Annealing, in metallurgy and materials science, is a heat treatment wherein a material is altered, causing changes in its properties such as hardness and ductility. It is a process that produces conditions by heating to above the critical temperature, maintaining a suitable temperature, and then cooling. Annealing is used to induce ductility, soften material, relieve internal stresses, refine the structure by making it homogeneous, and improve cold working properties.
Normalizing is a type of heat treatment applicable to ferrous metals only. It differs from annealing in that the metal is heated to a higher temperature and then removed from the furnace for air cooling. The purpose of normalizing is to remove the internal stresses induced by heat treating, welding, casting, forging, forming, or machining. Stress, if not controlled, leads to metal failure; therefore, before hardening steel, you should normalize it first to ensure the maximum desired results.
The hardening treatment for most steels consists of heating the steel to a set temperature and then cooling it rapidly by plunging it into oil, water, or brine. Most steels require rapid cooling (quenching) for hardening but a few can be air-cooled with the same results. Hardening increases the hardness and strength of the steel, but makes it less ductile. Generally, the harder the steel, the more brittle it becomes.
Tempering is a heat treatment technique applied to ferrous alloys, such as steel or cast iron, to achieve greater toughness by decreasing the hardness of the alloy. The reduction in hardness is usually accompanied by an increase in ductility, thereby decreasing the brittleness of the metal. Tempering is usually performed after quenching, which is rapid cooling of the metal to put it in its hardest state. Tempering is accomplished by controlled heating of the quenched work-piece to a temperature below its "lower critical temperature". This is also called the lower transformation temperature or lower arrest temperature; the temperature at which the crystalline phases of the alloy, called ferrite and cementite, begin combining to form a single-phase solid solution referred to as austenite. Heating above this temperature is avoided, so as not to destroy the very-hard, quenched microstructure, called martensite.
To remove some of the brittleness, you should temper the steel after quenching. Quenching and tempering provides the steel with high strength and ductility. Quenching and tempering is a heat-treatment method for high-quality heavy plates. Quenching and tempering consists of a two-stage heat-treatment process. Stage 1 includes hardening, in which the plate is austenitized to approximately 900°C and then quickly cooled. The material is water-quenched in a quench unit, in which the plate is clamped to avoid warpage. Stage 2 consists of tempering the material to obtain the desired material properties. Quenching and tempering achieves an extremely fine-grained and homogeneous microstructure. Quenched and tempered steel is characterized by high strength and good ductility.
Solution heat treatment involves the heating of an alloy to a suitable temperature, holding at that temperature for sufficient time to induce one or more of the constituents to enter into solid solution and then cooling rapidly enough to hold these constituents in solution. This kind of hear treatment mainly used for the Stainless steel, Aluminium alloys, Titanium alloys, Beryllium copper alloys, Inconel and associated heat resisting alloys, Nickel base alloys, Maraging steels products.
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