1.3 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.
1.3.1 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,
1.3.1.1 - Alloying Elements
• A Short list of elements is given below,
• Typical elements that are left over from the manufacturing processes leave behind detrimental elements,
- antimony
- arsenic
- hydrogen
- nitrogen
- oxygen
- tin
1.3.2 Making Steels
• The basic process is,
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.
3. A refining furnace is then used to burn off the excess carbon, leaving a good quality steel.
1.3.2.1 - The Ores
• The ores come in a number of forms,
- taconite -
-hermatite - iron oxide mineral
- limonite - iron oxide and water
• 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.
1.3.2.2 - Coke
• The classic recipe for Coke begins with bituminous coal. It is then heated to 2100°F and then cooled with water.
• Coke will
- generate higher level of heat during steel making
- generate carbon monoxide which reacts with oxygen in iron oxide and leaves iron
1.3.2.3 - Flux, Slag
• Some materials are used as a flux, and to create slag,
- limestone
- dolomite
• 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.
1.3.3 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.
1.3.4 How To Make Steel
• Making steel is a process of reducing the following impurities in pig iron,
- manganese
- silicon
- carbon
- phosphorous
- sulphur
- etc
• This operation is commonly done in one of three furnaces,
- open hearth - flames are directly applied to the metal, and can be seen from the open hearth.
- electric
- basic oxygen - a blast of pure oxygen reacts with impurities
• The basic procedures with all of these furnaces is,
1. Charge (pour in) scrap iron
2. Pour in molten (pig) iron
3. Add lime
4. Run the furnace
5. Tap the furnace to remove the steel - care must be used not to pour the slag on the surface
6. Pour off the slag off
• 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.
1.3.4.1 - Electric Furnaces
• There are two basic types,
- induction
- arcing
• In 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.
1.3.5 Forming The Steel
• There are options after the steel has been processed,
- ingots - the steel is poured into ingots, and stored to be formed later
- continuous casting - the steel is poured, and immediately formed to bars, rolls, etc.
• Continuous casting uses a slow pour that when running smoothly,
1. Is liquid at the top where it is being poured.
2. It solidifies, still at forming temperatures, and typically moving at 5 fpm. A pulling action keeps a continuous rate.
3. It is rolled, bent, formed, and cut.
• 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.
1.3.6 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
• General families of stainless steels include,
Austenitic (2xx, 3xx) -
Ferritic (4xx) -
Martensitic (4xx and 5xx) -
Precipitation Hardened (PH) -
Duplex -
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