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|PRODUCT DATA of 06: Stainless Steels|
|Material||06: Stainless Steels|
|General Information||Stainless steels - also known as corrosion-resistant steels - have alloying additions specifically to provide a continuous, adherent, self-healing oxide film and so reduce the attack of corrosive media. In addition to corrosion resistance, they also exhibit a number of other properties making them useful engineering materials (oxidation resistance, creep resistance, toughness at low temperature, magnetic or thermal characteristics). This section concentrates on those materials, normally aircraft grades, which can be considered for use in space and introduces precautions for their applications.|
|Use in Spacecraft||Use of stainless steels in spacecraft centre on applications requiring corrosion resistance (e.g. storage and handling of liquids and waste), components within some thermal protection systems and fasteners such as high-reliability, high strength bolts.)|
|Main Categories||Stainless steels contain chromium (at least 12 %) which provides the protective oxide film, plus a number of other alloying elements to enable a range of characteristics. Stainless steels are normally grouped by their metallurgical structure:
|Processing and Assembly||Most conventional processing techniques are applied to steels (e.g. machining,welding and fastening). Care shall be taken with some alloys that the processing does not degrade the microstructure, hence properties. Welding can affect the corrosion resistance of the weld and heat-affected zone (localized reduction of Cr-content) and produce heat distortion of the assembly.Correct choice of filler rod is important. Aircraft specifications for heat-treatments and processing are used.|
|Precautions||Alloys have generally been developed to have maximum corrosion resistance to specific environments. Careful evaluation of the service conditions is needed for successful alloy selection.
Chromium within the alloy can react with carbon and form localized Cr-depleted areas and brittle compounds, normally at grain boundaries. This effect is known as “sensitization” and can have serious consequences for corrosion resistance, especially stress corrosion cracking. “Stabilized” stainless steels have alloying additions (Ti, Mo, Nb) specifically to “tie-up” carbon as carbides and so prevent sensitization (also known as weld decay). Unstabilized, austenitic steels have a service temperature limit of 370 ºC.With the exception of stabilized or low-carbon grades (e.g. 321, 347, 316L, 304L), welded assemblies need solution treatment and quenching after welding.
Austenitic stainless steels can suffer stress corrosion cracking in chloride environments and they can be prone to pitting, crevice corrosion and weld decay unless composition, heat-treatment and service conditions are carefully controlled.
Austenitic stainless steels of the 300-series are generally resistant to stress corrosion cracking. Martensitic stainless steels of the 400-series are more or less susceptible, depending on composition and heat treatment. Precipitation hardening stainless steels vary in susceptibility from extremely high to extremely low, depending on composition and heat treatment. The susceptibility of these materials is particularly sensitive to heat treatment, and special vigilance is required to avoid problems due to SCC.
Stainless steel parts and fabrications shall be cleaned carefully prior to operation in service. Heat treatment can thicken the oxide film to produce scale or deplete the subscale metal of chromium. Welding spatter and flux residues can promote localized corrosion. Embedded carbon-rich materials from machining can react with chromium at high temperatures. Cleaning processes are normally chemical pickling using various combinations of acids, the residues of which also have to be removed thoroughly. Some grades can be susceptible to hydrogen embrittlement resulting from hydrogen pick-up during pickling processes.
|Hazardous and Precluded||Alloys prone to sensitization need careful consideration of their stress corrosion characteristics and service at elevated temperatures.|
|Effects of Space environment||Vacuum poses no special problems. All metals in contact under vacuum conditions or in inert gas have a tendency to cold weld. This phenomenon is enhanced by mechanical rubbing or any other process that can remove or disrupt oxide layers.
|Some Representative Products||European suppliers provide a wide range of stainless steels, all of which are denoted by national and international specifications and standards, including series specifically for aerospace grade materials.
Stainless steels that were evaluated and shown to have a high resistance to stress corrosion cracking are listed in Table A-6 (from ECSS-Q-ST-70-36).
Fastener fabricators or suppliers include: