| Steel is a metal alloy whose major | | | | similar but less beautiful bainite. |
| component is iron, with carbon content | | | | Perhaps the most important allotrope is |
| between 0.02% and 1.7% by weight. Carbon | | | | martensite, a chemically metastable |
| is the most cost effective alloying | | | | substance with about four to five times |
| material for iron, but many other | | | | the strength of ferrite. A minimum of |
| alloying elements are also used.[1] | | | | 0.4 wt% of carbon is needed in order to |
| Carbon and other elements act as a | | | | form martensite. When the austenite is |
| hardening agent, preventing dislocations | | | | quenched to form martensite, the carbon |
| in the iron atom crystal lattice from | | | | is "frozen" in place when the cell |
| sliding past one another. Varying the | | | | structure changes from FCC to BCC. The |
| amount of alloying elements and their | | | | carbon atoms are much too large to fit |
| distribution in the steel controls | | | | in the interstitial vaccancies and thus |
| qualities such as the hardness, | | | | distort the cell structure into a Body |
| elasticity, ductility, and tensile | | | | Centered Tetragonal (BCT) structure. |
| strength of the resulting steel. Steel | | | | Martensite and austenite have an |
| with increased carbon content can be | | | | identical chemical composition. As such, |
| made harder and stronger than iron, but | | | | it requires extremely little thermal |
| is also more brittle. The maximum | | | | activation energy to form. |
| solubility of carbon in iron is 1.7% by | | | | The heat treatment process for most |
| weight, occurring at 1130° Celsius; | | | | steels involves heating the alloy until |
| higher concentrations of carbon or lower | | | | austenite forms, then quenching the hot |
| temperatures will produce cementite | | | | metal in water or oil, cooling it so |
| which will reduce the material's | | | | rapidly that the transformation to |
| strength. Alloys with higher carbon | | | | ferrite or pearlite does not have time |
| content than this are known as cast iron | | | | to take place. The transformation into |
| because of their lower melting point.[1] | | | | martensite, by contrast, occurs almost |
| Steel is also to be distinguished from | | | | immediately, due to a lower activation |
| wrought iron with little or no carbon, | | | | energy. |
| usually less than 0.035%. It is common | | | | Martensite has a lower density than |
| today to talk about 'the iron and steel | | | | austenite, so that the transformation |
| industry' as if it were a single thing; | | | | between them results in a change of |
| it is today, but historically they were | | | | volume. In this case, expansion occurs. |
| separate products. | | | | Internal stresses from this expansion |
| Currently there are several classes of | | | | generally take the form of compression |
| steels in which carbon is replaced with | | | | on the crystals of martensite and |
| other alloying materials, and carbon, if | | | | tension on the remaining ferrite, with a |
| present, is undesired. A more recent | | | | fair amount of shear on both |
| definition is that steels are iron-based | | | | constituents. If quenching is done |
| alloys that can be plastically formed | | | | improperly, these internal stresses can |
| (pounded, rolled, etc.). | | | | cause a part to shatter as it cools; at |
| Iron, like most metals, is not found in | | | | the very least, they cause internal work |
| the Earth's crust in an elemental state. | | | | hardening and other microscopic |
| Iron can be found in the crust only in | | | | imperfections. It is common for quench |
| combination with oxygen or sulfur. | | | | cracks to form when water quenched, |
| Typically Fe2O3—the form of iron oxide | | | | although they may not always be visible. |
| (rust) found as the mineral hematite, | | | | At this point, if the carbon content is |
| and FeS2—Pyrite (fool's gold). Iron | | | | high enough to produce a significant |
| oxide is a soft sandstone-like material | | | | concentration of martensite, the result |
| with limited uses on its own. Iron is | | | | is an extremely hard but very brittle |
| extracted from ore by removing the | | | | material. Often, steel undergoes further |
| oxygen by combining it with a preferred | | | | heat treatment at a lower temperature to |
| chemical partner such as carbon. This | | | | destroy some of the martensite (by |
| process, known as smelting, was first | | | | allowing enough time for cementite, |
| applied to metals with lower melting | | | | etc., to form) and help settle the |
| points. Copper melts at just over 1000 | | | | internal stresses and defects. This |
| °C, while tin melts around 250 °C. | | | | softens the steel, producing a more |
| Steel melts at around 1370 °C. Both | | | | ductile and fracture-resistant metal. |
| temperatures could be reached with | | | | Because time is so critical to the end |
| ancient methods that have been used for | | | | result, this process is known as |
| at least 6000 years (since the Bronze | | | | tempering, which forms tempered steel. |
| Age). Since the oxidation rate itself | | | | Other materials are often added to the |
| increases rapidly beyond 800 °C, it is | | | | iron-carbon mixture to tailor the |
| important that smelting take place in a | | | | resulting properties. Nickel and |
| low-oxygen environment. Unlike copper | | | | manganese in steel add to its tensile |
| and tin, liquid iron dissolves carbon | | | | strength and make austenite more |
| quite readily, so that smelting results | | | | chemically stable, chromium increases |
| in an alloy containing too much carbon | | | | the hardness and melting temperature, |
| to be called steel. | | | | and vanadium also increases the hardness |
| Even in the narrow range of | | | | while reducing the effects of metal |
| concentrations that make up steel, | | | | fatigue. Large amounts of chromium and |
| mixtures of carbon and iron can form | | | | nickel (often 18% and 8%, respectively) |
| into a number of different structures, | | | | are added to stainless steel so that a |
| or allotropes, with very different | | | | hard oxide forms on the metal surface to |
| properties; understanding these is | | | | inhibit corrosion. Tungsten interferes |
| essential to making quality steel. At | | | | with the formation of cementite, |
| room temperature, the most stable form | | | | allowing martensite to form with slower |
| of iron is the body-centered cubic (BCC) | | | | quench rates, resulting in high speed |
| structure ferrite or ?-iron, a fairly | | | | steel. On the other hand sulfur, |
| soft metallic material that can dissolve | | | | nitrogen, and phosphorus make steel more |
| only a small concentration of carbon (no | | | | brittle, so these commonly found |
| more than 0.021 wt% at 910 °C). Above | | | | elements must be removed from the ore |
| 910 °C ferrite undergoes a phase | | | | during processing. |
| transition from body-centered cubic to a | | | | When iron is smelted from its ore by |
| face-centered cubic (FCC) structure, | | | | commercial processes, it contains more |
| called austenite or ?-iron, which is | | | | carbon than is desirable. To become |
| similarly soft and metallic but can | | | | steel, it must be melted and reprocessed |
| dissolve considerably more carbon (as | | | | to remove the correct amount of carbon, |
| much as 2.03 wt% carbon at 1154 °C)[2]. | | | | at which point other elements can be |
| As carbon-rich austenite cools, the | | | | added. Once this liquid is cast into |
| mixture attempts to revert to the | | | | ingots, it usually must be "worked" at |
| ferrite phase, resulting in an excess of | | | | high temperature to remove any cracks or |
| carbon. One way for carbon to leave the | | | | poorly mixed regions from the |
| austenite is for cementite to | | | | solidification process, and to produce |
| precipitate out of the mix, leaving | | | | shapes such as plate, sheet, wire, etc. |
| behind iron that is pure enough to take | | | | It is then heat-treated to produce a |
| the form of ferrite, and resulting in a | | | | desirable crystal structure, and often |
| cementite-ferrite mixture. Cementite is | | | | "cold worked" to produce the final |
| a stoichiometric phase with the chemical | | | | shape. In modern steelmaking these |
| formula of Fe3C. Cementite forms in | | | | processes are often combined, with ore |
| regions of higher carbon content while | | | | going in one end of the assembly line |
| other areas revert to ferrite around it. | | | | and finished steel coming out the other. |
| Self-reinforcing patterns often emerge | | | | These can be streamlined by a deft |
| during this process, leading to a | | | | control of the interaction between work |
| patterned layering known as pearlite due | | | | hardening and tempering. |
| to its pearl-like appearance, or the | | | | |