Various heat treatment processes involve carefully controlled heating and cooling of metals. For example,
stainless steel plate is commonly heat treated for use in various commercial applications. The common goals of heat treatment are: increase hardness, improve toughness, improve processing, improve formability, increase ductility, and improve elasticity.
The cooling stage has different effects depending on the material and process of the stainless steel plate. When stainless steel is cooled quickly, it hardens, whereas the rapid cooling phase of solution annealing will soften the material. While there are various types of heat treatment, the two important types are annealing and tempering.
Annealing involves heating a stainless steel plate to a specified temperature and then cooling it at a very slow and controlled rate. Annealing is commonly used to: soften metal cold work, improve machinability, and enhance electrical conductivity.
Annealing also restores the ductility of the material. During cold working, metals are able to harden. By pre-annealing stainless steel sheets, cold working can be performed without any risk of cracking, as the annealing relieves the mechanical stresses created during machining or grinding.
Large furnaces are used to anneal small pieces of stainless steel sheet. The interior of the oven must be large enough to allow air to circulate around the metal. For large pieces, a gas delivery furnace is used, while a bottom furnace is more practical for smaller metal pieces.
During annealing, a stainless steel plate is heated to a specific temperature at which recrystallization can occur. At this stage, any defects caused by deformation of the metal are repaired. The stainless steel plate is kept at this temperature for a fixed period of time and then cooled to room temperature.
The cooling process must be done very slowly to create a fine microstructure that maximizes softness. This is usually done by dipping the hot steel into sand, ash, or other low thermal conductivity substance, or by turning off the oven and allowing the steel to cool with the furnace.
Tempering is used to improve the toughness of stainless steel. Quenching is usually done after hardening to reduce excess hardness. Tempering is used to modify the hardness, ductility, toughness, strength and structural stability of materials.
Tempering involves heating the stainless steel plate to a precise temperature below the critical point, and is usually performed in air, vacuum or an inert atmosphere. Adjust the temperature according to the desired hardness reduction. Although it varies depending on the metal type, in general, low temperatures reduce brittleness while maintaining most of the hardness, while higher temperatures reduce hardness, which increases the material's elasticity and plasticity, but causes some yielding and stretching loss of strength.
Gradually heating the metal is essential to avoid cracking the stainless steel plate. The metal is then held at this temperature for a fixed period of time. A rough guideline is one hour per inch of thickness. During this time, internal stresses in the metal are relieved. The metal is then cooled in still air.
