Annealing
Annealing include Solution Annealing and Bright Annealing, the term denoting heat treatment that consists of heating to and holding at a suitable range of temperature followed by cooling at an appropriate rate, primarily for the softening of metallic materials. Generally, in plain carbon steel, annealing produces a ferrite-pearlite microstructure.
Steels may be annealed to facilitate cold working or machining, to improve mechanical or electrical properties, or to promote dimensional stability. The choice of an annealing treatment that will provide an adequate combination of such properties at minimum expense often involves a compromise. Terms used to denote specific types of annealing applied to steel are descriptive of the method used, the equipment used, or the condition of the material after treatment.
A fully annealed 1020 steel showing a ferrite-pearlite microstructure. Etched in 4% picral plus 2% nital. 5OO×
Stages of annealing: There are three stages in the annealing process, with the first being the recovery phase, which results in softening of the metal through removal of crystal defects (the primary type of which is the linear defect called a dislocation) and the internal stress which they cause. Recovery phase covers all annealing phenomena that occur before the appearance of new strain-free grains.
The second phase is recrystallization, where new strain-free grain nucleate and grow to replace those deformed by internal stresses. If annealing is allowed to continue once recrystallization has been completed, grain growth will occur, in which the microstructure starts to coarsen and may cause the metal to have less than satisfactory mechanical properties.
Annealing in a controlled atmosphere(Solution Annealing), high temperature of annealing will result in oxidation of the metal’s surface, resulting in scale. If want the scale be avoided, annealing is carried out in an oxygen, carbon, and nitrogen-free atmosphere (to avoid oxidation, carburization, and nitriding respectively) such as endothermic gas (a mixture of carbon monoxide, hydrogen gas, and nitrogen.The magnetic properties of mu-metal (Espey cores) are introduced by annealing the alloy in a hydrogen atmosphere Bright Annealing .
The heat treatment most frequently applied to gray iron, with the possible exception of stress relieving, is annealing. The annealing of gray iron consists of heating the iron to a temperature high enough to soften it and/or to minimize or eliminate massive eutectic carbides, thereby improving its machinability. This heat treatment reduces mechanical properties substantially. It reduces the grade level approximately to the next lower grade: for example, the properties of a class 40 gray iron will be diminished to those of a class 30 gray iron. The degree of reduction of properties depends on the annealing temperature, the time at temperature, and the alloy composition of the iron.
Gray iron is commonly subjected to one of three annealing treatments, each of which involves heating to a different temperature range. These treatments are ferritizing annealing, medium (or full) annealing, and graphitizing annealing.
Ferritizing Annealing. For an unalloyed or low-alloy cast iron of normal composition, when the only result desired is the conversion of pearlitic carbide to ferrite and graphite for improved machinability, it is generally unnecessary to heat the casting to a temperature above the transformation range. Up to approximately 595°C (1100°F), the effect of short times at temperature on the structure of gray iron is insignificant. For most gray irons, a ferritizing annealing temperature between 700 and 760°C (1300 and 1400°F) is recommended.
Medium (full) annealing. It is usually performed at temperatures between 790 and 900°C (1450 and 1650°F). This treatment is used when a ferritizing anneal would be ineffective because of the high alloy content of a particular iron. It is recommended, however, to test the efficacy of temperatures below 760°C (1400°F) before a higher annealing temperature is adopted as part of a standard procedure.
Holding times comparable to those used in ferritizing annealing are usually employed. When the high temperatures of medium annealing are used, however, the casting must be cooled slowly through the transformation range, from about 790 to 675°C (1450 to 1250°F).
Graphitizing Annealing. If the microstructure of gray iron contains massive carbide particles, higher annealing temperatures are necessary. Graphitizing annealing may simply serve to convert massive carbide to pearlite and graphite, although in some applications it may be desired to carry out a ferritizing annealing treatment to provide maximum machinability.
The production of free carbide that must later be removed by annealing is, except with pipe and permanent mold castings, almost always an accident resulting from inadequate inoculation or the presence of excess carbide formers, which inhibit normal graphitization; thus, the annealing process is not considered part of the normal production cycle.
To break down massive carbide with reasonable speed, temperatures of at least 870°C (1600°F) are required. With each additional 55°C (100°F) increment in holding temperature, the rate of carbide decomposition doubles. Consequently, it is general practice to employ holding temperatures of 900 to 955°C (1650 to 1750°F).
Related References:
1. Annealing of Aluminum and Aluminum Alloys
2. AP Tube
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