Metallurgy is a domain of materials science that studies the physical and chemical behavior of metallic elements, their intermetallic compounds, and their compounds, which are called alloys. It is also the technology of metals: the way in which science is applied to their practical use. Metallurgy is commonly used in the craft of metalworking.
Properties of metals
The five most used metals are:[citation needed]
The general physical properties of metals are:
- They are strong and hard.
- They are solids at room temperature (except for Mercury, which is the only metal to be liquid at room temperature)
- They have a shiny luster when polished.
- They make good heat conductors and electrical conductors.
- They are dense
- They produce a sonorous sound when struck.
- They have high melting points
- They are malleable
The properties of metals make them suitable for different uses in daily life.
- Copper is a good conductor of electricity and is ductile. Therefore Copper is used for electrical cables.
- Gold and Silver are very malleable, ductile and very nonreactive. Gold and silver are used to make intricate jewelry which does not tarnish. Gold can also be used for electrical connections.
- Iron and Steel are both hard and strong. Therefore they are used to construct bridges and buildings. A disadvantage of using Iron is that it tends to rust, whereas most steels rust, they can be formulated to be rust free.
- Aluminum is a good conductor of heat and is malleable. It is used to make saucepans and tin foil, and also aeroplane bodies as it is very light.
Pure elemental metals are often too soft to be of practical use which is why much of metallurgy focuses on formulating useful alloys.
Important common alloy systems
Common engineering metals include aluminium, chromium, copper, iron, magnesium, nickel, titanium and zinc. These are most often used as alloys. Much effort has been placed on understanding the iron-carbon alloy system, which includes steels and cast irons. Plain carbon steels are used in low cost, high strength applications where weight and corrosion are not a problem. Cast irons, including ductile iron are also part of the iron-carbon system.
Stainless steel or galvanized steel are used where resistance to corrosion is important. Aluminium alloys and magnesium alloys are used for applications where strength and lightness are required.
Cupro-nickel alloys such as Monel are used in highly corrosive environments and for non-magnetic applications. Nickel-based superalloys like Inconel are used in high temperature applications such as turbochargers, pressure vessels, and heat exchangers. For extremely high temperatures, single crystal alloys are used to minimize creep.
Production engineering of metals
In production engineering, metallurgy is concerned with the production of metallic components for use in consumer or engineering products. This involves the production of alloys, the shaping, the heat treatment and the surface treatment of the product. The task of the metallurgist is to achieve balance between material properties such as cost, weight, strength, toughness, hardness, corrosion and fatigue resistance, and performance in temperature extremes. To achieve this goal, the operating environment must be carefully considered. In a saltwater environment, ferrous metals and some aluminium alloys corrode quickly. Metals exposed to cold or cryogenicstress at elevated temperatures can creep. conditions may endure a ductile to brittle transition and lose their toughness, becoming more brittle and prone to cracking. Metals under continual cyclic loading can suffer from metal fatigue. Metals under constant
Metal working processes
Metals are shaped by processes such as casting, forging, flow forming, rolling, extrusion, sintering, metalworking, machiningfabrication. With casting, molten metal is poured into a shaped mould. With forging, a red-hot billet is hammered into shape. With rolling, a billet is passed through successively narrower rollers to create a sheet. With extrusion, a hot and malleable metal is forced under pressure through a die, which shapes it before it cools. With sintering, a powdered metal is compressed into a die at high temperature. With machining, lathes, milling machines, and drills cut the cold metal to shape. With fabrication, sheets of metal are cut with guillotines or gas cutters and bent into shape.
"Cold working" processes, where the product’s shape is altered by rolling, fabrication or other processes while the product is cold, can increase the strength of the product by a process called work hardening. Work hardening creates microscopic defects in the metal, which resist further changes of shape.
Various forms of casting exist in industry and academia. These include sand casting, investment casting (also called the “lost wax process”), die casting and continuous casting.
Joining
Welding
Welding is a technique for joining metal components by melting the base material. A filler material of similar composition may also be melted into the joint.
Brazing
Brazing is a technique for joining metals at a temperature below their melting point. A filler with a melting point below that of the base metal is used, and is drawn into the joint by capillary action. Brazing results in a mechanical and metallurgical bond between work pieces.
Soldering
Soldering is a method of joining metals below their melting points using a filler metal. Soldering results in a mechanical joint and occurs at lower temperatures than brazing, specifically below 450 C (840 F)[7].
Heat treatment
Metals can be heat treated to alter the properties of strength, ductility, toughness, hardness or resistance to corrosion. Common heat treatment processes include annealing, precipitation strengthening, quenching, and tempering. The annealing process softens the metal by allowing recovery of cold work and grain growth. Quenching can be used to harden alloy steels, or in precipitation hardenable alloys, to trap dissolved solute atoms in solution. Tempering will cause the dissolved alloying elements to precipitate, or in the case of quenched steels, improve impact strength and ductile properties.
Surface treatment
Plating
Electroplating is a common surface-treatment technique. It involves bonding a thin layer of another metal such as gold, silver, chromium or zinc to the surface of the product. It is used to reduce corrosion as well as to improve the product's aesthetic appearance.
Thermal spray
Thermal spraying techniques are another popular finishing option, and often have better high temperature properties than electroplated coatings.
Case hardening
Case hardening is a process in which an alloying element, most commonly carbon or nitrogen, diffuses into the surface of a monolithic metal. The resulting interstitial solid solution is harder than the base material, which improves wear resistance without sacrificing toughness.
Electrical and electronic engineering
Metallurgy is also applied to electrical and electronic materials where metals such as aluminium, copper, tin, silver, and goldprinted circuit boards and integrated circuits. are used in power lines, wires,
Metallurgical techniques
Metallurgists study the microscopic and macroscopic properties using metallography, a technique invented by Henry Clifton Sorby. In metallography, an alloy of interest is ground flat and polished to a mirror finish. The sample can then be etched to reveal the microstructure and macrostructure of the metal. A metallurgist can then examine the sample with an optical or electron microscope and learn a great deal about the sample's composition, mechanical properties, and processing history.
Crystallography, often using diffraction of x-rays or electrons, is another valuable tool available to the modern metallurgist. Crystallography allow the identification of unknown materials and reveals the crystal structure of the sample. Quantitative crystallography can be used to calculate the amount of phases present as well as the degree of strain to which a sample has been subjected.
The physical properties of metals can be quantified by mechanical testing. Typical tests include tensile strength, compressive strength, hardness, impact toughness, fatigue and creep life.
Chemical properties of metals
Substances on the Earth's surface will come in contact with air, water or acids. A major concern for the use of metals is their corrosion. The shiny surface of many metals becomes dull in time. This is due to a slow chemical reaction between the surface of the metal and oxygen in the air; this is typically a surface coating of the metal oxide. The general word equation is:
metal+oxygen → metal oxide
For example: The dull appearance of the metal lead is due to a coating of lead oxide.
If the surface is scratched then the shiny lead metal can be seen underneath.
Heating can speed up the reaction with oxygen. If a piece of copper is heated it quickly becomes coated in black copper oxide. The word equation is:
copper + oxygen → copper oxide
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