METAL ANNEALING and HARDENING

Post 634 –by Gautam Shah

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Interior Designers often handle metal heating without understanding the repercussions of it. Metals are intentionally heat treated to cut profiles, remove parts-sections from large structures, to shape or un-shape them, to machine, grind, draw, or sheer cut. Metals are unintentionally heat treated on close distance exposure to high heat sources, gas flame cutting or welding, accidental fires and extremities of weather cycles. The heat related exposures, slow or extremely rapid cooling, sustained hot or cold environments, etc. cause changes in the structure of the metals. These may be seen in structural failures, creep, rusting and failure of surface coatings. Here in this article Metal annealing and Hardening processes are explained in very simple terms.

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Cycle frames are stress relieved post-assembly > Wikipedia image by Thewalrus at en.wikipedia

Technically heat-treatment takes place, when metal items are hot-rolled, formed or processed. Here a rise of temperature is inevitable, and items are allowed to cool naturally or in a controlled manner. In general, alloy steels that have a lower heat-conductivity compared to carbon steels are heated more slowly to avoid the internal stresses.

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Gas cutting > Pixabay image by skeeze USA

Metals are heated without reaching to a melting stage, to provide ‘increased hardness, strength, toughness, softness, ductility, elasticity, electrical conductivity, improved formability, better machinability, stress relief, and improved dimensional stability’. On cooling the metal materials or formations change the surface and body structure, but without changing the size and shape. All metals and alloys are heat treated at some stage during the production. These thermo-mechanical processes are known as ‘annealing, normalizing, quench hardening, tempering, nitriding, martempering, austempering, carburizing, solution annealing, aging, etc.’

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Tempering colours for steels > Wikipedia image by Zaereth

Metal grains or crystals are rarely perfect, due to metal and alloy making and post-product forming. Such imperfections, endow capacities to take shock loads and reversal of stress. For some critical components, like boilers, jet engines, power house turbines, axles, hot discharge nozzles, however, some predictable behaviour is necessary. Copper and silver, are annealed by heating and cooling quickly, then immersing in water. Copper is nominally a malleable metal, and so can be shaped by hammering while cold. But it also hardens it allowing formation of a sharp edge. The excessive hardness can be removed by heating the material and plunging it into cold water.

Normalizing is a basic process of heat treatment for reducing stresses of manufacturing processes such as excessive hardness. For normalizing the temperature range is 65-100C lower, in comparison to annealing heat treatments. The rate of cooling is lowered by covering the item in sand, ashes or other substances of low heat conductivity, or by allowing it to cool inside and with the oven. This creates a softer product.

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Welded truss assembly cannot be undone (by fire torches for cutting) without disturbing the removed or remaining entity > Wikipedia US Navy photo by John E Peters

Tempering is a follow-up process to achieve a desirable balance between hardness and toughness of the item. Items hardened by quenching oil or water, are reheated to a lower temperature to decreases the hardness slightly, but to improve the toughness. The metal is held at the temperature for a fixed period, during which period the internal stresses in the metal are relieved. The term tempering is also used for low-carbon steels and nonferrous metals, which are cold worked to increases the hardness. Plates, tubular and linear products treated by quench-and-temper process. Heavy-walled structural shapes are sometimes water-quenched directly after the last roll in the mill, and tempered by the heat retained in inner section of the body.

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US WW-II Liberty ships failed due to brittle fracture of steel, that was too ductile when ships were deployed in frigid Atlantic. 

Metals can be surface-treated with or without heat treatments to harden, gain resistance to abrasion and wear, and to achieve fracture resistance, while leaving the interior soft and tougher. These methods include carburizing, cyaniding or nitriding by adding carbon, cyanide or nitrogen, respectively.

Carburizing of steel is a heat treatment for introducing carbon into the surface. It is carried out in a furnace that contains more carbon than the steel. The strength of hardened steel increases rapidly as the percentage of carbon is increased, but at the same time the steel’s toughness decreases. Often the most useful part is one in which the surface is higher in carbon and thus hard, while the interior is lower in carbon and thus tough. Such a combination of properties can be obtained by carburizing, or annealing the parts in a gas rich in carbon. Similarly De-carburization of steel is an opposite process. The steel is heated in an environment deficient in carbon.

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Cracking in Cast forms due to none or inadequate heat treatment processes

Cyaniding of steel is a heat treatment mainly used on low-carbon steels. Small articles like bolts, nuts, screw and small gears and sprockets are casehardened by heating red hot in a bath of sodium cyanide and then are quenched and rinsed, in water or oil, to remove any residual cyanide.

Nitriding is alternative process of hardening. The steel parts are heated in an atmosphere of ammonia and hydrogen but to a lower temperature, so the crystal structure remains ferritic. Nitrogen from the ammonia gets diffused into the steel.

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Annealed wire Nails > Pixabay Image by Josef17 -Josef Juchem

Annealing involves heating to a specified temperature, and then cooling it at a controlled rate. The temperature is adjusted depending on the degree softening that is required, or the amount of hardness to be reduced. It also varies according to metal type. Low temperatures reduce the brittleness, yet holding the hardness, High temperature treatments reduce hardness and increase elasticity and plasticity. The rate of cooling also affects the hardness. Steel hardens on rapid cooling may soften aluminum. Annealing is an integral part of making materials softer for forming or machining. Annealing precipitates and coagulates the carbides and results in large ferrite crystals.

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Sword making entails very mature Heat Treatment processes

Annealing is used for steel, however, other metals including copper, aluminum and brass can be subject to a process called solution annealing. It is a high temperature heat treating process for stainless steel, nickel and titanium alloys. Most austenitic stainless steels are annealed at a minimum temperature of 1038° C followed by water quenching or rapid cooling. Martensitic steels are annealed at lower temperature of 760° C and slow cooled. The items are held at a temperature and for time to bring the carbon in the steel into a solid solution.

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Padlock rings are toughened to make them less prone to cut and keys are hardened to prevent their bending. > Wikipedia image by Tomasz Sienicki

Direct Heat hardening: Surfaces can be hardened with induction or laser heating. Original or coated surfaces are heat treated for surface hardening, or for forming an alloy on the surface. Solid-solution hardening, here the additives are distributed uniformly throughout the crystalline grains. In comparison for precipitation hardening, the metal is heated to a temperature where one of the substance dissolves, then it is rapidly cooled to avoid precipitation. With steel contains aluminum, the nitrogen combines to form fine particles to harden the steel. Case Hardening, is used to make the surfaces of steel resistant to abrasion and wear, while leaving the interior soft and therefore tougher and more fracture-resistant. Case hardening is important in the manufacture of gears, axles, and other machine parts subject to wear. Aging is done at an elevated temperature that is still well below the temperature at which the precipitate will dissolve.

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Casting fresh from heat treatment furnace > Flickr image by Goodwin Steel Castings

Quenching is the cooling of the material from the higher temperature to room-temperature. Sudden cooling by quenching in oil or water, causes the surface to cool much faster then the inner or core mass. Frequent heating and graduated cooling anneal the metal mass more ductile or softer. Similarly This makes a surface of metal objects harder. Oil is the mildest medium, salt brine has the strongest quenching effect but water is between the two. In special cases, steel is cooled and held for some time in a molten salt bath. Heating and Quenching or cooling are essentially workshop based processes. These are difficult to implement at site.

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COATING of METALS -Part-I

Post 627 –by Gautam Shah

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A coating is a thin surfacing that is applied or attached to the surface with various degrees of ‘chemical’ integration or amalgamation. Metal coatings are of four types: ‘Organic’ coatings like paints, organosols or other polymeric compounds, Non-Organic coatings of metals, Coatings of metalloids reaching to ceramic states, and Gas reaction-deposition systems.

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Horses of Basilica San Marco > Wikipedia image by Author Tteske

Metals need a coating to alter the physical and chemical properties of a surface, such as to endow desired quality of texture, colour, patterns, electrical properties, surface reactivity, strength properties, etc. Often a surface treatment is in preparation for another treatment, as a temporary or permanent application. Maintenance of specific surface properties during sub-processing, transit, fabrication, installation and repairs require special coating systems.

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Copper Bronze spouted flagon 320 BC > Wikipedia image by Rosemania

Metal items require some form of ‘surface-deep’ preparatory ‘work’ to receive the coating, but such ‘work’ for small-or-thin-body entities like plates, sheets, foils, wires, threads may involve entire mass of the body. Post such preparatory work, involving heat leaves some stresses in the item. Small or thin body items have uniform stresses. But heavy items and assembled work can have differential stresses, which affects the final coating.

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Reclining Figure : Arched Legs 1969-1970 by Henry Moore > Flickr image by Pedro Ribeiro Simoes

One of the first Metal coating realized by Man was Patina. Metal rusting is nominally a crust like degradation products, but some metals like Bronze, acquire a surface layer over a period of time. Verdigris is the natural patina. Metal artifacts exposed to different environments such air, sea-water, soils acquire patina, a layer consisting of oxides, carbonates, sulfides, or sulfates. Patinas are products of surface mass degradation, and so etch or reduce thickness of the surfaces. Some patinas, however, curtail further degradation of the surface, and so are encouraged. Patinas are often visually appealing and so desired. Effects similar to patina can be achieved by designed exposure and by treating with various chemicals. Patinas are commonly green, but may vary in colour such as of red, brown, black, blue, or gray. Its surface may be smooth, glossy, or crusty. Newly made objects are deliberately patinated to simulate the antiquity in a process is often called distressing.

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Michelangelo’s Pieta in Bronze by Ferdinando Merinelli 1932 > Wikipedia image

Patina over copper alloys, such as bronze, due to the chlorides leads to green, while sulfur compounds are brown. The basic palette for patinas on copper alloys is blue-black due to ammonium sulfide, brown-black with liver of sulfur, blue-green for cupric nitrate, and yellow-brown due to ferric nitrate. For new artefacts accelerated patination carried out by applying chemicals with heat. Colours range from matte sandstone yellow to deep blues, greens, whites, reds and various blacks. Some patina colours are achieved by the mix of pigments and chemicals. The surface is enhanced by waxing, oiling, or other types of lacquers or clear-coats. French sculptor Auguste Rodin used to instruct assistants to urinate over bronzes stored or buried in the yard. A temporary-washable patina, is produced on copper, by the vinegar (acetic acid).

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Bronze busts –with and without patina > Wikipedia image by MatthiasKabel

In architecture, metals, like copper, bronze, etc. have been used for a very long time, for wall cladding, door panelling, ceiling tiles, and roof covering. Copper provides excellent corrosion resistance. Copper surfaces form tough oxide-sulfate patina coating that protects underlying copper mass and resists further corrosion. Copper corrosion products are less toxic. Copper sheets have been used in many building to cover rounded domes, and articulated roof surfaces. Architectural copper is, though susceptible to oxidizing acids, heavy-metal salts, alkali, sulfur and nitrogen oxides, ammonia, sulfur and ammonium compounds. Brass, an alloy of copper and zinc, has good resistance to atmospheric corrosion, alkali, and organic acids.

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Thracian plaque > Wikipedia image by Ivorrusev

Metal to metal cladding systems were precursors to Plating technology. Such dual metal structures were created by beating, rolling, rivetting or co-forming. The chief purpose was to add strength to a weaker metal. Forging a metal over metal in some cases created partial homogeneity. It was easier to forge soft metals like tin, lead and copper over harder metals like Bronze, Iron etc. Tin and lead could be softened through heat and used for coating. Metal coating by cladding or sheathing, were useful for corrosion resistance, wear resistance, improved electrical/thermal conductivity and better handling (touch-feel).

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Gold foil processing at a workshop in Kanazawa Japan > Wikipedia image by Eckhard Pecher

Some of the simplest methods of sheathing used Gold, Silver and their amalgams with mercury. Gold, Silver, Tin and mercury based amalgams were used as liquidized coatings, where as Silver and Gold were fused as thin sheets or foils. Tin coatings were used for mirror making.

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METALS TREATMENTS for CORROSION RESISTANCE

METALS TREATMENTS for CORROSION RESISTANCE

Post 344 ⇒   by Gautam Shah 

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Corrosion is oxidation of metals in reaction with an oxidant such as oxygen. Rusting is formation of iron oxides through gradual destruction of the material by chemical reaction with the environment. Corrosion or degradations as a process of wearing away of the mass also occurs in ceramics (efflorescence of bricks and terracotta), stones, glass and polymers. Corrosion can be local or more widespread affecting the entire surface. Corrosion is a diffusion process and occurs on the exposed surfaces. One of the main methods of preventing such destruction is to isolate the surface from its environment, including the atmosphere and contact with other passive materials.

Glass disease or corrosion

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Bishnupur W Bengal, India Corroded Brick work

Both, Ferrous and Non ferrous metals, have a tendency to corrode that is the atom lose electrons and become ions. This is called an anodic reaction, and for the corrosion process to proceed there must be a corresponding cathodic perceptor that adsorbs the electrons.

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Reasons for corrosion could be:

External (environmental)

Internal (constitutional)

Structural (joint system)

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There are several ways prevent, reduce or terminate the corrosion.

Metals get corroded by the environment. In most cases, to create an ideal environment for surrounding the metal will require a very large and extensive system, and possibly large quantity of energy to maintain it.

Corrosion occurs due to the discharge of electrons. Metal electrons migrate due to electrical potential that builds up amongst different metals and between the metal and an electric conductive substance, of an assembly. If cathodic polarity is imposed by supplying an electric DC current to reverse the direction or by attaching sacrificial anodes such as aluminium or magnesium, whose atomic relationship with steel is such that a current is generated without external assistance (the anodes are earthed and have electrolytic continuity with the structure).

Next course is to isolate the metal from corrosion conducive aspects of environment (such as moisture, water, acidic fumes, chlorides etc.). Coatings and plating provide the required isolation. The most appropriate way would be to place the isolating element as close to the metal surface, as possible, and if feasible even integrate it into the metal itself. Electroplating, metalizing etc., are some of the techniques that are on the verge of integration.

Copper roof on the Minneapolis City Hall, coated with patina

Corrosion can also be controlled by constitutionally upgrading the metal, by one of the following techniques.

Mechanical -stress induction and relief techniques at Normal, Hot and Cold temperatures,

  Chemical -additive and subtractive treatments

  Alloying – solid solution, precipitation, amalgamation.

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