Steel vs Iron
- Iron is a naturally occurring metallic element. It is almost never found in its native form (pure iron) in nature. It is usually found as an oxide or sulfide, with many other impurity elements mixed in. The difference between iron and steel is the amount of carbon: Iron that contains more than 0.35 per cent carbon becomes steel.
- Wrought iron is the purest form of iron generally encountered or produced in quantity. It may contain as little as 0.04% Carbon (by weight). From its traditional method of manufacture, wrought iron has a fibrous internal texture. Quality wrought-iron blacksmithing takes the direction of these fibers into account during forging, since the strength of the material is stronger in line with the grain, than across the grain. Most of the remaining impurities from the initial smelting become concentrated in silicate slag trapped between the iron fibers. This slag produces a lucky side effect during forge-welding. When the silicate melts, it makes wrought-iron self-fluxing. The slag becomes a liquid glass that covers the exposed surfaces of the wrought-iron, preventing oxidation which would otherwise interfere with the successful welding process.
- Cast iron is iron that contains between 2.0% to 6% Carbon by weight. There is so much carbon present, that the hardness cannot be switched off. Hence, cast iron is a brittle metal, which can break like glass. Cast iron cannot be forged without special heat treatment to convert it to malleable iron.
- Steel is a mixture of Iron and between 0.3% to 1.7% Carbon by weight. The presence of carbon allows steel to assume one of several different crystalline configurations. Macroscopically, this is seen as the ability to "turn the hardness of a piece of steel on and off" through various processes of heat-treatment. If the concentration of carbon is held constant, this is a reversible process. Steel with a higher carbon percentage may be brought to a higher state of maximum hardness.
Steel with below 0.6% Carbon content cannot be hardened by simple heat-treatment enough to make useful hardened-steel tools. Hence, in what follows, wrought-iron, low-carbon-steel, and other soft unhardenable iron varieties will be referred to indiscriminately as just iron.
When iron ore is smelted into usable metal, a certain amount of carbon is usually alloyed with the iron. (Charcoal is almost pure carbon.) The amount of carbon significantly affects the properties of the metal. If the carbon content is over 2%, the metal is called cast iron, because it has a relatively low melting point and is easily cast. It is quite brittle, however, and is therefore not used for blacksmithing. If the carbon content is between 0.25% and 2%, the resulting metal is tool steel, which can be heat treated as discussed above. When the carbon content is below 0.25%, the metal is either "wrought iron" or "mild steel." The terms are never interchangeable. In preindustrial times, the material of choice for blacksmiths was wrought iron. This iron had a very low carbon content, and also included up to 5% of glassy slag. This slag content made the iron very tough, gave it considerable resistance to rusting, and allowed it to be more easily "forge welded," a process in which the blacksmith permanently joins two pieces of iron, or a piece of iron and a piece of steel, by heating them nearly to a white heat and hammering them together. Forge welding is more difficult to do with modern mild steel. Modern steel production, using the blast furnace, cannot produce true wrought iron, so this material is now a difficult-to-find specialty product. Modern blacksmiths generally substitute mild steel for making objects that were traditionally of wrought iron.
Hot Rolled vs Cold Rolled Steel
| Hot rolled steel | Cold rolled steel | |
|---|---|---|
| Hot-rolled steel is put through the rolling process to make its final dimensions at temperatures above 1000° | The rolling process for cold-rolled steel is done at temperatures close to room temperature. This procedure makes the material stronger and harder. | |
| Finish The two processes leave the metal with different colors and finishes. |
Hot-rolled has a rougher, blue-gray finish. | Cold-rolled is smooth and gray. |
| Tolerances | Hot-rolled finished material will have looser tolerances applied to it because it reconfigures itself during the cooling process. | Cold rolling allows a more precise dimensional finished product because it has already gone through the cooling process and it is closer to the finished dimension. |
| Finished products |
When steel is heated to the point it is malleable, it is possible to force it through a variety of shapes. This allows manufacture of I beams and other structural components. | Cold steel is limited to few shapes, mostly flat, round, square and variations of those. It is straighter, has a better finish and tighter tolerances. |
How to Identify Cold Rolled Steel
By Lawrence Koenig, eHow Contributor
- Measure the steel. If it seems to be a bit short on one side and is not completely squared up, then you are more than likely dealing with hot rolled steel. This is because hot rolled steel is much harder to calculate to accurate measurements once it begins to recrystallize. The steel will shrink considerably and in most cases unevenly.
- Look at the color and the finish. Hot rolled steel always has a rough blue-gray finish, while cold rolled steel has a smooth gray finish. This is because hot rolled steel takes a considerably longer amount of time to process. As it has to be continually heated, its surface is exposed to the air more, causing it to oxidize faster than cold rolled steel. This accounts for its rougher surface and bluish color.
- Assess the number of shapes available. If several different sizes of steel are available in the store's selection, chances are they are hot rolled instead of cold rolled. This is because it is much easier to forge hot steel than cold steel -- so stores carry fewer shapes and sizes of cold rolled steel.
- Check how the product bends. Hot rolled steel is much more flexible than cold rolled steel. If the steel you're looking at has very little give, chances are it is cold rolled.
- Examine the edges. Hot rolled steel has rounded edges more often than not, while cold rolled steel has exact right angles.
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