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Heat Treatment is a group of manufacturing techniques used to alter the hardness and toughness of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass.
The techniques include annealing, case hardening, induction hardening, precipitation strengthening, tempering and quenching.
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 aluminum alloys, 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, there by achieving maximum strength and hardness.
Annealing
Annealing is a technique used to recover cold work and relax stresses within a metal. During annealing, small grains recrystallize to form larger grains, and precipitates dissolve into the matrix. The result is a soft, ducile 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.
Quenching
To harden by quenching, a metal (usually steel or cast iron) must be heated into the austenitic crystal phase and then quickly cooled in oil, water, or brine. Upon being rapidly cooled, the austentite will transform to martensite, a hard brittle crystalline structure. Often a quenched metal is too hard, and must be tempered (heat treated at a low temperature) to impart some ductility.
If a precipitation hardend alloy is quenched, its alloying elements will be trapped in solution, resulting in a soft metal. Tempering a "solutionized" metal will alloy 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.
Case hardening is a type of quenching heat treatment in which the surface of a part is heated (either with a flame or inductive heating) and then quenched. This imparts a hard surface to a part, without robbing it of ductility. Tools, blades, and bearings are often case hardened.
Heat treatment of swords and knives
Usually a quenched blade is too brittle for use until tempered. Depending on the alloy used, it will be evenly heated to 200 to 500 degrees Fahrenheit (90 to 260 °C), held at that temperature (soaked) for an appropriate time (seconds or hours), then cooled slowly over an appropriate duration (minutes or hours). This heat treatment will ensure a stong blade that will hold an edge but not break by balancing the amount of hard martensite with ductile ferrite and pearlite.
In certain cases, different areas of an object will be heat treated differently This is called differential hardening. It is common in high quality knives and swords. The Japanese katana is the best known for this. However, Chinese swords were traditionally done this way, as were Nepalese Khukuri, and many others.
See also
References
"Principles of Physical Metallurgy". Reed-Hill, Robert. 3rd edition. PWS Publishing, Boston. 1994.
"Heat treatment"