Annealing Under Protective Atmosphere

The annealing furnace way of heating calls for three different stages, namely the recovery stage, the recrystallization stage and the grain growth stage. Atmosphere furnaces or vacuum furnaces may also be used for the annealing process. 

An absolute control of accurate temperature and the authentic means of preventing decarburization, are some of the essential prerequisites of this unique heating equipment. 

It is important to note that in a box or roller hearth furnace, surface protection is often  attained through application of decarb-resistant coating or by packing the work pieces in pipes, in which they are then surrounded by non-decarburizing material, such as spent charcoal and mica, or cast iron chips. Furnaces with prepared atmospheres frequently are used for the annealing of tool steels.

The packing materials as mentioned above may carburize, decarburize, or just be neutral to the steel, completely depending on the heating temperature, the carbon content of the steel, and the density of the packing or even the particle size of the packing material. The primary task followed by such materials is to debar the decarburizing gases from coming in contact with the steel. If oxygen is to be kept out, the packing must be very tight and the sealing of the container perfect, for most gases can diffuse through ordinary seals at a surprisingly rapid rate at elevated temperatures. 

A precautionary step against the cracking up from thermal shock, is done by loading the furnace at a relatively low temperature in order to allow the furnace load to get heated up slowly with the furnace. Post soaking up at the annealing temperature, the work pieces are to be cooled down in the furnace at 10 to 25 °C/h to 540 °C or even lower. Anywhere below 540 °C (1000 °F), the cooling rate for most tool steels is no longer critical, and the work may be then cooled to the normal room temperature.