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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LIST of METALS related BLOGS

 

Post 628 –by Gautam Shah

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  1. FORGING > https://interiordesignassist.wordpress.com/2015/06/11/forging/
  2. COATING of METALS – 1 > https://interiordesignassist.wordpress.com/2016/10/07/coating-of-metals-part-i/
  3. METAL COATINGS > https://interiordesignassist.wordpress.com/2015/06/01/metal-coatings/
  4. SHAPING of MATERIALS > https://interiordesignassist.wordpress.com/2015/07/03/shaping-of-materials/
  5. PROCESSING of MATERIALS > https://interiordesignassist.wordpress.com/2014/11/27/processing-of-materials/
  6. METALWORKING PROCESSES > https://interiordesignassist.wordpress.com/2014/09/02/metalworking-processes/
  7. METAL WORKING Processes > https://interiordesignassist.wordpress.com/2014/08/12/metal-working-processes/
  8. PERCUSSIVE TOOLS > https://interiordesignassist.wordpress.com/2016/05/07/percussive-tools/
  9. HAMMERS > https://interiordesignassist.wordpress.com/2014/07/20/hammers/
  10. DRILLING > https://interiordesignassist.wordpress.com/2014/06/17/drilling/
  11. PLATES, SHEETS and FILM MATERIALS -part I > https://interiordesignassist.wordpress.com/2016/02/07/plates-sheets-and-film-materials-part-i/
  12. IRON MAKING > https://interiordesignassist.wordpress.com/2015/11/03/iron-making/
  13. SMITHY > https://interiordesignassist.wordpress.com/2015/04/16/smithy/
  14. CARBON and STEELS > https://interiordesignassist.wordpress.com/2015/03/15/carbon-and-steels/
  15. WROUGHT IRON LATTICES > https://interiordesignassist.wordpress.com/2014/10/27/wrought-iron-lattices/
  16. STAINLESS STEELS > https://interiordesignassist.wordpress.com/2015/04/09/stainless-steels/
  17. IRON or STEEL -technologies through history > https://interiordesignassist.wordpress.com/2014/06/18/iron-or-steel-technologies-through-history/
  18. METAL CLEANING PROCESSES > https://interiordesignassist.wordpress.com/2016/01/11/metal-cleaning-processes/
  19. ALLOYS > https://interiordesignassist.wordpress.com/2016/01/05/alloys/
  20. FERROUS ALLOYS > https://interiordesignassist.wordpress.com/2015/05/11/ferrous-alloys/
  21. ALUMINIUM BRONZE alloys > https://interiordesignassist.wordpress.com/2015/02/03/aluminium-bronze-alloys/
  22. METAL EMBELLISHMENTS -PAINTED ENAMELS > https://interiordesignassist.wordpress.com/2015/08/20/metal-embellishments-painted-enamels/
  23. GILDING > https://interiordesignassist.wordpress.com/2015/07/12/gilding/
  24. FUSION JOINING SYSTEMS > https://interiordesignassist.wordpress.com/2015/04/11/fusion-joining-systems/
  25. METALS and ROOFS -Part – I > https://interiordesignassist.wordpress.com/2015/03/30/metals-and-roofs-%e2%97%8f-part-i/
  26. CORROSION PROOFING TREATMENTS > https://interiordesignassist.wordpress.com/2015/03/17/corrosion-proofing-treatments/
  27.  METAL TREATMENTS for CORROSION RESISTANCE > https://interiordesignassist.wordpress.com/2015/02/22/metals-treatments-for-corrosion-resistance/
  28. PATINA > https://interiordesignassist.wordpress.com/2015/03/08/patina/
  29. COPPER -1 > https://interiordesignassist.wordpress.com/2015/07/19/copper-1/
  30. COPPER -2 Copper Compounds > https://interiordesignassist.wordpress.com/2015/07/21/copper-2-copper-compounds/
  31. COPPER 3 -Bronze alloys > https://interiordesignassist.wordpress.com/2015/08/02/copper-3-bronze-alloys/
  32. TIN > https://interiordesignassist.wordpress.com/2015/03/02/tin/
  33. PEWTER > https://interiordesignassist.wordpress.com/2015/11/05/pewter/

enclume

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METAL CLEANING PROCESSES

Post 571by Gautam Shah

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Metals parts need surface treatments and cleaning for conservation, restoration,  decoration, reflectivity (shine), dulling (removal of gloss), better hardness, surface integrity, prevention of corrosion and in preparation for the next treatment. Metal components get both organic and inorganic substances deposited from various lubricating oils, corroding environments and substances, and deposits from hard water etc. Metal surfaces need cleaning for removal of residual products from earlier processing, weathering, depositions during storage and transport and environmental contaminations. A cleaned metal is comparatively more active, and so may need, immediate next treatment, to prevent corrosion and contamination. So metal surfaces often receive simultaneously the processes of cleaning, surface activation and protection.

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Metal surfaces needs to be smoothed, scrubbed or scoured to receive next set of treatments. Cleaning processes are employed to discharge electrical ions from the component. Cleaning processes are designed to serve several purposes, such as cleaning, scoring and protective coating. Cleaning occurs at a raw material stage, product formation stage, in preparation to other finishes, or just prior to a marketable finish. But there are two distinct stages, at manufacturing plant and on-site applications.

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Center-less grinding  Wikipedia image by Wizard191

Alkali cleaning is typically done with caustic soda (NaOH) which removes all polar substances such as fats and oils. The vegetable and animal oils are converted to glycerol and soap, and the petroleum-based oils and grease are emulsified.

Acid cleaning, like alkali removes ionic deposits from the metal surface. Depending on the secondary products like salts determine the quality of acid to be used. Pickling process removes oxides from the surface by a dissolving liquid. For steel, a warm dilute sulphuric acid, and in some cases hydrochloric acid, hot or at normal temperature, is used. However, for some alloy steels nitric acid, is needed. Acid and alkali both processes require post treatment ph. balancing rinse or treatment.

Solvent cleaning: the articles are washed with a petroleum solvent or undergo cleaning (de-greasing) by vapour, in which a solvent such as tri-or tetra chloroethylene is heated in a closed system, and its vapours are condensed on the metal surface.

Emulsion cleaning: the metal parts are immersed in a warm mixture of kerosene, a wetting agent, and an alkaline solution.

Mechanical cleaning use force of air or water jet and sonar waves to remove partially attached particles. The process may include fine particles like sand, silica, metal grit etc. to blast the surface for the same purpose.

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Electrolytic cleaning: the articles are immersed in an alkaline solution and a direct current is passed between them and the other electrode which is usually steel.

Ultrasonic cleaning is used for blind holes or gears packed with soils etc. Small particles embedded in crevices get detached due to ultrasonic waves.

Plasma treatments bombard a surface with electron ions to break the surface level chemical bonds and change the chemical composition of the surface. Plasma treatments may also remove variety of material types such as paints, polymers, glass and ceramics.

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Statue of Perseus Piazza della Signoria Florence After cleaning Image by Jrousso at Wikipedia En

Metal cleaning Tools and Techniques

The metal products are small items like nails, rivets, long rolled sections, drawn items like wires, woven items like lattices, entwined items like ropes and barbed fencing, hollow pipes and tubes, or solid castings. Other products include assemblies like automobile bodies, boxes, cages, machine housings and structures like a truss. Metal components are single or multi-metal items, composites with polymers, ceramics, etc. Art restorers have to deal with metal objects of unknown composition, ageing, and coated with natural or applied substances. Art restorations require reversible processes, so if a treatment is found unviable it can be fully reverted. Metal products in continuous line productions are moved through several sets of electrodes, which, submerged in a cleaning liquid, electrolytically generate hydrogen gas at the steel surface for lifting residues off the material.

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Sand-blasting Wikipedia image by National Institute for occupational safety and health NIOSH USA

Hard substances like metal nodules or slags are removed by abrasion or high pressure air or water jet. Soft materials like soil etc. are removed by brushing. Brushing may be carried out dry or wet and with scrubbing and scourager agents. Brushes with natural hair, synthetic fiber, metal fibres and wires are used. Abrasive materials like A Carborundum, sand grit, metal particles etc. are blasted on the surface to clean up the surface. In some instances like manufacturing of nails, abrasive materials rotated in ball mills with nails, to achieve high polish. Ultrasonic sound waves are also used to remove particles.

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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.

Autosave-File vom d-lab2/3 der AgfaPhoto GmbH

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.

Roller chain Kettenvergleich

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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METALS and ROOFS ● Part – I

Post 379 – by Gautam Shah 

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Roofs of buildings have been formed of smaller components such as ceramic tiles, slate stones, stone slabs, and grasses (thatched) or of masonry domes. Leakage of water and air were chief problems. These were solved by provision of acute slopes for drainage, design of joints, and use of joints’ sealants. Other problems included structural integrity against vibrations (earthquake and usage), accommodation of stresses (thermal, wind and loadings) and maintenance through repairs and replacement.

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Metals are the lightest and thin body materials for roofing. Lead and copper, have been most widely exploited-metals, till coated steel products became available. Metals are used for roofing elements such as for main body cover, flashing or joint making, and drainage system including, gutters, spout and pipes. A metal roofing system can have all components made from different metals. Such multi metal systems are used for mechanical advantages of installation but not for galvanic protection. Performance, maintenance, service life, and recovery costs determine the cost effectiveness of metal roofing components.

Warsaw Palace Copper Roof with vertical seams

Copper has been a favoured roofing material for two main reasons: 1. It is fairly durable when it can form bluish-green patina and, 2. It can be bent, stretched or moulded into difficult -doubly curved shapes. Copper sheets used for roofing are easy to shape over curved structures such as cupolas and domes. Copper roofing sheets can be forged joined for greater lengths. Copper sheets or panels are lighter than wooden shingles, and much lighter than slate, ceramic tiles, or stone slabs.

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Copper roofs are prone to galvanic erosion, unless isolated from other conductive elements. Copper is a universal roofing material as it is used for cover, gutters, flashing, coping and spouts. Copper is an ideal for flashing as it is malleable and easily workability, solder-able with tin, and is resistant to Alkaline environments. It is easy to hammer or work into watertight joints, without any caulk or the gasket. Copper is reusable material.

Copper covered cupola St Maria Florence

Copper roofing system is essentially how joints are formed. One of the simplest ways is to create a lapped joint with a soldered rim. But such a joint of tin, may not architecturally match the green-blue patina. So, concealed seam joints filled with a tin solder lining are preferred. To accommodate the seam thickness, the joints are used for nailing to batten and the seam folded over to form horizontal steps or vertical line marks. For flatter or low-pitched roofs the seams are folded downward and the groove filled with a solder or caulking compound.

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A copper roof can stain adjacent building materials of lighter colours. Lead-coated copper can result in a black or gray stain. Proper design can avert such a situation. It may not be necessary to isolate copper from lead, tin or many stainless steels under most conditions. Some isolation can be achieved by using coatings including paints.

Vancouver Sun Tower Dome

Metal roofs do not add to risks of lightning strikes, as the metal disperses the electricity throughout the structure, lowering the damage by flash fire.

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