Origins and production showing the conditions necessary to form different phases steel workpiece in a
blacksmith's art 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 under reductive conditions, where oxygen reacts with carbon in the fuel to produce carbon monoxide, which then reduces the iron oxide into metallic iron. This process, known as
smelting, was first applied to metals with lower
melting points, such as
tin, which melts at about , and
copper, which melts at about , and the combination, bronze, which has a melting point lower than . In comparison, iron melts at about , a temperature not attainable at the start of the
Iron Age. Small quantities of iron were smelted in ancient times in a semi-liquid state by repeatedly heating the ore in a
charcoal fire and then
welding the resulting clumps together with a hammer. The process eliminated much of the impurities, resulting in the production of
wrought iron. As furnaces reached higher temperatures due to
bellows improvements leading to increased airflow, iron with higher carbon contents were able to be produced. 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 , it is important that smelting takes 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. Other materials are often added to the iron/carbon mixture to produce steel with the 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 can be 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. The addition of
lead and
sulphur decrease grain size, thereby making the steel easier to
turn, but also more brittle and prone to corrosion. Such alloys are nevertheless frequently used for components such as nuts, bolts, and washers in applications where toughness and corrosion resistance are not paramount. For the most part, however,
p-block elements such as sulphur,
nitrogen,
phosphorus, and lead are considered contaminants that make steel more brittle and are therefore removed from steel during the melting processing. Even in a narrow range of concentrations of mixtures of carbon and iron that make steel, several 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-centred 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 and 0.021 wt% at . The inclusion of carbon in alpha iron is called
ferrite. At 910 °C, pure iron transforms into a
face-centred 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), at , 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). 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 hypoeutectoid 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-centred 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-centred tetragonal (BCT) structure. There is no thermal
activation energy barrier which prevents transformation from austenite to martensite. There is no compositional change, so the atoms generally retain their same neighbours. 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, such as
annealing,
quenching, and
tempering. 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. ==History==