MATERIAL SCIENCE

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. Low alloy steels are defined as having an alloy contents between 1 and 4% and high alloy steels have 4 to 50% alloying contents.[1] 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.

Ferrite (α-iron, δ-iron; soft) Austenite (γ-iron; harder) Spheroidite Pearlite (88% ferrite, 12% cementite) Bainite Martensite Ledeburite (ferrite-cementite eutectic, 4.3% carbon) Cementite (iron carbide, Fe3C; hardest) Steel classes Carbon steel (≤2.1% carbon; low alloy) Stainless steel (+chromium) Maraging steel (+nickel) Alloy steel (hard) Tool steel (harder) Other iron-based materials Cast iron (>2.1% carbon) Ductile iron Wrought iron (contains slag)

Carbon steel Iron alloy phases v • d • e Ferrite (α-iron, δ-iron; soft) Austenite (γ-iron; harder) Spheroidite Pearlite (88% ferrite, 12% cementite) Bainite Martensite Ledeburite (ferrite-cementite eutectic, 4.3% carbon) Cementite (iron carbide, Fe3C; hardest) Steel classes Carbon steel (≤2.1% carbon; low alloy) Stainless steel (+chromium) Maraging steel (+nickel) Alloy steel (hard) Tool steel (harder) Other iron-based materials Cast iron (>2.1% carbon) Ductile iron Wrought iron (contains slag) 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 per cent; 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."[1] 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 toweld. 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 ofdislocations, 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. 85% of all steel used in the U.S. is carbon steel.[1] Type::Carbon steel is broken down in to four classes based on carbon content: Mild and low carbon steel Mild steel is the most common form of steel as its price is relatively low while it provides material properties that are acceptable for many applications. Low carbon steel contains approximately 0.05–0.15% carbon[1]and mild steel contains 0.16–0.29%[1] carbon, therefore it is neither brittle nor ductile. Mild steel has a relatively low tensile strength, but it is cheap and malleable; surface hardness can be increased throughcarburizing.[2] It is often used when large amounts of steel is needed, for example as structural steel. The density of is 7,861.093 kg/m³ (0.284 lb/in³), the tensile strength is a maximum of 500 MPa (73,000 psi) and the Young's modulus is 210,000 MPa (30,000,000 psi).[citation needed] Low carbon steels suffer from yield-point runout where the materials has two yield points. The first yield point (or upper yield point) is higher than the second and the yield drop dramatically after the upper yield point. If a low carbon steel is only stressed to some point between the upper and lower yield point then the surface may develop luder bands.[3] Higher carbon steels Carbon steels which can successfully undergo heat-treatment have a carbon content in the range of 0.30–1.70% by weight. Trace impurities of various other elements can have a significant effect on the quality of the resulting steel. Trace amounts of sulfur in particular make the steel red-short. Low alloy carbon steel, such as A36 grade, contains about 0.05% sulfur and melts around 1426–1538 °C (2600–2800 °F).[4]Manganese is often added to improve the hardenability of low carbon steels. These additions turn the material into a low alloy steel by some definitions, but AISI's definition of carbon steel allows up to 1.65% manganese by weight. Medium carbon steel Approximately 0.30–0.59% carbon content.[1] Balances ductility and strength and has good wear resistance; used for large parts, forging and automotive components.[5] High carbon steel Approximately 0.6–0.99% carbon content.[1] Very strong, used for springs and high-strength wires.[6] Ultra-high carbon steel Approximately 1.0–2.0% carbon content.[1] Steels that can be tempered to great hardness. Used for special purposes like (non-industrial-purpose) knives, axles or punches. Most steels with more than 1.2% carbon content are made using powder metallurgy. Steel can be heat treated which allows parts to be fabricated in an easily-formable soft state. If enough carbon is present, the alloy can be hardened to increase strength, wear, and impact resistance. Steels are often wrought by cold working methods, which is the shaping of metal through deformation at a low equilibrium or metastable temperature. Heat treatment Iron-carbon phase diagram, showing the temperature and carbon ranges for certain types of heat treatments. The purpose of heat treating carbon steel is to change the mechanical properties of steel, usually ductility, hardness, yield strength, or impact resistance. Note that the electrical and thermal conductivity are slightly altered. As with most strengthening techniques for steel, Young's modulus is unaffected. Steel has a higher solid solubility for carbon in theaustenite phase; therefore all heat treatments, except spheroidizing and process annealing, start by heating to an austenitic phase. The rate at which the steel is cooled through the eutectoid reaction affects the rate at which carbon diffuses out of austenite. Generally speaking, cooling swiftly will give a finer pearlite (until the martensite critical temperature is reached) and cooling slowly will give a coarser pearlite. Cooling a hypoeutectoid (less than 0.77 wt% C) steel results in a pearlitic structure with α-ferrite at the grain boundaries. If it is hypereutectoid (more than 0.77 wt% C) steel then the structure is full pearlite with small grains of cementite scattered throughout. The relative amounts of constituents are found using the lever rule. Here is a list of the types of heat treatments possible: Spheroidizing: Spheroidite forms when carbon steel is heated to approximately 700 °C for over 30 hours. Spheroidite can form at lower temperatures but the time needed drastically increases, as this is a diffusion-controlled process. The result is a structure of rods or spheres of cementite within primary structure (ferrite or pearlite, depending on which side of the eutectoid you are on). The purpose is to soften higher carbon steels and allow more formability. This is the softest and most ductile form of steel. The image to the right shows where spheroidizing usually occurs.[7] Full annealing: Carbon steel is heated to approximately 40 °C above Ac3 or Ac1 for 1 hour; this assures all the ferrite transforms into austenite (although cementite might still exist if the carbon content is greater than the eutectoid). The steel must then be cooled slowly, in the realm of 38 °C (100 °F) per hour. Usually it is just furnace cooled, where the furnace is turned off with the steel still inside. This results in a coarse pearlitic structure, which means the "bands" of pearlite are thick. Fully-annealed steel is soft and ductile, with no internal stresses, which is often necessary for cost-effective forming. Only spheroidized steel is softer and more ductile.[8] Process annealing: A process used to relieve stress in a cold-worked carbon steel with less than 0.3 wt% C. The steel is usually heated up to 550–650 °C for 1 hour, but sometimes temperatures as high as 700 °C. The image rightward shows the area where process annealing occurs. Isothermal annealing: It is a process in which hypoeutectoid steel is heated above the upper critical temperature and this temperature is maintained for a time and then the temperature is brought down below lower critical temperature and is again maintained. Then finally it is cooled at room temperature. This method rids any temperature gradient. Normalizing: Carbon steel is heated to approximately 55 °C above Ac3 or Acm for 1 hour; this assures the steel completely transforms to austenite. The steel is then air-cooled, which is a cooling rate of approximately 38 °C (100 °F) per minute. This results in a fine pearlitic structure, and a more-uniform structure. Normalized steel has a higher strength than annealed steel; it has a relatively high strength and ductility.[9] Quenching: Carbon steel with at least 0.4 wt% C is heated to normalizing temperatures and then rapidly cooled (quenched) in water, brine, or oil to the critical temperature. The critical temperature is dependent on the carbon content, but as a general rule is lower as the carbon content increases. This results in a martensitic structure; a form of steel that possesses a super-saturated carbon content in a deformed body-centered cubic (BCC) crystalline structure, properly termed body-centered tetragonal (BCT), with much internal stress. Thus quenched steel is extremely hard but brittle, usually too brittle for practical purposes. These internal stresses cause stress cracks on the surface. Quenched steel is approximately three to four (with more carbon) fold harder than normalized steel.[10] Martempering (Marquenching): Martempering is not actually a tempering procedure, hence the term "marquenching". It is a form of isothermal heat treatment applied after an initial quench of typically in an oil or brine solution at a temperature right above the "martensite start temperature". At this temperature, residual stresses within the material are relieved and some bainite may be formed from the retained ferrite which did not have time to transform into anything else. In industry, this is a process used to control the ductility and hardness of a material. With longer marquenching, the ductility increases with a minimal loss in strength; the steel is held in this solution until the inner and outer temperatures equalize. Then the steel is cooled at a moderate speed to keep the temperature gradient minimal. Not only does this process reduce internal stresses and stress cracks, but it also increases the impact resistance.[11] Quench and tempering: This is the most common heat treatment encountered, because the final properties can be precisely determined by the temperature and time of the tempering. Tempering involves reheating quenched steel to a temperature below the eutectoid temperature then cooling. The elevated temperature allows very small amounts of spheroidite to form, which restore ductility, but reduces hardness. Actual temperatures and times are carefully chosen for each composition.[12] Austempering: The austempering process is the same as martempering, except the steel is held in the brine solution through the bainite transformation temperatures, and then moderately cooled. The resulting bainite steel has a greater ductility, higher impact resistance, and less distortion. The disadvantage of austempering is it can only be used on a few steels, and it requires a special brine solution.[13] Case hardening Case hardening processes harden only the exterior of the steel part, creating a hard, wear resistant skin (the "case") but preserving a tough and ductile interior. Carbon steels are not very hardenable; therefore wide pieces cannot be thru-hardened. Alloy steels have a better hardenability, so they can thruharden and do not require case hardening. This property of carbon steel can be beneficial, because it gives the surface good wear characteristics but leaves the core tough.

CAST IRON Iron alloy phases v • d • e Ferrite (α-iron, δ-iron; soft) Austenite (γ-iron; harder) Spheroidite Pearlite (88% ferrite, 12% cementite) Bainite Martensite Ledeburite (ferrite-cementite eutectic, 4.3% carbon) Cementite (iron carbide, Fe3C; hardest) Steel classes Carbon steel (≤2.1% carbon; low alloy) Stainless steel (+chromium) Maraging steel (+nickel) Alloy steel (hard) Tool steel (harder) Other iron-based materials Cast iron (>2.1% carbon) Ductile iron Wrought iron (contains slag) Iron-Cementite meta-stable diagram. Cast iron usually refers to grey cast iron, but identifies a large group of ferrous alloys, which solidify with a eutectic. The color 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 %wt of the alloy material, while the main alloying elements are carbon (C) and silicon (Si). The amount of carbon in cast irons is 2.1-4 %wt. Cast irons contain appreciable amounts of silicon, normally 1-3 %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 1154 °C and 4.3 wt% carbon. Since cast iron has nearly this composition, its melting temperature of 1150 to 1200 °C is about 300 °C 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 and wear resistance, cast irons have become an engineering material with a wide range of applications, including pipes, machine and car parts. Production Cast iron is made by remelting pig iron, often along with substantial quantities of scrap iron and scrap steel, and taking various steps to remove undesirable contaminants such asphosphorus and sulfur. Depending on the application, carbon and silicon content are reduced to the desired levels, which may be anywhere from 2% to 3.5% and 1% to 3% respectively. Other elements are then added to the melt before the final form is produced by casting.[citation needed] Iron is most commonly melted in a small blast furnace known as a cupola (see blast furnace for more details). After melting is complete, the molten iron is removed or ladled from the forehearth of the blast furnace. This process was devised by the Chinese, whose innovative ideas revolutionized the field of metallurgy. Previously, iron was melted in an air furnace, which is a type of reverberatory furnace Varieties of cast iron Grey cast iron Cast iron drain, waste and vent piping in a Canadian timber-frame building in Mission, British Columbiain the 1980s. Main article: Grey iron Silicon is essential to making grey cast iron as opposed to white cast iron. When silicon is alloyed with ferrite and carbon in amounts of about 2 percent, the carbide of iron becomes unstable. Silicon causes the carbon to rapidly come out of solution as graphite, leaving a matrix of relatively pure, soft iron. Weak bonding between planes of graphite lead to a high activation energy for growth in that direction, resulting in thin, round flakes. This structure has several useful properties. The metal expands slightly on solidifying as the graphite precipitates, resulting in sharp castings. The graphite content also offers good corrosion resistance. Graphite acts as a lubricant, improving wear resistance. The exceptionally high speed of sound in graphite gives cast iron a much higher thermal conductivity. Since ferrite is so different in this respect (having heavier atoms, bonded much less tightly) phonons tend to scatter at the interface between the two materials. In practical terms, this means that cast iron tends to “damp” mechanical vibrations (including sound), which can help machinery to run more smoothly. All of the properties listed in the paragraph above ease the machining of grey cast iron. The sharp edges of graphite flakes also tend to concentrate stress, allowing cracks to form much more easily, so that material can be removed much more efficiently. Easier initiation of cracks can be a drawback once an item is finished, however: grey cast iron has less tensile strength and shock resistance than steel. It is also difficult to weld. Grey cast iron's high thermal conductivity and specific heat capacity are often exploited to make cast iron cookware and disc brake rotors. Other cast iron alloys Furnace bellows operated bywaterwheels, from the Nong Shu, byWang Zhen, 1313 AD, during the Chinese Yuan Dynasty. 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 tomartensite). 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 (conceivably?) balls for rolling-element bearings and the teeth of a backhoe's digging bucket (although the latter two applications would normally use high quality wrought high-carbon martensitic steels and cast medium-carbon martensitic steels respectively). 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 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. Malleable iron starts as a white iron casting, that is then heat treated at about 900 °C. 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. A more recent development is nodular or ductile cast iron. Tiny amounts of magnesium or cerium added to these alloys slow down the growth of graphite precipitates by bonding to the edges of the graphite planes. Along with careful control of other elements and timing, this allows the carbon to separate as spheroidal particles as the material solidifies. The properties are similar to malleable iron but parts can be cast with larger sections. Cast iron is melted in furnaces usually in half ton measures. The metal melted usually consists of discs and drums and its properties are changed by adding an inoculant. This alters the characteristics of the metal to various grades and between grey and SG iron. Comparative qualities of cast irons[1] Name Nominal composition [% by weight] Form and condition Yield strength [ksi (0.2% offset)] Tensile strength [ksi] Elongation [% (in 2 inches)] Hardness [Brinell scale] Uses Cast grey iron (ASTM A48) C 3.4, Si 1.8, Mn 0.5 Cast — 25 0.5 180 Engine blocks, fly-wheels, gears, machine-tool bases White C 3.4, Si 0.7, Mn 0.6 Cast (as cast) — 25 0 450 Bearing surfaces Malleable iron (ASTM A47) C 2.5, Si 1.0, Mn 0.55 Cast (annealed) 33 52 12 130 Axle bearings, track wheels, automotive crankshafts Ductile or nodular iron C 3.4, P 0.1, Mn 0.4, Ni 1.0, Mg 0.06 Cast 53 70 18 170 Gears, cams, crankshafts Ductile or nodular iron (ASTM A339) — Cast (quench tempered) 108 135 5 310 — Ni-hard type 2 C 2.7, Si 0.6, Mn 0.5, Ni 4.5, Cr 2.0 Sand-cast — 55 – 550 Strength Ni-resist type 2 C 3.0, Si 2.0, Mn 1.0, Ni 20.0, Cr 2.5 Cast — 27 2 140 Resistance to heat and corrosion [edit]Historical uses A cast iron wagon wheel Original Tay Bridge from the north Fallen Tay Bridge from the north Because cast iron is comparatively brittle, it is not suitable for purposes where a sharp edge or flexibility is required. It is strong under compression, but not under tension. Cast Iron was first invented in China (see also: Du Shi), and poured into molds to make weapons and figurines. Historically, its earliest uses included cannon and shot. In England, theironmasters of the Weald continued producing these until the 1760s, and this was the main function of the iron industry there after the Restoration, though probably only a minor part of the industry there earlier. Cast iron pots were made at many English blast furnaces at that period. In 1707, Abraham Darby patented a method of making pots (and kettles) thinner and hence cheaper than his rivals could. This meant that his Coalbrookdale Furnaces became dominant as suppliers of pots, an activity in which they were joined in the 1720s and 1730s by a small number of other coke-fired blast furnaces. The development of the steam engine by Thomas Newcomen provided a further market for cast iron, since this was considerably cheaper than the brass of which the engine cylinders were originally made. A great exponent of cast iron was John Wilkinson, who amongst other things cast the cylinders for many of James Watt's improved steam enginesuntil the establishment of the Soho Foundry in 1795. Cast iron bridges The Eglinton Tournament Bridge,North Ayrshire, Scotland, built from cast iron. The Iron Bridge over the River Severn at Coalbrookdale, England The major use of cast iron for structural purposes began in the late 1770s when Abraham Darby IIIbuilt the Iron Bridge, although short beams had been used prior to the bridge, such as in the blast furnaces at Coalbrookdale. This was followed by others, including Thomas Paine, who patented one; cast iron bridges became common as the Industrial Revolution gathered pace. Thomas Telfordadopted the material for his bridge upstream at Buildwas, and then for a canal trough aqueduct atLongdon-on-Tern on the Shrewsbury Canal. The Pontcysyllte Aqueduct,Llangollen, Wales, viewed from the ground It was followed by the Chirk Aqueduct and the Pontcysyllte Aqueduct, both of which remain in use following recent restorations. Cast iron beam bridges were used widely by the early railways, such as the Water street bridge at the Manchester terminus of the Liverpool and Manchester Railway. However, problems arose when such a bridge collapsed shortly after opening in 1846. The Dee bridge disaster was caused by excessive loading at the centre of the beam by a passing train, and many similar bridges had to be demolished and rebuilt, often in wrought iron. The bridge had been under-designed, being trussed with wrought iron straps, which were wrongly thought to reinforce the structure. The centres of the beams were put into bending, with the lower edge in tension, where cast iron is very weak. The best way of using cast iron was by using arches, so that all the material is in compression, where it is very strong. Nevertheless, cast iron continued to be used for structural support, until the Tay Rail Bridge disaster of 1879 created a crisis of confidence in the material. Crucial lugs for holding tie bars and struts had been cast integral with the columns, and they failed during the early stages of the accident. In addition, the bolt holes were also cast and not drilled, so that all the tension from the tie bars was placed on a corner, rather than being spread over the length of the hole. The replacement bridge was built in wrought iron and steel. Further bridge collapses occurred, however, culminating in the Norwood Junction rail accident of 1891. Thousands of cast iron rail under-bridges were eventually replaced by steel equivalents. Buildings Cast-iron architecture Cast iron columns enabled architects to build tall buildings without the enormously thick walls required to construct masonry buildings of any height. This allowed tall buildings to have large windows, in large cities, manufacturing buildings and early department stores were built with cast iron columns to allow daylight to enter. Examples can be seen in New York City's SoHo Cast Iron Historic District. Architects also liked cast iron because slender cast iron columns could suppport the weight that would require thick masonry columns or piers, opening up floor space in practical building like factories, and sight lines in houses of worship and auditoriums. Textile mills Another important use was in textile mills. The air in these contained flammable fibres from the cotton, hemp, or wool being spun. As a result, textile mills had an alarming propensity to burn down. The solution was to build them completely of non-combustible materials, and it was found convenient to provide the building with an iron frame, largely of cast iron. This replaced flammable wood. The first such building was at Ditherington in Shrewsbury, Shropshire. Many other warehouses were built using cast iron columns and beams, although there were many collapses owing to faulty designs, flawed beams or overloading. During the Industrial Revolution, cast iron was also widely used for the frame and other fixed parts of machinery, including spinning and later weaving machinery in the textile mills. Cast iron became a widespread material, and many towns had foundries producing machinery, not only for industry but also agriculture.