Mumbai University > Mechanical Engineering > Sem 4 > Material Technology

Marks: 5M

Year: Dec 2014

Iron is an incredibly useful substance. It's less brittle than stone yet, compared to wood or copper, extremely strong. If properly heated iron is also relatively easy to shape into various forms as well as refine using simple tools. And speaking of those tools unlike wood iron can handle high temperatures allowing us to build everything from fire tongs to furnaces out of it. In contrast to most substances, you can also magnetize iron making it useful in the creation of electric motors and generators. The Earth's crust is 5 percent iron and in some areas the element concentrates in ores that contain as much as 70 percent iron.

When you compare iron and steel with something like aluminum, you can see why it was so important historically. To refine aluminum, you need access to huge quantities of electricity. Furthermore to shape aluminum you have to either cast it or extrude it. Iron however is much easier to manipulate.

The only real problem with iron and steel is rust. Fortunately you can control rust by painting, galvanizing, chrome plating or coating the iron with a sacrificial anode, which corrodes faster than the stronger metal. Think of this last option as hiring a bodyguard to take a bullet for the president. The more active metal has to almost completely corrode before the less active iron or steel begins the process. Humans have come up with countless uses for iron from carpentry tools and culinary equipment to complicated machinery and instruments of torture. Before iron can be put to any of these uses however it has to be mined from the ground.

Properties of Iron

Iron is a shiny, bright white metal that is soft, malleable, ductile and strong. Its surface is usually discolored by corrosion, since it combines readily with the oxygen of the air in the presence of moisture. In absolutely dry air, it does not rust. The oxide that is produced is crumbly and soft, giving no protection to the base metal, which eventually rusts away. It is found in nature as the metal only in meteorites and in very rare circumstances where iron minerals have been reduced by environmental factors.

Importance of Iron

At room temperature, iron is in the form of ferrite, or α-iron, a body-centered cubic structure. The density of α-iron is 7.86 g/cc. At 910°C it changes to γ-iron, which is face-centered cubic and somewhat softer. At 1535°C iron melts, and boils at 3000°C .Cobalt melts at 1480°C, nickel at 1455°C. The specific heat of any of the three metals is about 0.107 cal/g-K. The thermal conductivities of Fe, Co and Ni are 3.37, 3.81 and 4.19 cal/s-cm-K. Their electrical resistivities are 9.71, 6.24 and 6.84 μΩ-cm. These are "worse" than those of copper by factors of only 4 to 6, so the iron metals are very good conductors of electricity and heat. Comparing the numbers shows how similar these metals are in their physical properties. Cobalt and nickel do not make useful alloys with carbon, as iron does. They are much too expensive to use as structural metals other than as alloying elements or coatings.