Stainless steels are steels with at least 10.5 % of chromium and max. 1.2 % of carbon necessary to ensure the build-up of a self-healing oxide layer which provides the alloy’s corrosion resistance.
The composition of the alloying elements greatly influences the metallurgical structure of stainless steel and defines four main families, each with its typical mechanical, physical and chemical properties.
- Austenitic stainless steels: Fe-Cr-Ni, C < 0.1 % (non-magnetic)
- Ferritic stainless steels: Fe-Cr (> 10.5 %), C < 0.1 % (magnetic)
- Duplex stainless steels: Fe-Cr-Ni, combined austenitic-ferritic structure, (magnetic)
- Martensitic stainless steels: Fe-Cr, C > 0.1 % (magnetic and hardenable)
Furthermore, there are also precipitation hardening grades and superalloy grades.
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 austenitic grades are annealed under pure hydrogen or hydrogen/nitrogen mixtures. Depending on the chemical composition for ferritic, duplex and martensitic grades argon, argon/hydrogen, hydrogen/nitrogen mixtures are used.
Care should be taken when selecting the protective or reactive gas as the steels can pick-up hydrogen and/or nitrogen. This may lead to embrittlement phenomena and cracking.
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"
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 (argon)/hydrogen mixtures or hydrogen produce 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"
After surface activation, low temperature plasma carburizing is possible for some austenitic grades to improve performance in applications demanding resistance to wear, erosion, and fatigue.
To enhance hardness and wear resistance, for some austenitic grades, for ferritic and martensitic grades plasma nitrocarburizing is possible.
Stainless steels can be nitrided in plasma at temperatures below 420 °C.
In so-called solution nitriding, nitrogen is dissolved in the surface of the stainless steel at a temperature of about 1100 °C and a nitrogen pressure of 0.1 to 3 bar.
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, 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 hydrogen/nitrogen mixtures, hydrogen/argon or pure hydrogen is recommended.
Messer processes = "Neutrotherm" and "Hydrotherm"
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 that 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.
Messer processes = "Neutrotherm" and "Hydrotherm"
Cold treatment of steel consists of exposing higher-carbon martensitic grades 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 stresses are among the benefits of the cold treatment of steel.
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.