heat treatment - Heat Treatment of Metals and Alloys, Quenching, Surface Hardening, Heat treatment of swords and knives

Subjecting a metal component to a cycle of heating and cooling so as to modify its internal crystalline structure and therefore promote desirable physical and mechanical properties. The form and rate of the cycle is important: for example, the rapid quenching of a heated alloy may preserve at the low temperature the crystalline structure or chemical composition characteristic of the high temperature.

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 of Metals and Alloys

Metallic materials consist of a microstructure of small crystals called "grains" or crystallites. 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 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. In precipitation hardening alloys, precipitates dissolve into the matrix, "solutionizing" the alloy. 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. 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.

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.

Surface Hardening

Surface hardening describes a variety of quenching processes that harden only the surface of the part. Tools, blades, and bearings are often heat treated with surface hardening processes.

Induction hardening quickly austentizes a part before quenching.The surface endures a martensitic transformation while the core remains ductile to a heat transfer gradient through the cross section of the part. Flame hardening (heating with an open flame before quenching) may be used to heat the surface of the part, although it can also be used to harden a part all the way through, or selectively harden only a portion of the part. These alloying elements diffuse into the surface of the part, creating a hard, wear resistant layer.

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 between 200 and 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 strong 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.