An alloy is a partial or complete solid solution
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Published: Mon, 5 Dec 2016
- An alloy is a partial or complete solid solution of one or more elements in a metallic matrix. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may be homogeneous in distribution depending on thermal heat treatment. Alloys usually have different properties from those of the component elements.
- Alloying one metal with other metal(s) or non metal(s) often enhances its properties. For example, steel is stronger than iron, its primary element. It is sometimes due to the sizes of the atoms in the alloy, since larger atoms exert a compressive force on neighbouring atoms, and smaller atoms exert a tensile force on their neighbours, helping the alloy resist deformation.
- Sometimes alloys may exhibit marked differences in behaviour even when small amounts of one element occur. For example, impurities in semi-conducting ferromagnetic alloys lead to different properties, as first predicted by White, Hogan, Suhl, Tian Abrie and Nakamura.
- Some alloys are made by melting and mixing two or more metals. Bronze, an alloy of copper and tin, was the first alloy discovered, during the prehistoric period now known as the Bronze Age; it was harder than pure copper and originally used to make tools and weapons, but was later superseded by metals and alloys with better properties.
- In later times bronze has been used for ornaments, bells, statues, and bearings. Brass is an alloy made from copper and zinc.
- Unlike pure metals, most alloys do not have a single melting point, but a melting range in which the material is a mixture of solid and liquid phases. The temperature at which melting begins is called the solidus, and the temperature when melting is just complete is called the liquidus. However, for most alloys there is a particular proportion of constituents (in rare cases two)-the eutectic mixture-which gives the alloy a unique melting point.
IMPORTANT OF ALLOYS
- Some alloys are used so predominantly with respect to their base metals that the name of the primary constituent is also used as the name of the alloy. For example, 14 karat gold is an alloy of gold with other elements. Similarly, the silver used in jewellery and the aluminium used as a structural building material are also alloys.
- The term “alloy” is sometimes used in everyday speech as a synonym for a particular alloy. For example, automobile wheels made of an aluminium alloy are commonly referred to as simply “alloy wheels”, although in point of fact steels and most other metals in practical use are also alloys.
ALLOY OF IRON
- Anthracite iron (carbon)
- Cast iron (carbon)
- Pig iron (carbon)
- Wrought iron (carbon)
- Fernico (nickel, cobalt)
- Elinvar (nickel, chromium)
- Invar (nickel)
- Kovar (cobalt)
- Spiegeleisen (manganese, carbon, silicon)
Anthracite iron is the substance created by the smelting together of anthracite coal and iron ore. Research into the smelting of iron using anthracite coal began in the 1820s. Initial experiments, most notably by Gueymard and Robin at Vizille in 1827, attempted to gradually substitute anthracite for other fuels, such as coke or charcoal, but all failed due to the use of cold blast techniques, which generated insufficient heat to keep the anthracite in combustion.
In the United States, where the Lehigh Coal and Navigation Company (LC&N) had begun shipping anthracite to Philadelphia in 1820, there was great interest in exploiting the great anthracite deposits of Schuylkill County for iron making.
The Franklin Institute, in 1830, offered a gold medal to the manufacturer of the greatest quantity of anthracite iron, and Nicholas Biddle and his associates offered a prize of $5,000 to the first individual to smelt a certain quantity of iron ore within a given time, using anthracite.
The Lehigh Coal and Navigation also offered free water power and discount rates on coal and shipping to encourage the development of the process.
HISTORY OF ANTHRACITE IRON
The key breakthrough occurred in 1829, when James Beaumont Neilson patented the hot blast, which he had conceived in an attempt to improve the efficiency of conventionally-fueled furnaces.
The first person to employ the hot blast technique to anthracite smelting was Dr. Frederick W. Gesenhainer, who filed for a patent on the process in 1831 and received it in 1833. In 1836, he tried smelting anthracite iron on a practical scale at his property, Valley Furnace, near Pottsville, Pennsylvania. He produced a small quantity of iron, but due to mechanical breakdowns, could not keep the furnace in operation for more than two months.
While distinguished visitors, including Governor Joseph Ritner, acknowledged his success, he sold out his share in Valley Furnace and went to New York City.
Research was proceeding along parallel lines across the Atlantic. George Crane, owner, and David Thomas, supterintendent of the Yniscedwyn Iron Works, had themselves conceived of the idea of using hot blast to smelt anthracite.
Thomas was sent to Scotland to examine Neilson’s installation and reproduced it at Yniscedwyn. Crane filed for a British patent on smelting iron with anthracite and hot blast in 1836, and received it in 1837. By the time the patent was sealed, Yniscedwyn was producing about 35 tons of iron using anthracite only as a fuel.
Inspired both by Geisenhainer and Crane (whose success was closely followed by the LC&N), experiments in the US continued. Baughman, Guiteau and Company used an old furnace near Mauch Chunk to produce some anthracite iron during late 1837.
They built another experimental furnace nearby, which was worked for about two months during fall and winter 1838 and for some time in 1839, but mechanical deficiencies led them to abandon the furnace at the end of 1839. In the meantime, Pioneer Furnace, in Pottsville, was blown in using anthracite fuel in 1839.
It was built by William Lyman obtained the aid of a Welsh emigrant, Benjamin Perry, who was familiar with Neilson’s process and the Yniscedwyn works, for the blowing-in. The furnace ran for three months on anthracite alone and fulfilled the conditions to win the $5,000 prize. In the design of Pioneer Furnace, Lyman had also been assisted by David Thomas, who had arrived in the United States in May 1839.
Thomas was engaged by the LC&N to set up the Lehigh Crane Iron Company and its first furnace at Catasauqua, which went into blast in 1840, along with five other anthracite furnaces. This marked the commercial establishment of anthracite iron production in the United States.
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 4wt%. Cast irons contain appreciable amounts of silicon, normally 1 to 3wt%, 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 4.3wt% carbon. Since cast iron has nearly this composition, its melting temperature of 1,150 to 1,200°C (2,102to 2,192°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 (declining usage), cylinder blocks, and gearbox cases (declining usage). It is resistant to destruction and weakening by oxidisation (rust).
Pig iron is the intermediate product of smelting iron ore with coke, usually with limestone as a flux. Pig iron has a very high carbon content, typically 3.5-4.5%, which makes it very brittle and not useful directly as a material except for limited applications.
The traditional shape of the molds used for these ingots was a branching structure formed in sand, with many individual ingots at right angles to a central channel or runner. Such a configuration is similar in appearance to a litter of piglets suckling on a sow.
When the metal had cooled and hardened, the smaller ingots (the pigs) were simply broken from the much thinner runner (the sow), hence the name pig iron. As pig iron is intended for remelting, the uneven size of the ingots and inclusion of small amounts of sand was insignificant compared to the ease of casting and of handling.
Wrought iron is an iron alloy with a very low carbon content, in comparison to steel, and has fibrous inclusions, known as slag. This is what gives it a “grain” resembling wood, which is visible when it is etched or bent to the point of failure. Wrought iron is tough, malleable, ductile and easily welded.
Historically, it was known as “commercially pure iron however it no longer qualifies because current standards for commercially pure iron require a carbon content of less than 0.008 wt%.
Before the development of effective methods of steelmaking and the availability of large quantities of steel, wrought iron was the most common form of malleable iron. A modest amount of wrought iron was used as a raw material for manufacturing of steel, which was mainly to produce swords, cutlery and other blades.
Demand for wrought iron reached its peak in the 1860s with the adaptation of ironclad warships and railways, but then declined as mild steel became more available.
Before they came to be made of mild steel, items produced from wrought iron included rivets, nails, chains, railway couplings, water and steam pipes, nuts, bolts, horseshoes, handrails, straps for timber roof trusses, and ornamental ironwork.
Wrought iron is no longer produced on a commercial scale. Many products described as wrought iron, such as guard rails, garden furniture and gates, are made of mild steel. They retain that description because they were formerly made of wrought iron or have the appearance of wrought iron. True wrought iron is required for the authentic conservation of historic structures.
Ferrosilicon, or ferrosilicium, is a ferroalloy an alloy of iron and silicon with between 15% and 90% silicon. It contains a high proportion of iron silicides. Its melting point is about 1200 °C to 1250 °C with a boiling point of 2355 °C. It also contains about 1% to 2% of calcium and aluminium.
Ferrosilicon is used as a source of silicon to deoxidize steel and other ferrous alloys. This prevents the loss of carbon from the molten steel (so called blocking the heat); ferromanganese, spiegeleisen, silicides of calcium, and many other materials are used for the same purpose.
It can be used to make other ferroalloys. Ferrosilicon is also used for manufacture of silicon, corrosion-resistant and high-temperature resistant ferrous silicon alloys, and silicon steel for electromotors and transformer cores.
In manufacture of cast iron, ferrosilicon is used for inoculation of the iron to accelerate graphitization. In arc welding, ferrosilicon can be found in some electrode coatings.
Ferrosilicon is a basis for manufacture of prealloys like magnesium ferrosilicon (FeSiMg), used for modification of melted malleable iron. FeSiMg contains 3-42% magnesium and small amounts of rare earth metals. Ferrosilicon is also important as an additive to cast irons for controlling the initial content of silicon.
Ferrosilicon is also used in the Pidgeon process to make magnesium from dolomite.
In contact with water, ferrosilicon may slowly produce hydrogen.
Ferrosilicon is produced by reduction of silica or sand with coke in presence of scrap iron, millscale, or other source of iron. Ferrosilicons with silicon content up to about 15% are made in blast furnaces lined with acid fire bricks.
Ferrosilicons with higher silicon content are made in electric arc furnaces. An overabundance of silica is used to prevent formation of silicon carbide. Microsilica is a useful byproduct.
The usual formulations on the market are ferrosilicons with 15%, 45%, 75%, and 90% silicon. The remainder is iron, with about 2% consisting of other elements like aluminium and calcium.
Ferromolybdenum is an important iron molybdenum alloy, with a molybdenum content of 60-70% It is the main source for molybdenum alloying of HSLA steel. The molybdenum is mined and is subsequently transformed into the molybdenum(VI) oxide.
? Ferrotitanium is a ferroalloy, an alloy of iron and titanium with between 10-20..45-75% titanium and sometimes a small amount of carbon. It is used in steelmaking as a cleansing agent for iron and steel; the titanium is highly reactive with sulfur, carbon, oxygen, and nitrogen, forming insoluble compounds and sequestering them in slag, and is therefore used for deoxidizing, and sometimes for desulfurization and denitrification.
? In steelmaking the addition of titanium yields metal with finer grain structure.ferrotitanium rowmaterial name is Ilmenitesant.making process of ferro titanium:- ilmenite-100%600kge MoO3. This oxide is mixed with iron oxide and aluminium and is reduced in the an aluminothermic reaction to molybdenum and iron.
? The ferromolybdenum can be purified by electron beam melting or used as it is. For alloying with steel the ferromolybdenum is added to molten steel before casting. Among the biggest suppliers of Ferromolybdenum in Europe is the German trading house Grondmet in Düsseldorf, Germany.
Spiegeleisen is a ferromanganese alloy containing approximately 15% manganese and small quantities of carbon and silicon.
Historically, this was the standard form in which manganese was traded and used in steel making (see Bessemer process); today, manganese is usually traded and used in more concentrated form, 80% manganese content being typical.
Spiegeleisen is sometimes also referred to as specular pig iron, Spiegel iron, just Spiegel, or Bisalloy.
- Ferrochrome (FeCr) is an alloy of chromium and iron containing between 50% and 70% chromium. The ferrochrome is produced by electric arc melting of chromite, an iron magnesium chromium oxide and the most important chromium ore.
- Most of the world’s ferrochrome is produced in South Africa, Kazakhstan and India, which have large domestic chromite resources. Increasing amounts are coming from Russia and China.
- The production of steel is the largest consumer of ferrochrome, especially the production of stainless steel with chromium content of 10 to 20% is the main application of ferrochrome.
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