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Annealing of Steel
Annealing is a thermal process in which all types of steels, including stainless steel and carbon steel, are exposed to a heat treatment process for softening. The process involves heating steel to above the recrystallization temperature, maintaining the temperature for a pre-determined amount of time and cooling the steel slowly to room temperature. Annealing is used to produce an advantageous combination of ductility, machinability and strength in steel while removing hydrogen embrittlement.
Annealing of steel begins with the steel being heated to a temperature that is above the recrystallization temperature of the steel. The steels particular recrystallization temperature depends on the grade and makeup of the steel. After the steel is heated to the recrystallization temperature it must be held at that temperature for a specific amount of time; this time is dependent on the amount, size, and type of steel being annealed. The purpose of annealing is to allow sufficient time for the a-phase transformation to occur, the transformation allows for the grains to re-align and thus softening the steel.
The highest temperatures required for annealing typically occur just after the steel reaches its recrystallization temperature. Once the highest temperatures have been reached, the steel should be allowed to cool slowly to annealing temperature. This temperature will be lower that the recrystallization temperature and can range from 540°C (1004°F) to 680°C (1256°F) based on the grade and makeup of the steel. How the steel is cooled can directly effect the properties of the steel; cooling should be done at a rate of no more than 45C (6.7F) per hour to prevent cracking or contamination. The time and rate of cooling can be used to adjust the hardness and the elasticity of the steel.
The annealing process is used to soft the steel; this allows the steel to be machined more easily and allows for improved ductility and greater formability. The increased ductility makes it possible to bend or stretch the steel into a range of shapes as well as smoothing any stress cracks. The slower cooling time will produce steel with more elongation allowing it to be bent more easily. The annealing process also produces an oxide layer on the steel that helps to keep it from rusting and oxidizing.
