Glossary of terms used in heat treatment of steel


          Glossary of terms used in heat treatment of steel

Various terms used in the heat treatment of steels are described below:

Ageing – It describes a time temperature dependent change in the properties of certain alloy steels. It is a change in properties that may occur gradually at atmospheric temperature (Natural ageing) and more rapidly at higher temperature (Artificial ageing).

Quench ageing – It is a change in properties that may occur gradually at atmospheric temperature and more rapidly at higher temperature following rapid cooling (Precipitation hardening).

Strain ageing – It is a change in properties that may occur gradually at atmospheric temperature and more rapidly at higher temperature following plastic straining.

Annealing – It is a term denoting a treatment, consisting of heating to and holding at a suitable temperature followed by cooling at a suitable rate, used primarily to soften but also to simultaneously produce desired changes in other properties or in microstructure. The purpose of such changes may be, but is not confined to:

  • Inducing softness
  • Improving machinability
  • Improving cold working properties
  • Improving mechanical or electrical properties
  • Increasing stability of dimensions
  • Obtaining a desired structure
  • Removing stresses

The time temperature cycle used vary widely both in maximum temperature attained and in cooling rate employed, depending on the composition of the steel, its condition, and the result desired. Various types of annealing processes are as follows:

Bright annealing– It is annealing in a protective medium to prevent discoloration of the bright surface.

Cycle annealing – It is an annealing process employing a predetermined and closely controlled time temperature cycle to produce specific properties or microstructure.

Flame annealing – It is an annealing process in which the heat is applied directly by a flame.

Full annealing – It is heating to and holding at some temperature above the transformation range, followed by cooling slowly at a rate through the transformation range such that that the hardness of the steel approaches a minimum.

Graphitizing – In this type of annealing process the annealing of steel is done in such a way that some or all of the carbon is precipitated as graphite.

Intermediate annealing – It consists of annealing of steel at one or more stages during manufacture and before final thermal treatment.

Isothermal annealing – It is heating to and holding at some temperature above the transformation range, then cooling to and holding at a suitable temperature until austenite to pearlite (Ferrite – carbide) transformation is complete and finally cooling freely.

Process annealing – It is an imprecise term which is used to denote various treatments that improve workability.

Quench annealing – It is an annealing of austenitic steel by solution heat treatment.

Spheroidizing – It is heating and cooling in a cycle designed to produce a spheroidal or globular form of carbide.

Sub critical annealing – It is heating to and holding at some temperature below the transformation range, followed by cooling at a suitable rate.

Austempering – It is quenching from a temperature above the transformation range to some temperature above the upper limit of martensite formation, and holding at this temperature until the austenite is completely transformed to the desired intermediate structure, for the purpose of conferring certain mechanical properties. It is carried out in a medium having a rate of heat abstraction high enough to prevent the formation of high temperature transformation products.

Austenitizing – It is forming austenite by heating into the transformation range (partial austenitizing) or above the transformation range (complete austenitizing). When used without qualification, the term implies complete austenitizing.

Blueing – It is a treatment of the surface of steels usually in the form of sheet or strip, on which, by the action of air or steam at a suitable temperature, a thin blue oxide film is formed on the initially scale free surface, as a means of improving appearance and resistance to corrosion. This term is also used to denote a heat treatment of springs after fabrication, to reduce the internal stress created by coiling and forming.

Carbon potential – It is a measure of the ability of an environment containing active carbon to alter or maintain, under prescribed conditions, the carbon content of the steel exposed to it. In any particular environment, the carbon level attained will depend on such factors as temperature, time, and steel composition.

Carbon restoration – It is the replacement of the carbon lost in the surface layer from previous processing by carburizing the layer to the original carbon level.

Carbonitriding – It is a case hardening process in which steel part is heated above the lower transformation temperature in a gaseous atmosphere of such composition (Mixture of carburizing and nitriding gases) as 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 steel part.

Carburizing – It is a process in which carbon is introduced into solid steel by heating it and holding it above the transformation temperature range while in contact with a carbonaceous material that may be a solid, liquid, or gas. Carburizing is frequently followed by quenching to produce a hardened case.

Case hardening – In this treatment the surface layer of steel has been suitably altered in composition and can be made substantially harder than the interior or core. Normally case hardening is followed by suitable heat treatment. It is also used to designate the hardened surface layer of a piece of steel that is large enough to have a distinctly softer core or centre.

Cold treatment – It is exposing the steel to suitable subzero temperature for the purpose of obtaining desired conditions or properties, such as dimensional or microstructural stability. When the treatment involves the transformation of retained austenite, it is usually followed by a tempering treatment.

Conditioning heat treatment – It is a preliminary heat treatment used to prepare a material for a desired reaction to a subsequent heat treatment.

Controlled cooling – It is a term used to describe a process by which a steel product is cooled from an elevated temperature, usually from the final hot forming operation in a predetermined manner of cooling to avoid hardening, cracking, or internal damage.

Core – It is the interior portion of steel which is substantially softer than the surface layer (case) after case hardening. The term core is also used to designate the relatively soft central portion of certain hardened tool steels.

Critical range and critical points – It is synonymous with the term transformation range. These are arrest points, critical temperatures, change points and transformation points at which changes occur in the constitution of steels.

Cyaniding – It is the introduction of carbon and nitrogen into the surface of steel by heating to and holding at a suitable temperature in contact with molten cyanides.

Decarburization: it is the loss of carbon from the surface of steel as the result of heating in a medium that reacts with the carbon.

Differential heating – Any method of heating so controlled as to produce a desired non uniform temperature distribution in a steel object. It is a heating process by which the temperature is made to vary throughout the steel object being heated so that on cooling different portions, may have such different physical properties as may be desired.

Differential quenching – It is the selective quenching of the different parts of the same steel object.

Drawing – It is drawing the temper and is synonymous with tempering.

Eutectic alloy – It is the alloy composition that freezes at constant temperature similar to a pure metal. It is the lowest melting (or freezing) combination of two or more metals. The alloy structure (homogeneous) of two or more solid phases formed from the liquid eutectically.

Flame hardening – It is a process of heating the surface layer of steel above the transformation temperature range by means of a high temperature flame followed by quenching.

Grain refining – It is heating from some temperature below the transformation range to a suitable temperature above that range followed by cooling at a suitable rate.

Hardenability – It is the property in steel that determines the depth and distribution of hardness induced by quenching.

Hardening – It is a process of increasing hardness of metal by suitable treatment usually involving heating and cooling. This usually implies quenching from a temperature either within or above the transformation range.

Heat treatment – It is a combination of heating and cooling operations applied to steel to obtain desired conditions or properties. Heating for the sole purpose of hot working is excluded from the meaning of this definition.

Heat treatment solution: It is a treatment in which an alloy is heated to a suitable temperature and held at this temperature for a sufficient length of time to allow a desired constituent to enter into solid solution, followed by rapid cooling to hold the constituent in solution. The material is then in a supersaturated unstable state which may subsequently exhibit age hardening.

Homogenizing – It is a high temperature heat treatment process intended to eliminate or to decrease chemical segregation by diffusion.

Induction hardening – It is heating by means of an alternating magnetic field to a temperature within or above the transformation range followed immediately by quenching.

Isothermal transformation – It is a change in a phase of steel at constant temperature.

Malleablizing – It is a process of annealing white cast iron in which the combined carbon is wholly or partly transformed to graphitic or free carbon and, in some cases, part of the carbon is removed completely.

Maraging – It is a precipitation hardening treatment applied to special steel to precipitate one or more of intermetallic compounds.

Martempering – It is quenching from a temperature above the transformation range to some temperature above the upper limit of martensite formation, holding at that temperature long enough to permit equalisation of temperature without transformation of the austenite followed by cooling in air. This results into the formation of martensite which may be tempered as desired.

It is a hardening procedure in which an austenitized steel object is quenched into an appropriate medium whose temperature is maintained substantially at the Ms temperature of the steel object, held in the medium until its temperature is uniform throughout but not long enough to permit bainite to form, and then cooled in air. The treatment is followed by tempering.

Nitriding – It is a process of case hardening in which a steel of special composition is heated in an atmosphere of ammonia or in contact with nitrogenous material. Surface hardening is produced by the absorption of nitrogen without quenching.

Normalizing – It is a process in which steel is heated to a temperature above the transformation range and subsequently cooled in still air at room temperature.

Overheated – Steel is said to have been overheated if, after exposure to an unduly high temperature, it develops an undesirably coarse grain structure but is not permanently damaged.  The structure damaged by overheating can be corrected by suitable heat treatment or by mechanical work or by a combination of the two. In this respect it differs from a burnt structure.

Patenting – It is heating to a suitable temperature well above the transformation range, followed by cooling to a temperature below that range in air or in a bath of molten lead or salt maintained at a suitable temperature to produce a structure which will facilitate subsequent cold working and give the desired mechanical properties in the finished state.

Precipitation hardening – It results in hardening due to the precipitation of a constituent from a super saturated solid solution which occurs in certain suitably quenched steel on heating to and holding at some temperature below the transformation range.

Preheating – It is heating of steel to an appropriate temperature immediately prior to austenitizing when hardening high hardenability construction steel, several types of tool steels and heavy sections.

Quenching – It is the rapid cooling of steel. When applicable the following more specific terms are used.

Direct quenching – It is quenching of carburized steel part directly from the carburizing operation.

Fog quenching – It is quenching in an air mist.

Hot quenching – It is a term used to cover a variety of quenching procedures in which a quenching medium is maintained at a prescribed temperature above 71 deg C.

Interrupted quenching – It is a quenching procedure in which the steel is removed from the first quench at a temperature substantially higher than that of the quenching medium and is then subjected to a second quenching system having a different cooling rate than the first.

Selective quenching – It is quenching of only certain portion of steel.

Slack quenching – The incomplete hardening of steel due to quenching from the austenitizing temperature at a rate slower than the critical cooling rate for the particular steel. It results into the formation of one or more transformation products in addition to martensite.

Spray quenching – It is quenching of steel in a spray of liquid.

Time quenching – It is interrupted quenching in which the duration of holding of the steel in the quenching medium is controlled.

Secondary hardening – It is a hardening effect which occurs on cooling certain previously hardened steel from a particular range of tempering temperature.

Soaking – It is prolonged heating of a metal at a selected temperature.

Spheroidizing – It is subjecting steel to a selected temperature cycle usually within or near the transformation range in order to produce a suitable globular form of carbide for such purpose as:

  • Improving machinability
  • Facilitating subsequent cold working
  • Obtaining a desired structure for subsequent heat treatment

Stabilizing Treatment – It is treatment applied to stabilize the dimensions of steel or the structure of the steel material. For example

  1. Before finishing to final dimensions, heating the steel to or somewhat beyond its operating temperature and then cooling to room temperature a sufficient number of times to ensure stability of dimensions in service
  2. Transforming retained austenite in those materials that retain substantial amounts when quench hardened (see cold treatment)
  3. Heating a solution treated austenitic stainless steel that contains controlled amounts of titanium or niobium plus tantalum to a temperature below the solution heat treating temperature to cause precipitation of finely divided, uniformly distributed carbides of those elements, thereby substantially reducing the amount of carbon available for the formation of chromium carbides in the grain boundaries on subsequent exposure to temperature in the sensitizing range.

Stress relieving – It is a process to reduce internal residual stresses in steel by heating the steel to a suitable temperature and holding for a proper time at that temperature and then cooling slowly.  This treatment is applied to relieve internal stresses induced during casting, quenching, normalizing, machining, cold working, or welding.

Sub zero treatment – It is cooling hardened steel to a temperature sufficiently below zero deg C to promote the transformation of any retained austenite to martensite.

Temper carbon – It is the free or graphitic carbon that comes out of solution usually in the form of rounded nodules in the structure during Graphitizing or Malleablizing.

Tempering – It is heating hardened, normalized or mechanical worked steel to some temperature below the transformation range and holding for a suitable time at that temperature, followed by cooling at a suitable rate. The process is usually applied for the purpose of producing a desired combination of mechanical properties.

Double tempering – It is a treatment in which quenched hardened steel is given two complete tempering cycles at substantially the same temperature for the purpose of ensuring completion of the tempering reaction and promoting stability of the resulting microstructure.

Snap temper – It is a precautionary interim stress relieving treatment applied to high hardenability steel immediately after quenching to prevent cracking because of delay in tempering them at the prescribed higher temperature.

Temper brittleness – It is the brittleness that results when certain steel is held within, or is cooled slowly through, a certain range of temperatures below the transformation range.   The brittleness is revealed by notched bar impact test at or below room temperature.

Transformation range or Transformation temperature range: These are those ranges of temperature within which austenite form during heating and transforms during cooling. The two ranges are distinct, sometimes overlapping but never coinciding. The limiting temperatures of the ranges depend on the composition of the alloy and on the rate of change of temperature particularly during cooling.

Transformation temperature – The temperature at which a change in phase occurs. The term is sometimes used to denote the limiting temperature of a transformation range. The following symbols are used for iron and steel:

Accm – In hypereutectoid steel, the temperature at which the solution of cementite in austenite is completed during heating

Ac1 – The temperature at which austenite begins to form during heating

Ac3 – The temperature at which transformation of ferrite to austenite is completed during heating

Ac4 – The temperature at which austenite transforms to delta ferrite during heating

Ae1, Ae3, Aecm, Ae4 – The temperatures of phase changes at equilibrium

Arcm – In hypereutectoid steel, the temperature at which precipitation of cementite starts during cooling

Ar1 – The temperature at which transformation of austenite to ferrite or to ferrite plus cementite is completed during cooling

Ar3 – The temperature at which austenite begins to transform to ferrite during cooling

Ar4 – The temperature at which delta ferrite transforms to austenite during cooling

Ms – The temperature at which transformation of austenite to martensite starts during cooling

Mf – The temperature, during cooling, at which transformation of austenite to martensite is substantially completed

All these changes except the formation of martensite occur at lower temperatures during cooling than during heating, and depend on the rate of change of temperature.