Glossary | C & D (The vocabulary of the coating and heat-treating industries—explained)

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C

Cadmium Ion Plating

The deposition of cadmium by a vacuum process to provide galvanic corrosion protection.

Cadmium Plating

The electrolytic deposition of cadmium to provide galvanic corrosion protection. Restricted by environmental considerations.

Calcium (Ca)

An element that is used in certain steels to control shape, size and the distribution of oxide and/or sulfide inclusions. It can also increase ductility, impact strength and machinability.

Calorizing

The process of imparting oxidation resistance to an iron or steel surface by heating in aluminum powder at 800–1000°C (1470–1830°F).

Capped Steel

A type of steel, similar to rimmed steel, that is usually cast in a bottle-top ingot.The application of a mechanical or chemical cap causes the top metal to solidify, rendering the rimming action incomplete.

Car Furnace

A batch-type furnace that uses a car on rails to enter and leave the furnace area. Car furnaces are used for lower stress-relieving ranges.

Carbon (C)

The most important alloying element essential to the formation of cementite, pearlite, spheriodite, bainite, and iron-carbon martensite. Steels with higher carbon content have higher strength, hardness, hardenability, and ductile-to-brittle transition temperatures by up to 60% compared with steels of similar microstructure. Toughness and ductility of pearlitic steels, however, are decreased with increasing carbon content.

Carbon Potential

A measure of the stability of an environment containing active carbon to alter or maintain, under prescribed conditions, the carbon level of the steel. Note: In any particular environment, the carbon level attained will depend on such factors as temperature, time, and steel composition.

Carbon Restoration

The process of replacing the carbon lost in the surface layer from previous processing by carburizing the surface. Sometimes called recarburizing.

Carbon Steel

Steel that has no specified minimum quantity for any alloying element (other than the commonly accepted amounts of manganese, silicon, and copper) and containing only an incidental amount of any element other than carbon, silicon, manganese, copper, sulfur, and phosphorus.

Carbonitriding

A case hardening process in which a suitable ferrous material is heated (about 900°C by pack, gas, salt bath or plasma process) to above the lower transformation temperature in a gaseous atmosphere that is designed to cause simultaneous absorption of carbon and nitrogen by the surface and, by diffusion, create a concentration gradient. The process is completed by cooling at a rate that produces the desired properties in the workplace. Compare to carburizing.

Carbonization

The conversion of an organic substance into elemental carbon. (Should not be confused with carburization.)

Carburizing

The absorption and diffusion of carbon into solid ferrous alloys by heating (about 900°C by pack, gas, salt bath or plasma process), to a temperature usually above Ac3, in contact with a suitable carbonaceous material. It is 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 temperatures or by cooling to room temperature, then reaustenitizing and quenching.

Carburizing Flame

A gas flame that will introduce carbon into some heated metals, as during a gas welding operation. A carburizing flame is a type of reducing flame.

Carrier Gas

A gas, usually nitrogen or argon, that carries powder into the thermal spray process.

Case

The portion of a ferrous alloy, extending inward from the surface, that been prepped for case hardening. The case is typically considered to be the portion of the alloy (a) whose composition has been measurably altered from the original composition, (b) that appears dark on an etched cross section, or (c) that has hardness (after hardening) equal to or greater than a specified value. Contrast with core.

Case Depth

The depth to which the case extends on carburized or nitrided parts.

Case Hardening

A general term that refers to any process applicable to steel that changes the chemical composition of the surface layer by (a) causing it to absorb carbon and/or nitrogen and (b) creating a concentration gradient using diffusion. The processes commonly used are carburizing and quench hardening, cyaniding, nitriding and carbonitriding.

Casting

(object) An object that is at or near its finished shape, obtained by the solidification of a substance in a mold.
(process) Pouring molten metal into a mold to produce an object of a desired shape.

Cathode

The negative electrode in a plasma torch (gun). Generally made from thoriated tungsten (thorium is added to reduce the metal work function aiding electron release). See also anode.

Cathodic Coating

Coatings which become the cathode in an electrochemical cell with the substrate (anode). This type of coating protects the substrate from corrosion only by being a complete barrier. If the coating allows the environment to reach the substrate, accelerated corrosion of the substrate will occur.

Cathodic Protection

A technique to reduce the corrosion rate of a metal by making it the cathode of an electrochemical cell. Thermal spray zinc and aluminum coatings provide this protection to steel substrates, the coating being the anode and the steel being the cathode.

Cation

A positively charged ion.

Caustic Quenching

Quenching with aqueous solutions of 5–10% sodium hydroxide (NaOH).

Cavitation

The formation and rapid collapse within a liquid of cavities or bubbles that contain vapor and/or gas.

Cavitation Erosion

A form of erosion that removes material with the action of vapor bubbles in a very turbulent liquid.

Cementation

The introduction of one or more elements to the outer portion of a metal object via diffusion at high temperature.

Cementite

A compound of iron and carbon, known chemically as iron carbide and having the approximate chemical formula Fe3C. It is characterized by an orthorhombic crystal structure. When it occurs as a phase in steel, the chemical composition will be altered by the presence of manganese and other carbide-forming elements.

Cermet Powders

A composite powder of metal and ceramic constituents produced by methods such as agglomeration, sintering and spray drying. Examples include WC-Co and TiC-Ni.

Checks

Numerous, very fine cracks in a coating or at the surface of a metal part. Checks may appear during processing or during service and are most often associated with thermal treatment or thermal cycling. Also called check marks, checking and heat checks.

Chemical Conversion Coating

A protective/decorative, nonmetallic coating produced by chemical reaction of a metal with a chosen environment. It is often used to prepare the surface prior to the application of an organic coating.

Chemical Vapor Deposition (CVD)

The deposition of a coating onto a substrate via a chemical reaction between gases in a chamber. The chemical reaction produces components that adhere to / are deposited onto the substrate.

Chromating

Chromate conversion is a process which completely degreases an object and removes any traces of an oxide film, which it replaces through immersion with a chromate coating that can be painted. It is used as a post-treatment for cadmium, zinc and aluminum coatings.

Chromic Acid Anodizing

A gray/brown anodic coating (1–2 micron) produced on aluminum alloys in chromic acid at room temperature. It acts as an undercoat for paint.

Chromium (Cr)

An element used in low-alloy steels to increase (a) resistance to corrosion and oxidation, (b) high-temperature strength, (c) hardenability, and (d) abrasion resistance in high-carbon alloys. Straight chromium steels can be brittle and are susceptible to temper embrittlement.

Chromium Nitride

A ceramic, of metallic coloration, that is often applied as a thin coating by PVD. It has an extremely high hardness, 3500Hv, and is used to increase a surface's resistance to wear.

Chromizing

The high-temperature (approx. 900°C) pack / gaseous diffusion of chromium into the surface of a component to enhance high-temperature corrosion/oxidation resistance.

Cladding

The application of a thick (generally above 1mm) coating which melts or diffuses into the substrate. Processes include weld cladding and plasma transferred arc (PTA).

Close Annealing

Another term for box annealing.

Coalescence

The growth of grains, via absorption, at the expense of the remainder. Also, the growth of a phase or particle, via absorption or re-precipitation, at the expense of the remainder.

Coarsening

An increase in grain size—usually, but not necessarily, caused by grain growth.

Coating

(substance) A thin (generally less than 1mm) layer of material.
(process) The application of a thin layer of material onto the surface of a substrate.

Coating Thickness

By measuring the parameters X and Y, the thickness of the coating can be calculated by a simple geometrical equation. Optical inspection of the depression reveals the projected surfaces of the abraded coating and substrate sections.

Coating Wear

Wear can be determined by using a slurry composed of water and SiC particles falling continuously onto the ball at the contact region. The slurry wears the substrate in a controlled manner and thus assures highly reproducible results. By comparing the geometry of the crater for different periods of wear time, the wear rate of the coating and the substrate can be determined precisely.

Coatings: Corrosion Protective

Protection from an aggressive chemical environment can be accomplished in several ways. The surface can be coated with an inert material or with a material that forms a protective surface after reacting with the environment or with a material that will be sacrificially removed to protect the underlying material. Tantalum, platinum, and carbon are inert in many chemical environments. For example, carbon coatings are used on metals that are implanted in the human body to provide compatibility. In the aerospace industry, parts are PVD-coated with aluminum of ion vapor deposition (IVD) so as to prevent galvanic corrosion of dissimilar materials in contact. Chromium, aluminum, silicon, and the MCrAlY alloys will react with oxygen to form a coherent, protective, oxide layer on the surface. If the metal ions (Fe, Cu) diffuse more rapidly than the oxygen through the oxide, a thick oxide will form on the surface. If the oxygen diffuses more rapidly through the oxide than the metal ions (Al, Si, Ti, Zr—the “valve” metals), oxidation will occur at the interface and a thin oxide will be formed. The MCrAlY alloy coatings are used as protective coatings on aircraft engine turbine blades. Cadmium, aluminum, and Al:Zn alloys are used as galvanic sacrificial coatings on steel. Vacuum cadmium (“vac cad”) plating has the advantage over electroplated cadmium in that there is no possibility of hydrogen embrittlement of high-strength steel when vacuum deposition processing is used.

Coatings: Decorative and Decorative/Wear

Metallization for strictly decorative purposes is a large market. Applications vary from coating polymer webs—which are then converted to decorative uses such as balloons and labels—to metallization of three-dimensional articles, such as sports trophies, zinc die cast and molded polymer decorative fixtures, and cosmetic containers. Often these coatings consist of a reflective aluminum coating that is deposited on a smooth base coat, then over-coated with a dyed lacquer to give the coating the desired color and texture as well as corrosion and wear resistance. In some applications, in addition to the decorative aspects of the coating, the coating is required to withstand wear. For example, titanium nitride (TiN) is gold colored, and titanium carbonitride (TiCxNy) can vary in color from gold to purple to black, depending on the composition. Zirconium nitride (ZrN) has the color of brass and is much more wear and scratch resistant than brass. Decorative/wear coatings are used on door hardware, plumbing fixtures, fashion items, marine hardware, and other such applications.

Coatings: Hard and Wear-Resistant

Hard coatings are often called metallurgical coatings and are a type of tribological coating. The hard coatings are used to increase the cutting efficiency and operational life of cutting tools and to maintain the dimensional tolerances of components used in applications where wear can occur, such as injection molds. In addition, the coatings can act as a diffusion barrier where high temperatures are generated by motion between surfaces, or corrosion protection in aggressive environments. There are various classes of hard coating materials. They include: ionically bonded metal oxides (Al2O3, ZrO2, and TiO2), covalently bonded materials (SiC, boron carbon [B4C], diamond, diamond-like-carbon [DLC], TiC, AlN, CrC, mixed carbide, vanadium carbide, nitride and carbonitride compound alloys, and cubic boron nitride), and some metal alloys (cobalt chromium aluminum yttrium [CoCrAlY], NiAl, NiCrBSi). In some cases the coatings may be layered to combine properties. Hard coatings also are used to minimize fatigue wear, such as is found in ball bearings. Wear-resistant coatings also may be applied to surfaces where there is a light or periodic load. For example, hard coatings are deposited on plastics to improve scratch resistance. Applications are on molded plastic lenses and plastic airplane canopies. In some cases, wear coatings, such as SiO2 or Al2O3, may be applied to already hard surfaces, such as glass, to increase the scratch resistance.

Coatings: Packaging

Barrier coatings are used on flexible polymer films and paper for food packaging to reduce the water vapor transmission rate (WVTR) and the oxygen transmission rate (OTR) through the paper/polymer film. The most common barrier coating material is aluminum, which is deposited on rolls of polymer film (web), then supplied to “converters”, which fabricate the packaging. In some cases, the metal coatings are first deposited onto one surface and then “transferred” to the packaging film. Transparent barrier coatings are desirable in many instances. Layers of SiO2-x, by reactive evaporation and PACVD and composite coatings of SiO2:30% Al2O3 by E-beam co-evaporation are used to form transparent barrier layers. The composite coating material is more dense and flexible than the SiO2 or Al2O3 deposited material alone. Aluminum films are used on polymer, helium-filled balloons to reduce the loss of helium.

Coatings: Reflector

Metal films are widely used for reflector surfaces. Silver is often used when corrosion is not a problem, such as for back-surface mirrors. Aluminum can be used either as a front-surface or back-surface reflector. Often, aluminized front-surface reflectors, such as headlight reflectors, are over-coated with a protective polymer film (top coat). Chromium is used on front-surface reflectors when corrosion is a problem, even though its reflectivity in the visible (60%) is less than that of aluminum (> 90%). Reflector films are used in numerous commonly encountered applications, such as on compact discs for video and music storage, lamp reflectors, and visual mirrors such as the rear-view mirrors for cars. In some cases, multilayer films—similar to multilayer optical films—are used to selectively reflect certain wavelengths. Examples are “cold mirrors”, which reflect the visible radiation but not infrared wavelengths, and “heat mirrors”, which reflect the infrared but not the visible. Heat mirrors are used to raise the internal temperature of halogen lamps. Cold mirrors are used to reduce the heat of stage-lighting on performers.

Coatings: Solid Film Lubricants / Low-Friction Coatings

NASA (National Aeronautics and Space Administration) pioneered the use of vacuum-deposited, solid, thin-film lubricants. These lubricants are of two types: the low-shear metal lubricants (e.g. silver and lead) and the laminar-shearing compound materials (e.g. molybdenum disulfide). The low-shear metal lubricants are used in high-torque applications such as the rotating anodes in X-ray tubes. Low-shear compound materials are used in mechanical-bearing applications, in vacuum and where lubricant “creep” can be a problem. Because only a very thin film is needed for lubrication, the application of the lubricant film does not result in significant changes to dimensions. Low friction coatings of metal-containing carbon (Me-C) are used to reduce wear in mechanical contact applications.

Coatings: Thermal Control

The composition of the thermal control coatings on windows differs from the desired end result. If the object is to keep solar radiation from entering through the window, solar control coating (a multilayer film of glass-TiO2-Cr-TiO2) may be used. If the object is to keep heat in the room, low-E coating (a thin film of silver) can be used to reflect 85% to 95% of the low-temperature infrared radiation back into the room. One such “double-E coating” is glass-ZnO-Ag-(Ti)-ZnO-Ag-(Ti)-ZnO-TiO2. The ZnO provides an antireflective coating. Other types of thermal control coatings include solar absorbers, which absorb solar radiation, selective solar absorbers, which selectively adsorb solar radiation and do not emit infrared radiation, and others which have high emissivity to enhance cooling by radiation. Thermal barrier coatings are used to reduce the thermal transport from a hot environment to the substrate. Zirconium oxide (ZrO2) stabilized with calcium oxide (CaO), MgO, or Y2O3 is used as a thermal barrier coating on aircraft engine turbine blades.

Coherent Precipitate

A crystalline precipitate that forms from solid solution. It has an orientation that maintains continuity between the crystal lattice of the precipitate and the lattice of the matrix, and is usually accompanied by some strain in both lattices. Because the lattices fit at the interface between precipitate and matrix, there is no discernible phase boundary.

Cold Die Quenching

A quench utilizing cold, flat, or shaped dies to extract heat from a part. Cold die quenching is slow, expensive, and is limited to smaller parts with large surface areas.

Cold Treatment

Treatment carried out after quenching that transforms retained austenite into martensite. It involves cooling and holding at a temperature below ambient.

Cold Welding

Cohesion between two surfaces of a metal, generally under the influence of externally applied pressure at room temperature.

Cold Working

Plastic deformation of a metal at a temperature low enough so that recrystallization does not occur during cooling.

Columnar Structure

A coarse structure of parallel, elongated grains that is formed by unidirectional grain growth. It is most often observed in castings, but also sometimes in the structures resulting from the combination of diffusional growth and solid-state transformation.

Combined Carbon

The part of the total carbon in steel or cast iron that is not free carbon.

Composite

A mixture of two or more materials. Nearly all have a reinforcing material called filler (e.g. wood, glass, etc) and a natural or artificial resin, called matrix, to achieve specific characteristics and required properties.

Composite Coating

Any composite that is being used as a coating. Many thermal spray coatings can be considered composite coatings.

Composite Powder

A powder in which each particle is, itself, a composite. Contrast with blended powder.

Compound Layer

A non-etching layer of iron nitrides. It forms on the surface of ferrous materials during nitriding or nitrocarburizing. It is normally removed through grinding before a component is installed.

Computational Fluid Dynamics (CFD)

A method of simulating a flow field on a computer. The Navier-Stokes Equations (governing nonlinear partial differential equations) are replaced by numbers, which are advanced in space/time in order to obtain a final, numerical description of the flow.

Conditioning Heat Treatment

This term encompasses any heat treatment that is used to prepare a material to react in a specific way to a subsequent heat treatment.

Congruent Transformation

Any isothermal or isobaric phase change in which the phases' compositions remain equal to one another throughout the process.

Constitution Diagram

See phase diagram.

Constricted Arc (Plasma)

A column of plasma arc obtained by a constricted nozzle (usually water-cooled and in copper) and connected to the positive polarity (anode), while the negative polarity is connected to the cathodic electrode (usually tungsten added with thorium oxides).

Continuous Casting

An operation in which a cast shape is continuously drawn through the bottom of the mold as it solidifies. The length is not determined by the mold's dimensions.

Continuous Cooling Transformation (CCT) Diagram

A diagram that represents the beginning and end of the transformation of the initial phase using a set of curves drawn depending on the variables time (logarithmically represented) and temperature (linearly represented).

Continuous Precipitation

Precipitation from a supersaturated solid solution in which the precipitate particles grow by long-range diffusion and without recrystallization of the matrix. Continuous precipitates grow from nuclei that are distributed more-or-less uniformly throughout the matrix. They are usually randomly oriented, but may form a Widmanstätten structure. Also called general precipitation. Compare with discontinuous precipitation and localized precipitation.

Continuous-Type Furnace

A furnace, used for heat treating, wherein the materials being treated progress continuously through the furnace; entering one door and being discharged from another. Related terms: belt furnace, direct-fired tunnel-type furnace, rotary retort furnace, shaker-hearth furnace.

Controlled Cooling

Cooling from an elevated temperature in a manner predetermined either to avoid hardening, cracking, and internal damage, or to produce desired microstructure or mechanical properties.

Convergence

The steady decrease, with each successive iteration of a numerical solution, of the solution's relative error figure. When a certain predefined limited has been reached, the solution is considered to have converged; meaning that the solution has become stable and correct.

Cooling Curve

A curve that describes a material's temperature over time as the material cools.

Cooling Stresses

Residual stresses resulting from non-uniform distribution of temperature during cooling.

Copper (Cu)

An element that is used in certain steels to improve resistance to atmospheric corrosion. Hot working this element is detrimental to surface quality. For foundry applications, copper is meant to include all alloys containing 98% or more copper. Used for conductivity castings. Its melting point is 1083°C (1981.4°F).

Copper Plating

The electrolytic deposition of copper for the purpose of either providing a corrosion barrier (often as an undercoat for hard chrome plate) or reclaiming worn parts.

Core

In a ferrous alloy that has been prepared for case hardening, the part of the alloy that is not the case. Typically considered to be any portion that (a) appears light on an etched cross section, (b) has an essentially unaltered chemical composition, or (c) has a hardness, after hardening, that is less than a specified value.

Corrosion

(1) Gradual chemical or electrochemical attack on a metal by atmosphere, moisture or other agents.
(2) Chemical attack of furnace linings by gases, slags, ashes or other fluxes occurring in various melting practices.

Corrosion Fatigue

The contribution to premature fracture of metal that simultaneously involves both corrosion and repeated cyclic loading at the fewer cycles / lower stress levels which become necessary with a corrosive atmosphere.

Corrosion Index

A number expressing the maximum depth in microns or mils to which corrosion would penetrate in one year on the basis of a linear extrapolation of the penetration occurring during the lifetime of a given test or service.

Corrosion Potential

The potential of a corroding surface in an electrolyte in comparison to a reference electrode measured under open-circuit conditions.

Corrosive Wear

Wear caused by a chemical or electrochemical reaction to environment.

Cracking

Breaks or splits in the coating's surface.

Cracking Strip

A fin of metal, molded on the surface of a casting to prevent cracking.

Crevice Corrosion

The localized corrosion of a metal surface at / immediately adjacent to an area that is shielded from full exposure to the environment by the surface of another material in close proximity.

Critical Cooling Rate

The rate of continuous cooling required to prevent undesirable transformation. For steel, it is the minimum rate at which austenite must be continuously cooled to suppress transformations above Ms temperature.

Critical Diameter (D)

The diameter of the bar that can be fully hardened with 50% martensite at its center.

Critical Load (Lc)

The value that translates the complex intrinsic properties of a specific coating system into very reproducible figures of great practical significance. Scratch testers provide cross-referenced data on Lc by simultaneously recording three different effects: tangential force variations, acoustic emission fluctuations, and microscopic deformations.

Critical Point

(1) The temperature or pressure at which a change in crystal structure, phase, or physical properties occurs. Same as transformation temperature.
(2) In an equilibrium diagram, the specific value of composition, temperature, pressure or combination thereof at which the phases of a heterogeneous system are in equilibrium.

Critical Range

The temperature range between an upper and lower critical point for a given material.

Critical Strain

The amount of strain sufficient for recrystallization. Because the strain is small—usually only a few percent—recrystallization takes place from only a few nuclei, producing a recrystallized structure consisting of very large grains.

Critical Temperature

(1) Synonymous with critical point if the pressure is constant.
(2) The temperature above which the vapor phase cannot be condensed to liquid by an increase in pressure.

Critical Temperature Ranges

Synonymous with Transformation Ranges, which is the preferred term.

Cr-Ni Coatings

Anticorrosion coatings resistant to high temperatures.

Crushed Powder

Powder formed from a solid that has been crushed to the appropriate size for spraying.

Cryogenic Treatment

See Cold Treatment.

Curie Temperature

The temperature of magnetic transformation below which a metal or alloy is ferromagnetic and above which is paramagnetic.

Cyaniding

A case-hardening process in which a ferrous material is heated to within the transformation range in a molten salt that contains cyanide. This causes the simultaneous absorption of carbon and nitrogen at the surface and the creation via diffusion of a concentration gradient. Quench hardening completes the process.

Cycle Annealing

An annealing process employing a predetermined and closely-controlled time-temperature cycle to produce specific properties or microstructures.

D

Dead Soft

A temper of nonferrous alloys, and some ferrous alloys, that corresponds to the condition of minimum hardness and tensile strength produced by full annealing.

Decalescence

A phenomenon associated with the transformation of alpha iron to gamma iron on the superheating of iron and steel. Revealed by the darkening of the metal surface owing to the sudden decrease in temperature caused by the fast absorption of the latent heat of transformation. Contrast with recalescence.

Decarburization

Loss of carbon from the surface layer of a carbon-containing alloy due to reaction with one or more chemical substances in a medium that contacts the surface.

Degreasing

The removal of grease and oil from a surface.

Degrees of Freedom

The number of independent variables (such as temperature, pressure, or concentration within the phases present) that may be altered at will without causing a phase change in an alloy system at equilibrium—the number of such variables that must be fixed arbitrarily to define the system completely.

Delta Ferrite

See ferrite.

Density

The mass-per-unit volume of a substance, usually expressed in grams per cubic centimeter or in pounds per cubic foot.

Deposit Corrosion

Localized corrosion under or around a deposit or collection of material on a metal surface. See also crevice corrosion.

Detonation Gun

A thermal spraying process in which the coating material is heated and accelerated to the work piece by shock waves from a series of detonations or explosions from gas mixtures. Also known as D-Gun (Praxair).

Dew Point

The temperature and pressure at which a gas begins to condense to a liquid.

Dew Point Analyzer

An atmosphere-monitoring device that measures the partial pressure of water vapor in an atmosphere.

DI (Ideal Diameter)

The diameter of a round steel bar that will harden at the center to a given percent of martensite when subjected to an ideal quench (i.e. Grossman quench severity H = infinity)

Diamond-Like Carbon

A thin film coating applied by a PVD or CVD process. It possesses some of the hardness of diamond, but with the low-friction properties of graphite (Friction Coefficient ≈ 0.1). Used on cutting and forming tools, on medical implants and for low-friction surfaces.

Die

A metal block used when forming materials by casting, molding, stamping, threading, or extruding.

Die Assembly

The parts of a die-stamp or press that hold the die and locate it for the punches.

Die Casting

A rapid, water-cooled, permanent mold casting process that is applied to nonferrous metals. Three types: (1) the plunger-type operated hydraulically, mechanically or by compressed air with or without a gooseneck, (2) the direct-air injection which forces metal from a goose-neck into the die, (3) the Cold-Chamber Machine. All force the metal into the die with a pressure greater than that of gravity flow.

Die Coating

A material (e.g. silicone, stearate, oil, or wax) used as lubrication for a die pattern or core box to facilitate easy removal of a casting, mold or core.

Differential Heating

Heating that intentionally produces a temperature gradient within an object such that, after cooling, a desired stress distribution or variation in properties is present within the object.

Diffusion

(1) The spreading of a constituent in a gas, liquid, or solid, tending to make the composition of all parts uniform.
(2) The spontaneous movement of atoms or molecules to new sites within a material.

Diffusion Coefficient

A factor of proportionality that represents the amount of a substance that is diffusing across a unit area through a unit concentration gradient in unit time.

Dilatometer

An instrument that measures the linear expansion or contraction of a metal resulting from changes in factors such as temperature and allotropy.

Dip Coat

In solid and shell mold investment casting, a fine ceramic coating applied as a slurry to the pattern to produce maximum surface smoothness, followed by a cheaper conventional investment.

Direct Chill (DC) Casting

A continuous method of making ingots or billets. Also, extrusion by pouring the metal into a short mold. Sometimes called semi-continuous casting.

Direct-Fired Tunnel-Type Furnace

A continuous-type furnace where the work is conveyed through a tunnel-type heating zone and the parts are hung on hooks or fixtures to minimize distortion.

Direct Quenching

(1) Quenching carburized parts directly from the carburizing operation.
(2) Quenching pearlitic, malleable parts directly from the malleablizing operation.

Dirty Casting

A casting containing an excessive amount of nonmetallic inclusions in the body of the metal.

Discontinuous Precipitation

Precipitation from a supersaturated solid solution in which the precipitate particles grow by short-range diffusion, accompanied by recrystallization of the matrix in the region of precipitation. Discontinuous precipitates grow into the matrix from nuclei near grain boundaries, forming cells of alternate lamellae of precipitate and depleted (and recrystallized) matrix. Often referred to as cellular or nodular precipitation. Compare with continuous precipitation and localized precipitation.

Dissociation

As applied to heterogeneous equilibria, the transformation of one phase into more phases of different composition. Compare with order-disorder transformation.

Disturbed Metal

The cold-worked metal that has formed on a polished surface during the processes of grinding and polishing.

Double Aging

Using two different aging treatments to control the type of precipitate formed from a supersaturated matrix in order to obtain the desired properties. The first aging treatment, sometimes referred to as intermediate or stabilizing, is usually carried out at higher temperature than the second.

Double Tempering

A treatment in which a quench-hardened, ferrous metal is subjected to two complete tempering cycles, usually at similar temperatures, for the purpose of ensuring completion of the tempering reaction and promoting stability of the resulting microstructure.

Drawing

A misnomer for tempering.

Dry Cyaniding

(obsolete) See carbonitriding.

Dry Sand Casting

A process in which sand molds are dried at above 100°C (212°F) before use.

Ductile Cast Iron

A cast iron that has been treated, while molten, with an element such as magnesium or cerium to induce the formation of free graphite as nodules or spherulites. This imparts a measurable degree of ductility to the cast metal. Also known as nodular cast iron, spherulitic graphite cast iron, and SG iron.

Ductile Fracture

Fracture characterized by tearing of metal accompanied by appreciable gross plastic deformation and the expenditure of considerable energy. Compare with brittle fracture.

Ductility

The ability of a material to deform plastically without fracturing. This can be measured in a few ways, including by the elongation or reduction of the material's area in a tensile test, as well as by height of cupping in an Erichsen test.

Dust

Small, solid particles created by the break-up of larger particles.


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