Alloy Steel

Alloy steel is steel alloyed with other elements in amounts of between 1 and 50% by weight to improve its mechanical properties. Alloy steels are broken down into two groups: low alloy steels and high alloy steels. The differentiation between the two is somewhat arbitrary; Smith and Hashemi define the difference at 4%, while Degarmo, et al., define it at 8%. However, most commonly alloy steel refers to low alloy steel.

These steels have greater strength, hardness, hot hardness, wear resistance, hardenability, or toughness compared to carbon steel. However, they may require heat treatment in order to achieve such properties. Common alloying elements are molybdenum, manganese, nickel, chromium, vanadium, silicon and boron

ASTM Steel

ASTM International (ASTM), originally known as the American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, and services.

Austenitic Steel

Steels containing high percentages of certain alloying elements such as manganese and nickel which are austenitic at room temperature and cannot be hardened by normal heat-treatment but do work harden. They are also non-magnetic. Typical examples of austenitic steels include the 18/8 stainless steels and 14% manganese steel.

Austenite (or gamma phase iron) is a metallic non-magnetic allotropes of iron or a solid solution of iron, with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 1000 K (about 727 °C); other alloys of steel have different eutectoid temperatures.

Carbon Steel

Carbon steel, also called plain carbon steel, is steel where the main alloying constituent is carbon. The AISI defines carbon steel as: "Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt, columbium [niobium], molybdenum, nickel, titanium, tungsten, vanadium or zirconium, or any other element to be added to obtain a desired alloying effect; when the specified minimum for copper does not exceed 0.40 percent; or when the maximum content specified for any of the following elements does not exceed the percentages noted: manganese 1.65, silicon 0.60, copper 0.60."

The term "carbon steel" may also be used in reference to steel which is not stainless steel; in this use carbon steel may include alloy steels.

Steel with a low carbon content has properties similar to iron. As the carbon content rises, the metal becomes harder and stronger but less ductile and more difficult to weld. In general, higher carbon content lowers the melting point and its temperature resistance. Carbon content influences the yield strength of steel because carbon atoms fit into the interstitial crystal lattice sites of the body-centered cubic (BCC) arrangement of the iron atoms. The interstitial carbon reduces the mobility of dislocations, which in turn has a hardening effect on the iron. To get dislocations to move, a high enough stress level must be applied in order for the dislocations to "break away". This is because the interstitial carbon atoms cause some of the iron BCC lattice cells to distort.

Chrome Moly Steel

Alloying elements include chromium and molybdenum, and as a result these materials are often referred to as chromoly steel, or cro-mo, or CRMO. They have an excellent strength to weight ratio, are easily welded and are considerably stronger and harder than standard 1020 steel.

While these grades of steel do contain chromium, it is not in great enough quantities to provide the corrosion resistance found in stainless steel.

Examples of applications for 4130 include structural tubing, bicycle frames, AK-47 receivers, clutch and flywheel components, and roll cages. They are also used in aircraft and therefore 41xx grade structural tubing is sometimes referred to as "aircraft tubing".

One of the important characteristics of the 41xx class of steels is their ability to be case hardened by carburization of the surface. The core of the material retains its bulk properties, while the outside is significantly hardened to reduce wear and tear on the part. This makes this grade of steel an excellent material for such uses as gears, piston pins, and crankshafts.

Duplex & Super Duplex

In the duplex and super duplex alloys, pitting corrosion, crevice corrosion, and stress corrosion cracking resistance is considerably higher than for conventional austenitic stainless steels. Duplex stainless steel has been designed to exceed the commonly specified PREN of 40 and provide, for example, castings suitable for pumps and valves on offshore oil and gas platforms. Duplex and Super Duplex are a high-tech materials for use in many situations where safer more cost effective, high strength, corrosion resistant materials are needed.

Electronic / Magnetic Alloy

There are many magnetic alloys. There three magnetic substances that you will find on the Periodic Table of elements. These are iron (Fe), nickel (Ni), and cobalt (Co). Iron is the most highly magnetic material. Magnetic alloys are alloys consisting two or more of these metals. Most magnetic alloys are made to make extra powerful magnets. Steel and alnico are two of the most important alloys. Alnico is made up of iron, nickel, cobalt, and aluminum. Steel is made up of iron and carbon. Another magnetic alloy is permalloy, a mixture of iron and nickel.

Hastelloy

Hastelloy is the registered trademark name of Haynes International, Inc. The trademark is applied as the prefix name of a range of twenty two different highly corrosion-resistant metal alloys loosely grouped by the metallurgical industry under the material term “superalloys” or “high-performance alloys”.

The predominant alloying ingredient is typically the transition metal nickel. Other alloying ingredients are added to nickel in each of the subcategories of this trademark designation and include varying percentages of the elements molybdenum, chromium, cobalt, iron, copper, manganese, titanium, zirconium, aluminum, carbon, and tungsten.

The primary function of the Hastelloy super alloys is that of effective survival under high-temperature, high-stress service in a moderately to severely corrosive, and/or erosion prone environment where more common and less expensive iron-based alloys would fail, including the pressure vessels of some nuclear reactors, chemical reactors, and pipes and valves in chemical industry.

Heat Resisting Steel

Heat resistant steel is designed typically for use in heat treat furnaces and heat exchangers - high nickel and chrome steels. Typical parts include heat treat trays, grids and hangers.

Inconel

Inconel is a registered trademark of Special Metals Corporation that refers to a family of austenitic nickel-chromium-based superalloys. Inconel alloys are typically used in high temperature applications. It is often referred to in English as "Inco" (or occasionally "Iconel"). Common trade names for Inconel include: Inconel 625, Chronin 625, Altemp 625, Haynes 625, Nickelvac 625 and Nicrofer 6020.

Inconel alloys are oxidation and corrosion resistant materials well suited for service in extreme environments. When heated, Inconel forms a thick, stable, passivating oxide layer protecting the surface from further attack. Inconel retains strength over a wide temperature range, attractive for high temperature applications where aluminum and steel would succumb to creep as a result of thermally-induced crystal vacancies (see Arrhenius equation). Inconel's high temperature strength is developed by solid solution strengthening or precipitation strengthening, depending on the alloy. In age hardening or precipitation strengthening varieties, small amounts of niobium combine with nickel to form the intermetallic compound Ni3Nb or gamma prime (γ'). Gamma prime forms small cubic crystals that inhibit slip and creep effectively at elevated temperatures.

Manganese Steel

Mangalloy is made by alloying steel, containing 0.8 to 1.25% carbon, with 11% to 15% manganese.Mangalloy is a unique non-magnetic steel with extreme anti-wear properties. The material is very resistant to abrasion and will achieve up three times its surface hardness during conditions of impact, without any increase in brittleness which is usually associated with hardness. This allows mangalloy to retain its toughness.

Most steels contain 0.15% - 0.8% manganese. High strength alloys often contain 1% - 1.8% manganese. At about 1.5% manganese content, the steel becomes brittle, and this trait increases until about 4 to 5% manganese content is reached. At this point, the steel will pulverize at the strike of a hammer. Further increase in the manganese content will increase both hardness and ductility. Both reach their highest points around 12%, depending on other alloying agents.

Mangalloy has been used in the mining industry, cement mixers, rock crushers, and other high impact and abrasive environments. These alloys are finding new uses as cryogenic steels, due to their high strength at very low temperatures. Mangalloy is heat treatable, but the manganese lowers the temperature at which austenite transforms into ferrite. Most grades are ready for use after annealing and quenching, with no further need of tempering, and usually have a normal Brinell hardness of around 200 HB, (roughly the same as 304 stainless steel), but, due to its unique properties, this number has very little effect on determining the abrasion and impact resistance of the metal.

Maraging Steel

Maraging steels (a portmanteau of martensitic and aging) are iron alloys which are known for possessing superior strength and toughness without losing malleability. These steels are a special class of low carbon ultra-high strength steels which derive their strength not from carbon, but from precipitation of inter-metallic compounds. The principal alloying element is 15 to 25% nickel. Secondary alloying elements are added to produce intermetallic precipitates, which include cobalt, molybdenum, and titanium. Original development was carried out on 20 and 25% Ni steels to which small additions of Al, Ti, and Nb were made.

The common, non-stainless grades contain 17–19% nickel, 8–12% cobalt, 3–5% molybdenum, and 0.2–1.6% titanium. Stainless grades rely on chromium not only to prevent their rusting, but to augment the hardenability of the alloy as their nickel content is substantially reduced. This is to ensure they can transform to martensite when heat treated, as high-chromium, high-nickel steels are generally austenitic, and unable to undergo such a transition.

Due to the low carbon content maraging steels have good machinability. Prior to aging, they may also be cold rolled to as much as 80–90% without cracking. Maraging steels offer good weldability, but must be aged afterward to restore the properties of heat affected zone.

When heat treated the alloy has very little dimensional change, so it is often machined to its final dimensions. Due to the high alloy content the alloys have a high hardenability. Since ductile FeNi martensites are formed upon cooling, cracks are non-existent or negligible. They can also be nitrided to increase case hardness. They can be polished to a fine surface finish.

Non-stainless varieties of maraging steels are moderately corrosion resistant and resist stress corrosion and hydrogen embrittlement. More corrosion protection can be gained by cadmium plating or phosphating.

Martensitic Steel

The austenite that exists during hot-rolling or annealing is transformed almost entirely to martensite during quenching on the run-out table or in the cooling section of the continuous annealing line. The Martensitic steels are characterized by a martensitic matrix containing small amounts of ferrite and/or bainite. Within the group of multiphase steels, Martensitic steels show the highest tensile strength level. This structure can also be developed with post-forming heat treatment. Martensitic steels provide the highest strengths, up to 1700 MPa ultimate tensile strength. Martensitic steels are often subjected to post-quench tempering to improve ductility, and can provide adequate formability even at extremely high strengths.

Carbon is added to Martensitic steels to increase hardenability and for strengthening the martensite. Manganese, silicon, chromium, molybdenum, boron, vanadium, and nickel are also used in various combinations to increase hardenability. Martensitic steels are produced from the austenite phase by rapid quenching to transform most of the austenite to martensite. CP steels also follow a similar cooling pattern, but here the chemistry is adjusted to produce less retained austenite and form fine precipitates to strengthen the martensite and bainite phases.

Resulfurized Steel

A plain carbon steel that contains added sulfur for improved machinability.

Resulfurized and Rephosphorized steel

A plain carbon steel that contains added sulfur and phosphorus for improved machinability.

Stainless Steel

In metallurgy, stainless steel, also known as inox steel or inox, is defined as a steel alloy with a minimum of 11% chromium content by mass. Stainless steel does not stain, corrode, or rust as easily as ordinary steel (it stains less, but it is not stain-proof). It is also called corrosion-resistant steel or CRES when the alloy type and grade are not detailed, particularly in the aviation industry. There are different grades and surface finishes of stainless steel to suit the environment to which the material will be subjected in its lifetime. Common uses of stainless steel are cutlery and watch cases and bands.

Stainless steel differs from carbon steel by the amount of chromium present. Carbon steel rusts when exposed to air and moisture. This iron oxide film (the rust) is active and accelerates corrosion by forming more iron oxide. Stainless steels have sufficient amounts of chromium present so that a passive film of chromium oxide forms which prevents further surface corrosion and blocks corrosion from spreading into the metal's internal structure.

Tool Steel

Tool steel refers to a variety of carbon and alloy steels that are particularly well-suited to be made into tools. Their suitability comes from their distinctive hardness, resistance to abrasion, their ability to hold a cutting edge, and/or their resistance to deformation at elevated temperatures (red-hardness). Tool steel is generally used in a heat-treated state.

With a carbon content between 0.7% and 1.4%, tool steels are manufactured under carefully controlled conditions to produce the required quality. The manganese content is often kept low to minimize the possibility of cracking during water quenching. However, proper heat treating of these steels is important for adequate performance, and there are many suppliers who provide tooling blanks intended for oil quenching.

Tool steels are made to a number of grades for different applications. Choice of grade depends on, among other things, whether a keen cutting edge is necessary, as in stamping dies, or whether the tool has to withstand impact loading and service conditions encountered with such hand tools as axes, pickaxes, and quarrying implements. In general, the edge temperature under expected use is an important determinant of both composition and required heat treatment. The higher carbon grades are typically used for such applications as stamping dies, metal cutting tools, etc.

Tool steels are also used for special applications like injection molding because the resistance to abrasion is an important criterion for a mold that will be used to produce hundreds of thousands of parts, chemical reactors, and pipes and valves in chemical industry.

Alloy Cast Iron

Cast iron usually refers to grey iron, but also identifies a large group of ferrous alloys, which solidify with a eutectic. The colour of a fractured surface can be used to identify an alloy. White cast iron is named after its white surface when fractured, due to its carbide impurities which allow cracks to pass straight through. Grey cast iron is named after its grey fractured surface, which occurs because the graphitic flakes deflect a passing crack and initiate countless new cracks as the material breaks.

Iron (Fe) accounts for more than 95% by weight (wt%) of the alloy material, while the main alloying elements are carbon (C) and silicon (Si). The amount of carbon in cast irons is 2.1 to 4 wt%. Cast irons contain appreciable amounts of silicon, normally 1 to 3 wt%, and consequently these alloys should be considered ternary Fe-C-Si alloys. Despite this, the principles of cast iron solidification are understood from the binary iron-carbon phase diagram, where the eutectic point lies at 1,154 °C (2,109 °F) and 4.3 wt% carbon. Since cast iron has nearly this composition, its melting temperature of 1150 to 1200 °C (2100–2200 °F) is about 300 °C (572 °F) lower than the melting point of pure iron.

Cast iron tends to be brittle, except for malleable cast irons. With its low melting point, good fluidity, castability, excellent machinability, resistance to deformation, and wear resistance, cast irons have become an engineering material with a wide range of applications, including pipes, machine and automotive industry parts, such as cylinder heads (depreciated usage), cylinder blocks, and gearbox cases (depreciated usage). It is resistant to destruction and weakening by oxidisation (rust).

Ductile Iron

Ductile iron, also called ductile cast iron, spheroidal graphite iron, or nodular cast iron, is a type of cast iron invented in 1943 by Keith Millis. While most varieties of cast iron are brittle, ductile iron is much more flexible and elastic, due to its nodular graphite inclusions.

Grey iron was the original "cast iron", and is an iron alloy characterized by its relatively high carbon content (usually 2% to 4%). When molten cast iron solidifies some of the carbon precipitates as graphite, forming tiny, irregular flakes within the crystal structure of the metal. While the graphite enhances the desirable properties of cast iron (improved casting & machining properties and better thermal conductivity), the flakes disrupt the crystal structure and provide a nucleation point for cracks, leading to cast iron's characteristic brittleness. In ductile iron the graphite is in the form of spherical nodules rather than flakes, thus inhibiting the creation of cracks and providing the enhanced ductility that gives the alloy its name. The formation of nodules is achieved by addition of "nodulizers" (for example, magnesium or cerium) into the melt. Yttrium has also been studied as a possible nodulizer.

Graphite iron having spherical graphite are produced from grey cast iron by a malleabilizing heat treatment. The heat treatment consists of heating grey cast iron at around 900 °C for almost 24 hours and cooling slowly or moderately in air. This treatment converts the graphite from flakes to a spherical shape. Cooling slowly gives a ferrite matrix while cooling moderately gives a pearlitic matrix.

A recent development in ductile iron metallurgy is austempered ductile iron where the metallurgical structure is manipulated through a sophisticated heat treating process.

Much of the annual production of ductile iron is in the form of ductile iron pipe, used for water and sewer lines. Ductile iron pipe is stronger and easier to tap, requires less support and provides greater flow area compared with pipe made from other materials. In difficult terrain it can be a better choice than PVC, concrete, polyethylene, or steel pipe.

Ductile iron is specifically useful in many automotive components, where strength needs surpass that of aluminum but do not necessarily require steel. Other major industrial applications include off-highway diesel trucks, class 8 trucks, agricultural tractors, and oil well pumps.

Grey Cast Iron

Gray iron, or grey iron, was the original "cast iron". It is an alloy of carbon (C), silicon (Si) and iron (Fe) and contains 1.7 to 4.5% carbon and 1 to 3% silicon.

It is relatively easy and inexpensive to make. Compared to the more modern engineered irons, gray iron has a lower tensile strength and lower ductility. In other words, it will fail more easily and its mode of failure will be sudden fracture (it will not bend). It is used for housings where tensile strength is non-critical, such as internal combustion engine cylinder blocks, pump housings, valve bodies, electrical boxes, and decorative castings.

In gray cast iron a large part or all of the carbon is in the form of flakes of graphite. In three dimensions, these appear as plate-like structures. In two dimensions, as polished surface will appear under a microscope, the graphite flakes appear as lines. The presence of graphite flakes gives capacity to damp vibrations caused by internal friction and, consequently, the ability to dissipate energy. Graphitic cast iron has a dark gray to almost black fracture. After a small degree of super cooling graphite forms when the cast iron solidifies from its liquid state. Slow cooling promotes graphitization. Rapid cooling partly or completely suppresses graphitization and leads to formation of cementite. Rapidly cooled gray iron in which the carbon does not form graphite flakes, but rather combines with iron to form cementite is called white iron.

Malleable Iron

Malleable iron starts as a white iron casting, that is then heat treated at about 900 °C (1,650 °F). Graphite separates out much more slowly in this case, so that surface tension has time to form it into spheroidal particles rather than flakes. Due to their lower aspect ratio, spheroids are relatively short and far from one another, and have a lower cross section vis-a-vis a propagating crack or phonon. They also have blunt boundaries, as opposed to flakes, which alleviates the stress concentration problems faced by grey cast iron. In general, the properties of malleable cast iron are more like mild steel. There is a limit to how large a part can be cast in malleable iron, since it is made from white cast iron.

Through an annealing heat treatment the brittle as cast structure is transformed. Carbon agglomerates into small roughly speherical aggregates of graphite leaving a matric of ferrite or pearlite according to the exact heat treat used. Three basic types of malleable iron are recognized within the casting industry, Blackheart malleable iron, Whiteheart malleable iron and Pearlitic malleable iron.

Like other similar irons with the carbon formed into spherical or nodular shapes, malleable iron exhibits good ductility. Incorrectly considered by some to be an "old" or "dead" material, malleable iron still has a legitimate place in the design engineer's toolbox. Malleable is a good choice for small castings or castings with thin cross sections (less than 0.25 inch). Other nodular irons produced with graphite in the spherical shape can be difficult to produce in these applications due to the formation of carbides from the rapid cooling.

Malleable iron is also exhibits good fracture toughness properties in low temperature environments better than other nodular irons due to its lower silicon content. The ductile to brittle transformation temperature is lower than many other ductile iron alloys.

In order to form properly the spherical-shaped nodules of graphite in the annealing process, during the casting process care must be taken to insure the iron casting will solidify with an entirely white iron cross section. Heavier sections of a casting will cool slowly and with the slow cooling some primary graphite may form. This graphite will form random flake-like structures and will not transform in heat treatment. When stress is applied to such a casting in application the fracture will be lower than normal and the large particles of primary graphite can be seen. Such iron is said to have a 'mottled' appearance. Some countermeasures can be applied to enhance forming the all white structure, but malleable iron foundries often avoid producing heavy sections due to the constraint of slow cooling times causing the formation of the primary graphite.

After the casting and heat treat process malleable iron can be shaped through cold working, such as stamping for straightening, bending or coining operations. This is possible due to malleable iron property of being less strain rate sensitive than other materials.

It is often used for small castings requiring good tensile strength and the ability to flex without breaking (ductility). Electrical fittings, hand tools, pipe fittings, washers, brackets, fence fittings, power line hardware, farm equipment, mining hardware, and machine parts.

White Cast Iron

With a lower silicon content and faster cooling, the carbon in white cast iron precipitates out of the melt as the metastable phase cementite, Fe3C, rather than graphite. The cementite which precipitates from the melt forms as relatively large particles, usually in a eutectic mixture, where the other phase is austenite (which on cooling might transform to martensite). These eutectic carbides are much too large to provide precipitation hardening (as in some steels, where cementite precipitates might inhibit plastic deformation by impeding the movement of dislocations through the ferrite matrix). Rather, they increase the bulk hardness of the cast iron simply by virtue of their own very high hardness and their substantial volume fraction, such that the bulk hardness can be approximated by a rule of mixtures. In any case, they offer hardness at the expense of toughness. Since carbide makes up a large fraction of the material, white cast iron could reasonably be classified as a cermet. White iron is too brittle for use in many structural components, but with good hardness and abrasion resistance and relatively low cost, it finds use in such applications as the wear surfaces (impeller and volute) of slurry pumps, shell liners and lifter bars in ball mills and autogenous grinding mills, balls and rings in coal pulverisers, and the teeth of a backhoe's digging bucket (although cast medium-carbon martensitic steel is more common for this application).

It is difficult to cool thick castings fast enough to solidify the melt as white cast iron all the way through. However, rapid cooling can be used to solidify a shell of white cast iron, after which the remainder cools more slowly to form a core of grey cast iron. The resulting casting, called a 'chilled casting”, has the benefits of a hard surface and a somewhat tougher interior.

White cast iron can also be made by using a high percentage of chromium (Cr) in the iron; Cr is a strong carbide-forming element, so at high enough percentages of chrome, the precipitation of graphite out of the iron is suppressed. High-chrome white iron alloys allow massive castings (for example, a 10-tonne impeller) to be sand cast, i.e., a high cooling rate is not required, as well as providing impressive abrasion resistance.