Metallurgy is a domain of materials science that studies the
physical and chemical behavior ofmetallic elements, their intermetallic compounds, and their mixtures,
which are called alloys. It is also the technology of metals: the way in which
science is applied to their practical use. Metallurgy is distinguished from
the craft of metalworking.
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 in this sense used by the 1797Encyclopaedia
Britannica.[1] In the late 19th century it was
extended to the more general scientific study of metals and alloys and related
processes.[1] The roots are borrowed 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
Gold headband from
Thebes 750–700 BC
See also: Chalcolithic Bronze Age History of
ferrous metallurgy Metallurgy
in pre-Columbian AmericaMetallurgy
in pre-Columbian Mesoamerica History of metallurgy in the Indian subcontinent Prehistory
of Non-Ferrous Extractive Metallurgy
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 latePaleolithic period, c. 40,000
BC.[2] Silver, copper, tin and
meteoric iron can also be found native, allowing a
limited amount of metalworking in
early cultures.[3] Egyptian weapons made
from meteoric iron in
about 3000 BC were highly prized as "Daggers from Heaven".[4]
Certain metals can be
recovered from their ores by simply heating the rocks in a fire: notably
tin, leadand
(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 copper smelting is found at
the Belovode site,[5] these examples include a copper
axe from 5500 BC belonging to theVinča culture.[6] Other signs of early metals are
found from the third millennium BC in places likePalmela (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 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. By combining copper and tin, a superior metal
could be made, an alloy called bronze, a major technological shift which began the Bronze Age about 3500 BC.
The extraction
of iron from
its ore into a workable metal is much more difficult than copper or tin. It
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.[4][7]
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 andNear 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.[8][9]
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".[10]
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
Casting bronze
Common
engineering metals include aluminium, chromium, copper, iron, magnesium, nickel, titaniumand 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 asturbochargers, 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, fatigueresistance,
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
Metals are shaped by
processes such as:
- 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 calledwork 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,.[11] 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
Metallography allows
the metallurgist to study the microstructure of metals.
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.
References
3.
^ Photos, E., 'The Question of Meteoritic versus Smelted
Nickel-Rich Iron: Archaeological Evidence and Experimental Results' World
Archaeology Vol. 20, No. 3, Archaeometallurgy (February 1989), pp.
403–421. Online version accessed on 2010-02-08.
4.
^ a b W. Keller (1963) The
Bible as History, p. 156 ISBN 0-340-00312-X
5.
^ Radivojević, Miljana; Rehren, Thilo; Pernicka, Ernst;
Šljivar, Dušan; Brauns, Michael; Borić, Dušan (2010). "On the origins of
extractive metallurgy: New evidence from Europe". Journal of
Archaeological Science 37 (11): 2775. doi:10.1016/j.jas.2010.06.012.
9.
^ Temple, Robert K.G. (2007). The Genius of China:
3,000 Years of Science, Discovery, and Invention (3rd edition).
London: André Deutsch. pp.
44–56. ISBN 978-0-233-00202-6.
11.
^ Arthur Reardon (2011), Metallurgy for the
Non-Metallurgist (2nd edition), ASM International,ISBN 978-1-61503-821-3
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