From quenching to vacuum hardening — the key technical terms of heat treatment, briefly explained.
From quenching to vacuum hardening — the key technical terms of heat treatment, briefly explained.
Austenite – microstructure of the steel at hardening temperature, i.e. shortly before quenching (exception: stainless steel).
Austenitizing – reaching the hardening temperature and holding this temperature for a specified period of time.
Bainite hardening / Austempering – after austenitizing, the workpieces are cooled in a hot medium at approx. 240–350 ºC and then held at this temperature for an extended period. They are subsequently cooled further in air to room temperature in order to achieve maximum toughness.
Carbon – an essential element required for hardening steel. Soft carbon (graphite) is not present as such in the steel; instead, it combines with the iron present (ferrite) to form iron carbide.
Carbonitriding – the incorporation of carbon and nitrogen into the steel, with carbon absorption predominating. The operating temperature is approx. 870 ºC, and a layer thickness of approx. 0.2 mm can be achieved quickly.
Carburizing – (part of the case-hardening process) Carbon is introduced into the surface layer of a low-carbon steel at approx. 900 °C (via gas, powder, or salt). After subsequent hardening by rapid cooling, a hard layer up to 3 mm thick is produced.
Chamber furnaces – heated by gas or electricity. These furnace types are widely used in toolmaking, annealing, and many other industries.
Cryogenic treatment – immersing the steel in liquid nitrogen immediately after hardening, e.g. to achieve the most complete possible transformation of austenite into martensite.
Decarburization – burning carbon out of the steel surface while in the heated state, which causes, among other things, an unattractive appearance (flaking) and insufficient hardness. Therefore, the available air should be kept away from the steel surface as much as possible, e.g. by heating under protective gas or under vacuum.
Ferrite – structural constituent of steel or cast iron.
Gas nitriding – only nitrogen is introduced to a depth of approx. 0.5 mm. The process takes a very long time and is only applicable to special nitriding steels.
Gas nitrocarburizing – only nitrogen and carbon dioxide are introduced to a depth of approx. 0.5 mm. The process takes a very long time and is only applicable to special nitriding steels.
Gas nitrocarburizing & oxidation – only nitrogen and carbon are introduced to a depth of approx. 0.5 mm. The process takes a very long time and is only applicable to special nitriding steels. Oxidation is then carried out.
Hardening – austenitizing and cooling at a rate such that a significant portion of the workpiece experiences an increase in hardness through the formation of martensite.
Hardening depth – depending on the alloy, workpiece size, and cooling medium, the steel is hardened either through to the core or to a specific depth. The hardening depth is determined using the Jominy test.
Heating – heating until the desired temperature is reached in the outer layer.
Induction hardening – heat is generated via an inductor or copper coil through the alternating magnetic field and the electrical resistance of the steel, allowing the steel to be hardened to a specific depth below the surface (approx. 1 to 5 mm).
Martensite – microstructure of steel after hardening.
Pearlite – initial structure of steel in the soft condition, consisting of ferrite and iron/carbon carbides (so-called cementite).
Quenching – rapid cooling in water, oil, or polymer.
Retained austenite – during hardening, not all of the austenite is transformed into hard martensite (depending on the alloy and selected hardening temperature), so a certain percentage of retained austenite remains.
Salt bath furnaces – after preheating, the steel is immersed in a crucible containing a special molten salt and then brought to hardening temperature. Direct contact with the salt provides particularly intensive and precise heat transfer, and decarburization cannot occur during quenching either, as a protective salt film always remains on the workpiece. The medium can be regulated effectively, allowing both an inert and a carburizing atmosphere to be created.
Shaft furnaces – furnaces with a cylindrical cross-section that are often recessed into the floor. They are particularly suitable for the heat treatment of long and rod-shaped workpieces.
Stress relieving – during stress relieving, residual stresses present in components are reduced. These can be caused by manufacturing processes such as welding, milling, turning, or forming. These stresses can lead to distortion, cracking, or component failure.
Surface oxidation – this refers to scaling, which occurs when steel surfaces react with oxygen at temperatures from 200°C onward.
Tempering – the stresses created during hardening are relieved by reheating at approx. 200–300 ºC for about half an hour to two hours, resulting in a slight loss of hardness.
Vacuum hardening – the component is heated in a furnace with a vacuum retort. Since the air is pumped out of the retort, the surface remains completely clean and bright. This environmentally friendly hardening method is becoming increasingly widespread. After the workpiece has been austenitized under vacuum, it is quenched under high pressure with nitrogen. This achieves cooling rates at which oil cooling can be replaced. For carburizing, reactive gases such as acetylene can also be introduced.