Metal Properties, Structures, and Processing Techniques
Metal Characteristics
- High thermal and electrical conductivity
- Mechanical strength
- Plasticity
- High malleability
- Recyclability
Crystalline Structures
- Crystalline: When atoms are perfectly arranged in space (metals, ceramics).
- Amorphous: Only presents a short-range order (glass, glassy polymers).
Types of Crystallization
- Body-centered cubic
- Face-centered cubic
- Hexagonal close-packed
Solid Solutions
Two solid metals are soluble in one another if, in the lattice of one of them, some of its atoms can be replaced by other metal atoms (substitution) or can be inserted into the spaces (interstitial).
Defects in the Crystal Lattice
- Point defects: Due to atoms of the same or another metal located at a point that does not belong to the lattice.
- Line defects: Decrease the mechanical strength of metals.
- Surface defects: Their respective crystallographic axes do not match each other.
Grain Formation
During solidification, when the liquid metal cools, there comes a moment when atoms begin to arrange themselves in space, simultaneously from several points. Its shape is equiaxed: there is no dimension that prevails over the others.
Hardening Methods
Cold Deformation Hardening
Plastic deformation causes hardening and fragility; it is subjected to heat treatment (annealing).
Grain Refinement Hardening
There is an experimental relationship between the yield strength of a material and the average size of its grains; the smaller the average grain size, the better the yield strength.
Solid Solution Hardening
It is due to the mechanical deformation that occurs when the sizes of the solvent and solute atoms do not exactly match.
Heat Treatments
Operations of heating and cooling metals to alter their crystalline structure.
- Annealing: Heated for some time at a determined temperature and then slowly cooled, giving greater plasticity.
- Quenching: Heat the metal, often followed by rapid cooling, yielding a hard metal with high mechanical strength.
- Tempering: Improves the toughness of quenched metal, diminishing its hardness.
Steelmaking Process
All operations necessary to obtain a ferrous metal from ore reserves with certain characteristics. The ore extracted from iron mines contains oxides and carbonates, accompanied by non-ferrous substances (gangue).
Mechanical Treatments
Enhance the properties of metals by mechanical deformation.
Hot Mechanical Treatments
Involves heating a metal to the deformation temperature, then forging it. This refines the grain size and removes blowholes.
Cold Mechanical Treatments
Consists of deforming the metal at ambient temperature, forging it, and subjecting it to drawing or lamination, to increase hardness and mechanical resistance.
Surface Treatments
- Metallization: A molten metal is projected, sprayed on the surface of another.
- Chrome Plating: Chromium is deposited electrolytically on the metal to reduce the coefficient of friction and increase wear resistance.
Ferrous Metals
Base element: iron. Magnetic metallic material, bluish color, ductile, melting point 1535°C, good conductor of heat and electricity. Exposed to air, it forms rust and corrodes (reddish-brown scaly substance).
Industrial Iron
Low carbon content, has some inadequate mechanical characteristics.
Steel
- Ductile and malleable.
- Mechanical resistance, hardness, and brittleness increase with carbon content.
- Weldability decreases with increasing carbon percentage.
- Easily oxidized.
Steel Alloying Elements
- Sulfur: Imparts great fragility; it is detrimental and is counteracted by adding manganese.
- Chromium: Increases the hardness, corrosion resistance, and toughness of steel.
- Manganese: Increases the hardness of hardened steels.
- Nickel: Is a key component of stainless steel and increases its resistance to traction.
- Lead: Favors steel machining by metal removal procedures because lead acts as a lubricant.
- Silicon: Removes excess oxygen and provides great elasticity.
Cast Irons
Iron-carbon alloy containing between 1.67% and 6.67% carbon. They are easily fusible, non-ductile, and non-malleable. Advantages: Simpler manufacturing than steel, possess acceptable mechanical characteristics, and are cheaper than steel.