They are essential processes in metal manufacturing which increase desirable characteristic of metal, while allowing for further processing to take place. Various heat treatment processes involve carefully controlled heating and cooling of metal. Steel, for example, is commonly heat treated for use in a variety of commercial applications. The cooling stage has different effects depending on the metal and process.
When steel is cooled quickly it hardens, whereas the rapid cooling stage of solution annealing will soften aluminum. Annealing involves heating steel to a specified temperature and then cooling at a very slow and controlled rate. Annealing also restores ductility. During cold working, the metal can become hardened to the extent that any more work will result in cracking.
By annealing the metal beforehand, cold working can take place without any risk of cracking, as annealing releases mechanical stresses produced during machining or grinding. Annealing is used for steel, however, other metals including copper , aluminum and brass can be subject to a process called solution annealed. Large ovens are used for annealing steel. The inside of the oven must be large enough to allow air to circulate around the metal.
The cooling can be either a quenching or an air cooling operation. A sub-category of tempering is austempering. It is mainly applied to ferrous metals such as steel and ductile iron. It is used to improve mechanical properties of metal alloys by reducing or eliminating distortion. Annealing: Annealing is the process of softening a material to obtain desired chemical and physical properties. Hardening: Hardening or quenching is the process of increasing the hardness of a material.
Tempering: Tempering is the process of heating a substance to a temperature below its critical range, holding and then cooling. Annealing: Annealing process involves the heating of a metal to or near the critical temperature followed by cooling to room temperature very slowly in an oven. Hardening: In hardening process, the metal is heated into austenitic crystal phase and then quickly cooled. Tempering: Tempering is done by re-heating the metal alloy to a temperature lower than the critical temperature, holding for some time and cooling.
Hardening: Hardening increases the hardness and strength of materials such as metal alloys. Tempering: Tempering reduces the brittleness of metals. Annealing: Annealing is used for metals and metal alloys. Hardening: Hardening is used for metal alloys containing sufficient carbon and alloy content. Tempering: Tempering is used mainly for steel. Annealing, hardening and tempering are heat treatment processes. Reheating of Steel during tempering temperature decreases the hardness Somewhat But improve the toughness.
Nitriding is a process of surface hardening in which nitrogen gas is used to obtain a hard surface for the Steel. In this process, the Steel parts are heated in an atmosphere of ammonia NH 3 for a prolonged period and then cooled slowly. During this process, when Ammonia comes in contact with steel is diffuses into nascent hydrogen and nascent nitrogen. This nascent nitrogen so produced diffuses into the surface of the workpiece forming hard nitrites which increase surface hardness.
Beside increasing surface hardness and wear resistance nitriding provides good resistance to corrosion due to water, air, and steam. Nitriding is generally employed to Steel parts which are moving like engine parts such a cylinder, crankshaft, etc. Cyaniding is also a surface hardening process in which the heated parts to be surface hardened are immersed in a bath of molten sodium or potassium cyanide. Then the parts are taken out of the bath and Queens in lime water to neutralize the particles of Cyanide salt sticking to the surface of the steel parts.
The cyanide yield carbon monoxide and nitrogen both of which behaves as active carburizing agents in hardening the surface of the Steel. This surface hardening is particularly suitable for small parts like a small gear, Bush pins, screws pins, and small hand tool which required thin and Hard-wear resisting surface. Quenching is the rapid cooling of a workpiece in water, oil or air to obtain certain material properties.
When hardness can be sacrificed, mineral oils are often used. These oil-based fluids often oxidize and form sludge during quenching, which consequently lowers the efficiency of the process. The quenching velocity cooling rate of oil is much less than water. Annealing and tempering are both heat treatment processes. It improves some properties of metals and alloys. But there is a fundamental difference between them. Here we will try to find the answer to a very common question that which heat treatment process used for softening hardened steel?
Before understanding the basic difference between annealing and tempering, one has to understand the general process of heat treatment. The heat treatment can complete in the following 3 steps. These three phases will determine the heat treatment process and improve some mechanical properties in the metal or alloy.
Annealing and Tempering, both are used mostly for softening the steel. But both are the two different methods of heat treatment. In the annealing process, steel is heated above the recrystallization temperature.
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