Glossary

Heat Treating Definitions

If you are unfamiliar with our processes, or would like to learn more, the glossary below covers some of our primary heat treatment terms and definitions. Still have questions? Please contact us.

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  Age Hardening

  A process of aging, usually following rapid cooling or cold working, that increases hardness and strength and ordinarily decreases ductility.

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  ALLOY

  A material composed of two or more chemical elements that exhibit metallic characteristics.

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  ALLOY STEEL

  A steel which achieves specific properties by the addition of alloying elements in solid solution.

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  ANNEALING

  Heating to and holding at a suitable temperature, often at or above transformation, and then cooling at a rate suitable to produce specific microstructure results. This can result in reduced hardness, improved machinability, easier cold working, a desired microstructure, or other mechanical or physical properties.
  

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  ATMOSPHERE (PROTECTIVE)

  The gases surrounding the work in high temperature apparatus in metallurgical practice. The character of the protective atmosphere in usually to prevent high temperature oxidation of the work surface. The atmosphere may also be enriched with component gasses to produce specific chemical reactions with the work surface, such as carburizing and nitriding, also known as Controlled Atmospheres.

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  AUSTENITE

  A solid solution of one or more elements in face centered cubic iron. Unless indicated otherwise, the solute is generally assumed to be carbon.

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  AUSTENITIZING

  Heating a ferrous alloy above the A3 or Acm transformation temperatures to form 100% austenite.

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  BRAZING

  Joining metals by melting and flowing a filler metal into the joint, the filler metal having a lower melting point than the adjoining metal. This may be accomplished by means of a torch (torch brazing), in a furnace (furnace brazing), or by induction brazing.

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  BRIGHT ANNEALING

  Annealing performed in a controlled atmosphere or vacuum so that surface oxidation is reduced, and the surface remains relatively bright. A common bright annealing atmosphere is hydrogen or dissociated ammonia (nitrogenhydrogen).

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  CARBIDE

  A carbon compound with one or more metallic elements.

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  CARBON STEEL

  A metal alloy of iron and carbon, without substantial amounts of other alloying elements. Carbon content usually limited to 1.00%, silicon limited to 0.60%, and manganese to 1.65%. Also called plain carbon steel.

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  CARBURIZING

  The controlled atmosphere process of adding carbon to the surface of steel with the intent of making the surface layer harder to a specific case depth, often in the range of 0.005 to 0.080 inch depth. The controlled atmosphere component achieving this surface reaction is carbon monoxide, usually provided in the atmosphere gas and increased by the addition of methane.

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  CASE HARDENING

  A heat treatment in which the surface layer of a piece is made substantially harder than the core either by (a) altering its composition and properties with a controlled atmosphere heat treatment, or (b) by selectively heating only the part surface by means of flame or induction, in both cases leaving the interior soft and ductile and thus creating a hard metal “case”.

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  CAST IRON

  An iron and carbon alloy, generally containing 1.7-3.5 % carbon.

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  CASTING

  A part produced by solidifying molten metal in a mold.

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  CHROMIUM

  An element added to ferrous alloys which results in increased hardenability and corrosion resistance.

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  COLD ROLLED

  Rolling performed at temperatures below 30 % of the melting temperature for a given material.

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  CONTROLLED COOLING

  The manipulation of cooling rate from an elevated temperature, usually through a transition temperature, to achieve specific properties.

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  CORE

  The internal portion of the metal part, as distinguished from the surface. Distinction is important in parts that have surface treatments like carburizing or nitriding, or in pieces with a large cross-sectional area.

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  CORROSION

  The conversion of refined metal to its more stable oxide resulting in the gradual destruction of materials.

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  COUPON

  A sample piece of metal used to determine the properties of the more elaborate metal part after heat treating through hardness testing and microanalysis.

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  DECARBURIZATION

  The result of heating in an atmosphere that reacts with the carbon in the part surface, causing the loss of carbon from the surface of a ferrous alloy.

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  DEFORMATION

  Change of dimensions resulting from applied stress.

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  DIFFUSION

  Spontaneous movement of new atoms into the metallic crystal structure, from a high concentration area to a low concentration area.

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  DUCTILE IRON (NODULAR IRON)

  A gray cast iron with magnesium added to its chemical composition. This allows for spherical graphite nodules, which enhance ductile properties.

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  DUCTILITY

  The ability of a material to deform without fracturing, measured by elongation or reduction of area in a tensile test, or by other means.

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  ELONGATION

  The increase in gage length upon tensile testing fracture, usually expressed as a percentage of the original gage length.

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  EMBRITTLEMENT

  The loss of a material's ductility, and an increased likelihood of fracture.

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  FATIGUE

  A weakness that develops in metal structures leading to fracture under repeated or fluctuating stresses, in some cases measured in millions of cycles.

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  FATIGUE STRENGTH

  Range of maximum stress that can be applied to the material for a specific number of cycles without causing fatigue failure, the stress being completely reversed within each cycle unless otherwise stated.

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  FILE HARDNESS

  Surface hardness determined by the use of a file of standardized hardness under the assumption that a metal which cannot be scored is as hard as, or harder than the file. This is a quick and relatively effective way of measuring surface hardness, although one of the the least accurate forms of hardness testing.

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  FILLET

  Two surfaces with a concave joint.

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  FLAME HARDENING

  The process of hardening the surface of a metal part by heating it with a high temperature flame followed by rapid cooling.

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  FLUX

  A chemical cleaning, flowing, and/or purifying agent. Often used as a protective covering to prevent the formation of, or facilitate the removal of, undesirable substances.

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  FORGING

  Hammering or pressing hot metal into a desired shape.

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  GRAIN

  A component of a solid metal microstructure in which the space lattice pattern is continuous. The boundary of a grain is at the intersection with another grain of different lattice orientation.

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  GRAIN SIZE

  An index number corresponding to the average dimension of a grain when the individual sizes are fairly uniform. Grain sizes are reported in terms of number of grains per unit area or volume, average diameter, or as a grain size number determined by area measurements.

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  GRAPHITE

  One of the crystal forms of carbon.

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  HARDENABILITY

  Ability of steel to form martensite, not specifically to harden.

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  HARDNESS

  The physical property of a material to resist abrasion, deformation, or indentation from another material. Typically measured in Rockwell or Brinell scales.

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  HEAT TREATMENT

  Heating and cooling a solid metal or alloy to obtain desired conditions or properties.

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  IMPACT TEST

  Fracturing a test bar at high velocity to determine the energy absorbed during failure

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  INDUCTION HARDENING

  Using electrical induction to heat the surface of steel above the transformation range, followed by rapid cooling.

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  ISOTHERMAL ANNEALING

  A thermal process in which a steel is heated above A1 or A3 depending on the given carbon content. This produces a uniform microstructure and a ductile material that is more easily cold worked or machined.

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  KNOOP HARDNESS

  Microhardness determined from the resistance of a material to indentation by an extended pyramid diamond indenter. This test is used for parts of small sizes and cross sections.

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  MACROGRAPH

  A photographic reproduction of any object that may be slightly reduced, of actual size, or magnified, not exceeding ten diameters.

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  MALLEABILITY

  The quality that determines the ease of shaping metal.

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  MARTENSITE

  This microstructure occurs on cooling when steel is quenched from its austenite range fast enough to suppress upper transformation products.

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  MECHANICAL PROPERTIES

  Physical properties of materials that can be manipulated by thermal processing. These include but are not limited to hardness, strength, ductility, and fracture toughness.

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  METALLOGRAPHY

  Visual examination by microscope and study of microstructures of metals and alloys.

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  METALLURGY

  The science of working or heating metals so as to give them certain desired characteristics or properties.

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  MICROHARDNESS TEST

  Hardness test used when test samples are very small or thin, or when small regions in a composite sample or plating are to be measured.

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  MICROSTRUCTURE

  The grain structure of a material as a result of prior processing. This can be viewed under a microscope on an etched sample.

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  NITRIDING

  Surface hardening a ferrous alloy by absorption of nitrogen in the surface at subcritical temperatures.

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  NORMALIZING

  This process is used to eliminate all effects of prior processes and to render the part uniform in structure. The process consists of heating a steel to a temperature about 100F above the critical range and cooling in still air.

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  PHYSICAL PROPERTIES

  Any property that is measurable, whose value describes a physical system, such as density, conductivity, thermal expansion, etc.

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  PICKLE

  Using chemical or electrochemical processes to clean metal surfaces.

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  POROSITY

  A casting defect that forms due to liquid metal cooling and reducing in size. This can be seen on a macro scale as visual holes, or a micro scale between dendrite growths.

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  POST WELD HEAT TREATMENT

  Heating welded metal immediately after welding resulting in tempering, stress relief, or providing a controlled rate of cooling to minimize formation of a hard or brittle structure.

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  PRECIPITATION HARDENING

  Also known as Age Hardening, the material is heated to a specific temperature and held for a certain time period. This increases the hardness and yield strength of the material by means of a change in microstructure accomplished by the formation of metal alloy precipitates. It can be very beneficial for hardening finished machined parts with critical tolerances.

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  PREHEATING

  Used to minimize thermal shock on complex parts. This process consists of heating parts to a temperature below the austenitizing temperature prior to austenitizing.

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  PROCESS ANNEALING

  A thermal process performed below A1 for a given steel, with the goal of refining grain size after cold work.

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  QUENCHING

  Rapid cooling from an elevated temperature in various mediums.

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  REDUCTION IN AREA

  The difference between the original crosssectional area of a tensile test piece and that of the smallest necked area at the point of fracture. This result is usually stated as percentage of the original area and is a measure of ductility.

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  SCALE

  The surface oxidization formed on a metal by heating or casting in air or other oxidizing atmospheres.

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  SHEAR STRENGTH

  Maximum shear load a material can withstand before failure occurs divided by its cross sectional area. A shear load is a force that produces a sliding failure along a plane that is parallel to the direction of the force.

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  SOAK

  Assuring the desired metallurgical reaction occurs to all parts in a furnace load by holding them at temperature for a sufficient time.

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  SOLDERING

  Joining two or more metals by the melting and flowing of alloys that have relatively low melting points into a joint, most commonly leadbased or tin base solders.

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  SPHEROIDIZING

  A thermal process that cycles above and below A1 to allow spheroidal carbides to form in the material. Forms the most ductile structure that has excellent cold formability and machinability.

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  STEEL

  An alloy of iron and carbon in which the carbon content is between about 0.05% and 1.0% and is malleable at some temperature in the solid state.

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  STRESS RELIEVING

  A thermal process performed below the recrystallization temperature and with sufficient time to allow dislocation recovery to occur to relive stresses from cold working, machining, quenching, or welding.

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  STRESS CORROSION CRACKING

  Spontaneous failure of metals, usually by slow crack propagation under corrosion and stress conditions.

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  STRESS RISERS

  A location in an object where stress is concentrated, as a result of sharp changes in contour or surface defects such as cracks.

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  RESIDUAL STRESSES

  Stresses that remain in a material after the cause of the stress has been removed.

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  SUBCRITICAL ANNEAL

  A soft annealing process that forms an even distribution of spheroidal carbides in the steel, which will make the material softer and tougher. This is used to increase the material’s machinability.

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  TEMPERING

  A process of heat treating, which is used to increase the toughness of iron-based alloys. Tempering is usually performed after hardening to reduce the excess hardness in the material. This is done by heating the metal to a temperature below the materials critical point for a certain period of time, then allowing it to cool in still air. The exact temperature determines the amount of hardness removed, and depends on both the specific composition of the alloy and on the desired properties in the finished product. For example, very hard tools are often tempered at low temperatures, while springs are tempered to much higher temperatures.

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  TENSILE STRENGTH

  A measurement of the amount of stress required to induce necking in a tensile sample.

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  THERMAL STRESSES

  Stressed induced in a material due to non-uniform heating or cooling. In pieces with large differences in cross-section in close proximity to each other, this can result in thermal shock and fracture.

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  TOUGHNESS

  The ability of a metal to deform plastically and to absorb energy in the process before fracture. Toughness is a combination of strength and ductility.

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  THERMAL EXPANSION

  The tendency of material to change in volume in response to a change in temperature.

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  VACUUM BRAZING

  A high-end joining process that can result in an extremely strong joint with no residual flux. The process joins two base metals such as inconels, steels, or other common alloys, using a dissimilar brazing alloy in a vacuum furnace.

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  VACUUM FURNACE

  A furnace using low atmospheric pressures rather than a protective gas atmosphere like typical heat-treating furnaces. Vacuum furnaces are categorized as hot wall or cold wall, depending on the location of the heating and insulating components.