HARDEN TERMS
Alphabetical listing of technical terms and process names from the hardening world.
Various technical terms and concepts as used in the heat treatment of tool steels. Some are not directly related, but are included because the toolmaker may be indirectly affected.
Air circulation ovens – equipped with a fan to ensure good heat transfer even at lower temperatures. Temperature range approx. 50 – 650 ºC.
Austenite – structure of steel at curing temperature, so just before startle (exception is stainless steel).
Bainite harden – after austenitizing, cool in a warm medium of approx. 240 – 350 ºC and then keep the workpieces at this temperature for a long time (60 – 180 minutes). Then cool further in the air to room temperature, with the aim of achieving maximum toughness. Tempering is no longer necessary after this.
Carbon – burning the carbon from the steel surface in a red-hot state, resulting in, among other things, an unsightly appearance (flaking) and a hardness that is too low. Therefore, the air present must be kept as far away from the steel surface as possible, for example by heating under shielding gas or under vacuum.
Carboning – (also called setting-hardening or cementation), low-carbon steel is diffused into the skin at approx. 900 ºC carbon (through a gas, powder or salt). After rapid cooling, a hard layer with a thickness of up to approx. 3 mm is formed.
Carbonitration – absorption of both carbon and nitrogen in the steel, where carbon absorption dominates. The working temperature is approx. 870ºC and a layer thickness of approx. 0.2 mm is quickly obtained.
Cementite – connection between iron and carbon. Many elements can bond with carbon, however a limited number form so-called carbides, the name of which is reserved for compounds of carbon with metals (eg chromium carbides, vanadium carbides, etc.).
Chamber ovens – gas or electrically heated. These oven types are widely used in tool shops, annealing companies and many other industries. CARBON – indispensable element required to harden steel. The soft carbon (graphite) is not present as such in the steel, but has combined with the iron (ferrite) present to form iron carbide.
Cleared – the stresses created by curing are relieved by heating for another half to two hours at about 200-300 ºC with little loss of hardness.
Curing depth – depending on the alloy, workpiece size and cooling medium, steel hardens to the core or to a certain depth. The curing depth is determined using the Jominy test.
Deter – cool at high speed in water or oil.
Deep cooling – transfer steel immediately after hardening into, for example, liquid nitrogen to achieve the most complete transformation of austenite into martensite.
Ferriet – soft iron crystals.
Ferritration – Nitrocarburizing in a whirlpool bath oven.
Finishing – Highly tempered after curing (at approx. 500 – 630 ºC), resulting in a “tough-hard” fine breeding structure. The process finds a huge area of application in machine and engine construction. AGING – expose hardened steel to temperatures up to approx. 120 ºC. Cool down in cold water in the meantime to avoid as much as possible slow changes in the fine structure, which could lead to small dimensional changes in the long term (eg calibers). A more powerful effect is achieved with deep cooling.
Flame hardening – where the steel surface to a depth ranging from 2 to 10 mm is quickly heated with a burner and immediately quenched with water sprinklers. The process is usually used in machine building with lower alloy steels such as C45 or 1.7033. Edge knives and malleable or nodular cast iron can also be flame- or induction-hardened.
Gas nitration – only nitrogen uptake to a depth of approx. 0.5 mm, the process takes a long time and can only be used with special nitriding samples.
Hardening – austenitizing and cooling at such a speed that hardness increases in a large part of the workpiece due to martensite formation.
Heating – heating until the desired temperature in the outer layer is reached.
Induction hardening – via a current coil, due to the changing magnetic field and the resistance of the steel, heat is generated and the steel can be hardened to a certain depth below the surface (roughly from 1 to 5 mm). This process is suitable for relatively low curing depths.
Ledeburite – a surplus of carbon that is present outside the cementite as so-called double or complex carbides. Ledeburite is difficult to dissolve and very hard.
Martensite – structure of steel after it has hardened.
Nitrocarboning – incorporation of both nitrogen and carbon into the steel, with nitrogen uptake dominating. Many variants are possible here. These processes are referred to as salt bath nitriding, soft nitriding, powder nitriding, nikotrating, tenifering, cyanating, ferriding, etc.
Nitrate – allow nitrogen to diffuse into the surface of steel, during a annealing process (approx. 500-550 ºC) from a nitrogen-emitting medium, creating a thin (in micron range), particularly wear-resistant layer.
Perlite – starting structure of mild steel, consisting of Ferrite and iron / carbon carbides (the so-called Cementite).
Protected hardening – In ovens equipped with a retort, the workpiece is hardened under a protective gas atmosphere, so that the surface is not affected. Instead of inert gases, so-called active gases can also be supplied with which, for example, it can be nitrated or carbonated.
Polymers – synthetic deterrents with adjustable cooling speed.
Restaustenite – During curing, not all austenite is converted to hard martensite (depending on alloy and chosen curing temperature), leaving a certain percentage of residual austenite.
Salt bath ovens – where the steel is preheated in a hot air oven at approx. 500 ºC in a crucible with a special liquid salt, after which it is brought to a curing temperature. Particularly intensive and accurate heat transfer is achieved due to the direct contact with the salt and no decarburization can take place, even during quenching. A protective salt film always remains on the workpiece. The medium can be well regulated and provide both an inert and a carburizing atmosphere.
Shaft ovens – Cylindrical section ovens that are often recessed into the floor. Specifically suitable for heat treating long and rod-shaped workpieces.
Shape change (warping) – change in the size or shape of a workpiece by heat treatment.
Silver steel – has nothing to do with silver, but is on high-gloss ground rod material from approx. 1 to 30 mm round, which is used for the production of special drills, shafts and small parts in instrument and equipment construction. Usually available in two grades: unalloyed with approx. 1% C (eg 1.1545) and alloyed as 1.2210
Stepped curing – After austenitizing, cool in a warm medium of approx. 180-200 ° C, let it warm for a short time (approx. 5-15 minutes) and then allow to cool in air. After this still has to be tempered. The aim of this method is: minimal deformation and avoidance of crack risks.
Swirl devices – comparable in heat transfer to salt baths. Instead of liquid salt, aluminum oxide powder is used, which is flushed with compressed air after it has been heated. The powder starts to move (starts to swirl) and then behaves like a liquid. It is an environmentally friendly process and it is also possible to supply active gases with which it is possible to carbonate or nitrocarbonate.
Thermochemic hardening processes – collective term for processes in which other elements diffuse from the outside into the steel surface e.g. nitriding, carburizing, inchromer etc.