Glossary of Transformation Terms

Ac1
The temperature at which austenite begins to form on heating.

Ac3
In hypoeutectoid steel, the temperature at which transformation of ferrite into austenite is completed upon heating.

Accm
In hypereutectoid steel, the temperature at which cementite goes into complete solution with austenite.

Ac1b
The beginning of the transformation of alpha + carbides to gamma, the end by Acle. The indices b and e mean the beginning and the end of transformation.

Acicular ferrite (AF)
Interganular product of fine interlocking ferrite grains separated by high-angle boundaries, and aspect ratio from ~ 3:1-10:1.

Austenite
The face-centered-cubic phase of iron and steel, also referred to as gamma iron. In steel, a solid solution in which gamma iron is the solvent. One of the allotropes of iron, also known as gamma iron. It is formed when iron is between 912º C and 1394º C and has a face-centred cubic structure. Also found in carbon steel.

Bainite
In steel, an acicular aggregate of ferrite and carbide, resulting from an isothermal transformation of austenite at a temperature below the pearlitic range and above Ms. A non-equilibrium phase, usually in steel, which is formed by quenching from the austenite phase. The rate of quenching required is slower than that necessary to form martensite but faster than that which produce the equilibrium phase of pearlite. The mechanism of formation is a displacive i.e. diffusionless transformation. Two types of bainite are recognised; upper and lower. Upper bainite forms at higher temperature and consequently the carbon present has sufficient to diffuse out forming carbides outside the bainite laths. Lower bainite forms at lower temperature and contains carbides within the laths as the carbon cannot diffuse out rapidly enough.

Carbide
A compound of carbon with one or more metallic elements.

Cementite
A compound of iron and carbon commonly known as iron carbide and having the approximate chemical structure, Fe3C. Cementite is characterized by an orthorhombic crystal structure. Iron carbide, Fe3C. Harder and stronger than ferrite, but not as malleable.

Coarse accicular ferrite (CAF)
Refers to the intergranular product formed at slower cooling rates than acicular ferrite with larger grain size and may be associated with carbides.

Continuous cooling transformation (CCT) diagrams
measure the extent of transformation as a function of time for a continuously decreasing temperature. In other words a sample is austenitised and then cooled at a predetermined rate and the degree of transformation is measured, for example by dilatometry. Obviously a large number of experiments is required to build up a complete CCT diagram.

Continuous-heating-transformation (CHT) diagram

CPH
Close-packed hexagonal describes a way for atoms (considered as hard spheres) to pack together to fill space. The first layer (A) consists of an hexagonal array of atoms. The next layer (B) sits in the hollows of the first layer. The third layer duplicates layer A, giving an ABAB… structure.

Dilatometer
An instrument for measuring the length of a metal sample during heating and cooling.

DCCT diagram
Deformation continuous cooling transformation

Eutectoid
1) An isothermal reversible reaction in which a solid solution on cooling is converted into two or more intimately mixed solids. The number of solids formed are the same number of components in the system. 2) An alloy having the same chemical composition indicated by the eutectoid point on a equilibrium diagram.

Ferrite
A solid solution of one or more elements in the body-center-cubic phase of iron or steel. Pure iron up to 912º C has a bcc structure and is known as alpha ferrite. Between 1394º C and the melting point of iron the bcc structure is now known as delta ferrite. Also found in carbon steel.

Ferrite with non-aligned second phase (FS(NA))
Ferrite completely surrounding either 1) microphases which are approximately equiaxed and randomly distributed, 2) isolated laths of acicular ferrite.

Ferrite sideplates (FSP)
Sideplates structures growing directly from polygonal ferrite or grain boundary allotriomorphs, i.e. Widmannstatten secondary sideplates.

FCC
Face-centred cubic describes a way in which atoms pack together to fill space. The first layer (A) consists of an hexagonal array of atoms. The next layer (B) sits in the hollows of the first layer. The third layer (C) does not duplicate either A or B layer, giving an ABCABC… structure FCC.

Hardness
Resistance of a material to indentation as measured by such methods as Brinell, Rockwell, and Vickers. The term hardness also refers to stiffness of a material, or its resistance to scratching, abrasion, or cutting.

Isothermal
Pertaining to changes or other phenomena occurring at a constant temperature.

Lath ferrite (LF)
Refers to a predominantly intergranular product resembling bainite which sometimes forms qmongst acicular ferrite or sideplate structures.

Martensite
A generic term used for microstructures formed by diffusionless phase transformations. A constituent found in hardened steel; has a needle like microstructure. Very rapid cooling (quenching) of steel (at about 1000° C per minute) produces a new microstructure, martensite. It is the hardest and most brittle form of steel. Subsequent reheating to about 400° C and holding it for a time (tempering) produces a strong and tough steel with lower hardness and brittleness.

Mf
The temperature at which martensite formation finishes during cooling.

Microhardness
The hardness of microconstituents of a material.

Microstructure
The structure of polished and etched metal and alloy specimens as revealed by the microscope at magnifications over 10 diameters.

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

Pearlite
A lamella aggregate of ferrite and carbide, the structure of pearlite can appear fine or coarse depending on processing. When steel is cooled at the rate of about 400º C per minute austenite crystals change into pearlite (a fine lamellar structure of alternating platelets of ferrite and iron carbide) at about 727º C. Faster cooling produces martensite.

Phase Diagram
A graphic representation of the equilibrium temperature and composition limits of phase fields reactions in an alloy system. In a binary system, temperature is usually the ordinate and composition the abscissa. Ternary and more complex systems require several two-dimensional diagrams to show the temperature - composition variables completely. In alloy systems, pressure is usually considered constant, although it may be treated as an additional variable.

Polygonal ferrite (PF)
Polygonal or equiaxed at low cooling rates. Grain boundary allotriomorph at higher coolig rates.

Proeutectoid
The constituent that separates out of a solid solution before the formation of eutectoid.

Retained austenite (RA)
Hardening of steels requires that the material be heated to a high temperature followed by a quenching and tempering process. During the heating cycle, the room temperature phase is transformed into a face-centered cubic structure known as Austenite. During quenching, the Austenite will then transform into fresh Martensite, which is a very hard, but brittle phase. Thus, a tempering process is almost always undertaken to reduce the brittleness of the steel at the expense of a slight loss in hardness. In real life, however, the heat treeatment process is not as ideal as this. Often, some of the Austenite will be retained after quenching and tempering, which can lead to a degradation in the materials performance.This is due to the fact that the retained Austenite can be transformed into fresh, untempered Martensite by applied stresses while in use. Also, the transformation of the retained Austenite will cause a dimensional instability in the part, leading to QC problems.

Spheroidite
another structure of ferrite and cementite. It is formed by reheating pearlite or bainite to just below the eutectoid temperature to get a softer and more ductile material. The cementite becomes sphere-like particles in a continuous ferrite matrix. There is no change in composition or amounts of the ferrite and cementite.

Sorbite
Structure of steel, resulting from the tempering of martensite. In a truly sorbitic structure, the cementite is completely dispersed in the matrix. The trend is to call this structure tempered martensite.

Tempered martensite
Martensite that has been heated to produce to BCC iron and a fine dispersion of iron carbide.

Transformation diagrams
There are two main types of transformation diagram that are helpful in selecting the optimum steel and processing route to achieve a given set of properties. These are time-temperature transformation (TTT) and continuous cooling transformation (CCT) diagrams. CCT diagrams are generally more appropriate for engineering applications as components are cooled (air cooled, furnace cooled, quenched etc.) from a processing temperature as this is more economic than transferring to a separate furnace for an isothermal treatment.

Time-temperature transformation (TTT) diagrams
measure the rate of transformation at a constant temperature. In other words a sample is austenitised and then cooled rapidly to a lower temperature and held at that temperature whilst the rate of transformation is measured, for example by dilatometry. Obviously a large number of experiments is required to build up a complete TTT diagram.

Time-temperature-austenitisation (TTA) diagram

Time-temperature embrittlement (TTE) diagrams

Time-temperature precipitation (TTP) diagrams

Time-temperature sensitization (TTS) diagrams

Widmanstatten structure
Plate-like structure seen in grains of steel in the course of transformation of a solid solution. A form of microstructure characterised by wedge shaped plates of the lower temperature phase growing at the grain boundaries of the higher temperature phase or from allotriomorphs of the lower temperature phase. Widmanstätten is an example of a displacive transformation. It can form at high temperatures, hence very low driving forces. The formation involves the cooperative growth of two plates which are separated by a low misorientation boundary. The two plates cancel out each other's strain energy.

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