Chromium and manganese alloyed steels contain, amongst others, alloyed Fe-C alloys. This steel group includes hot and cold work steels and ball and roller bearing steels, respectively.
Bright annealing
Definition according to DIN EN 10052: „Annealing carried out in m medium that allows the original metallic surface finish to be maintained by preventing oxidation of the metal.“
Annealing is a „heat treatment consisting of heating and soaking at a suitable temperature followed by cooling under conditions such that, after return to ambient temperature, the metal will be in a structural state closer to that of equilibrium.“
A medium is an „environment in which the product is placed during a heat treatment operation. The medium can be solid, liquid or gaseous.“
The protective gas consists exclusively or at least predominantly of an inert gas, such as nitrogen - in some cases with an addition of reactive gas components, for example hydrogen, propane or natural gas. The maximum concentration of the reactive gas fractions present in the furnace in this process is 5 %. We use the brand name Neutrotherm.
The hydrogen content in nitrogen-, argon- or helium-hydrogen mixtures exceeds 5 %. We use the brand name Hydrotherm.
Nitrogen can be blended with small percentages of hydrocarbons (methane, propane) or combined with methanol (CH3OH) to create a suitable atmosphere provided it is properly controlled.
For certain annealing processes CO is needed in addition to hydrogen. For this reason, the Endolin process was developed and patented by Messer. Endo gas is blended with nitrogen to get the desired concentration of active gas components and the mixture is fed directly into the furnace.
Neutral annealing
During annealing, the work piece is not chemically affected by the surrounding medium.
The protective gas consists exclusively or at least predominantly of an inert gas, such as nitrogen - in some cases with an addition of reactive gas components, for example hydrogen, propane or natural gas. The maximum concentration of the reactive gas fractions present in the furnace in this process is 5 %. We use the brand name Neutrotherm.
The hydrogen content in nitrogen-, argon- or helium-hydrogen mixtures exceeds 5 %. We use the brand name Hydrotherm.
Nitrogen can be blended with small percentages of hydrocarbons (methane, propane) or combined with methanol (CH3OH) to create a suitable atmosphere provided it is properly controlled.
For certain annealing processes CO is needed in addition to hydrogen. For this reason, the Endolin process was developed and patented by Messer. Endo gas is blended with nitrogen to get the desired concentration of active gas components and the mixture is fed directly into the furnace. Endolin mixtures have about 10 to 15 % higher reduction potential compared to exo gas. Contrary to exo gas, the carbon potential can be regulated in the furnace. A substantial advantage is, however, that endo gas and nitrogen can be fed separately into different ranges of the furnace plant. Thereby substantially higher concentrations of hydrogen and carbon monoxide are adjustable in the heating zone of the furnace and small amounts in the cooling zone, so that in these colder parts of the furnace the Endolin mixture clearly remains below the explosion limit. In this case, even carbon restoration becomes possible.
Definition according to DIN EN 10052: Carbon restoration is a „thermochemical treatment intended to restore the carbon content of the surface layer, decarburized during an earlier treatment.“
Decarburizing, blueing, blackening
Definition according to DIN EN 10052: „Decarburizing is a thermochemical treatment intended to produce decarburization of a ferrous product.“
Definition according to DIN EN 10052: „Blueing is an operation carried out in an oxidizing medium at a temperature such that the polished surface of a ferrous product becomes covered with a thin, continuous, adherent film of blue-colored oxide.“
Definition according to DIN EN 10052: „Blackening is an operation carried out in an oxidizing medium at a temperature such that the polished surface of a ferrous product becomes covered with a thin, continuous, adherent film of dark-colored oxide.“
Steam added to nitrogen/hydrogen mixtures, hydrogen or mixtures of nitrogen with air produce a black protective oxide layers on the surface of component parts as well as blue rust-resisting oxides between 300 and 650 °C.
A special process developed by Messer allows blackening at hardening temperature.
Carburizing
Definition according to DIN EN 10052: „Thermochemical treatment which is applied to a ferrous product in the austenitic state, to obtain a surface enrichment in carbon, which is in solid solution in the austenite. The carburized ferrous product undergoes quench hardening (immediately or later).“
To produce a gaseous carburizing atmosphere, three processes are used industrially:
endothermically produced gas (endo gas),
mixtures of nitrogen and liquid methanol,
mixtures of a gaseous hydrocarbon (natural gas, propane or others) and air.
Sometimes, the high carbon monoxide content of endo gas leads to an internal oxidation of Cr, Mn and Si. In order to avoid these unwanted reactions, a process free of oxygen containing gases was developed.
Low pressure carburizing does not use a “carrier gas”. The process takes place in a vacuum furnace. The carburization is carried out by adding a small controlled amount of a hydrocarbon, like C2H2 or C3H8.
Carbonitriding
Definition according to DIN EN 10052: „Thermochemical treatment which is applied to a ferrous product heated to a temperature above Ac1, to obtain a surface enrichment in carbon and nitrogen, which are in solid solution in the austenite. Generally, this operation is followed immediately by quench hardening.“
Nitrocarburizing
Definition according to DIN EN 10052: „Thermochemical treatment which is applied to a ferrous product in order to produce surface enrichment in nitrogen and carbon, which forms a compound layer. Beneath this compound layer there is a diffusion zone enriched in nitrogen.“
The compound layer is a „surface layer formed during a thermochemical treatment and made up of the chemical compounds formed by the element(s) introduced during the heat treatment and certain elements from the base metal.“
The diffusion zone is a “surface layer formed during a thermochemical treatment containing, in solid solution or where appropriate partially precipitated, the element(s) introduced during the heat treatment. The content of these elements diminishes continuously as the core is approached.“
Messer offers a process particularly adapted to the increasing requirements of the modern gas nitrocarburizing technology. The special feature of this process is a synthetic atmosphere in the furnace with high and well adjustable carbon activity.
Nitriding
Definition according to DIN EN 10052: „Thermochemical treatment which is applied to a ferrous product in order to produce surface enrichment in nitrogen.“
Sintering
Definition according to ISO 3252: Sintering is a ”thermal treatment of a powder or compact at a temperature below the melting point of the main constituent, for the purpose of increasing its strength by bonding together of the particles.”
Powder metal parts sintered in controlled atmospheres result in efficient binder removal, precise carbon control, size control, less sooting and bright surface finish.
It is important to understand the significance of introducing gases into the specific furnace areas where they are most effective. Called zoning, these special atmosphere injection techniques control both the flow pattern and the chemistry of the atmosphere.
Depending on the material to be sintered, in addition to nitrogen/hydrogen mixtures, nitrogen/hydrogen/hydrocarbon or pure hydrogen is recommended.
The Endolin process was developed and patented by Messer. Endo gas is blended with nitrogen to get the desired concentration of active gas components and the mixture is fed directly into the furnace.
Brazing and high-temperature brazing
Brazing is a joining process wherein metals are bonded together using a filler metal with a melting temperature greater than 450 °C, but lower than the melting temperature of the base metal.
High-temperature brazing is flux-free brazing under exclusion of air (vacuum, protective gas) with filler metals whose melting temperature is above 900 °C.
Depending on the base material, two different types of gas atmospheres are used in furnace brazing using flux and inert gas and in high-temperature brazing:
Chemically inert atmospheres which protect the parts being brazed from coming into contact with other gaseous elements, which might react with the metals being joined thereby producing surface films, which might inhibit flowing of, and wetting by the molten brazing alloy.
Chemically active atmospheres, which will react, during the brazing cycle, with any surface films present on either the parts to be joined, or the brazing alloy preform, removing them in the process.
The Endolin process was developed and patented by Messer. Endo gas is blended with nitrogen to get the desired concentration of active gas components and the mixture is fed directly into the furnace.
Cold & Cryogenic Treatment
Cold treatment of steel consists of exposing the ferrous material to subzero temperatures to either impart or enhance specific conditions or properties of the material. Increased strength, greater dimensional or microstructural stability, improved wear resistance, and relief of residual stress are among the benefits of the cold treatment of steel.
Cryogenic treatment, also referred to as deep cryogenic treatment (DCT), is a distinct process that uses extreme cold to modify the performance of materials.
Quenching
Definition according to DIN EN 10052: „Operation which consists of cooling a product more rapidly than in still air.“
For steels with an austenite–ferrite transformation, by varying the cooling rate from extremely slow to extremely fast, the yield strength can be changed from 200 MPa (microstructure of ferrite and carbide after soft annealing) to 2500 MPa (martensitic microstructure). Therefore, to obtain sufficient predictability and reproducibility of the service performance of steel components, correct selection of the cooling rate during heat treatment is important.
Gas quenching, is an alternative to quenching in molten salts, metals or in vaporizable liquids. Factors contributing to the continued interest in gas-quenching technology development include the following: increased legislation for environmental protection, energy savings, no production of toxic or combustible gases, increasing needs for process automation, and greater process flexibility to vary cooling rates. Distortion is reduced due to more uniform cooling rates. A significant advantage of gas quenching is that decarburization is eliminated thus reducing manufacturing cost.
