Metallurgy is a domain of materials science and engineering
that studies the physical and chemical behavior of metallic elements,
their intermetallic compounds, and their mixtures, which are
called alloys.
Metallurgy is also the technology of metals: the way in which science is applied
to the production of metals, and the engineering of metal components for use in
products for consumers and manufacturers. The production of metals involves the
processing of ores to
extract the metal they contain, and the mixture of metals, sometimes with other
elements, to produce alloys. Metallurgy is distinguished from the craft of metalworking.
Metallurgy is subdivided into ferrous metallurgy (sometimes also known as
black metallurgy) and non-ferrous
metallurgy or colored metallurgy. Ferrous metallurgy involves
processes and alloys based on iron while non-ferrous metallurgy involves processes and alloys
based on other metals. The production of ferrous metals accounts for 95 percent
of world metal production.
Etymology and pronunciation
The word was originally an alchemist's
term for the extraction of metals from minerals, the ending -urgy
signifying a process, especially manufacturing: it was discussed in this sense
in the 1797 Encyclopaedia Britannica. In the late 19th
century it was extended to the more general scientific study of metals, alloys,
and related processes. The roots of metallurgy derive from Ancient
Greek: μεταλλουργός, metallourgós, "worker in metal", from
μέταλλον, métallon, "metal" + ἔργον, érgon,
"work". In English, the /meˈtælədʒi/ pronunciation is the more common
one in the UK and Commonwealth. The /ˈmetələrdʒi/ pronunciation is the more
common one in the USA, and is the first-listed variant in various American
dictionaries (e.g., Merriam-Webster Collegiate, American Heritage).
History
The earliest recorded metal employed by humans appears to be
gold which can be
found free or "native". Small amounts of natural gold have been found
in Spanish caves used during the late Paleolithic
period, c. 40,000 BC. Silver, copper, tin
and meteoric iron
can also be found native, allowing a limited amount of metalworking
in early cultures. Egyptian weapons made from meteoric
iron in about 3000 BC were highly prized as "Daggers from
Heaven".
Certain metals can be recovered from their ores by simply
heating the rocks in a fire: notably tin, lead and (at a higher
temperature) copper, a process known as smelting. The
first evidence of this extractive metallurgy dates from the 5th and 6th
millennium BC and was found in the archaeological sites of Majdanpek, Yarmovac and Plocnik, all
three in Serbia.
To date, the earliest evidence of copper smelting is found at the Belovode site, including a copper
axe from 5500 BC belonging to the Vinča
culture.Other signs of early metals are found from the third
millennium BC in places like Palmela (Portugal), Los
Millares (Spain), and Stonehenge (United Kingdom). However, as often happens
with the study of prehistoric times, the ultimate beginnings cannot be
clearly defined and new discoveries are both continuous and ongoing.
Mining areas of the ancient Middle East.
Boxes colors: arsenic
is in brown, copper
in red, tin in grey,
iron in reddish brown, gold in yellow, silver in white and lead in black. Yellow
area stands for arsenic bronze, while grey area stands for tin bronze.
These first metals were single ones or as found. About 3500
BC, it was discovered that by combining copper and tin, a superior metal could
be made, an alloy
called bronze,
representing a major technological shift which began the Bronze Age.
The extraction of iron from its ore into a workable metal is much more difficult
than for copper or tin. The process appears to have been invented by the Hittites in
about 1200 BC, beginning the Iron Age. The
secret of extracting and working iron was a key factor in the success of the Philistines.
Historical developments in ferrous metallurgy can be found
in a wide variety of past cultures and civilizations. This includes the ancient
and medieval kingdoms and empires of the Middle East
and Near
East, ancient Iran,
ancient Egypt,
ancient Nubia, and
Anatolia (Turkey), Ancient Nok,
Carthage,
the Greeks and Romans
of ancient Europe,
medieval Europe, ancient and medieval China, ancient and
medieval India,
ancient and medieval Japan,
amongst others. Many applications, practices, and devices associated or
involved in metallurgy were established in ancient China, such as the
innovation of the blast furnace, cast iron, hydraulic-powered
trip
hammers, and double acting piston bellows.
A 16th century book by Georg
Agricola called De re metallica describes the highly developed and
complex processes of mining metal ores, metal extraction and metallurgy of the
time. Agricola has been described as the "father of metallurgy".
Extraction
Extractive metallurgy is the practice of
removing valuable metals from an ore and refining the extracted raw metals into a purer form. In
order to convert a metal oxide or sulfide to a purer metal, the ore must be reduced physically, chemically,
or electrolytically.
Extractive metallurgists are interested in three primary
streams: feed, concentrate (valuable metal oxide/sulfide), and tailings
(waste). After mining, large pieces of the ore feed are broken through crushing
and/or grinding in order to obtain particles small enough where each particle
is either mostly valuable or mostly waste. Concentrating the particles of value
in a form supporting separation enables the desired metal to be removed from
waste products.
Mining may not be necessary if the ore body and physical
environment are conducive to leaching.
Leaching dissolves minerals in an ore body and results in an enriched solution.
The solution is collected and processed to extract valuable metals.
Ore bodies often contain more than one valuable metal.
Tailings of a previous process may be used as a feed in another process to
extract a secondary product from the original ore. Additionally, a concentrate
may contain more than one valuable metal. That concentrate would then be
processed to separate the valuable metals into individual constituents.
Alloys
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 (those that contain essentially only carbon as an alloying
element) 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.
Copper-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
vessel, and heat exchangers. For extremely high temperatures, single
crystal alloys are used to minimize creep.
Production
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, 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 cryogenic
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 stress at elevated temperatures can creep.
Metalworking processes
- casting – molten metal is poured into a shaped mold.
- forging – a red-hot billet is hammered into shape.
- flow forming
- rolling – a billet is passed through successively narrower rollers to create a sheet.
- Laser cladding – metallic powder is blown through a movable laser beam (e.g. mounted on a NC 5-axis machine). The resulting melted metal reach a substrate to form a melt pool. By moving the laser head, it is possible to stack the tracks and build up a 3D piece.
- extrusion – a hot and malleable metal is forced under pressure through a die, which shapes it before it cools.
- sintering – a powdered metal is heated in a non-oxidizing environment after being compressed into a die.
- metalworking
- machining – lathes, milling machines, and drills cut the cold metal to shape.
- fabrication – sheets of metal are cut with guillotines or gas cutters and bent and welded into structural 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.
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 heating it and then allowing it to cool very slowly, which
gets rid of stresses in the metal and makes the grain structure large and
soft-edged so that when the metal is hit or stressed it dents or perhaps bends,
rather than breaking; it is also easier to sand, grind, or cut annealed metal. Quenching
is the process of cooling a high-carbon steel very quickly after you have
heated it, thus "freezing" the steel's molecules in the very hard
martensite form, which makes the metal harder. There is a balance between
hardness and toughness in any steel, where the harder it is, the less tough or
impact-resistant it is, and the more impact-resistant it is, the less hard it
is. Tempering relieves stresses in the metal that were caused by the
hardening process; tempering makes the metal less hard while making it better
able to sustain impacts without breaking.
Often, mechanical and thermal treatments are combined in
what is known as thermo-mechanical treatments for better properties and more
efficient processing of materials. These processes are common to high alloy
special steels, super alloys and titanium alloys.
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 spraying
Thermal spraying techniques are another popular finishing
option, and often have better high temperature properties than electroplated
coatings.
Microstructure
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. The
sample is then examined in an optical or electron microscope, and the image contrast
provides details on the 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
allows 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.
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