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Heat treatment is an important step in the final fabrication of many engineering products. It plays a pivotal role in manufacturing where a superior blend of material properties is required. Heat treatment process can be defined as an operation or combination of operations involving heating at a definite temperature, soaking for a particular period of time and cooling at some specified rate. The primary objective of heat treatment is to increase the service life of a product by altering its mechanical properties like strength, ductility, toughness, wear resistance, hardness, etc. and prepare the material for improved manufacturability. The change in the metal properties can be attributed to the phase transformations and the micro structural changes which take place during heat treatment services.
Stages of Heat Treatment
Industrial heat treating operation involves the following three basic stages irrespective of the type of process.
Heating - The metal is heated slowly to ensure uniform temperature throughout the surface. If uneven heating occurs, one section of a part can expand faster than another resulting in distortion or cracking. The heating rate depends mainly on the heat conductivity of the material.
Soaking - After the metal is heated to a specified temperature, it is held for a particular period of time at that temperature for the desired internal structural changes to take place. The length of time, called as the soaking period depends on the chemical analysis of the part and the weight of the metal treated.
Cooling – The metal is cooled to room temperature after soaking period to complete the heat treatment process. To cool the metal, it can be placed in contact with a cooling medium such as air, water, oil etc. The rate of cooling depends on the metal and the properties desired.
Some of the basic heat treatment processes are discussed below.
Annealing is the process of heating a metal to a specific temperature, holding it at that temperature for a definite length of time, and then cooling the metal to room temperature. Metals are annealed to relieve internal stresses, soften them and make them more ductile. To produce maximum softness, the metal must be cooled slowly which is ensured by burying the hot part in sand, ashes, or some other substance that does not conduct heat readily or by allowing the furnace and part to cool together called as furnace cooling.
Normalizing is almost similar to annealing except that the metal is cooled using still air at a faster rate after soaking period. Normalizing is less expensive than annealing and also normalized steels are harder and stronger than annealed steels. In fact highest degree of hardness can be achieved by normalizing among the other heat treatment processes. But the primary purpose of normalizing is to remove the internal stresses induced by heat treatment machines, welding, casting, forging, forming, or machining.
Hardening is basically the opposite of annealing. A metal is hardened by heating it to a higher temperature, holding the same for a set time and then cooling it suddenly. The rapid cooling is achieved by plunging the material into quench media such as water, oil, brine solution etc. Hence, hardening is also called as quenching as the metal at higher temperature is brought immediately to room temperature by immersing it in cooling medium. It increases the hardness and strength of the material but at the same time it adversely affects the brittleness.
Tempering is usually performed after hardening as rapid cooling of the metal results in high amount of internal stresses. Also it reduces the brittleness and increases the ductility of the hardened material.Tempering involves heating of metal to a lower critical temperature (much lower than the hardening temperature) and then cooling at a prescribed rate. Heating temperature majorly influences the hardness of the material, higher the temperature softer the metal. The rate of cooling hardly affects the microstructure of the metal.
Heat Treatment Equipment
The equipment required for the process heat treatment furnaces or other heat treating machines such as induction heat treating equipment, quenching baths or other cooling devices, temperature controls and other indicators required for the proper operation of the equipment or for the control of the process. In addition, heat treating equipment includes tongs and other devices for handling and holding the work while it is being heated or cooled. Equipment designed for the heating of metals includes electric furnaces, fuel-fired furnaces, bath furnaces, and devices for the measurement and control of temperature. Different types of heat treatment devices include oxyacetylene torches, hauck burners, forges and temporary heat treating oven constructed with firebrick and sheet asbestos. Electric furnaces are the most commonly heat treating furnaces and equipment used in the industries.
Some of the heat treatment equipment suppliers in the world are CARBOLITE, ELMETHERM, UTTIS, NABERTHERM, CAN-ENG Furnaces International, GADDA Industrie, KOPAR, CERADEL Industries, DESPATCH Industries and WESMAN. On TradeMachines, you have access to the largest selection of heat treatment equipment for sale at auction.