Hardening steel means increasing its mechanical resistance by selective modification and conversion of its microstructure. The treatment is conducted by heating and subsequent rapid cooling. If metal is plastically deformed, dislocations spread out within the workpiece. In order to increase strength, measures must be taken to impede the movement of dislocations.
The most important hardening process is the transformation hardening. The workpiece is heated to an extent that α-iron (ferrite) is transformed into γ-iron (austenite) at room temperature. In austenite much more carbon can be solved than in ferrite. By dissolving the existing cementite (Fe3C) its carbon dissolves in austenite. If the carbon-rich austenite is quenched, there won’t be a segregation in ferrite and cementite because of kinetic inhibition (diffusion takes time). The iron lattice can’t transform into the body-centered cubic α-iron because of the „trapped“ carbon atoms. It is shearing instead into a distorted tetragonal body-centered cubic lattice (martensite) to which it is braced by the carbon.
For this kind of hardening the cooling rate is most important. The larger the supercooling (temperature difference), the more martensite is formed. The conversion speed is controlled by different cooling media (water, oil or air). The chemical composition of the steel is also important. Because of its high diffusion rate carbon contributes best to the hardening capacity. However, the substitutional alloying elements, such as chromium, determine the hardenability of the material. With small parts/large quenching rates a hardenability covering the entire cross section of the workpiece is achieved. To be hardened steel must contain at least 0.2 % carbon.
The third mechanism is strain hardening which arises during cold forming. By increasing the dislocation density in the structure the sliding processes are hindered. This increases strength and is therefore called strain hardening. Strain hardening is particularly used for non-ferrous metal alloys (e. g. bronze) and solid solution alloys.
Heat treatment is a combination of hardening and tempering steel in order to make it strong and tough. The hardening is carried out by rapid cooling from the austenite. The material should be heated at least 4 degrees Celsius per minute. The temperatures for hardening are above the temperature of the complete dissolution of ferrite: for hypoeutectoid materials 30 to 50 degrees Celsius above the transition line Ac3, for hypereutectoid materials above the Ac1 transformation point (about 750 to 900 degrees Celsius). The holding time at the hardening temperature is about 20 + (workpiece thickness D (mm)/2) minutes. The following quenching process usually produces a martensitic structure, in some materials, it is also bainite or a mixture of martensite and bainite. These structures achieve the highest possible hardness for the respective kind of steel. Depending on the material either water, oil, salt bath or air are suitable for quenching. In general, steels with low carbon and low alloy content are cooled more abruptly than those with more carbon and a higher content of alloying elements.
Depending on the steel and end use the tempering process starts at temperatures between 150 and 700 degrees Celsius. Depending on the required mechanical properties of the workpieces they are held at that temperature for varying periods of time and they are generally air quenched. As a rule of thumb for tempering: For hardening twice the holding period as the holding period of the material on the austenitizing temperature.
For isothermal heat treatments both operations are summarized: The workpieces are brought directly from the hardening temperature to the tempering bath, left there a certain period of time and are then quenched definitely.
With induction hardening mainly complicated shaped parts can be hardened partially before quenching. Possibly, quenching isn’t even necessary if the heat can drain fast enough into the rest of the still cool workpiece. Mainly tempered steels reach values that come close to conventional curing. In terms of accuracy, controllability and availability, it is surpassed only by the laser beam hardening. Induction hardening is primarily used in tool making. With pliers for example, only the cutting edges are induction hardened, as they require a higher hardness than the complete tool.
There is also the precipitation by the temperature-dependent solubility of the iron lattice for certain foreign atoms. They are precipitated during quenching and brace the crystal lattice, such as cementite.
Carburizing is a process of steel heat treatment. The carbon content of steel is low which is why they are difficult to harden. Carburizing therefore is meant to enrich them with carbon and thus make hardening possible. Usually only the surface layer is enriched with carbon to gain more martensite here than in the core and to achieve a hard surface layer. The core mostly is to remain viscous and soft.
Tempering is a heat treatment process, in which a material is heated selectively to affect its properties, particularly, to reduce tension. Large-scale tempering is found in machining steel, aluminium and other non-ferrous metals or alloys as well as in glass production.
Nitriding is a process for surface hardening. Nitrogen is used. The result is a surface layer that is resistant to about 500 degrees Celsius. Possible methods: baking nitriding, gas nitriding, plasma nitriding.
The manufacturing process is generally carried out at temperatures from 500 to 520 degrees Celsius for treatment times from 1 to 100 hours. The core of the material remains ferritic, the formation of austenite near to the surface is avoided by diffusing nitrogen into the core. An extremely hard superficial compound layer (ε- and γ-iron nitrides) forms and – depending on the treatment period – can become 10 to 30 µm thick, with more or less strongly formed pores which in turn can be used e. g. as carrier for lubricants. Nitriding without compound layer formation, for instance, is possible for later chemical or galvanic coating. Under the compound layer there is a diffusion zone in which nitrogen is stored at a certain depth of the ferritic metal matrix. This nitrogen which is stored in solid solution leads to an increase in the endurance limit. The so-called nitriding depth is defined by the limit hardness. The limit hardness is 50 HV higher than the core hardness of the workpiece. A particularly high hardness in the diffusion zone can be reached with so-called nitriding steels.
It is possible to oxidate the compound layer to increase the protection against corrosion. This generally happens through steam which corrodes the iron content and forms an oxidic protection layer.
There are three main methods: Gas nitriding, liquid or salt bath nitriding and plasma nitriding. In salt bath nitriding it is possible to nitrate fractionally as the workpieces can be dipped partially. In plasma nitriding it can be covered mechanically by a clamping device, for example.