Glossary of Terms for Heat Treating

Glossary of Terms for Heat Treating:


Age Hardening:

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


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 used to soften the metal and produce desired changes in its microstructure. Annealing is used to improve machinability, dimensional stability, relieve stresses and define the crystalline grain structure of the material.


Austempering: 

A heat treating process for ferrous metals which increase hardness, fatigue strength, and impact resistance while reducing distortion of the parts.  Typically the part is heated high enough to change the microstructure to the austenitic phase and cooled rapidly to avoid the pearlite phase.  When it cools to a temperature slightly above the martensitic start temperature an isothermal hold is set up for a specified time in order to achieve the desired microstructure (bainite for steel).


Brazing:

A metal joining process whereby a filler metal is heated above the solidus and the liquid is distributed between two or more close-fitting parts via capillary action.  The filler metal is brought slightly above its melting temperature in a protective atmosphere; this can be in vacuum or in inert gas atmosphere free of oxygen.


Bright Annealing:

Annealing in a protective atmosphere or vacuum to prevent discoloration of a bright surface.


Boronizing:

a thermo-chemical surface hardening process used to diffuse boron atoms into the surface of the metal to form complex metal borides (i.e. – FeB/FeB2).  The benefits of boronizing include good corrosion resistance, reduced coefficient of friction, and increased fatigue life.


Carbide Coating:

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


Carbonitriding:

A case hardening process that introduces carbon and nitrogen into the surface of a solid ferrous alloy by heating the alloy in contact with a gaseous carbonaceous material and  ammonia.  The heat treating process is completed by quenching at a set rate to produce the desired properties of the work piece.


Carbon Potential:

A measure of the capacity of an environment containing active carbon to alter or maintain, under prescribed conditions, the carbon concentration in steel.


Carburizing:

A surface hardening process that introduces carbon into the surface of a solid ferrous alloy by heating the alloy in contact with a carbonaceous material to a temperature above the transformation range and holding at that temperature until a desired level of carbon is absorbed and diffused inward from the surface.  This is generally followed by quenching to produce a hardened case.


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.


Chemical Vapor Deposition:

A chemical process used to produce a non-volatile solid thin film of various materials on a substrate. In a typical CVD process the substrate is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface to produce the desired thin film. Frequently, volatile byproducts are also produced, which are removed by gas flow through the reaction chamber.


Cold Treatment:

A secondary cooling process that exposes the ferrous material to subzero temperatures (typically –84°C (-120°F) to either impart or enhance specific conditions or properties of the material. Increased strength, greater dimensional or micro-structural stability, improved wear resistance, and relief of residual stress are among the benefits of the cold treatment of steel.


Cryogenic Treatment:

A secondary cooling process that exposes the ferrous material to subzero temperatures (typically –190°C (-310°F) to either impart or enhance specific conditions or properties of the material. This is typically used to eliminate retained austenite in carburized parts.  Increased strength, greater dimensional or micro-structural stability, improved wear resistance, and relief of residual stress are among the benefits of the cold treatment of steel.


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 in intimate contact with another metal part, 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.

Ferriticnitrocarburizing:

A case hardening process that diffuses nitrogen and carbon into ferrous alloys at sub-critical temperatures.  The resultant hardened surface show increased resistance to wear, corrosion, and fatigue.


Gas Carburizing:

Carburizing in the presence of hydrocarbon gases which react on the surface of the steel to release nascent carbon which diffuses into the surface of the steel to improved wear and fatigue resistance.


Gas Nitriding:

A process of case hardening in which ferrous alloys, metals or non ferrous alloys, usually of special composition, is heated in an atmosphere of ammonia to produce absorption of nitrogen on the surface which hardens the material without quenching.


Glass:

A super-cooled liquid ceramic material commonly composed of silicates and oxides that form 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.


Hardfacing:

A process where harder or tougher material is applied to the surface a base metal.


High Pressure Gas Quenching:

Gas quenching at pressures greater than 1 bar; typically 2-20 bar.  HPGC is used to eliminate distortion typically found with oil quenching, and to produce a clean esthetically desirable surface.


Homogenizing: 

Heating a metal to high temperature in order to ensure uniform distribution of components.  This process is typically used to reduce chemical segregation and coring in cast structures and to produce a more homogeneous microstructure in hot worked materials.


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.


Ion Nitriding:

A vacuum heat treatment process in which 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, hardened surface.


Low Pressure Carburizing:

Low Pressure Vacuum Carburizing, also known as LPVC, which is a heat treat process that supplies additional carbon to the surface of the steel.  Performed in a vacuum furnace, this vacuum carburizing process is designed to achieve optimum case hardening properties for various alloy materials.


Magnetic Annealing:

An annealing process utilized on certain ferrous materials to achieve uniform magnetic properties.  This is usually done in vacuum or in a hydrogen atmosphere in order to avoid decarburization.  This process requires tight temperature uniformity, extended soak times and accurate control of the cooling rates to control the microstructure of the material without loss of magnetic properties.


Marquenching/Martempering:

A heat treatment involving transformation of austenite to martensite followed by step quenching, at a rate fast enough to avoid the formation of ferrite, pearlite or bainite.   This is typically performed at temperatures slightly above the Ms point. Soaking must be short enough to avoid the formation of bainite before the temperature is lowered below Ms.  The advantage of martempering is the reduction of thermal stresses compared to normal quenching. This prevents cracking and minimizes distortion.


Melting Point:

The temperature upon which a chemically pure substance, eutectic or maximum melting point material changes from a solid state to a liquid state.


Metal Oxide:

A 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.


Nitriding:

A chemical process which diffuses nascent nitrogen into the surface layers of low carbon steels, other metals, or alloys in order to improve surface hardness and wear resistance.


Nitriding Potential:

The nitriding potential is directly proportional to the nitrogen activity in the gas mixture containing ammonia as the nitriding species.  It is defined as K = pNH3/pH23/2, where “p” is the partial pressure of the ammonia and hydrogen.


Normalizing:

Heating a ferrous alloy to a temperature above the transformation range in order to create a phase change in the metal’s microstructure and refinement of the grain structure.  After heating for the required time the material is cooled in air, or in vacuum at a rate equivalent to air cooling.

Overaging:

Aging at a higher temperature, and/or for a longer time than required for formation of a coherent precipitate, thus causing loss of coherency in the precipitating phase (coarsening) and loss of strength and hardness.


Pack Carburizing:

A carburizing process where Parts are packed in a high carbon medium such as carbon powder or cast iron shavings and heated in a furnace for 12 to 72 hours at 900 ºC (1652 ºF).  Carbon monoxide gas (CO) is produced and reacts on the surface of the steel part releasing nascent carbon, which is then diffuses into the surface.


Partial Pressure:

The partial pressure of a gas introduced into a vacuum furnace is the force exerted by the gas (or gases) constrained in the vacuum vessel. If only a single gas is present, the partial pressure of the system is the same as the total pressure.  In vacuum systems, partial pressure usually refers to the operation of a vacuum furnace at or above 1 torr (1000 microns). The chamber is usually evacuated to a higher vacuum level; commonly between 10-3 torr and 10-5 torr (0.1 to 0.01 microns) then an inert gas is introduced at a controlled rate to a set partial pressure range which is maintained during the heating cycle.


Phase:

A homogeneous portion of matter separated from other matter by an interface.


Physical Vapor Deposition:

A vaporization coating process, involving transfer of solid material on an atomic level.  The coatings are generally used to improve hardness, wear resistance and oxidation and corrosion resistance.


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.


Press Quenching:

An oil quench process using a quench press that holds the object under pressure as it is cooled. These are used to quench large gears and other circular or flat parts to prevent distortion and maintain the part integrity.


Recrystallizing Anneal:

A heat treatment process used to transfer heavily distorted, cold worked grains into a uniform equiaxed grain stress-free structure.


Refractory Metal:

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


Sintering:

A heating process used to bonds adjacent particles in a “green” compacted powder mass; in order 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 even higher for refractory metals.


Soldering:

A process designed for joining more than one metal.  This is accomplished by the fusion of a filler metal alloy that has a melting point less than about 800°F (425°C).


Solid Solution:

A constituent of alloys is formed when atoms of one metal are incorporated into the crystals of another metal as a single phase.


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.


Spheroidize Annealing:

A heat treating process used to soften metals for ease of stamping and forming.  Heating to a set temperature followed by cooling very slowly changes the metallurgical structure such that the end product looks like spheres or balls


Spring Temper:

A steel temper process which results in an increased upper limit of elasticity; obtained by hardening and tempering in the usual way followed by reheating until the steel turns blue.


Stopping Off:

Applying a physical barrier (copper plate, specialized paints) to selected areas of a part to prevent unwanted hardening during carburization, or to prevent adverse reactions between the parts and the furnace supports.


Subcritical Anneal:

A heat treatment process typically in the temperature range of 1300 F-1400 F used to produce a softened condition with less microstructural refinement than a full anneal process.  Also referred to as a “process anneal” it is used to improve ductility to the material for subsequent cold-working/forming operations.


Straightening:

A post-heat treatment process utilized to ensure that elongated parts meet straightness requirements.  This is typically done with the use of weights.  A special case is “tempering plasticity”, where distorted thin steel products (i.e. quenched circular saw blades) are clamped flat and tempered.  The result is flat saw blades.


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.

Temper:

Tempering is a post-heat treat process used after a hardening process.  The secondary heating cycle reduces hardness slightly and increases ductility.


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, a liquid that solidifies, creates a metallurgical bond between the metal components; finished parts are bright and clean. Vacuum brazing is usually performed at temperatures above 1000°F, but always below the solidus of the base metals.


Vacuum Heat Treating:

The process of heating alloys in a vacuum chamber at pressures down to 10-6 Torr. 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.


Vapor Pressure:

The gas pressure exerted when a substance is in equilibrium with its own vapor.  The vapor pressure is a function of the substance and the temperature.