IRON MAKING

Post 540 — by Gautam Shah

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First iron used by ancient people was of a meteoric source, an iron alloy with nickel. This was used for everal millenniums before the actual iron age. It was a natural Iron in metallic state and so required no smelting of ores. This nearly pure iron is softer than bronze, and therefore tools formed of it had soft wearing edge.

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Primitive age iron was smelted by mixing iron ore with charcoal, and burning in bloomeries, a type of furnaces where bellows were used to force in the air. The carbon monoxide produced by the burning charcoal, reduced the iron oxide ore to metallic iron. The apparatus, however, did not achieve a temperature of 1540° C, to completely melt the iron. The metal collected in the bottom of the furnace remained as a spongy non homogeneous mass or bloom. It had high proportion of intermingled slag. The blooms were repeatedly heated, beaten and folded to remove the slag. This produced wrought iron (=worked iron), a malleable, but fairly soft material. Iron age Irons were not castable products but required hot forming (forging). This was mainly due to inability to fully melt the material. Hot forming was a labourious process, requiring skill and experience. In comparison to bronze, iron ore was procurable everywhere and cheaper to process.

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Wrought iron shows high resistance to corrosion due to the trapped slag in the metal. The presence of slag in the iron helps fusion joining by hammering or forging. Wrought iron is no longer produced commercially, because low-carbon steel is less expensive and is of more uniform quality. Wrought iron, however, is still produced for certain craft-based uses such as making intricate craft objects balustrades, gates, garden accessories, etc.

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Simulated form of wrought iron is made by melting scrap mild steel in small furnaces, blowing air through the melt to remove carbon, and pouring the molten metal into a ladle containing molten slag, which is usually prepared by melting iron ore, mill scale, and sand together. When the molten iron carrying a large amount of gas in solution, is poured into the molten slag (kept at a lower temperature than iron), the metal solidifies almost instantly, releasing the dissolved gas. The force exerted by the gas shatters the metal into minute particles that are heavier than the slag and settle at the bottom of the ladle, agglomerating into a spongy mass.

Silla iron armor, en:Three Kingdoms of Korea, 3rd century Wikipedia image

It was Chinese (1200 BC or earlier) who designed kilns that could raise the temperature for iron making. These kilns, used upgraded coal and had high volume air supply for efficient burning. Chinese were able to melt the Iron and cast it into desired forms. Casting was less labourious, and allowed multiple items with same die form. It was accurate than forging each piece. Chinese smiths melted wrought iron and cast iron together to produce steel -a material of controlled carbon content. The process was called ‘harmonizing the hard and the soft’. This was widely used for casting cooking pots and iron statuettes. A cast iron is harder than wrought iron, but maintains the cutting edge.

Casting pig iron, Iroquois smelter, Chicago, between 1890 and 1901. Wikipedia image

Perhaps as early as 500 BC, although certainly by 200 AD, high quality steel was also produced in southern India by the crucible technique. In this system, high-purity wrought iron, charcoal, and glass were mixed in a crucible and heated until the iron melted and absorbed the carbon.

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Carbon content of iron is a major factor that creates harder material. It was necessary to absorb more carbon in the iron. This required higher ratio of fuel to ore, and push in a lot more volume of air. The strength of iron begins to increase with carbon contents of 0.5 percent. To heat treat iron a carbon content of 1.2% was necessary. Wrought iron which contained less than this proportion had no qualitative effect due to heat treatments. A higher carbon content creates a brittle material but allows heat hardening. ‘Iron hardening by quenching was not practised because it made iron very brittle, unless followed by tempering, or reheating at a lower temperature, to restore toughness’. Simple fire, 600-700° C, based technique of repeated cold forging and annealing was used.

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In the pre-Christian portion of the period, the first important steel production was started in India, using a process called Wootz steel. It was prepared as sponge (porous) iron. This was hammered while hot to expel slag, broken into smaller pieces, and placed with wood chips in clay containers, and heated. On melting, an iron composition containing 1 to 1.6% carbon was produced. The pieces were reheated to form articles that required a hard body and sharp edge. Such steel products were exported to Middle East and other countries. It was known as Faulad (Persian). (Faulad or wootz steel has a Kannada term, ukku, a Language of Indian region of Karnataka).

Elevator screen from the Chicago Stock Exchange cast iron electroplated with copper. Wikipedia image by Joe Mabel

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Nowadays commercial steel plants produce ingots or pig iron. It has very limited use. It goes to casting foundries or to steel mills. At both the places it is remelted to reduce its carbon content and for allying by adding various elements such as manganese and nickel. Often scrap steels are also added for the same purposes.

Melting points for various forms of Irons

Iron, Wrought     1482 – 1593

Iron, Gray Cast   1127 – 1204

Iron, Ductile       1149

Steel, Carbon      1425 – 1540

Steel, Stainless   1510

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FERROUS ALLOYS

Post 420 – by Gautam Shah

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Ferrous alloys refer to metals, where the chief constituents are Iron and carbon. Ferrous alloys are formed with metallic and nonmetallic compounds that enter into the structure and occupy the interstices of compounds. The metallic compounds include: Chromium, Manganese, Molybdenum, Nickel, Silicon, Titanium, Tungsten, Vanadium, etc. The non-metallic compounds include elements of smaller atomic numbers like Carbon, Nitrogen, and Boron.The word ‘Ferro-alloy’ generally refers to alloys of iron with a high proportion of one or more other metal elements. Such alloys have distinctive qualities.

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Metals are alloyed because these become far more suitable for various uses, than in their pure state, or without the alloying agents. Alloys are formed when a metal element and its alloying compound form a solution at certain high temperature and solidify to form a solid solution. Sometimes the intermingling is so close that dissolved substance cannot be distinguished or separated by mechanical means. This in someway, is a result of the differing softening and melting point, and mechanical processes of amalgamation.

Cast iron grills

In some alloys the metals do not show complete solubility, and separate constituents may be recognized. Capacity of one metal to accommodate another metal varies with the temperature. In an aluminium copper alloy, the aluminium at 530° C can hold 5 % of copper in solution, but at room temperature it can hold only 0.5 % of copper. So if a 5 % copper alloy is rapidly cooled from 530° C, the excess copper cannot go out of the mass, but remains in the alloy, well dispersed in the mass.

Wrought iron gate

In alloys where inter-metallic compounds predominate, the alloy shows toughness of the solid solution and hardness of the inter-metallic compound. But the alloy with such inter-metallic compounds, may be hard but very brittle.

When a component of an alloy melts at a temperature lower than all other constituents, than that alloy is called eutectic. Such alloys have thin layers of the metal or small globules of one metal embedded in a matrix of another metal.

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The physical properties of various types of steel and of any given steel alloy at varying temperatures depend primarily on the amount of carbon present, and on how it is distributed in the iron. Before heat treatment most steels are a mixture of three substances: ferrite, pearlite, and cementite.

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Ferrite is iron containing small amounts of carbon and other elements in solution, and is soft and ductile.

Cementite, is a compound of iron containing about 7 % carbon. It is extremely brittle and hard.

Pearlite is an intimate mixture of ferrite and cementite having a specific composition, characteristic structure, and physical properties intermediate between its two constituents.

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The toughness and hardness of a steel that is not heat-treated depend on the proportions of these three ingredients. As the carbon content of a steel increases, the amount of ferrite present decreases and the amount of pearlite increases. The process lasts till the steel has 0.8 per cent of carbon, then it is entirely composed of pearlite. Steel with still more carbon is a mixture of pearlite and cementite.

Excavator bucketRaising the temperature of steel changes ferrite and pearlite to an allotropic form of iron-carbon alloy known as austenite, which has the property of dissolving all the free carbon present in the metal. If the steel is cooled slowly, the austenite reverts to ferrite and pearlite, but if cooling is sudden the austenite is frozen or changes to martensite, which is an extremely hard allotropic modification that resembles ferrite but contains carbon in solid solution.

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CORROSION PROOFING TREATMENTS

Post 367 –  by Gautam Shah 

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Corrosion is degradation of a material due to an electrochemical oxidation process with the environment. The process is more common with metals, but can also cause disintegration of a polymer due to sunlight exposure, or in case of ceramics and stones as seen as efflorescence.

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Stable metals like copper and precious metals like Gold, Silver, Platinum, are less prone to disintegration. Some metals form their own protective cover on the surfaces to prevent, or slow down the corrosion.

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There are several ways to stop or retard the corrosion. One, is to isolate the metal object from environment and other metal objects, and Two, is to constitutionally alter the metal to reduce its vulnerability to rusting. In both the cases the availability of electrons for displacement is reduced. Metals are protected by formation of a barrier. The barrier could be an applique coating like paint or an integrated one like plating or galvanizing. Integrated barriers are also formed by metalizing, surface alloying and ceramic formation. The barrier could be generated by the material itself such as the Patina on bronze. The Barrier could remain on the surface forever, or covered by other coating systems.

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Barrier protection: One of the oldest methods of protecting the metal surface is plating with a metal of stable nature such as Tin, Silver or Gold. Such plating processes were expensive and used for small objects. Post middle ages, metal household objects of iron were covered with a layer of coloured ceramic-glass called enamel. Enamel is inert, and adheres tightly to the steel, protecting it from corrosion while providing attractive appearance. Later chromium plating via electrolytic compounds began to be used as a protective-barrier coating on steel. To get better adherence, the steel is first electroplated with layers of copper or nickel. Today many other types of barrier protections of organic nature such as paints and polymers are used.

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The oxide layer that forms on metals when they are exposed to air also constitutes a protective barrier. Bronze, Stainless steel and aluminum form the most stable and protective of such films. The thickness of the oxide film on aluminum is often increased by making the part function as the anode in an electrolytic cell. This process, called anodizing, enhances the corrosion resistance and makes it easier to colour the surface. The films that form on copper and steel as a result of corrosion (commonly known as tarnish and rust) are somewhat thicker and show a characteristic colour that is often incorporated into the design of the part.

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Galvanic protection: Applique protective films (like paint) on steel are susceptible to being broken at scratches and sharp dents. This occurs in automobiles and other entities, as the applique films have no ability for self-healing. A protection application of zinc metal which has greater capacity to donate electrons then the steel, if forms a prime (first layer on steel) surface, then the objects can be protected from effects of corrosion. This is called galvanic protection. A layer of zinc can be placed on a steel surface by either by hot-dipping or electroplating. Galvanized steel is much more resistant to corrosion than un-galvanized steel. Where a galvanized coating is cut or scratched, the zinc flows in over the exposed area and provides continuous protection. Cadmium can also be deposited for galvanic protection of steel. Hot-dip aluminum-coated steel is used in the exhaust systems of automobiles. At low temperatures its action is galvanic, but at high temperatures the oxidation forms a barrier layer. Galvanic protection is also provided by imposing an electrical potential on steel structures.

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Other coating techniques

Among other methods for applying a metal layer to metal is thermal spray coating, a generic term for processes in which a metal wire is melted by a plasma arc or a flame, atomized, and sprayed onto a surface in an inert gas. A process similar to this is vacuum coating wherein metal is evaporated and deposited as coating in high vacuum. High-temperature bearing super alloy components, such turbines are given oxidation protection by annealing them in a chamber containing volatile aluminum chloride.

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Metal coatings are surface treatments that form the first coatings in a multi coat set. Conversion coatings are of basic two types: phosphating and chromating. These are  temporary, but provide an adequate substrate for subsequent applications.

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Phosphate coatings are used for ferrous, zinc metals, aluminium, tin and cadmium metal surfaces. It is a thin, porous, insulating and adherent application that allows keying of the applied paint film. The electrical inertness of the coating arrests corrosion spread to local spots. Phosphate coatings are applied by immersion, brush application or spraying. Zinc phosphate coatings are smooth and fine-grained treatments, used for reducing the corrosion creep under the paint. Coatings containing manganese phosphate are less widely used as paint pre-treatment because they have a large coarse crystal structure although these heavy coatings are very useful as oil-carriers and have good wear resistance, which is advantageous for engineering components.

Steel structure showing residues of inner coatings

Chromating is formation of a chromium oxide film on the metal surface. It is used to increase corrosion resistance of metals like aluminium, magnesium, tin, zinc and cadmium. It is also used to enhance the tarnish resistance of copper and silver.

Diffusion Coatings are also known as cementation coatings as part of the applied material interacts and forms alloys with the substrate. Cementation coating process is very similar to carburising of iron to produce surface-hardened steel (iron heated with carbon particles for the diffusion to occur). Common processes falling in this category, are: aluminizing (calorising), chromising and Sherardising (zinc cementation coating), siliconising and borating. Processes like hot dip galvanizing, tinning, aluminizing and terneplating also form alloy, but technology is different from diffusion coating. Such alloyed coatings are used where high corrosion and abrasion resistance, in very active environments are needed.

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Metal cladding can cover a metal or other surfaces to form a barrier against corrosion. The thickness of a cladding metal could be few microns (metal leaf) to few millimeters (metal sheets or plates).

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Chromising is the term applied to the formation of a diffusion coating on iron or steel by chromium to produce a surface with enhanced oxidation, corrosion and wear resistance. Gas phase chromising is performed, when the articles heated in a powdered mixture of chromium, alumina or kaolin and an ammonium halide in a hydrogen atmosphere.

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CARBON and STEELS

CARBON and STEELS

Post 365 ⇒   by Gautam Shah 

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Iron as a metal is very ancient material. It was difficult to process (smelt), unlike materials with lower melting temperatures, such as copper and its alloys. Iron is rarely obtainable in pure form. The impurities in iron derive from the ore, and carbon through the smelting process. Carbon is one of the most important of impurities, varying between 0.002% and 2.1%. Presence of Carbon makes the Iron up to 1000 times a harder material. Technically more than 90 per cent of all steels are carbon steels. Presence of small amounts of carbon changes the quality of steel. It affects strength, hardness, mechanical properties (machining, forming, etc.). With very high percentage of carbon workability and impact strength are reduced, whereas with lower carbon content hardness and tensile strength are higher.

Iron of meteorite -similar to Earth’s inner core

Iron ore pellets

Crude iron or Pig iron metal is produced in a furnace, by mixing ore with coke. The high carbon content of crude iron can be further reduced by refining it with air or oxygen, to turn it into steel. A carbon content metal is commonly called Cast Iron. The carbon content of cast iron is 2.1 percent or more. Gray cast iron is relatively soft. It can be easily machined and welded. It is used for engine cylinder blocks, pipe, and machine tool structures. White cast iron is hard, brittle, but not weldable. When annealed, it becomes malleable cast iron. Malleable cast iron can be welded and machined. It is ductile material. Ductile cast iron is sometimes called nodular or graphite cast iron. It is ductile malleable and weldable.

Iron Ore

Pig Iron billets

Besides carbon other elements present are, manganese, silicon, copper, nickel, chromium, molybdenum, vanadium, tungsten, tin, niobium, zirconium, and non metals like sulphur, phosphorus. These materials mostly find their way through the scrape that is partly used for steel making or through an intentional quality markup. The additions of these materials take steel to the category of Alloy steel. Such alloying elements are added to gain properties like better strength, hardness, durability, or corrosion resistance. These are often called specialty steels.

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Adjusting the carbon content is most common tool to control quality of steel. Other quality determinant is the rate at which the steel is cooled. Steel properties are also modified by heat treatments, mechanical working it at hot or cold temperatures and by adding other alloying elements besides carbon.

Steel with high carbon content is hard and strong, but not ductile enough for common uses. In carbon steels, the higher carbon content lowers the melting point and reduces weldability.

Mild steel bars

Low carbon steel has approximately 0.05% to 0.25% carbon content with other materials like manganese. Mild steel, is also known as plain-carbon steel or low-carbon steel. Its very common form of steel, and its material properties are adequate for many applications. It is ductile and malleable. It has a relatively low tensile strength, but is cheap and amenable to cold forming processes. Its surface hardness can be increased with carburizing. It is used for structural steel.

Steel forging

Medium carbon steel has approximately 0.29% to 0.54% carbon content (with 0.60 to 1.65% manganese content). It shows good wear resistance and used for large parts, forging and car parts.

High carbon steel has approximately 0.55% to 0.95% carbon content (with 0.30 to 0.90% manganese content). It is very strong material and used for springs and high-strength wires.

High end Steel – chef’s knife

Ultra high carbon steel has approximately 2.5–3.0% carbon content. These steels that can be tempered to great hardness and used cutting tools, knives, axles or punches. Steel with a carbon content above 2.14% is considered cast iron.

Clydach Gorge Iron Bridge Cast iron supports

Hardened steel usually refers quenched or quenched and tempered steel. Silver steel or high-carbon bright steel, gets its name from its appearance, due to the high carbon content. Silver steel is used for cutting edges and axle components.

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IRON or STEEL -technologies through history

Post —by Gautam Shah

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       Iron or Steel is one of the most complex of all metals used by man. In spite of its very large volume of use, it still remains a very enigmatic material.

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2   Iron changes the properties during manufacture, post processing, aging and usage. Many of the changes are known to man for years, but ignored. Some of these effects became apparent very late in the life span of the structure, or functional entities. Such realizations, though late have not affected us very severely, because superior technologies of later dates provided better guaranteed and efficient solutions.

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       Many of the Industrial revolution period steel structures, such as large span buildings, bridges and ships were formed of very inferior materials and fastening techniques (hind sight realizations). But ‘change solutions’ that became available 30 / 50 years later were better enough to have no regrets for replacement. In few cases there have been losses of life, such as Titanic or Liberty series of ships. The only regrets were that often such structures collapsed suddenly.

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4        The technological deficiencies that affected the structures and entities were due to ignorance and lack of inadequate knowledge, but similar problems have continued even today due to insincere applications.

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5       However, birth of steel fabrication was with cast shapes like parts of columns, capitals, brackets and sections of arches. These were components of compression. Tensile capacity was untested. Hollowed out brackets and arch forming sections had few subsections that were tensile stressed. Tensile behaviour of steel was not completely unknown quality. As the integrity of castings improved, through constitution and methods of cooling, the tensile reliability increased.

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6        One of the most widely used form of ferrous metal has been the sheets. Sheets are re-rolled, cut into strips and folded or formed into various sections.

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7        Compared to cast steels, drawn steels had better grain alignment and tensile strength was known. Mild steels produced through use of Bessemer process provided the much needed ductility and tensile stress capacity.

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       Steels were re-rolled into sheets, but in the manufacturing number of annealing, tempering, hardening processes were perfected.

9        Annealing and Hardening, are nominally considered two extreme processes, former a softening and the later its opposite method. But Tempering that is readjusting the quality of steel is now considered even more important. It is chiefly used in forming various sections, automobile bodies and cages for white-goods. Companies producing furniture, automobiles and white-goods have a selfish interest in replacement markets, and so design their product for 10 years life cycle. After that no one is bothered about the product.

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