Iron-based alloys, also called ferrous alloys, include cast irons and steels and are the most widely used structural metals. Steels consist primarily of iron and contain some carbon and manganese, and often additional alloying elements.

They are distinguished from nearly pure iron, which is called ingot iron, and also from cast irons, which contain carbon in excess of 2% and from 1 to 3% silicon. Irons and steels can be divided into various classes, depending on their alloy compositions and other characteristics, as indicated in Table 1. Some examples of particular irons or steels and their alloy compositions are given in Table 2.

A wide variation in properties exists for various steels, as illustrated in Fig. 1. Pure iron is quite weak, but is strengthened considerably by the addition of small amounts of carbon.

Additional alloying with small amounts of niobium, vanadium, copper, or other elements permits strengthening by grain refinement, precipitation, or solid solution effects. If sufficient carbon is added for quenching and tempering to be effective, a major increase in strength is possible. Additional alloying and special processing can be combined with quenching and tempering and/or precipitation hardening to achieve even higher strengths.

Naming Systems for Irons and Steels : A number of different organizations have developed naming systems and specifications for various irons and steels that give the required alloy composition and sometimes required mechanical properties. These include the American Iron and Steel Institute (AISI), the Society of Automotive Engineers (SAE International), and the American Society for Testing and Materials (ASTM International).

In addition, SAE and ASTM have cooperated to develop a new Unified Numbering System (UNS) that gives designations not only for irons and steels, but also for all other metal alloys. See the Metals Handbook: Desk Edition (Davis, 1998) for an introduction to various naming  systems, and the current publication on the UNS System (SAE, 2008) for a description of those designations and their equivalence with other specifications.

The AISI and SAE designations for various steels are coordinated between the two organizations and are nearly identical. Details for common carbon and low-alloy steels are given in Table 3. Note that in this case there is usually a four-digit number. The first two digits specify the alloy content other than carbon, and the second two give the carbon content in hundredths of a percent.

For example, AISI 1340 (or SAE 1340) contains 0.40% carbon with 1.75% manganese as the only other alloying element. (Percentages of alloys are given on the basis of weight.)

The UNS system has a letter followed by a five-digit number. The letter indicates the category of alloy, such as F for cast irons, G for carbon and low-alloy steels in the AISI–SAE naming system, K for various special-purpose steels, S for stainless steels, and T for tool steels.

For carbon and lowalloy steels, the number is in most cases the same as that used by AISI and SAE, except that a zero is added at the end. Thus, AISI 1340 is the same steel as UNS G13400. Some particular classes of irons and steels will now be considered.

Types of Iron and Steel

Cast Irons

Cast irons in various forms have been used for more than two thousand years and continue to be relatively inexpensive and useful materials. The iron is not highly refined subsequent to extraction from ore or scrap, and it is formed into useful shapes by melting and pouring into molds. The temperature required to melt iron in a furnace is difficult to achieve. As a result, prior to the modern industrial era, there was also considerable use of wrought iron, which is heated and forged into useful shapes, but never melted in processing.

Several different types of cast iron exist. All contain large amounts of carbon, typically 2 to 4% by weight, and also 1 to 3% silicon. The large amount of carbon present exceeds the 2% that can be held in solid solution at elevated temperature, and in most cast irons the excess is present in the form of graphite.

Gray iron contains graphite in the form of flakes, as seen in Fig. 2 (top). These flakes easily develop into cracks under tensile stress, so that gray iron is relatively weak and brittle in tension. In compression, the strength and ductility are both considerably higher than for tension.

Ductile iron, also called nodular iron, contains graphite in the more nearly spherical form of nodules, as seen in Fig. 2 (bottom). This is achieved by careful control of impurities and by adding small amounts of magnesium or other elements that aid in nodule formation. As a result of the different form of the graphite, ductile iron has considerably greater strength and ductility in tension than gray iron.

White iron is formed by rapid cooling of a melt that would otherwise form gray iron. The excess carbon is in the form of a multiphase network involving large amounts of iron carbide, Fe₃C, also called cementite. This very hard and brittle phase results in the bulk material also being hard and brittle.

For malleable iron, special heat treatment of white iron is used to obtain a result similar to ductile iron. In addition, various alloying elements are used in making special-purpose cast irons that have improved response to processing or desirable properties, such as resistance to heat or corrosion.

Carbon Steels

Plain-carbon steels contain carbon, in amounts usually less than 1%, as the alloying element that controls the properties. They also contain limited amounts of manganese and (generally undesirable) impurities, such as sulfur and phosphorus. The more specific terms low-carbon steel and mild steel are often used to indicate a carbon content of less than 0.25%, such as AISI 1020 steel.

These steels have relatively low strength, but excellent ductility. The structure is a combination of BCC iron, also called α-iron or ferrite, and pearlite. Pearlite is a layered two-phase structure of ferrite and cementite (Fe₃C), as seen in Fig. 3 (left).

Low-carbon steels can be strengthened somewhat by cold working, but only minor strengthening is possible by heat treatment. Uses include structural steel for buildings and bridges, and sheet metal applications, such as automobile bodies.

Medium-carbon steels, with carbon content around 0.3 to 0.6%, and high-carbon steels, with carbon content around 0.7 to 1% and greater, have higher strengths than low-carbon steels, as a result of the presence of more carbon. In addition, the strength can be increased significantly by heat treatment using the quenching and tempering process, increasingly so for higher carbon contents.

However, high strengths are accompanied by loss of ductility—that is, by more brittle behavior. Medium-carbon steels have a wide range of uses as shafts and other components of machines and vehicles. High-carbon steels are limited to uses where their high hardness is beneficial and the low ductility is not a serious disadvantage, as in cutting tools and springs.

In quenching and tempering, the steel is first heated to about 850^◦C so that the iron changes to the FCC phase known as γ -iron or austenite, with carbon being in solid solution. A supersaturated solution of carbon in BCC iron is then formed by rapid cooling, called quenching, which can be accomplished by immersing the hot metal into water or oil.

After quenching, a structure called martensite is present, which has a BCC lattice distorted by interstitial carbon atoms. The martensite exists either as groupings of parallel thin crystals (laths) or as more randomly oriented thin plates, surrounded by regions of austenite.

As-quenched steel is very hard and brittle due to the two phases present, the distorted crystal structure, and a high dislocation density. To obtain a useful material, it must be subjected to a second stage of heat treatment at a lower temperature, called tempering. This causes removal of some of the carbon from the martensite and the formation of dispersed particles of Fe₃C.

Tempering lowers the strength, but increases the ductility. The effect is greater for higher tempering temperatures and varies with carbon content and alloying, as illustrated in Fig. 4. The microstructure of a quenched and tempered steel is shown in Fig. 3 (right).

Low-Alloy Steels

In low-alloy steels, also often called simply alloy steels, small amounts of alloying elements totaling no more than about 5% are added to improve various properties or the response to processing. Percentages of the principal alloying elements are given for some of these in Table 3.

As examples of the effects of alloying, sulfur improves machineability, and molybdenum and vanadium promote grain refinement. The combination of alloys used in the steel AISI 4340 gives improved strength and toughness—that is, resistance to failure due to a crack or sharp flaw.

In this steel, the metallurgical changes during quenching proceed at a relatively slow rate so that quenching and tempering is effective in components as thick as 100 mm. Note that the corresponding plain-carbon steel, AISI 1040, requires very rapid quenching that cannot be achieved except within about 5 mm of the surface.

Various special-purpose low-alloy steels are used that may not fit any of the standard AISI–SAE designations. Many of these are described in the ASTM Standards, where requirements are placed on mechanical properties in addition to alloy content. Some of these are classified as high-strength low-alloy (HSLA) steels, which have a low carbon content and a ferritic-pearlitic structure, with small amounts of alloying resulting in higher strengths than in other low-carbon steels.

Examples include structural steels as used in buildings and bridges, such as ASTM A242, A441, A572, and A588. Note that use of the term “high-strength” here can be somewhat misleading, as the strengths are high for a low-carbon steel, but not nearly as high as for many quenched and tempered steels. The low-alloy steels used for pressure vessels, such as ASTM A302, A517, and A533, constitute an additional group of special-purpose steels.

Stainless Steels

Steels containing at least 10% chromium are called stainless steels because they have good corrosion resistance; that is, they do not rust. These alloys also frequently have improved resistance to high temperature. A separate system of AISI designations employs a three-digit number, such as AISI 316 and AISI 403, with the first digit indicating a particular class of stainless steels. The corresponding UNS designations often use the same digits, such as S31600 and S40300 for the two just listed.

The 400-series stainless steels have carbon in various percentages and small amounts of metallic alloying elements in addition to the chromium. If the chromium content is less than about 15%, as in types 403, 410, and 422, the steel in most cases can be heat treated by quenching and tempering to have a martensitic structure, so that it is called a martensitic stainless steel. Uses include tools and blades in steam turbines.

However, if the chromium content is higher, typically 17 to 25%, the result is a ferritic stainless steel that can be strengthened only by cold work, and then only modestly. These are used where high strength is not as essential as high corrosion resistance, as in architectural use.

The 300-series stainless steels, such as types 304, 310, 316, and 347, contain around 10 to 20% nickel in addition to 17 to 25% chromium. Nickel further enhances corrosion resistance and results in the FCC crystal structure being stable even at low temperatures.

These are termed austenitic stainless steels. They either are used in the annealed condition or are strengthened by cold work, and they have excellent ductility and toughness. Uses include nuts and bolts, pressure vessels and piping, and medical bone screws and plates.

Another group is the precipitation-hardening stainless steels. These are strengthened as the name implies, and they are used in various high-stress applications where resistance to corrosion and high temperature are required, as in heat-exchanger tubes and turbine blades. An example is 17-4 PH stainless steel (UNS S17400), which contains 17% chromium and 4% nickel—hence its name—and also 4% copper and smaller amounts of other elements.

Tool Steels and Other Special Steels

Tool steels are specially alloyed and processed to have high hardness and wear resistance for use in cutting tools and special components of machinery. Most contain several percent chromium, some have quite high carbon contents in the 1 to 2% range, and some contain fairly high percentages of molybdenum and/or tungsten. Strengthening generally involves quenching and tempering or related heat treatments. The AISI designations are in the form of a letter followed by a one- or two-digit number.

For example, tool steels M1, M2, etc., contain 5 to 10% molybdenum and smaller amounts of tungsten and vanadium; and tool steels T1, T2, etc., contain substantial amounts of tungsten, typically 18%.

The tool steel H11, containing 0.4% carbon, 5% chromium, and modest amounts of other elements, is used in various high-stress applications. It can be fully strengthened in thick sections up to 150 mm and retains moderate ductility and toughness even at very high yield strengths around 2100 MPa and above.

This is achieved by the ausforming process, which involves deforming the steel at a high temperature within the range where the austenite (FCC) crystal structure exists. An extremely high dislocation density and a very fine precipitate are introduced, which combine to provide additional strengthening that is added to the usual martensite strengthening due to quenching and tempering. Ausformed H11 is one of the strongest steels that has reasonable ductility and toughness.

Various additional specialized high-strength steels have names that are nonstandard trade names. Examples include 300 M, which is AISI 4340 modified with 1.6% silicon and some vanadium, and D-6a steel used in aerospace applications. Maraging steels contain 18% nickel and other alloying elements, and they have high strength and toughness due to a combination of a martensitic structure and precipitation hardening.

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