Glossary
Heat Treatment & Metal Finishing Glossary
Plain-language definitions of the metallurgy, hardening and plating terms used across our services and knowledge base.
A quick reference for engineers and buyers working with hardened and finished small parts — screws, nuts, bolts and washers. Terms are grouped by topic; follow a link to read more where we cover it in depth.
Microstructure & phases
- Austenite
- A face-centred-cubic (FCC) phase of iron that is stable at high temperature and can dissolve a large amount of carbon. Steel is heated into the austenite range before quenching so it can harden.Learn more
- Bainite
- A fine mixture of ferrite and carbide formed at cooling rates between those that give pearlite and martensite, offering a useful balance of strength and toughness.Learn more
- Cementite
- Iron carbide (Fe3C), a hard and brittle compound that adds strength and wear resistance when dispersed through the steel.Learn more
- Ferrite
- A soft, ductile body-centred-cubic (BCC) phase of iron that holds very little carbon. It is the main phase in low-carbon steel in the annealed condition.Learn more
- Grain size
- The average size of the crystals (grains) in a metal. Finer grains generally improve both strength and toughness.Learn more
- Martensite
- A very hard, brittle structure formed when austenite is cooled fast enough to trap carbon in a distorted lattice. It gives hardened steel its strength and is normally tempered afterwards to reduce brittleness.Learn more
- Microstructure
- The arrangement of phases and grains in a metal as seen under a microscope. It determines hardness, strength and toughness.Learn more
- Pearlite
- A layered mixture of ferrite and cementite formed when austenite cools at a moderate rate. It provides medium strength with reasonable ductility.Learn more
- Retained austenite
- Austenite that fails to transform to martensite during quenching and stays in the hardened steel. Too much can lower hardness and cause later dimensional change; sub-zero treatment reduces it.Learn more
Heat-treatment processes
- Annealing
- A slow heat-and-cool cycle that softens steel, relieves stress and improves machinability by producing a coarse, stable structure.Learn more
- Carbonitriding
- A case-hardening process similar to carburizing but with nitrogen added along with carbon, allowing lower temperatures and improved hardenability of the case.Learn more
- Carburizing
- A case-hardening process that diffuses carbon into the surface of low-carbon steel at high temperature (about 850-950 C), then quenches it to form a hard case over a tough core.Learn more
- Case hardening
- Hardening only the surface layer of a part while leaving a tough core, usually by adding carbon or nitrogen to the surface before quenching.Learn more
- Effective case depth
- The depth from the surface to the point where hardness falls to a defined limit (commonly 550 HV), used to specify and verify a hardened case.Learn more
- Hardenability
- The depth to which a steel can be hardened by quenching. It is a property of the alloy and is not the same as maximum hardness; higher-alloy steels harden deeper.Learn more
- Mesh-belt furnace
- A continuous conveyor furnace in which small parts travel through on a woven metal belt, ideal for high-volume, uniform heat treatment of fasteners.Learn more
- Nitriding
- A lower-temperature case-hardening process that diffuses nitrogen into the surface to form hard nitrides, giving good wear and fatigue resistance with little distortion and no quench.Learn more
- Normalizing
- Heating steel above its transformation range and cooling in still air to refine grain size and give uniform properties.Learn more
- Quenching
- Rapid cooling of steel from the austenite range in oil, water, polymer or gas to form hard martensite.Learn more
- Spheroidize annealing
- A long, low-temperature anneal that rounds the carbides into spheres, giving the softest and most machinable condition for high-carbon steel.Learn more
- Sub-zero treatment
- Cooling hardened steel well below 0 C (deep freeze) to convert retained austenite to martensite, stabilising dimensions and raising hardness.Learn more
- Tempering
- Reheating quenched steel to a moderate temperature to reduce brittleness and internal stress, trading a little hardness for much greater toughness.Learn more
- Through hardening
- Hardening a part uniformly from surface to core, used where the whole cross-section must carry load. Best suited to medium- and high-carbon alloy steels.Learn more
Hardness & testing
- HB (Brinell)
- A hardness scale using a hard ball indenter over a larger area, suited to softer metals and coarse-structured materials.Learn more
- HRC (Rockwell C)
- A hardness scale that measures how deep a diamond cone penetrates under load. It is widely used for hardened steel, typically in the 20-65 HRC range.Learn more
- HV (Vickers)
- A hardness scale using a diamond pyramid indenter. It is versatile across soft to hard materials and is used for case-depth and thin-coating measurement.Learn more
Plating & finishing
- Barrel plating
- A plating method in which bulk small parts tumble together in a rotating perforated barrel, efficient and low-cost for fasteners.Learn more
- Black oxide
- A conversion process that forms a thin black magnetite film on steel for a dark finish and mild corrosion resistance, with almost no dimensional change.Learn more
- Chromate conversion
- A post-plating chemical treatment that forms a thin protective film on zinc, boosting corrosion resistance and adding colour (white, blue or yellow/rainbow).Learn more
- Coating thickness
- The thickness of a plated or coated layer, often measured in micrometres. It is a key specification that affects corrosion life and thread fit.Learn more
- Cr3+ (trivalent chromium)
- The Cr(III) chemistry used for modern, RoHS/REACH-compliant chromate passivation of zinc, replacing the toxic hexavalent version.Learn more
- Cr6+ (hexavalent chromium)
- The older Cr(VI) passivation chemistry, now restricted under RoHS/REACH because it is toxic and carcinogenic, and replaced by Cr3+.Learn more
- Passivation
- A treatment that forms a thin protective oxide film. For stainless steel it restores corrosion resistance; in zinc plating the term refers to the chromate conversion step.Learn more
- Phosphate coating (zinc / manganese)
- A conversion coating of zinc or manganese phosphate that forms a crystalline layer for corrosion protection, wear resistance and paint/oil adhesion.Learn more
- Rack plating
- A plating method in which parts are held on a fixture (rack), used for larger, delicate or precisely-positioned parts to control coating uniformity.Learn more
- Zinc plating (electroplating)
- Electrodeposition of a zinc layer onto steel parts to provide sacrificial corrosion protection, usually followed by a chromate passivation.Learn more
Properties, standards & defects
- De-embrittlement baking
- Heating plated high-strength parts (typically around 200 C for several hours) soon after plating to drive out absorbed hydrogen and prevent embrittlement.Learn more
- Decarburization
- Loss of carbon from the steel surface during high-temperature processing, which lowers surface hardness and strength if it is not controlled.Learn more
- Distortion
- Dimensional change or warping of a part during heat treatment caused by thermal and transformation stresses; controlled by fixturing, part design and process choice.Learn more
- Hydrogen embrittlement
- A loss of ductility caused by hydrogen absorbed during pickling or plating, which can crack high-strength fasteners under load.Learn more
- Proof load
- The maximum tensile force a fastener must withstand without permanent stretch, defined by its property class.Learn more
- Property class (ISO 898-1)
- The strength grade of a steel fastener, such as 8.8, 10.9 or 12.9, defined by ISO 898-1 and encoding its tensile and yield strength.Learn more
- Quench crack
- A crack formed during quenching from thermal and transformation stresses, often linked to sharp corners, high carbon or too severe a quench.Learn more
- Red rust
- Reddish iron-oxide corrosion of the base steel that appears once the protective coating is consumed, and is used as the failure point in salt spray tests.Learn more
- Salt spray test (ASTM B117 / ISO 9227)
- An accelerated corrosion test that exposes parts to a salt-fog chamber. The time to white rust or red rust indicates the durability of the coating.Learn more
- Tensile strength
- The maximum stress a material can withstand before it fractures, usually given in megapascals (MPa).Learn more
- White rust
- White, powdery zinc corrosion product that appears first as a zinc coating begins to corrode, before the base steel is attacked.Learn more
- Yield strength
- The stress at which a material begins to deform permanently. Below it the part springs back to its original shape.Learn more