Heattreating: Process Description
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Tapping Molten Aluminum for casting from a tilting barrel furnace. |
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Process Description
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Common long products and hot rolled sheet are generally shipped as rolled. However, cold rolling of strip, surface coating, deep drawing of pipe and wire, stainless and high alloy steels, and large specialty forgings require heat treating steps. Heat treating changes the properties of steel being treated by affecting the size and alignment of the crystalline structure of the metal, the carbon, and other elements in the steel. The specific changes to crystalline structures and the temperature-time ranges required to achieve the desired physical properties are very complicated and are not discussed here. However, heat treating involves the following factors: heating, temperature control, atmosphere control, and controlled cooling. Heat treating may also involve maintaining a specific temperature for a period of time and reheating after an initial period of cooling. Heat treating furnaces are one of the major uses of natural gas in the steel industry. There are several types of heat treating commonly used.
Annealing
Annealing consists of heating the steel to or near the critical temperature (temperature at which crystalline phase change occurs) to make it suitable for fabrication. Annealing is performed to soften steel after cold rolling, before surface coating and rolling, after drawing wired rod or cold drawing seamless tube. Stainless steels and high alloy steels generally require annealing because these steels are more resistant to rolling.
Normalizing
Normalizing consists of heating the steel above the critical temperature and cooling in air. This treatment refines the grain size and improves the uniformity of microstructure and properties of hot rolled steel. Normalizing is used in some plate mills, in the production of large forgings such as railroad wheels and axles, some bar products.
Stress Relieving
Stress Relieving consists of heating the steel to a temperature below the critical range to relieve the stresses resulting from cold working, shearing, or gas cutting. It is not intended to alter the microstructure or mechanical properties significantly.
Spheroidize Annealing
Spheroidize annealing is a prolonged heating of the steel in a controlled atmosphere furnace at or near the lower critical point, followed by a very slow cooling. This process provides improvement in the performance of the steel in cold forming.
Accelerated Cooling
Accelerated cooling improves the resistance to impact and refines the grain size of certain grades of steel. The cooling is provided by fans or by a water spray or dip.
Quenching
Quenching consists of heating the steel above the critical point and holding at that temperature for enough time to change the crystalline structure. This heat is followed by quenching in a water or oil bath to bring the steel back through the critical temperature range without further changes to the microstructure. Quenching produces a very hard, very brittle steel.
Tempering
Tempering is carried out by preheating previously quenched or normalized steel to a temperature below the critical range, holding, and then cooling to obtain the desired mechanical properties. Tempering is used to reduce the brittleness of quenched steel. Many products that require hardness and resistance to breakage are quenched and tempered.
Strength – the ability to withstand mechanical stress
Ductility – Ability to be formed without rupture
Hardness – Resistance to deformation, abrasion, cutting, crushing, etc.
Toughness – ability to absorb shock without breaking
Fatigue Resistance – ability to undergo cyclic forces without failure