2.3 IRONS AND STEELS
Irons and steels are the most popular metals in use today. The production of iron was at one time a subject of mystic awe.
Any engineer involved with modern engineering should have at least a passing knowledge of steels to understand many of the processes.
2.3.1 Types of Steel
Various steel alloys are commonly identified with the SAE-AISI numbers,
Typical applications for plain steels (based on the SAE-AISI numbers) are,
2.3.1.1 - Alloying Elements
HòˆòÈA Short list of elements is given below,
Typical elements that are left over from the manufacturing processes leave behind detrimental elements,
2.3.2 Making Steels
1. Iron ore is mined, and crushed. At this point in contains iron, carbon, oxygen, and a variety of other minerals.
2. The ore is heated in a blast furnace with coke. This removes some of the elements, notably oxygen. Pig iron remains, and has high levels of carbon.
2.3.2.1 - The Ores
HòˆòÈThe ores come in a number of forms,
The ores are crushed to ease handling, and speed melting.
After crushing, iron rich ore can be separated using magnets.
the resulting ore is formed into pellets of about 65% iron.
2.3.2.2 - Coke
HòˆòÈThe classic recipe for Coke begins with bituminous coal. It is then heated to 2100°F and then cooled with water.
2.3.2.3 - Flux, Slag
HòˆòÈSome materials are used as a flux, and to create slag,
By adding a flux material, it will react with impurities, causing them to flow.
After the flux dissolves the impurities, it reacts with them to form a solid called slag. This floats to the top of the melt, where it is removed.
2.3.3 The Blast Furnace - How To Make Pig Iron
These furnaces are large heated vessels, and they are lined with bricks of refractory materials.
Iron pellets, limestone, and coke are mixed together and dumped into the top of the furnace.
Air is preheated to 2000°F and this is used to `blast' the mixture into the furnace. The coke ignites, and elevates the temperature of the mixture to 3000°F. This results in a reduction of the iron oxides, and separation of the slags.
After some period of time (a few hours) the furnace is tapped, and the iron is drawn off to large ladles. This `pig' iron typically has a impurity contents of 4% C, 1.5% Si, 1% Mn, 0.04% S, 0.4% P.
2.3.4 How To Make Steel
Making steel is a process of reducing the following impurities in pig iron,
This operation is commonly done in one of three furnaces,
The basic procedures with all of these furnaces is,
Any oxygen left in the steel when solidified will combined with carbon. The result is small voids that are actually pockets of carbon monoxide gas. A killed steel will have all oxygen removed.
2.3.4.1 - Electric Furnaces
HòˆòÈIn induction furnaces large coils are wound around the crucible. AC current is applied, and this induces heat in the metal inside. Vacuum can be applied to the melt to increase purities of the final steel.
Arcing furnaces use carbon electrodes at high potentials to create arcs. These act to heat the metal.
The furnaces reach temperatures up to 3500°F.
2.3.5 Forming The Steel
There are options after the steel has been processed,
Continuous casting uses a slow pour that when running smoothly,
2. It solidifies, still at forming temperatures, and typically moving at 5 fpm. A pulling action keeps a continuous rate.
The result of continuous casting is a single process that produces final steel sections without any of the intermediate problems that result from remelting ingots.
2.3.6 Stainless Steel
These steels use a high Chromium content(10 to 12%) to form a protective layer of chromium oxide over the surface of the work that is resistant to many forms of corrosion
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