Metal heat treating glossary
Common heat treating terms
Whether you’re new to the industry or a seasoned professional, understanding key terminology is essential to navigating the world of metal processing. In this guide, you’ll find clear definitions to help you understand the techniques and processes we use to ensure precision and quality in every project.
A
Aging
The process of soaking solution heat treated parts at a moderately elevated temperature (or at room temperature for some alloys and tempers) to enhance strength and corrosion resistance.
AISI
Abbreviation for the American Iron and Steel Institute.
Allotropy
The existence of an element in two or more physical forms. Common elements with this property include carbon, sulfur, and phosphorus.
Alloy
A substance with metallic properties composed of two or more metals.
Annealing
A thermal treatment process used to soften metal, typically for products that require bending or forming.
ASTM International
American Society for Testing and Materials (ASTM) is an international standards organization that develops and publishes voluntary consensus technical standards for materials, products, systems, and services.
Austenite
A solid solution of iron and carbon (and sometimes other elements) in which gamma iron, characterized by a face-centered cubic crystal structure, is the solvent. Austenite is a softer and more malleable state of non-magnetic steel that typically forms when steel is cooled at a slower rate from a transformation temperature.
Austenitizing
The process of forming austenite by heating a ferrous alloy above the transformation range.
AWS
Abbreviation for the American Welding Society.
B
Brittleness
The tendency of a material to break suddenly instead of deforming under load. Examples include cast iron and glass.
Bright Annealing / Bright Hardening
The process of annealing or hardening in a protective medium to prevent oxidation and maintain a bright surface finish.
C
Carburizing
A heat treatment process that introduces carbon into a heated solid ferrous alloy by contact with a carbonaceous material. Typically followed by quenching to produce a hardened surface.
Case Hardening
A process that hardens the surface layer, or "case," of a ferrous alloy to be harder than the core. Common methods include carburizing, cyaniding, carbonitriding, nitriding, flame hardening, and induction hardening.
Cast Iron
Iron containing 2 to 4.5 percent carbon, silicon, and other trace elements, commonly used for casting into molds. Cast iron is somewhat brittle.
Cementite
A compound of iron and carbon, also known as iron carbide.
Cryogenic Treatment
The process of exposing steel to subzero temperatures (typically -85°C or -120°F) to achieve desired properties such as dimensional stability and microstructural integrity. Also known as cold treatment or deep freezing.
D
Decarburization
The loss of carbon from the surface of a ferrous alloy when heated in the presence of a medium (such as oxygen) that reacts with the carbon.
Degreasing
The process of removing oil, grease, or other contaminants from surfaces.
Diffusion
The process of intermingling atoms or particles within a solution, typically moving from areas of high concentration to areas of low concentration.
Drawing
A term sometimes used for the process of tempering hardened steel.
Ductility
The ability of a material to undergo permanent deformation under tension without rupture.
E
Elasticity
The ability of a material to return to its original shape after a load is removed.
Equilibrium
A state of balance where opposing forces or processes are equal and cancel each other out.
F
Ferrite
A solid solution of one or more elements in body-centered cubic iron.
Flame Hardening
A process for hardening the surface of ferrous alloys using an intense flame to heat the surface above the upper critical temperature, followed by quenching.
Forging
The shaping of metal through the application of localized compressive forces, typically by hammering or pressing.
Fracture
A ruptured metal surface that typically shows a crystalline pattern, with fatigue fractures often displaying a smooth, clamshell appearance.
G
Grain
The individual crystals in metals that make up the material’s microstructure.
H
Hardenability
The ability of a ferrous alloy to develop hardness after being heated and quenched.
Hardening
A process of increasing a metal's hardness, typically involving heating and quenching. Specific methods include aging, case hardening, flame hardening, induction hardening, precipitation hardening, and quench hardening.
Hardness
The ability of a metal to resist permanent deformation. Categories include resistance to penetration, abrasion, and elastic deformation.
Homogeneous Carburizing
A process that converts a low-carbon ferrous alloy to one of uniform, higher carbon content throughout its section.
I
Inclusions
Impurities or particles, typically formed during solidification, such as silicates, sulfides, and oxides.
Induction Hardening
A surface hardening process in which the surface of a ferrous workpiece is heated by electromagnetic induction to above the upper critical temperature and then immediately quenched.
M
Macroscopic Details
Structural features on an object large enough to be observed by the naked eye or with low magnification (around 10x).
Macrostructure
The structure of metals as seen in macroscopic examination.
Martensite
An unstable constituent formed when steel is heated and rapidly quenched. Martensite is the hardest of the transformation products of austenite, with a needlelike microstructure.
Metallurgy
The study of metals, their behaviors, properties, and extraction from ores.
Microscopy
The use of a microscope to investigate or study small structures.
Microstructure
The structure of metals as observed under a microscope after polishing and etching.
N
Neutral Hardening
The process of hardening carbon steel by heating to the proper temperature in atmospheres such as nitrogen or nitrogen-methanol, followed by quenching.
Nitriding
A process that introduces nitrogen into the surface of a ferrous alloy at temperatures below Ac1, typically using ammonia or molten cyanide. Quenching is not required to produce a hard case.
Nonferrous
Metals that do not contain iron or iron alloys.
Normalizing
Heating a ferrous alloy to a suitable temperature above the transformation range and then cooling it below the transformation range.
O
Oxidation
The reaction of oxygen with other elements, often resulting in rust or corrosion. Overoxidation can damage metals.
P
Precipitation Hardening
A heat treatment process in which an alloy is hardened by allowing a constituent to precipitate from a supersaturated solid solution at room temperature or slightly elevated temperatures.
Q
Quenching
The rapid cooling of metal alloys to harden them, using media such as air, oil, water, molten metals, or fused salts.
S
SAE
Abbreviation for the Society of Automotive Engineers.
Scale
Surface oxidation that forms on metals when heated in air or in other oxidizing environments.
Soaking
Heating metal to a uniform temperature throughout its mass for a specified period.
Solubility
The extent to which one substance dissolves in another.
Solution Heat Treatment
Heating an alloy to a suitable temperature to dissolve one or more constituents, then rapidly cooling the alloy to hold these constituents in solution.
Spheroidizing
A process in which carbon steel is heated just below the transformation temperature, causing an aggregate of globular carbide to form from microstructures such as pearlite.
Stainless Steel
An alloy of iron containing at least 11 percent chromium, often with nickel, resistant to rusting and corrosion.
Steel
An alloy of iron and other elements, typically carbon. Carbon steel contains less than 2 percent carbon, while alloy steels contain other significant alloying elements like chromium or nickel.
Strain
The deformation of a metal when stress is applied.
Strength
The ability of a metal to resist external forces. This includes tensile, compressive, or shear strength.
Stress
The force applied per unit area on a material. It can be tensile (stretching), compressive (squeezing), shear (sliding), or torsional (twisting).
Stress Relief
Reducing residual stress in metal by heating it to a given temperature and holding it there for a specified time.
T
Tempering
A process of stress-relieving steels hardened by quenching to reduce brittleness and improve toughness.
Thermal Expansion
The increase in a material’s dimensions as it is heated due to the movement of atoms.
Thermal Stress
Shear stress induced by uneven heating or cooling, often occurring when there is a temperature difference between a material's interior and exterior surfaces.
Tool Steel
A group of steels designed for specific uses, such as heat resistance, and capable of being heat treated to produce hardness and wear resistance.
Toughness
The ability of a metal to resist rupture, particularly under impact or when a notch is present. Toughness is tested by measuring the energy absorbed in a standard impact test.
Transformation Temperature
The temperature at which a material transitions from one phase to another, such as from ferrite to austenite in steel.
V
Vacuum Annealing
A process of annealing carried out under subatmospheric pressure.