Heat Treating Glossary

Age Hardening
The hardening of a material via precipitation from solid solution of coherent transitional phases which produce a strain in the atomic lattice. Synonymous with Precipitation Hardening.

Alloy
A substance of metallic properties composed of two or more elements, at least one of which is a metal.

Annealing
A heat treatment process that softens the metal and produces desired changes in its microstructure. Vacuum annealing is referred to as bright annealing. The purposes of annealing include improving formability, machinability, dimensional stability, and various physical properties.

Carbide Coating
TiC and SiC coating is a vacuum deposition process where titanium or silicon carbide compound is formed on graphite components.

Carburizing
“Absorption and diffusion of carbon into solid ferrous alloy by heat, to a temperature usually above AC3, in contact with a suitable carbonaceous material. A form of case hardening that produces a carbon gradient extending inward from the surface, enabling the surface layer to be hardened either by quenching directly from the carburizing temperature or by cooling to room temperature, then reaustenitizing and quenching.” Heat Treater’s Guide, Glossary, page 861, 1995

Vacuum Carburizing. Low pressure vacuum carburizing (LPC) prevents microstructure surface intergranular oxidation (IGO) and decarburization, with precise case depth control. The presence or absence of carbides can be controlled according to specification requirements.

Ceramic
An inorganic, nonmetallic, thermally-stable material, usually in a crystalline structure. Ceramic consists of one or more metals in combination with a non-metal, usually oxygen, but can include nitrides, borides, carbides, silicides, and sulfides. They are characterized by a high temperature strength, good electro-thermal insulation, and high chemical stability.

Degassing
A heat treatment process that removes gas from the metal; vacuum processing is used to degas titanium, copper, and tantalum.

Diffusion Bonding
A joining process that involves atomic movement from one metal part to another, typically in the same alloy family, to form a strong metallurgical bond. This occurs at high temperature (but below the melting point) and pressure. The process is particularly facilitated in a vacuum because of the elimination of oxygen, allowing for a high integrity bond.

Emissivity
A measure of thermal emittance; measured by the ratio of the radiation emitted by a surface to the radiation emitted by a blackbody (a surface that emits all the heat energy that it absorbs) under the same conditions.

Eutectic Alloy
An alloy of two or more elements that exhibits isothermal freezing with the simultaneous precipitation of alternate phases at a lower temperature than any of the pure elements in the alloy. In the liquid form, a eutectic usually exhibits high fluidity.

Gas Carburizing
A process that introduces carbon into the surface of a solid ferrous alloy by heating the alloy in contact with a gaseous carbonaceous material to a temperature above the transformation range and holding at that temperature. This is generally followed by quenching to produce a hardened case

Gas Nitriding
A process of case hardening in which a ferrous alloy, usually of special composition, is heated in an atmosphere of ammonia to produce surface hardening by the absorption of nitrogen without quenching.

Glass
A super-cooled liquid ceramic material, commonly composed of silicates and oxides, that forms by cooling the viscous materials without sufficient time for a crystalline lattice structure to form. 

Hardening and Tempering
A common heating and cooling treatment used to harden and strengthen the metal. One way to measure is by using various hardness scales including Brinell and Rockwell. Hardened metal is reheated in order to temper it; hardness is lowered but the ductility is increased. Vacuum process will result in bright finishes. Temperature of the temper operation determines the final strength, hardness, and ductility.

Homogenizing
A heat treatment process that holds the metal at a high temperature to eliminate or decrease chemical segregation by diffusion.

Hydriding / Dehydriding
Hydriding is the addition of hydrogen to metal to embrittle it for pulverizing. After pulverizing, hydrogen is removed through dehydriding so the material becomes ductile.

Hydrogen Annealing
Annealing under a controlled atmosphere of hydrogen to prevent oxidation or discoloration of parts. Hydrogen is a reducing gas, so hydrogen annealing also can "surface clean" oxidized parts of some alloys by reducing the oxide. Sometimes referred to as "bright annealing". Hydrogen also will decarburize steel parts, so hydrogen annealing is used to improve the magnetic properties of carbon steel by decarburizing the steel. Sometimes referred to as "magnetic annealing".

Ion Nitriding
A high-voltage glow discharge is used to bombard low and medium alloy steels, tool steels, and stainless steel surfaces with nitrogen ions. The nitrogen ions form nitrides with the alloy elements and diffuse into the part resulting in clean, surface hardening.

Magnetic Annealing
Annealing to produce optimum magnetic properties within a material. See Hydrogen Annealing.

Melting Point
The temperature upon which a chemically pure substance changes from a solid state to a liquid state.

Metal Oxide
A binary compound of oxygen with a metal. Most metals react with oxygen under appropriate temperature and pressure conditions, and oxidation can occur slowly at room temperature to form a thin oxide coat that can serve to protect the metal. 

Mixtures or Alloys
Unlike pure metals, many alloys do not have a single melting point. Instead, they have a melting range in which the material is a mixture of solid and liquid phases. The temperature at which melting begins is called the solidus, and that at which melting is complete is called the liquidus. Special alloys can be designed with a single melting point, however, and these are called eutectic mixtures. See also Eutectic Alloy.

Nitriding
Introduces “nitrogen into the surface layer of solid ferrous alloy by holding at suitable temperature (below Ac1 for ferritic steels) in contact with a nitrogenous material, usually ammonia or molten cyanide of appropriate composition. Quenching is not required to produce a hard case.” Heat Treater’s Guide, Glossary, page 869, 1995.

Ion nitriding. A reaction caused by a high voltage glow discharge between the metal being processed (the cathode) and the vacuum chamber’s wall (the anode). Operating parameters such as applied voltage, vacuum level and nitrogen ions react with the metal alloys forming the nitrided surface.”

Vacuum gas nitriding. Surface hardens ferrous materials by processing parts below the austenitic temperature. The part surface is diffused with dissociated ammonia (nascent nitrogen, NH3) in the vacuum furnace with no cracking of the gas on the “inert” graphite hot zone surfaces; cracking only occurs on the work (no retort).

Normalizing
The process of heating a ferrous alloy to a temperature above the transformation range to create a phase change in the metal's microstructure, then cooling in air, or as in vacuum heat treating, at a rate equivalent to air cooling. The benefit of normalizing is the refinement of the grain structure.

Precipitation Hardening
A process of hardening by precipitating an element from a supersaturated solid solution to form coherent transitional precipitates, which produces a strain in the atomic lattice. Synonymous with Age Hardening.

Refractory Metal
A metal that has a melting point well above the common alloying bases (i.e. iron, cobalt, nickel). They include niobium (columbium), tantalum, molybdenum, tungsten, and rhenium. These metals maintain high strength and hardness at elevated temperatures.

Sintering
A heating process that bonds adjacent particles in a powder mass to increase strength and density.  High temperature vacuum sintering is performed at elevated temperatures below the melting point of the metal, up to 2650°F, and higher for refractory metals.

Soldering
A joining process by heating the metals to a suitable temperature below the solidus of the base metals and applying a filler metal having a liquidus not exceeding 840°F. The filler metal is distributed between the closely-fitted surfaces of the joint by capillary action.

Solid Solution
An alloy phase that is a homogeneous solid solution of the chemical elements in the alloy.

Solution Treating
A process of heating an alloy and holding it at a suitable temperature until one or more of the constituents enters a solid solution, at which point the solution is cooled rapidly to hold the alloy in a supersaturated condition.

Stress Relieving
A uniform heating process followed by slow, uniform cooling; the process reduces stresses in fabricated or machined parts, and results in dimensional stability.

Thermocouple
A device for measuring temperatures made of two dissimilar metal or alloy wires joined at one end and connected to a voltage-measuring instrument at the other end. Common types include J, K, N, R, S, and W (also known as C). 

Vacuum Brazing
A thermal vacuum process that joins metals when a braze alloy creates a metallurgical bond between the metal components; finished parts are bright and clean. Usually performed at temperatures above 1000°F, but always below the solidus of the base metals.

Vacuum Heat Treating
Heating alloys in a vacuum chamber at pressures down to 10-6 Torr, a feasible lower limit for commercial applications.  Cooling is performed with inert gas at pressures ranging from sub-atmospheric to as high as 10 atmospheres (10 bar). The controlled environment produces bright, clean, high-integrity parts, which eliminates the need for many finishing operations. Annealing in a vacuum is considered a “bright annealing” process.

Vapor Pressure
The vapor pressure of a substance (liquid or solid) is the equilibrium pressure of a vapor above the substance; in other words the vapor (formed from evaporation of the substance) above the substance when in a closed container.

Example: The vapor pressure of water at 77ºF is 22.8 torr; at 212ºF it is 760 torr. Vapor pressure can be used to determine the boiling point of a substance. For instance at 760 torr the boiling point is 212ºF, however if a vacuum is pulled to 22.8 torr the boiling point of water drops to 77ºF.

When working in vacuum one must recognize that even if the work is not at the boiling point for the substance there is always vapor pressure forming above the surface and excessive heating of a substance under vacuum has the possibility of evaporating the substance. For example at 760 torr and 122ºF, if water is heated for an extended period in an open container the water will eventually evaporate. Therefore care must be used when heating under vacuum at high temperatures if the material is at or near the known vapor pressure of a substance.

American Society of Material (ASM) and the Metal Treating Institute (MTI) links are an excellent source for metallurgical and processing information. Titanium industry related information is available on the International Titanium Associations’ (ITA) website.