Steels with a carbon content of up to 0.79 % contain all unalloyed and low-alloyed Fe-C alloys up to the eutectoid point. This steel group includes, amongst others, structural steels, case hardening and nitriding steels, free-cutting steels, heat-treatable steels but also some types of tool steels.
BRIGHT ANNEALING, NEUTRAL ANNEALING
Definition according to DIN EN 10052: „Annealing carried out in a 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.“
During neutral annealing, the work piece is not chemically affected by the surrounding medium.
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 exact and consistent composition of the protective gas is vital for the quality of metal surfaces. Any unwanted chemical reactions with the metal must be reliably prevented. The solution is the Neutrotherm process, which is based on a neutral gas mixture containing less than 5 % hydrogen.
With minimal investment in hardware, the process provides excellent protection against oxidation of the metal, ensuring the surface remains clean.
Messer process = "Neutrotherm"
The Hydrotherm process is based on a gas mixture containing up to 100 % hydrogen. Here, the hydrogen has not only a chemical action, but - thanks to its high thermal conductivity - also performs a physical function. With this process, the annealing capacity of high-convection furnaces can be increased by up to 50 %.
Messer process = "Hydrotherm"
Nitrogen can be blended with small percentages of hydrocarbons (methane, propane) or combined with methanol to create a suitable atmosphere, provided it is properly controlled.
Messer processes = "Variocarb direct" and "Variocarb therm"
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.“
Messer process = "Endolin"
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-coloured 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-coloured 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.
Messer process = "Blackrapid"
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, Messer process = "Variocarb therm"
- 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.
Messer process = "Hydrocarb"
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 acetylene or propane.
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.
Messer process = "Vario-Nitro-Carb Plus"
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.
Messer processes = "Neutrotherm, "Hydrotherm", "Variocarb direct" and "Hydrocarb"
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.
Messer process = "Endolin"
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.
Messer process = "Endolin"
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.
One mechanism, which influences the properties mentioned above and which can be initiated by a cold treatment is the so-called “Retained Austenite Transformation”.
The causes of the formation of retained austenite include:
- Too high a surface carbon content after carburizing has been completed. This is generally caused by a lack of control of the carbon potential of the furnace atmosphere or carburizing medium.
- Too slow of a cooling rate (quenching) of the carburized case.
- Quenching from too high austenitizing temperature.
Retained austenite is soft and has a much lower strength and wear resistance than is seen in freshly formed plate martensite. In addition to this, the fatigue resistance will also have a lower value. Further, the retained austenite at the surface will most certainly reduce the residual compressive stresses.
One condition that is not always recognized is that whatever retained austenite is present within the formed case will progressively transform in two ways:
- Dimensional change in size will be seen, simply because the retained austenite is continually transforming itself to fresh un-tempered martensite and will only stop transforming when there is little or no retained austenite.
- An increase in surface hardness will also occur should retained austenite be present as it transforms to martensite.
Cryogenic treatment, also referred to as deep cryogenic treatment (DCT), is a distinct process that uses extreme cold to modify the performance of materials.
"Cooling chamber"
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.
