iron is to change the properties of the iron. Carbon is a very hard substance and when it is added to iron, it becomes a much stronger material, depending on the amount of carbon added. Pure iron itself is relatively soft and ductile compared to carbon impregnated steels. By adding carbon to the iron you get a variety of strengths in your steel.

From the example of iron carbide, you may have noticed the percent carbon that is included in a specimen of iron. Iron has allotropic properties. This means it acquires a different crystal structure at different temperatures. Iron will transform to a BCC [more strength] structure or ferrite at cooling temperatures and then go to FCC [more ductile] at a lower temp also called austenite. Cementite is the name for iron carbide. It defines the point where the solubility of carbon in iron is at its maximum. This will yield a very hard and brittle steel which may not be suited for all applications.

Let's take a look at low (or mild), medium, and high carbon steels and see what conditions they are best suited for.

Low Carbon Steels Applications for low carbon steel are seen in joists in building construction, body panels for automobiles, wire and nails. These fabrications adapt low carbon steel for good reason. The low cost of production of this steel is beneficial because of the number of uses it supplies. Wire and nails are found almost anywhere something is built and there will always be a need for body panels on cars to be made. The ductility of low carbon steel allows for ease of manipulation and reparability (i.e. dents in car fenders, bending wire).

Medium Carbon Steels These steels will have a higher tensile strength and less ductility than mild steel. Applications for this include farm equipment, engine components, gears, and structural fixtures. Farm equipment and engine components are parts that you want to be very strong and durable yet not brittle. The medium carbon content allows the strength to be there and will hold up to many cycles of stress and strain. In an engine, the connecting rods and crankshaft endure a lot of loading and unloading. Too brittle a material could result in mechanical failures over time. And because they will be subject to high temperatures, too soft a steel will result in extreme elongation. The higher carbon content reduces elongation at high temperatures.

Figure 1. This image shows a layer of carbon applied toa gear. This is called "case hardening" which can be done by a process of heat treatment and diffusion. The reason for this is to have great wear resistance on the outside of the gear where it interfaces with another gear. The center portionis basic steel which retains good strength properties and will not be subject to cracking.
High Carbon Steels This form of steels has the best hardness, strength, and least ductility. The areas best suited for this steel is in tools, drills, saws, knife blades, and bearings. High carbon content reduces the wear and deformation of the steel. Many types of tools require this because you don't want to replace a hammer after three hits on a nail or wear out a drill bit too soon. Knife blades and axes follow the same principal. For these applications, the harder the steel, the better.

Charles Norwood 4/29/96


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