The heat treating industry represents a value of about $75 billion in the world, and in USA it’s a $20 billion industry [1]. Heat treating adds about $15 billion per year in value to metal products by imparting specific properties that are required if parts are to function successfully.

Heat treating is very closely linked to the manufacture of steel products: about 80 percent of heat treated parts are made of steel. These include steel mill output such as bar and tube, as well as parts that have been cast, forged, welded, machined, rolled, stamped, drawn or extruded. It is also a vital step in the manufacture of nonferrous products. For example, aluminum alloy automotive castings are heat treated to improve hardness and strength; brass and bronze items are heat treated to increase strength and prevent cracking; titanium alloy structures are heat treated to improve strength at high temperatures.

Heat treating can soften metal, to improve formability. It can make parts harder, to improve strength. It can put a hard surface on relatively soft components, to increase abrasion resistance. It can create a corrosion-resistant skin, to protect parts that would otherwise corrode. And, it can toughen brittle products. In addition, it can be used to modify material (e.g., stress relieving, or spring aging) to maximize its service life.

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 -120¼ 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.

Reference:

[1] A written address to the Heat Treating industry by Roger J. Fabian, 2000-2001 president of the ASM Heat Treating Socity.