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Friday, March 1, 2019

Self-replicating spacecraft

From Wikipedia, the free encyclopedia

The idea of self-replicating spacecraft has been applied – in theory – to several distinct "tasks". The particular variant of this idea applied to the idea of space exploration is known as a von Neumann probe, after being conceived by mathematician John von Neumann. Other variants include the Berserker and an automated terraforming seeder ship.

Theory

Von Neumann proved that the most effective way of performing large-scale mining operations such as mining an entire moon or asteroid belt would be by self-replicating spacecraft, taking advantage of their exponential growth. In theory, a self-replicating spacecraft could be sent to a neighbouring planetary system, where it would seek out raw materials (extracted from asteroids, moons, gas giants, etc.) to create replicas of itself. These replicas would then be sent out to other planetary systems. The original "parent" probe could then pursue its primary purpose within the star system. This mission varies widely depending on the variant of self-replicating starship proposed. 

Given this pattern, and its similarity to the reproduction patterns of bacteria, it has been pointed out that von Neumann machines might be considered a form of life. In his short story, "Lungfish", David Brin touches on this idea, pointing out that self-replicating machines launched by different species might actually compete with one another (in a Darwinistic fashion) for raw material, or even have conflicting missions. Given enough variety of "species" they might even form a type of ecology, or – should they also have a form of artificial intelligence – a society. They may even mutate with untold thousands of "generations". 

The first quantitative engineering analysis of such a spacecraft was published in 1980 by Robert Freitas, in which the non-replicating Project Daedalus design was modified to include all subsystems necessary for self-replication. The design's strategy was to use the probe to deliver a "seed" factory with a mass of about 443 tons to a distant site, have the seed factory replicate many copies of itself there to increase its total manufacturing capacity, over a 500-year period, and then use the resulting automated industrial complex to construct more probes with a single seed factory on board each.

It has been theorized that a self-replicating starship utilizing relatively conventional theoretical methods of interstellar travel (i.e., no exotic faster-than-light propulsion, and speeds limited to an "average cruising speed" of 0.1c.) could spread throughout a galaxy the size of the Milky Way in as little as half a million years.

Implications for Fermi's paradox

In 1981, Frank Tipler put forth an argument that extraterrestrial intelligences do not exist, based on the absence of von Neumann probes. Given even a moderate rate of replication and the history of the galaxy, such probes should already be common throughout space and thus, we should have already encountered them. Because we have not, this shows that extraterrestrial intelligences do not exist. This is thus a resolution to the Fermi paradox – that is, the question of why we have not already encountered extraterrestrial intelligence if it is common throughout the universe. 

A response came from Carl Sagan and William Newman. Now known as Sagan's Response, it pointed out that in fact Tipler had underestimated the rate of replication, and that von Neumann probes should have already started to consume most of the mass in the galaxy. Any intelligent race would therefore, Sagan and Newman reasoned, not design von Neumann probes in the first place, and would try to destroy any von Neumann probes found as soon as they were detected. As Robert Freitas has pointed out, the assumed capacity of von Neumann probes described by both sides of the debate are unlikely in reality, and more modestly reproducing systems are unlikely to be observable in their effects on our Solar System or the Galaxy as a whole.

Another objection to the prevalence of von Neumann probes is that civilizations of the type that could potentially create such devices may have inherently short lifetimes, and self-destruct before so advanced a stage is reached, through such events as biological or nuclear warfare, nanoterrorism, resource exhaustion, ecological catastrophe, or pandemics. 

Simple workarounds exist to avoid the over-replication scenario. Radio transmitters, or other means of wireless communication, could be used by probes programmed not to replicate beyond a certain density (such as five probes per cubic parsec) or arbitrary limit (such as ten million within one century), analogous to the Hayflick limit in cell reproduction. One problem with this defence against uncontrolled replication is that it would only require a single probe to malfunction and begin unrestricted reproduction for the entire approach to fail – essentially a technological cancer – unless each probe also has the ability to detect such malfunction in its neighbours and implements a seek and destroy protocol (which in turn could lead to probe-on-probe space wars if faulty probes first managed to multiply to high numbers before they were found by sound ones, which could then well have programming to replicate to matching numbers so as to manage the infestation). Another workaround is based on the need for spacecraft heating during long interstellar travel. The use of plutonium as a thermal source would limit the ability to self-replicate. The spacecraft would have no programming to make more plutonium even if it found the required raw materials. Another is to program the spacecraft with a clear understanding of the dangers of uncontrolled replication.

Applications for self-replicating spacecraft

The details of the mission of self-replicating starships can vary widely from proposal to proposal, and the only common trait is the self-replicating nature.

Von Neumann probes

A von Neumann probe is a spacecraft capable of replicating itself. The concept is named after Hungarian American mathematician and physicist John von Neumann, who rigorously studied the concept of self-replicating machines that he called "Universal Assemblers" and which are often referred to as "von Neumann machines".

If a self-replicating probe finds evidence of primitive life (or a primitive, low-level culture) it might be programmed to lie dormant, silently observe, attempt to make contact (this variant is known as a Bracewell probe), or even interfere with or guide the evolution of life in some way. 

Physicist Paul Davies of Arizona State University has even raised the possibility of a probe resting on our own Moon, having arrived at some point in Earth's ancient prehistory and remained to monitor Earth, which is very reminiscent of Arthur C. Clarke's The Sentinel and the Stanley Kubrick film 2001: A Space Odyssey. 

A variant idea on the interstellar von Neumann probe idea is that of the "Astrochicken", proposed by Freeman Dyson. While it has the common traits of self-replication, exploration, and communication with its "home base", Dyson conceived the Astrochicken to explore and operate within our own planetary system, and not explore interstellar space. 

Oxford-based philosopher Nick Bostrom discusses the idea that future powerful superintelligences will create efficient cost-effective space travel and interstellar Von Neumann probes.

Anders Sandberg and Stuart Armstrong argued that launching the colonization of the entire reachable universe through self-replicating probes is well within the capabilities of a star-spanning civilization, and proposed a theoretical approach for achieving it in 32 years, by mining planet Mercury for resources and constructing a Dyson Swarm around the Sun.

Berserkers

A variant of the self-replicating starship is the Berserker. Unlike the benign probe concept, Berserkers are programmed to seek out and exterminate lifeforms and life-bearing exoplanets whenever they are encountered.

The name is derived from the Berserker series of novels by Fred Saberhagen which describe a war between humanity and such machines. Saberhagen points out (through one of his characters) that the Berserker warships in his novels are not von Neumann machines themselves, but the larger complex of Berserker machines – including automated shipyards – do constitute a von Neumann machine. This again brings up the concept of an ecology of von Neumann machines, or even a von Neumann hive entity. 

It is speculated in fiction that Berserkers could be created and launched by a xenophobic civilization (see Anvil of Stars, by Greg Bear, in the section In fiction below) or could theoretically "mutate" from a more benign probe. For instance, a von Neumann ship designed for terraforming processes – mining a planet's surface and adjusting its atmosphere to more human-friendly conditions – might malfunction and attack inhabited planets, killing their inhabitants in the process of changing the planetary environment, and then self-replicate and dispatch more ships to attack other planets.

Replicating seeder ships

Yet another variant on the idea of the self-replicating starship is that of the seeder ship. Such starships might store the genetic patterns of lifeforms from their home world, perhaps even of the species which created it. Upon finding a habitable exoplanet, or even one that might be terraformed, it would try to replicate such lifeforms – either from stored embryos or from stored information using molecular nanotechnology to build zygotes with varying genetic information from local raw materials.

Such ships might be terraforming vessels, preparing colony worlds for later colonization by other vessels, or – should they be programmed to recreate, raise, and educate individuals of the species that created it – self-replicating colonizers themselves. Seeder ships would be a suitable alternative to Generation ships as a way to colonize worlds too distant to travel to in one lifetime.

In fiction

Von Neumann probes

  • 2001: A Space Odyssey: The monoliths in Arthur C. Clarke's book and Stanley Kubrick's film 2001: A Space Odyssey were intended to be self-replicating probes, though the artifacts in "The Sentinel", Clarke's original short story upon which 2001 was based, were not. The film was to begin with a series of scientists explaining how probes like these would be the most efficient method of exploring outer space. Kubrick cut the opening segment from his film at the last minute, however, and these monoliths became almost mystical entities in both the film and Clarke's novel.
  • Destiny's Road: Larry Niven frequently refers to Von Neumann probes in many of his works. In his 1998 book Destiny's Road, Von Neumann machines are scattered throughout the human colony world Destiny and its moon Quicksilver in order to build and maintain technology and to make up for the lack of the resident humans' technical knowledge; the Von Neumann machines primarily construct a stretchable fabric cloth capable of acting as a solar collector which serves as the humans' primary energy source. The Von Neumann machines also engage in ecological maintenance and other exploratory work.
  • No Man's Sky: In the video game No Man's Sky mysterious probes called Sentinels roam all the planets in the entire galaxy, analyzing the fauna and flora and protecting them from extinction. It is likely these probes have self-replicated for centuries and managed to spread through the galaxy.
  • Spin: In the novel by Robert Charles Wilson, Earth is veiled by a temporal field. Humanity tries to understand and escape this field by using Von Neumann probes. It is later revealed that the field itself was generated by Von Neumann probes from another civilization, and that a competition for resources had taken place between earth's and the aliens' probes.
  • Von Neumann's War: In Von Neumann's War by John Ringo and Travis S. Taylor (published by Baen Books in 2007) Von Neumann probes arrive in the solar system, moving in from the outer planets, and converting all metals into gigantic structures. Eventually, they arrive on Earth, wiping out much of the population before being beaten back when humanity reverse engineers some of the probes.
  • Grey Goo: In the video game Grey Goo, the "Goo" faction is composed entirely of Von Neumann probes sent through various microscopic wormholes to map the Milky Way Galaxy. The faction's units are configurations of nanites used during their original mission of exploration, which have adapted to a combat role. The Goo starts as an antagonist to the Human and Beta factions, but their true objective is revealed during their portion of the single-player campaign. Related to, and inspired by, the Grey Goo doomsday scenario.
  • We Are Legion (We Are Bob) by Dennis E. Taylor: Bob Johansson, the former owner of a software company, dies in a car accident, only to wake up a hundred years later as a computer emulation of Bob. Given a Von Neumann probe by America's religious replacement, he is sent out to explore, exploit, expand, and experiment for the good of the human race.

Berserkers

  • In the science fiction short story collection Berserker by Fred Saberhagen, a series of short stories include accounts of battles fought against extremely destructive Berserker machines. This and subsequent books set in the same fictional universe are the origin of the term "Berserker probe".
  • In the 2003 miniseries reboot of Battlestar Galactica (and the subsequent 2004 series) the Cylons are similar to Berserkers in their wish to destroy human life. They were created by humans in a group of fictional planets called the Twelve Colonies. The Cylons created special models that look like humans in order to destroy the twelve colonies and later, the fleeing fleet of surviving humans.
  • The Borg of Star Trek – a self-replicating bio-mechanical race that is dedicated to the task of achieving perfection through the assimilation of useful technology and lifeforms. Their ships are massive mechanical cubes (a close step from the Berserker's massive mechanical Spheres).
  • In the computer game Star Control II, the Slylandro Probe is an out-of-control self-replicating probe that attacks starships of other races. They were not originally intended to be a berserker probe; they sought out intelligent life for peaceful contact, but due to a programming error, they would immediately switch to "resource extraction" mode and attempt to dismantle the target ship for raw materials. While the plot claims that the probes reproduce "at a geometric rate", the game itself caps the frequency of encountering these probes. It is possible to deal with the menace in a side-quest, but this is not necessary to complete the game, as the probes only appear one at a time, and the player's ship will eventually be fast and powerful enough to outrun them or destroy them for resources – although the probes will eventually dominate the entire game universe.
  • In Iain Banks' novel Excession, hegemonising swarms are described as a form of Outside Context Problem. An example of an "Aggressive Hegemonising Swarm Object" is given as an uncontrolled self-replicating probe with the goal of turning all matter into copies of itself. After causing great damage, they are somehow transformed using unspecified techniques by the Zetetic Elench and become "Evangelical Hegemonising Swarm Objects". Such swarms (referred to as "smatter") reappear in the later novels Surface Detail (which features scenes of space combat against the swarms) and The Hydrogen Sonata.
  • The Inhibitors from Alastair Reynolds' Revelation Space series are self-replicating machines whose purpose is to inhibit the development of intelligent star-faring cultures. They are dormant for extreme periods of time until they detect the presence of a space-faring culture and proceed to exterminate it even to the point of sterilizing entire planets. They are very difficult to destroy as they seem to have faced every type of weapon ever devised and only need a short time to 'remember' the necessary counter-measures.
  • Also from Alastair Reynolds' books, the "Greenfly" terraforming machines are another form of berserker machines. For unknown reasons, but probably an error in their programming, they destroy planets and turn them into trillions of domes filled with vegetation – after all, their purpose is to produce a habitable environment for humans, however in doing so they inadvertently decimate the human race. By 10,000, they have wiped out most of the Galaxy.
  • The Reapers in the video game series Mass Effect are also self-replicating probes bent on destroying any advanced civilization encountered in the galaxy. They lie dormant in the vast spaces between the galaxies and follow a cycle of extermination. It is seen in Mass Effect 2 that they assimilate any advanced species.
  • Mantrid Drones from the science fiction television series Lexx were an extremely aggressive type of self-replicating Berserker machine, eventually converting the majority of the matter in the universe into copies of themselves in the course of their quest to thoroughly exterminate humanity.
  • The Babylon 5 episode "Infection" showed a smaller scale berserker in the form of the Icarran War Machine. After being created with the goal of defeating an unspecified enemy faction, the War Machines proceeded to exterminate all life on the planet Icarra VII because they had been programmed with standards for what constituted a 'Pure Icaran' based on religious teachings, which no actual Icaran could satisfy. Because the Icaran were pre-starflight, the War Machines became dormant after completing their task rather than spreading. One unit was reactivated on-board Babylon 5 after being smuggled past quarantine by an unscrupulous archaeologist, but after being confronted with how they had rendered Icara VII a dead world, the simulated personality of the War Machine committed suicide.
  • The Babylon 5 episode "A Day in the Strife" features a probe that threatens the station with destruction unless a series of questions designed to test a civilization's level of advancement are answered correctly. The commander of the station correctly surmises that the probe is actually a berserker and that if the questions are answered the probe would identify them as a threat to its originating civilization and detonate.
  • Greg Bear's novel The Forge of God deals directly with the concept of "Berserker" von Neumann probes and their consequences. The idea is further explored in the novel's sequel, Anvil of Stars, which explores the reaction other civilizations have to the creation and release of Berserkers.
  • In Gregory Benford's Galactic Center Saga series, an antagonist berserker machine race is encountered by Earth, first as a probe in In the Ocean of Night, and then in an attack in Across the Sea of Suns. The berserker machines do not seek to completely eradicate a race if merely throwing it into a primitive low technological state will do as they did to the EMs encountered in Across the Sea of Suns. The alien machine Watchers would not be considered von Neumann machines themselves, but the collective machine race could.
  • On Stargate SG-1 the Replicators were a vicious race of insect-like robots that were originally created by an android named Reese to serve as toys. They grew beyond her control and began evolving, eventually spreading throughout at least two galaxies. In addition to ordinary autonomous evolution they were able to analyze and incorporate new technologies they encountered into themselves, ultimately making them one of the most advanced "races" known.
  • On Stargate Atlantis, a second race of replicators created by the Ancients were encountered in the Pegasus Galaxy. They were created as a means to defeat the Wraith. The Ancients attempted to destroy them after they began showing signs of sentience and requested that their drive to kill the wraith be removed. This failed, and an unspecified length of time after the Ancients retreated to the Milky Way Galaxy, the replicators nearly succeeded in destroying the Wraith. The Wraith were able to hack into the replicators and deactivate the extermination drive, at which point they retreated to their home world and were not heard from again until encountered by the Atlantis Expedition. After the Atlantis Expedition reactivated this dormant directive, the replicators embarked on a plan to kill the Wraith by removing their food source, i.e. all humans in the Pegasus Galaxy.
  • In Stargate Universe Season 2, a galaxy billions of light years distant from the Milky Way is infested with drone ships that are programmed to annihilate intelligent life and advanced technology. The drone ships attack other space ships (including Destiny) as well as humans on planetary surfaces, but don't bother destroying primitive technology such as buildings unless they are harboring intelligent life or advanced technology.
  • In the Homeworld: Cataclysm video game, a bio-mechanical virus called Beast has the ability to alter organic and mechanic material to suit its needs, and the ships infected become self-replicating hubs for the virus.
  • In the SF MMO EVE Online, experiments to create more autonomous drones than the ones used by player's ships accidentally created 'rogue drones' which form hives in certain parts of space and are used extensively in missions as difficult opponents.
  • In the computer game Sword of the Stars, the player may randomly encounter "Von Neumann". A Von Neumann mothership appears along with smaller Von Neumann probes, which attack and consume the player's ships. The probes then return to the mothership, returning the consumed material. If probes are destroyed, the mothership will create new ones. If all the player's ships are destroyed, the Von Neumann probes will reduce the planets resource levels before leaving. The probes appear as blue octahedrons, with small spheres attached to the apical points. The mothership is a larger version of the probes. In the 2008 expansion A Murder of Crows, Kerberos Productions also introduces the VN Berserker, a combat oriented ship, which attacks player planets and ships in retaliation to violence against VN Motherships. If the player destroys the Berserker things will escalate and a System Destroyer will attack.
  • In the comic Transmetropolitan a character mentions "Von Neumann rectal infestations" which are apparently caused by "Shit-ticks that build more shit-ticks that build more shit-ticks".
  • In the anime Vandread, harvester ships attack vessels from both male- and female-dominated factions and harvest hull, reactors, and computer components to make more of themselves. To this end, Harvester ships are built around mobile factories. Earth-born humans also view the inhabitants of the various colonies to be little more than spare parts.
  • In Earth 2160, the Morphidian Aliens rely on Mantain strain aliens for colonization. Most Mantain-derived aliens can absorb water, then reproduce like a colony of cells. In this manner, even one Mantain Lady (or Princess, or Queen) can create enough clones to cover the map. Once they have significant numbers, they "choose an evolutionary path" and swarm the enemy, taking over their resources.
  • In the European comic series Storm No 20 & 21 goes over this subject. A kind of bezerk von Neumann is set to collision course with the pandarve system.
  • In PC role-playing game Space Rangers and its sequel Space Rangers 2: Dominators, a league of 5 nations battles three different types of Berserker robots. One that focuses on invading planets, another that battles normal space and third that lives in hyperspace.
  • In the Star Wolves video game series, Berserkers are a self-replicating machine menace that threatens the known universe for purposes of destruction and/or assimilation of humanity.
  • The Star Wars expanded universe features the World Devastators, large ships designed and built by the Galactic Empire that tear apart planets to use its materials to build other ships or even upgrade or replicate themselves.
  • The Tet in the 2013 film Oblivion is revealed to be a Berserker of sorts: a sentient machine that travels from planet to planet, exterminating the indigenous population using armies of robotic drones and cloned members of the target species. The Tet then proceeds to harvest the planet's water in order to extract hydrogen for nuclear fusion.
  • In Eclipse Phase, an ETI probe is believed to have infected the TITAN computer systems with the Exsurgent virus to cause them to go berserk and wage war on humanity. This would make ETI probes a form of berserker, albeit one that uses pre-existing computer systems as its key weapons.
  • In Herr aller Dinge by Andreas Eschbach, an ancient nano machine complex is discovered buried in a glacier off the coast of Russia. When it comes in contact with materials it needs to fulfill its mission, it creates a launch facility and launches a space craft. It is later revealed that the nano machines were created by a pre-historic human race with the intention of destroying other interstellar civilizations (for an unknown reason). It is purposed that the reason there is no evidence of the race is because of the nano-machines themselves and their ability to manipulate matter at an atomic level. It is even suggested that viruses could be ancient nano machines that have evolved over time.

Replicating Seeder Ships

  • Code of the Lifemaker by James P. Hogan describes the evolution of a society of humanoid-like robots who inhabit Saturn's moon Titan. The sentient machines are descended from an unmanned factory ship that was to be self replicating, but suffered radiation damage and went off course, eventually landing on Titan around 1,000,000 BC.
  • In the Metroid Prime subseries of games, the massive Leviathans are probes routinely sent out from the planet Phaaze to infect other planets with Phazon radiation and eventually turn these planets into clones of Phaaze, where the self-replication process can continue.
  • In David Brin's short story collection, The River of Time (1986), the short story "Lungfish" prominently features von Neumann probes.[12] Not only does he explore the concept of the probes themselves, but indirectly explores the ideas of competition between different designs of probes, evolution of von Neumann probes in the face of such competition, and the development of a type of ecology between von Neumann probes. One of the vessels mentioned is clearly a Seeder type.
  • In The Songs of Distant Earth by Arthur C. Clarke, humanity on a future Earth facing imminent destruction creates automated seedships that act as fire and forget lifeboats aimed at distant, habitable worlds. Upon landing, the ship begins to create new humans from stored genetic information, and an onboard computer system raises and trains the first few generations of new inhabitants. The massive ships are then broken down and used as building materials by their "children".
  • On the Stargate Atlantis episode "Remnants", the Atlantis team finds an ancient probe that they later learn was launched by a now-extinct, technologically advanced race in order to seed new worlds and re-propagate their silicon-based species. The probe communicated with inhabitants of Atlantis by means of hallucinations.
  • On Stargate Universe, the human adventurers live on a ship called Destiny. Its mission was to connect a network of Stargates, placed by preceding seeder ships on planets capable of supporting life to allow instantaneous travel between them.
  • The trilogy of albums which conclude the comic book series Storm by Don Lawrence (starting with Chronicles of Pandarve 11: The Von Neumann machine) is based on self-replicating conscious machines containing the sum of all human knowledge employed to rebuild human society throughout the universe in case of disaster on Earth. The probe malfunctions and although new probes are built, they do not separate from the motherprobe, which eventually results in a cluster of malfunctioning probes so big that it can absorb entire moons.
  • Humanity in Xeno series is creation of rogue Seeder (so technically it is also Berserker) known as Deus.
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Steel (updated)

From Wikipedia, the free encyclopedia
The steel cable of a colliery winding tower
 
Steel is an alloy of iron and carbon, and sometimes other elements. Because of its high tensile strength and low cost, it is a major component used in buildings, infrastructure, tools, ships, automobiles, machines, appliances, and weapons. 

Iron is the base metal of steel. Iron is able to take on two crystalline forms (allotropic forms), body centered cubic and face centered cubic, depending on its temperature. In the body-centered cubic arrangement, there is an iron atom in the center and eight atoms at the vertices of each cubic unit cell; in the face-centered cubic, there is one atom at the center of each of the six faces of the cubic unit cell and eight atoms at its vertices. It is the interaction of the allotropes of iron with the alloying elements, primarily carbon, that gives steel and cast iron their range of unique properties. 

In pure iron, the crystal structure has relatively little resistance to the iron atoms slipping past one another, and so pure iron is quite ductile, or soft and easily formed. In steel, small amounts of carbon, other elements, and inclusions within the iron act as hardening agents that prevent the movement of dislocations that are common in the crystal lattices of iron atoms. 

The carbon in typical steel alloys may contribute up to 2.14% of its weight. Varying the amount of carbon and many other alloying elements, as well as controlling their chemical and physical makeup in the final steel (either as solute elements, or as precipitated phases), slows the movement of those dislocations that make pure iron ductile, and thus controls and enhances its qualities. These qualities include such things as the hardness, quenching behavior, need for annealing, tempering behavior, yield strength, and tensile strength of the resulting steel. The increase in steel's strength compared to pure iron is possible only by reducing iron's ductility.

Steel was produced in bloomery furnaces for thousands of years, but its large-scale, industrial use began only after more efficient production methods were devised in the 17th century, with the production of blister steel and then crucible steel. With the invention of the Bessemer process in the mid-19th century, a new era of mass-produced steel began. This was followed by the Siemens–Martin process and then the Gilchrist–Thomas process that refined the quality of steel. With their introductions, mild steel replaced wrought iron. 

Further refinements in the process, such as basic oxygen steelmaking (BOS), largely replaced earlier methods by further lowering the cost of production and increasing the quality of the final product. Today, steel is one of the most common manmade materials in the world, with more than 1.6 billion tons produced annually. Modern steel is generally identified by various grades defined by assorted standards organizations.

Definitions and related materials

The noun steel originates from the Proto-Germanic adjective stahliją or stakhlijan (made of steel), which is related to stahlaz or stahliją (standing firm).

The carbon content of steel is between 0.002% and 2.14% by weight for plain iron–carbon alloys. These values vary depending on alloying elements such as manganese, chromium, nickel, tungsten, and so on. Basically, steel is an iron-carbon alloy that does not undergo eutectic reaction. In contrast, cast iron does undergo eutectic reaction. Too little carbon content leaves (pure) iron quite soft, ductile, and weak. Carbon contents higher than those of steel make a brittle alloy commonly called pig iron. While iron alloyed with carbon is called carbon steel, alloy steel is steel to which other alloying elements have been intentionally added to modify the characteristics of steel. Common alloying elements include: manganese, nickel, chromium, molybdenum, boron, titanium, vanadium, tungsten, cobalt, and niobium. Additional elements, most frequently considered undesirable, are also important in steel: phosphorus, sulfur, silicon, and traces of oxygen, nitrogen, and copper. 

Plain carbon-iron alloys with a higher than 2.1% carbon content are known as cast iron. With modern steelmaking techniques such as powder metal forming, it is possible to make very high-carbon (and other alloy material) steels, but such are not common. Cast iron is not malleable even when hot, but it can be formed by casting as it has a lower melting point than steel and good castability properties. Certain compositions of cast iron, while retaining the economies of melting and casting, can be heat treated after casting to make malleable iron or ductile iron objects. Steel is distinguishable from wrought iron (now largely obsolete), which may contain a small amount of carbon but large amounts of slag.

Material properties

Iron-carbon equilibrium phase diagram, showing the conditions necessary to form different phases
 
Iron is commonly found in the Earth's crust in the form of an ore, usually an iron oxide, such as magnetite or hematite. Iron is extracted from iron ore by removing the oxygen through its combination with a preferred chemical partner such as carbon which is then lost to the atmosphere as carbon dioxide. This process, known as smelting, was first applied to metals with lower melting points, such as tin, which melts at about 250 °C (482 °F), and copper, which melts at about 1,100 °C (2,010 °F), and the combination, bronze, which has a melting point lower than 1,083 °C (1,981 °F). In comparison, cast iron melts at about 1,375 °C (2,507 °F). Small quantities of iron were smelted in ancient times, in the solid state, by heating the ore in a charcoal fire and then welding the clumps together with a hammer and in the process squeezing out the impurities. With care, the carbon content could be controlled by moving it around in the fire. Unlike copper and tin, liquid or solid iron dissolves carbon quite readily. 

All of these temperatures could be reached with ancient methods used since the Bronze Age. Since the oxidation rate of iron increases rapidly beyond 800 °C (1,470 °F), it is important that smelting take place in a low-oxygen environment. Smelting, using carbon to reduce iron oxides, results in an alloy (pig iron) that retains too much carbon to be called steel. The excess carbon and other impurities are removed in a subsequent step. 

Other materials are often added to the iron/carbon mixture to produce steel with desired properties. Nickel and manganese in steel add to its tensile strength and make the austenite form of the iron-carbon solution more stable, chromium increases hardness and melting temperature, and vanadium also increases hardness while making it less prone to metal fatigue.

To inhibit corrosion, at least 11% chromium is added to steel so that a hard oxide forms on the metal surface; this is known as stainless steel. Tungsten slows the formation of cementite, keeping carbon in the iron matrix and allowing martensite to preferentially form at slower quench rates, resulting in high speed steel. On the other hand, sulfur, nitrogen, and phosphorus are considered contaminants that make steel more brittle and are removed from the steel melt during processing.

The density of steel varies based on the alloying constituents but usually ranges between 7,750 and 8,050 kg/m3 (484 and 503 lb/cu ft), or 7.75 and 8.05 g/cm3 (4.48 and 4.65 oz/cu in).

Even in a narrow range of concentrations of mixtures of carbon and iron that make a steel, a number of different metallurgical structures, with very different properties can form. Understanding such properties is essential to making quality steel. At room temperature, the most stable form of pure iron is the body-centered cubic (BCC) structure called alpha iron or α-iron. It is a fairly soft metal that can dissolve only a small concentration of carbon, no more than 0.005% at 0 °C (32 °F) and 0.021 wt% at 723 °C (1,333 °F). The inclusion of carbon in alpha iron is called ferrite. At 910 °C, pure iron transforms into a face-centered cubic (FCC) structure, called gamma iron or γ-iron. The inclusion of carbon in gamma iron is called austenite. The more open FCC structure of austenite can dissolve considerably more carbon, as much as 2.1% (38 times that of ferrite) carbon at 1,148 °C (2,098 °F), which reflects the upper carbon content of steel, beyond which is cast iron. When carbon moves out of solution with iron, it forms a very hard, but brittle material called cementite (Fe3C).

When steels with exactly 0.8% carbon (known as a eutectoid steel), are cooled, the austenitic phase (FCC) of the mixture attempts to revert to the ferrite phase (BCC). The carbon no longer fits within the FCC austenite structure, resulting in an excess of carbon. One way for carbon to leave the austenite is for it to precipitate out of solution as cementite, leaving behind a surrounding phase of BCC iron called ferrite with a small percentage of carbon in solution. The two, ferrite and cementite, precipitate simultaneously producing a layered structure called pearlite, named for its resemblance to mother of pearl. In a hypereutectoid composition (greater than 0.8% carbon), the carbon will first precipitate out as large inclusions of cementite at the austenite grain boundaries until the percentage of carbon in the grains has decreased to the eutectoid composition (0.8% carbon), at which point the pearlite structure forms. For steels that have less than 0.8% carbon (hypoeutectoid), ferrite will first form within the grains until the remaining composition rises to 0.8% of carbon, at which point the pearlite structure will form. No large inclusions of cementite will form at the boundaries in hypoeuctoid steel. The above assumes that the cooling process is very slow, allowing enough time for the carbon to migrate. 

As the rate of cooling is increased the carbon will have less time to migrate to form carbide at the grain boundaries but will have increasingly large amounts of pearlite of a finer and finer structure within the grains; hence the carbide is more widely dispersed and acts to prevent slip of defects within those grains, resulting in hardening of the steel. At the very high cooling rates produced by quenching, the carbon has no time to migrate but is locked within the face-centered austenite and forms martensite. Martensite is a highly strained and stressed, supersaturated form of carbon and iron and is exceedingly hard but brittle. Depending on the carbon content, the martensitic phase takes different forms. Below 0.2% carbon, it takes on a ferrite BCC crystal form, but at higher carbon content it takes a body-centered tetragonal (BCT) structure. There is no thermal activation energy for the transformation from austenite to martensite. Moreover, there is no compositional change so the atoms generally retain their same neighbors.

Martensite has a lower density (it expands during the cooling) than does austenite, so that the transformation between them results in a change of volume. In this case, expansion occurs. Internal stresses from this expansion generally take the form of compression on the crystals of martensite and tension on the remaining ferrite, with a fair amount of shear on both constituents. If quenching is done improperly, the internal stresses can cause a part to shatter as it cools. At the very least, they cause internal work hardening and other microscopic imperfections. It is common for quench cracks to form when steel is water quenched, although they may not always be visible.

Heat treatment

There are many types of heat treating processes available to steel. The most common are annealing, quenching, and tempering. Heat treatment is effective on compositions above the eutectoid composition (hypereutectoid) of 0.8% carbon. Hypoeutectoid steel does not benefit from heat treatment.

Annealing is the process of heating the steel to a sufficiently high temperature to relieve local internal stresses. It does not create a general softening of the product but only locally relieves strains and stresses locked up within the material. Annealing goes through three phases: recovery, recrystallization, and grain growth. The temperature required to anneal a particular steel depends on the type of annealing to be achieved and the alloying constituents.

Quenching involves heating the steel to create the austenite phase then quenching it in water or oil. This rapid cooling results in a hard but brittle martensitic structure. The steel is then tempered, which is just a specialized type of annealing, to reduce brittleness. In this application the annealing (tempering) process transforms some of the martensite into cementite, or spheroidite and hence it reduces the internal stresses and defects. The result is a more ductile and fracture-resistant steel.

Steel production

Iron ore pellets for the production of steel
 
When iron is smelted from its ore, it contains more carbon than is desirable. To become steel, it must be reprocessed to reduce the carbon to the correct amount, at which point other elements can be added. In the past, steel facilities would cast the raw steel product into ingots which would be stored until use in further refinement processes that resulted in the finished product. In modern facilities, the initial product is close to the final composition and is continuously cast into long slabs, cut and shaped into bars and extrusions and heat treated to produce a final product. Today only a small fraction is cast into ingots. Approximately 96% of steel is continuously cast, while only 4% is produced as ingots.

The ingots are then heated in a soaking pit and hot rolled into slabs, billets, or blooms. Slabs are hot or cold rolled into sheet metal or plates. Billets are hot or cold rolled into bars, rods, and wire. Blooms are hot or cold rolled into structural steel, such as I-beams and rails. In modern steel mills these processes often occur in one assembly line, with ore coming in and finished steel products coming out. Sometimes after a steel's final rolling, it is heat treated for strength; however, this is relatively rare.

History of steelmaking

Bloomery smelting during the Middle Ages

Ancient steel

Steel was known in antiquity and was produced in bloomeries and crucibles.

The earliest known production of steel is seen in pieces of ironware excavated from an archaeological site in Anatolia (Kaman-Kalehöyük) and are nearly 4,000 years old, dating from 1800 BC. Horace identifies steel weapons such as the falcata in the Iberian Peninsula, while Noric steel was used by the Roman military.

The reputation of Seric iron of South India (wootz steel) grew considerably in the rest of the world. Metal production sites in Sri Lanka employed wind furnaces driven by the monsoon winds, capable of producing high-carbon steel. Large-scale Wootz steel production in Tamilakam using crucibles and carbon sources such as the plant Avāram occurred by the sixth century BC, the pioneering precursor to modern steel production and metallurgy.

The Chinese of the Warring States period (403–221 BC) had quench-hardened steel, while Chinese of the Han dynasty (202 BC – 220 AD) created steel by melting together wrought iron with cast iron, gaining an ultimate product of a carbon-intermediate steel by the 1st century AD.

Wootz steel and Damascus steel

Evidence of the earliest production of high carbon steel in the Indian Subcontinent are found in Kodumanal in Tamil Nadu area, Golconda in Andhra Pradesh area and Karnataka, and in Samanalawewa areas of Sri Lanka. This came to be known as Wootz steel, produced in South India by about sixth century BC and exported globally. The steel technology existed prior to 326 BC in the region as they are mentioned in literature of Sangam Tamil, Arabic and Latin as the finest steel in the world exported to the Romans, Egyptian, Chinese and Arab worlds at that time – what they called Seric Iron. A 200 BC Tamil trade guild in Tissamaharama, in the South East of Sri Lanka, brought with them some of the oldest iron and steel artifacts and production processes to the island from the classical period. The Chinese and locals in Anuradhapura, Sri Lanka had also adopted the production methods of creating Wootz steel from the Chera Dynasty Tamils of South India by the 5th century AD.[32][33] In Sri Lanka, this early steel-making method employed a unique wind furnace, driven by the monsoon winds, capable of producing high-carbon steel. Since the technology was acquired from the Tamilians from South India, the origin of steel technology in India can be conservatively estimated at 400–500 BC.

The manufacture of what came to be called Wootz, or Damascus steel, famous for its durability and ability to hold an edge, may have been taken by the Arabs from Persia, who took it from India. It was originally created from a number of different materials including various trace elements, apparently ultimately from the writings of Zosimos of Panopolis. In 327 BC, Alexander the Great was rewarded by the defeated King Porus, not with gold or silver but with 30 pounds of steel. Recent studies have suggested that carbon nanotubes were included in its structure, which might explain some of its legendary qualities, though given the technology of that time, such qualities were produced by chance rather than by design. Natural wind was used where the soil containing iron was heated by the use of wood. The ancient Sinhalese managed to extract a ton of steel for every 2 tons of soil, a remarkable feat at the time. One such furnace was found in Samanalawewa and archaeologists were able to produce steel as the ancients did.

Crucible steel, formed by slowly heating and cooling pure iron and carbon (typically in the form of charcoal) in a crucible, was produced in Merv by the 9th to 10th century AD. In the 11th century, there is evidence of the production of steel in Song China using two techniques: a "berganesque" method that produced inferior, inhomogeneous steel, and a precursor to the modern Bessemer process that used partial decarbonization via repeated forging under a cold blast.

Modern steelmaking

A Bessemer converter in Sheffield, England
 
Since the 17th century, the first step in European steel production has been the smelting of iron ore into pig iron in a blast furnace. Originally employing charcoal, modern methods use coke, which has proven more economical.

Processes starting from bar iron

In these processes pig iron was refined (fined) in a finery forge to produce bar iron, which was then used in steel-making.

The production of steel by the cementation process was described in a treatise published in Prague in 1574 and was in use in Nuremberg from 1601. A similar process for case hardening armor and files was described in a book published in Naples in 1589. The process was introduced to England in about 1614 and used to produce such steel by Sir Basil Brooke at Coalbrookdale during the 1610s.

The raw material for this process were bars of iron. During the 17th century it was realized that the best steel came from oregrounds iron of a region north of Stockholm, Sweden. This was still the usual raw material source in the 19th century, almost as long as the process was used.

Crucible steel is steel that has been melted in a crucible rather than having been forged, with the result that it is more homogeneous. Most previous furnaces could not reach high enough temperatures to melt the steel. The early modern crucible steel industry resulted from the invention of Benjamin Huntsman in the 1740s. Blister steel (made as above) was melted in a crucible or in a furnace, and cast (usually) into ingots.

Processes starting from pig iron

A Siemens-Martin steel oven from the Brandenburg Museum of Industry.
 
White-hot steel pouring out of an electric arc furnace.
 
The modern era in steelmaking began with the introduction of Henry Bessemer's Bessemer process in 1855, the raw material for which was pig iron. His method let him produce steel in large quantities cheaply, thus mild steel came to be used for most purposes for which wrought iron was formerly used. The Gilchrist-Thomas process (or basic Bessemer process) was an improvement to the Bessemer process, made by lining the converter with a basic material to remove phosphorus.

Another 19th-century steelmaking process was the Siemens-Martin process, which complemented the Bessemer process. It consisted of co-melting bar iron (or steel scrap) with pig iron. 

These methods of steel production were rendered obsolete by the Linz-Donawitz process of basic oxygen steelmaking (BOS), developed in the 1950s, and other oxygen steel making methods. Basic oxygen steelmaking is superior to previous steelmaking methods because the oxygen pumped into the furnace limited impurities, primarily nitrogen, that previously had entered from the air used. Today, electric arc furnaces (EAF) are a common method of reprocessing scrap metal to create new steel. They can also be used for converting pig iron to steel, but they use a lot of electrical energy (about 440 kWh per metric ton), and are thus generally only economical when there is a plentiful supply of cheap electricity.

Steel industry

Steel production (in million tons) by country in 2007
 
The steel industry is often considered an indicator of economic progress, because of the critical role played by steel in infrastructural and overall economic development. In 1980, there were more than 500,000 U.S. steelworkers. By 2000, the number of steelworkers fell to 224,000.

The economic boom in China and India caused a massive increase in the demand for steel. Between 2000 and 2005, world steel demand increased by 6%. Since 2000, several Indian and Chinese steel firms have risen to prominence, such as Tata Steel (which bought Corus Group in 2007), Baosteel Group and Shagang Group. As of 2017, though, ArcelorMittal is the world's largest steel producer. In 2005, the British Geological Survey stated China was the top steel producer with about one-third of the world share; Japan, Russia, and the US followed respectively.

In 2008, steel began trading as a commodity on the London Metal Exchange. At the end of 2008, the steel industry faced a sharp downturn that led to many cut-backs.

Recycling

Steel is one of the world's most-recycled materials, with a recycling rate of over 60% globally; in the United States alone, over 82,000,000 metric tons (81,000,000 long tons; 90,000,000 short tons) were recycled in the year 2008, for an overall recycling rate of 83%.

As more steel is produced than is scrapped, the amount of recycled raw materials is about 40% of the total of steel produced - in 2016, 1,628,000,000 tonnes (1.602×109 long tons; 1.795×109 short tons) of crude steel was produced globally, with 630,000,000 tonnes (620,000,000 long tons; 690,000,000 short tons) recycled.

Contemporary steel

Bethlehem Steel (Bethlehem, Pennsylvania facility pictured) was one of the world's largest manufacturers of steel before its closure in 2003

Carbon steels

Modern steels are made with varying combinations of alloy metals to fulfill many purposes. Carbon steel, composed simply of iron and carbon, accounts for 90% of steel production. Low alloy steel is alloyed with other elements, usually molybdenum, manganese, chromium, or nickel, in amounts of up to 10% by weight to improve the hardenability of thick sections. High strength low alloy steel has small additions (usually l.t. 2% by weight) of other elements, typically 1.5% manganese, to provide additional strength for a modest price increase.

Recent Corporate Average Fuel Economy (CAFE) regulations have given rise to a new variety of steel known as Advanced High Strength Steel (AHSS). This material is both strong and ductile so that vehicle structures can maintain their current safety levels while using less material. There are several commercially available grades of AHSS, such as dual-phase steel, which is heat treated to contain both a ferritic and martensitic microstructure to produce a formable, high strength steel. Transformation Induced Plasticity (TRIP) steel involves special alloying and heat treatments to stabilize amounts of austenite at room temperature in normally austenite-free low-alloy ferritic steels. By applying strain, the austenite undergoes a phase transition to martensite without the addition of heat. Twinning Induced Plasticity (TWIP) steel uses a specific type of strain to increase the effectiveness of work hardening on the alloy.

Carbon Steels are often galvanized, through hot-dip or electroplating in zinc for protection against rust.

Alloy steels

Stainless steels contain a minimum of 11% chromium, often combined with nickel, to resist corrosion. Some stainless steels, such as the ferritic stainless steels are magnetic, while others, such as the austenitic, are nonmagnetic. Corrosion-resistant steels are abbreviated as CRES.

Some more modern steels include tool steels, which are alloyed with large amounts of tungsten and cobalt or other elements to maximize solution hardening. This also allows the use of precipitation hardening and improves the alloy's temperature resistance. Tool steel is generally used in axes, drills, and other devices that need a sharp, long-lasting cutting edge. Other special-purpose alloys include weathering steels such as Cor-ten, which weather by acquiring a stable, rusted surface, and so can be used un-painted. Maraging steel is alloyed with nickel and other elements, but unlike most steel contains little carbon (0.01%). This creates a very strong but still malleable steel.

Eglin steel uses a combination of over a dozen different elements in varying amounts to create a relatively low-cost steel for use in bunker buster weapons. Hadfield steel (after Sir Robert Hadfield) or manganese steel contains 12–14% manganese which when abraded strain-hardens to form an incredibly hard skin which resists wearing. Examples include tank tracks, bulldozer blade edges and cutting blades on the jaws of life.

Standards

Most of the more commonly used steel alloys are categorized into various grades by standards organizations. For example, the Society of Automotive Engineers has a series of grades defining many types of steel. The American Society for Testing and Materials has a separate set of standards, which define alloys such as A36 steel, the most commonly used structural steel in the United States. The JIS also define series of steel grades that are being used extensively in Japan as well as in developing countries.

Uses

A roll of steel wool
 
Iron and steel are used widely in the construction of roads, railways, other infrastructure, appliances, and buildings. Most large modern structures, such as stadiums and skyscrapers, bridges, and airports, are supported by a steel skeleton. Even those with a concrete structure employ steel for reinforcing. In addition, it sees widespread use in major appliances and cars. Despite growth in usage of aluminium, it is still the main material for car bodies. Steel is used in a variety of other construction materials, such as bolts, nails, and screws and other household products and cooking utensils.

Other common applications include shipbuilding, pipelines, mining, offshore construction, aerospace, white goods (e.g. washing machines), heavy equipment such as bulldozers, office furniture, steel wool, tools, and armour in the form of personal vests or vehicle armour (better known as rolled homogeneous armour in this role).

Historical

A carbon steel knife
 
Before the introduction of the Bessemer process and other modern production techniques, steel was expensive and was only used where no cheaper alternative existed, particularly for the cutting edge of knives, razors, swords, and other items where a hard, sharp edge was needed. It was also used for springs, including those used in clocks and watches.

With the advent of speedier and thriftier production methods, steel has become easier to obtain and much cheaper. It has replaced wrought iron for a multitude of purposes. However, the availability of plastics in the latter part of the 20th century allowed these materials to replace steel in some applications due to their lower fabrication cost and weight. Carbon fiber is replacing steel in some cost insensitive applications such as aircraft, sports equipment and high end automobiles.

Long steel

A steel bridge
 
A steel pylon suspending overhead power lines

Flat carbon steel

Weathering steel (COR-TEN)

Stainless steel

A stainless steel gravy boat

Low-background steel

Steel manufactured after World War II became contaminated with radionuclides by nuclear weapons testing. Low-background steel, steel manufactured prior to 1945, is used for certain radiation-sensitive applications such as Geiger counters and radiation shielding.
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