Post 634 –by Gautam Shah
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.
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.
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.’
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.
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.
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.
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.
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.
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.
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.
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.