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|PRODUCT DATA of 05: Steels|
|General Information||Steels, as a family of materials, offer a wide range of characteristics that find uses in many and varied applications. This section concentrates on those materials, normally aircraft grades, which can be considered for use in space and any precautions that shall be taken for their application.|
|Use in Spacecraft||Steels are used in structural items (e.g. rocket motor casings) and within engineering components (e.g. bearings and springs) in a variety of subsystems and devices.|
Steels are based on alloys of iron and carbon (between 0,05 % and 2 %C). All contain some level of other elements, i.e. even plain carbon steels (up to 1,7 % C) contain manganese up to about 1 % Mn. This results from excess Mn used for deoxidation and desulphurization during smelting. Impurity levels (e.g. phosphorus and sulphur) depend mainly on the smelting and melting processes used, although increased use of remelted scrap metal can introduce other problem elements such as copper. Alloy steels contain one or more additional alloying elements to improve properties and workability.
The tensile strength of plain carbon steels increases with carbon content up to approximately 0,8 %C, reaching a theoretical maximum of about 900 MPa, with a corresponding decrease in ductility. Hardness increases progressively with C-content, so that low- (0,1 % C-0,3 % C) to medium-carbon steels (0,3 % C-0,6 %C) are used for various "engineering" components, whereas high-carbon steels (0,6 % C-0,9 %C) are used for applications requiring hardness and wear resistance.
Alloying additions to plain carbon steels produce a wide range of alloy steels with improved performance. Alloying effects can be microstructure-related: for example, control of transformation effects, control of grain size, carbide precipitation; process-related: workability, heat-treatment, hardenability and weldability; corrosion-related: forming adherent oxide films on the surface . Depending on the level of additions, some elements have effect on all of these.
The tensile strengths attainable from alloy steels depend on the composition, mechanical working and heat-treatment processes. For engineering uses (i.e. materials having a combination of useful properties such as strength, toughness and processability) strengths rarely exceed 1250 MPa. The exceptions are some cold-worked products, e.g. wires, some hardened and tempered items such as ball bearings and some spring steels and “maraging” steels. Where the UTS exceeds 1250 MPa, stress corrosion becomes an issue.
“Maraging” steels (from “martensite-ageing”) contain Ni (either 12 % or 18 % typically) with various combinations of Cr, Co, Mo, Ti and Al and very low levels of carbon (0,03 %). These alloys have a number of benefits: very high tensile strengths (1175 MPa to 2450 MPa); high toughness which remains good at low temperatures; weldability; ease of heat-treatment and machinability. Low strength maraging steels have better resistance to stress corrosion than low alloy steels. However, fatigue and wear resistance tend to be lower than low alloy steels. They are also high-cost materials.
|Processing and Assembly||High quality aircraft steels are normally produced by electric-melting processes. Vacuum-melting is applied to grades for forged heavy-duty aircraft components.
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. Heat treatments can be applied to the bulk of thematerial or used to selectively harden the surface. A wide range of compositional and mechanical surface treatments are available to selectively improve surface properties (e.g. carburising, nitriding, shot peening and thread rolling). Aircraft specifications for heat-treatments and processing are used.
High-strength martensitic steels (UTS =1225 MPa) shall be carefully machined using carbide-tipped tools and other techniques to ensure that the formation of an untempered martensitic structure does not occur on surfaces.
|Precautions||Carbon and low-alloy steels with ultimate tensile strengths below 1225 MPa (180 ksi) are generally resistant to stress corrosion cracking. For applications where the primary loading is compressive or low tensile or with a history of satisfactory performance, materials with UTS =1225 MPa can be accepted providing that their stress corrosion properties were approved.
Some steels have a ductile-brittle transformation which, depending on the alloy composition, can occur within the normal service conditions for some space components. Specifications normally include a value for the impact energy.
Depending on the alloy, some steels exhibit poor weldability. This is linked to the carbon content (or carbon-equivalent value) and can produce brittleness in the weld affected zone. Steels are prone to corrosion in atmospheric and acidic aqueous solutions. Some strong acids can be handled by low-carbon steels (mild steel), although a careful evaluation of the concentration ranges is needed. Alkaline solutions have a slow corrosion rate (owing to a passivation-effect), but corrosion rates are fairly high in hot, high alkali concentrations. Low-alloy steels, depending on the composition, tend to have better resistance to atmospheric corrosion. High-alloy steels with nickel contents >3 % show improved resistance to atmospheric and marine environments, although Cr-levels can promote pitting in some conditions. Stress corrosion cracking occurs in steels in hot (>40 ºC approx.) caustic solutions and in some other chemical solutions (ammonia, nitrate, hydrogen-sulphide containing).
Higher strength steels are also prone to SCC in seawater and other chloride solutions. High-strength steels are susceptible to hydrogen embrittlement resulting from hydrogen pick-up during plating and pickling processes (or excessive cathodic protection). Such problems in parts are normally alleviated by a post-process baking procedure.
|Hazardous and Precluded||Platings on steels commonly used in terrestrial applications for improved corrosion resistance can be unsuitable for space. These include zinc, cadmium or other volatile metals|
|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 which can remove or disrupt oxide layers.
|Some Representative Products||European suppliers provide a wide range of steels, all of which are denoted by national and international specifications and standards, including series specifically for aerospace grade materials.
Steels that were evaluated and shown to have a high resistance to stress corrosion cracking are listed in Table A-5 (from ECSS-Q-ST-70-36