The temperature and time for annealing depend on the specific material and purpose of the annealing process.
For DNA amplification using PCR, the typical annealing temperature is 5°C below the lowest primer's melting temperature (Tm), which usually falls in the range of 50-60°C. The annealing time is typically 15-30 seconds.
For steel, there are different temperature ranges for annealing depending on the desired outcome. Subcritical annealing, which does not involve a crystal structure change, occurs at temperatures between 538°C – 649°C / 1000°F – 1200°F. Intermediate annealing, which involves some transformation to austenite, is carried out at temperatures between 649°C – 760°C / 1200°F – 1400°F. Full annealing, which completely austenitizes the work, is done at temperatures between 816°C – 927°C / 1500°F – 1700°F.
In some cases, parts can be annealed in a vacuum or reducing atmosphere to achieve a bright surface finish. Annealing in air is used when surface finish is not critical, and an endothermic/neutral atmosphere may be used to control decarburization.
For diffusion annealing, which aims to eliminate structure inhomogeneities or concentration differences in the workpiece, very high temperatures are used, typically between 1050 and 1250 °C, and the annealing duration can be up to 50 hours. This process is often used for nickel-based brazed joints to increase their strength and corrosion resistance.
Recrystallization annealing is performed to transform a structure that has been straightened due to cold forming and restore the original material properties. The temperatures for recrystallization annealing range between 450 and 600 °C for unalloyed steels and between 600 and 800 °C for medium- to high-alloy steels.
In general, annealing is a heat treatment process that involves heating a material above its recrystallization temperature, maintaining a suitable temperature for a specific duration, and then cooling it to alter its physical and sometimes chemical properties. The specific time-temperature cycles used depend on the material composition, condition, and desired results. Annealing can be used to relieve internal stresses, improve machinability, facilitate cold working, enhance mechanical or electrical properties, increase stability of dimensions, and produce a more uniform and homogeneous internal structure.
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