Heat treatment is a method used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve a desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering and quenching. It is noteworthy that while the term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur as incidental phases of other manufacturing processes such as hot forming or welding.
Heat Treatment of Metals and Alloys
Metallic materials consist of a microstructure of small crystals called "grains" or crystallites. The nature of the grains (i.e. grain size and composition) determine the overall mechanical behavior of the metal. Heat treatment provides an efficient way to manipulate the properties of the metal by controlling rate of diffusion, and the rate of cooling within the microstructure. In carbon and low alloy steels, fast rates of cooling result in a high degree of hardness. In precipitation hardened alloys like 2000 series, 6000 series, and 7000 series aluminium alloy, as well as some superalloys and some stainless steels, fast cooling rates result in a softer metal. At these fast cooling rates, the alloying elements are trapped in solution and require a tempering to precipitate intermetallic particles, thereby achieving maximum strength and hardness.
Annealing is a technique used to recover cold work and relax stresses within a metal. Annealing typically results in a soft, ductile metal. When an annealed part is allowed to cool in the furnace, it is called a "full anneal" heat treatment. When an annealed part is removed from the furnace and allowed to cool in air, it is called a "normalizing" heat treatment. During annealing, small grains recrystallize to form larger grains. In precipitation hardening alloys, precipitates dissolve into the matrix, "solutionizing" the alloy.
To harden by quenching, a metal (usually steel or cast iron) must be heated into the austenitic crystal phase and then quickly cooled. Depending on the alloy and other considerations (such as concern for maximum hardness vs. cracking and distortion), cooling may be done with forced air or other gas (such as nitrogen), oil, polymer dissolved in water, or brine. Upon being rapidly cooled, the austentite will transform to martensite, a hard brittle crystalline structure. Most applications require that quenched parts be tempered (heat treated at a low temperature, often three hundred degree Fahrenheit or one hundred fifty degrees Celsius) to impart some toughness. Higher tempering temperatures (may be up to thirteen hundred degrees Fahrenheit, depending on alloy and application) are sometimes used to impart further ductility, although some strength is lost.
If a precipitation hardened alloy is quenched, its alloying elements will be trapped in solution, resulting in a soft metal. Tempering a "solutionized" metal will allow the alloying elements to diffuse through the microstructure and form intermetallic particles. These intermetallic particles will fall out of solution and act as a reinforcing phase, there by increasing the strength of the alloy. This process is called "artificial aging". Conversely, some alloys may be "naturally aged" in which the intermetallic particles form at room temperature. Naturally aging alloys are often stored in a freezer to prevent them from aging until needed.
Complex heat treating schedules are often devised by metallurgists to optimize an alloy's mechanical properties. In the aerospace industry, a superalloy may undergo five or more different heat treating operations to develop the desired properties. This can lead to quality problems depending on the accuracy of the furnace's temperature controls and timer.
Quenched steel, while very hard and strong, is too brittle to be useful for most applications. A method for alleviating this problem is called tempering. For most steels, tempering involves heating to between 250 and 500 °C, holding that temperature (soaking) for an appropriate amount of time (on the order of seconds or hours), then cooling slowly over an appropriate length of time (minutes or hours). This heat treatment results in higher toughness and ductility, without sacrificing all of the hardness and tensile strength gained from rapid quenching. Tempering balances the amount of hard martensite with ductile ferrite and pearlite.
In some applications, different areas of a single object are given different heat treatments. This is called differential hardening. It is common in high quality knives and swords. The Chinese jian is one of the earliest known examples of this, and the Japanese katana the most widely known. The Nepalese Khukuri is another example.