Annealing is the process of heating metal to alter its strength and hardness.
When properly annealed, metal becomes more ductile and free of stress, reducing its cold working properties.
To accomplish this in steel, it is heated to temperature that is less than its melting point, held at that temperature for a period of time, and then slowly cooled.
Austenitic process annealing and ferrite sub-critical annealing are perhaps the most common heat treatments for industrial steel.
Austenitic Process Annealing Also known as normalizing, this process is conducted at a temperature above 720 degrees Celsius, depending on the metal's alloy.
In each case, the intent is to soften the metal in a short period of time-a process that commonly involves the transition of pearlite into more ductile austenite.
When observed microscopically, a cross section of pearlite steel that has been process annealed will show that austenite has replaced pearlite, causing a change in the metal's crystalline structure.
Ferrite Sub-critical Annealing Conducted at a temperature below 720 degrees Celsius and lasting for an extended period of time, sub-critical annealing in the ferrite temperature range produces the softest steel.
Unlike process annealing, sub-critical annealing heats steels until their iron carbide forms into globules, resulting in considerable ductility and stress relief.
Seen at the microscope level, sub-critical annealed steels have a marbled appearance that represents increased ductility.
Atmospheres for Annealing The atmosphere in which steels are annealed has a significant impact on decarburization, the process of decreasing carbon content in metal.
Most steels are annealed in nitrogen / carbon atmospheres, which are naturally decarburizing to steels.
To prevent carburization, some companies implement a Carboflex control system for furnaces that control nitrogen / hydrogen, nitrogen / natural gas, nitrogen / propane or a nitrogen / cracked methanol mixture that prevents carburization.
Preventing carburization in amateur work settings is difficult due to the difficulty of maintaining atmosphere mixtures in the absence of professional equipment.
In most instances, Steel annealing that requires carburization management should be outsourced to professionals.
Furnaces for Annealing Unlike heat treatments that focus on a section of a work piece, stainless steel annealing typically concentrates on the entire piece, hence the advantage of placing it in a furnace.
With that said, annealing furnaces can range in size and function significantly.
For example, short belt furnaces with small openings may be used to anneal jewelry, while multi-story furnaces are used for annealing industrial grade metal products such as steel stripping.
Conclusion As with other types of metal work, both amateurs and professionals practice annealing.
But there is a point when amateur work, however good, is inappropriate.
If a work piece will impact people's safety when it is implemented in a final assembly, a professional should anneal it.
Although the purpose of annealing is to soften metal, making it too soft-or leaving it too hard-could have critical consequences.
To learn more about heat treatments that strengthen steel and other types of metal, contact a provider of industrial metal strengthening services.