What Is Heat Treatment?What Happens When Metals After Heat Treating?

What Is Heat Treatment?

Heat treatment is the heating and cooling of metals to change their physical and mechanical properties, without letting it change its Heat Treatment shape. Heat treatment could be said to be a method for strengthening materials but could also be used to alter some mechanical properties such as improving formability, machining, etc. The most common application is metallurgical but heat treatment can also be used in manufacture of glass, aluminum, steel and many more materials.

Heat treatment processes include case hardening, tempering, solution and ageing treatment, Specialty Stainless Steel Processes (S3P), annealing and normalising.

Heat treated parts are essential to the operation of automobiles, aircraft, spacecraft, computers and heavy equipment of every kind. Saws, axes, cutting tools, bearings, gears, axles, fasteners, camshafts and crankshafts all depend on heat treating.

The Most Common Heat Treatment

Case Hardening：Case hardening is the process of hardening the surface of a metal by infusing elements into the material's surface, forming a thin layer of a harder alloy.

Annealing：Annealing is a form of heat treatment that brings a metal closer to its equilibrium state. Annealing softens metal making it more workable and providing for greater ductility. In this process, the metal is heated above its upper critical temperature to change its microstructure. Afterward, the metal is slow-cooled.

Normalising：Normalizing involves heating steel, and then keeping it at that temperature for a period of time, and then cooling it in air. The resulting microstructure is a mixture of ferrite and cementite which has a higher strength and hardness, but lower ductility. Normalizing is performed on structures and structural components that will be subjected to machining, because it improves the machinability of carbon steels.

The main difference between normalizing and annealing is that the cooling rate of normalizing is slightly faster,so the production cycle of normal heat treatment is short. Therefore, normalizing can be used as far as possible when annealing and normalizing can meet the performance requirements of parts.

The Basics of Heat Treating
Although iron and steel account for the vast majority of heat treated materials, alloys of aluminum, copper, magnesium, nickel and titanium may also be heat treated.

Heat treating processes require three basic steps:

1. Heating to a specified temperature.

2. Holding at that temperature for the appropriate amount of time.

3. Cooling according to prescribed methods.

Temperatures may range as high as 2400°F and time at temperature may vary from a few seconds to as many as 60 hours or more.

Some materials are cooled slowly in the furnace, but others must be cooled quickly, or quenched. Certain cryogenic processes require treatment at -120°F or lower. Quenching media include water, brine, oils, polymer solutions, molten salts, molten metals and gases. Each has specific characteristics that make it ideal for certain applications. However, 90 percent of parts are quenched in water, oil, gases or polymers.