Steel and Metal Alloys: Production, Properties, and Processes
Steel Production:
Iron (Fe) is extracted from blast furnaces, separating slag. The molten iron is then further processed in other ovens, such as the cubilot, to reduce impurities. Oxygen converters are used, which are cylindrical or conical vessels lined with ceramic refractory material. Molten iron and scrap are loaded, and pure oxygen is introduced under pressure to reduce impurities, maintaining high temperatures. Electric arc furnaces, with water-cooled walls and a ceramic refractory coating, are also used. These are closed with a graphite electrode cover. Iron, scrap, and lime are added, and the temperature is increased. Alloying elements are then added to achieve specific compositions. Temperature and composition are carefully controlled.
Steel Solidification:
Molten steel is poured into molds to cool, or into channels to form bars. These bars are then sent to hot rolling mills without needing to be reheated.
Heat Treatments for Steel:
Heat treatments are controlled processes that alter the properties of steel by changing the proportions of its constituents.
- Tempering: Achieves a martensitic structure by heating to austenite and rapidly cooling.
- Reheating: Heating below 723°C to avoid austenite formation, followed by air cooling, increases tenacity and mechanical resistance.
- Annealing: Reduces hardness and increases elasticity for better workability. Heating to a high temperature and cooling slowly in air.
- Types of Annealing: Regeneration (for >0.6% C), globular supercritical (for alloy and tool steel), and normalizing.
- Normalizing: Heating to austenitization temperature and air cooling to refine the grain structure, reduce internal stresses, and improve mechanical properties.
Metal Production Processes:
Mining: Extraction of mineral deposits.
Metallurgy: Separation of metals from other elements.
Metal Industries: Processing metals into useful items.
Metal Extraction: Minerals are extracted and separated from impurities, which are rich in metal oxides, sulfides, and carbonates. The ore is the mineral rich in usable metal. The gangue is the unusable part, poor in metal. Metal extraction is profitable when the metal content exceeds a minimum percentage.
Alloys:
An alloy is a product of two or more chemical elements, at least two of which must be metals. Alloys exhibit metallic characteristics and are identified by their components.
Solidification of Metals and Alloys:
Pure metals have a fixed melting point, while alloys do not. The solidification of alloys involves different phases:
- Liquid Phase: Above the liquidus line, the alloy is liquid.
- Solid + Liquid Phase: In this region, the alloy contains both solid and liquid phases.
- Solid Phase: Below the solidus line, the alloy is solid.
Advantages of Alloys:
Alloys solidify over a temperature range, not at a constant temperature. Their solidification or melting temperature is lower than that of their pure components. They are intimate mixtures of fine crystals, making them ideal for filling molds and creating homogeneous parts.
Pure Iron:
Pure iron has limited industrial applications due to its low mechanical strength and tendency to corrode. Iron-carbon alloys with less than 0.03% carbon retain magnetic properties, used in transformer cores.
Allotropic Forms of Iron:
- Delta Range: 1539°C to 1396°C.
- Gamma Range: 1390°C to 900°C.
- Beta Range: 900°C to 750°C.
- Alpha Range: 0°C to 750°C.
To obtain a specific iron variety, rapid cooling is needed at the corresponding temperature.
Constituents of Iron-Carbon Alloys:
- Ferrite: Alpha iron.
- Cementite: Iron carbide.
- Pearlite: 86.4% ferrite and 13.5% cementite.
- Austenite: Gamma iron.
- Martensite: Alpha iron.
Iron-Carbon Alloy Products:
- Steels: Carbon content between 0.1% and 1.76%.
- Cast Irons: Carbon content between 1.76% and 6.67% (typically 3% to 4.5%).
Forging:
A hot metal mass is placed between mold halves, and compression is applied to shape it.
Molding:
Molten metal is poured into a closed mold and allowed to solidify.
Types of Cast Iron:
- White Cast Iron: Contains carbon as iron carbide.
- Gray Cast Iron: Contains carbon as graphite in various forms:
- Laminar: Flake-like.
- Nodular: Branch or grape-like.
- Spheroidal: Sphere-shaped.
Reduction:
The separation of oxygen from a metal.
Phases for Obtaining Steel and Castings:
The process starts with obtaining cast iron in a blast furnace. Then, liquid converters are used to eliminate impurities and obtain the desired steel.
Blast Furnace:
A steel structure lined with refractory ceramic material. It has two conjoined trunks, about 30m high and 6m in diameter. Raw gases are extracted from the top, while hot air enters at the bottom. Cast iron and slag are collected at the base.
Blast Furnace Inputs:
- Iron Ore: Oxidized iron that, once reduced, becomes cast iron.
- Coke: Acts as fuel for high temperatures and provides carbon for iron reduction.
Blast Furnace Process:
Raw materials are introduced in layers at the top. Coke combustion is achieved by injecting hot air at the base. Cast iron is periodically removed from a hole slightly higher than the previous one.
Cast Iron from Blast Furnace:
An alloy of iron and carbon, with approximately 4% carbon, 2% silicon, and smaller amounts of phosphorus, sulfur, and oxygen.