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Austenitic Stainless Steel
Description of material
AISN is a Carbon-Chromium-Nickel austenitic stainless steel whose Nitrogen content increases strength with little influence on toughness. It is a variation of the well-known type 304 stainless steel, but offers more corrosion resistance.
AISN is suitable for the fabrication of many products such as flanges, fittings, valves, bolting, pump shafts and many organic chemicals and parts working in mild to medium corrosive environments. In addition, this grade is widely used in pressure vessels, chemical and petrochemical and petroleum refining plant. AISN is also used for many marine applications and boat propeller shafting, but for this last use see also Marinox 19.
Argon Oxygen Decarburization
Generally, AISN has the general corrosion resistance of type 304 but offers a better resistance to pitting and crevice corrosion. Nevertheless, pitting and crevice corrosion may occur in chloride environments if concentration, pH and temperature are at determinate levels. AISN, as with all standard austenitic grades, could suffer from stress corrosion cracking above certain levels of stress. Very strain hardened structures increase the risk of this form of corrosion mainly in aggressive mineral acid and hot alkaline solutions. It should be noted that this grade, as for every kind of stainless steel, surfaces should be free of contaminant and scale, heat tint, and passivated for optimum resistance to corrosion.
AISN is readily fabricated by cold working operations such as cold drawing and bending, but should only be used for a moderate amount of cold heading, because its chemical balance does not allow to obtain a soft strain hardening structure after cold deformation, due to a high CWHF (Cold Working Hardening Factor) mainly due to its high Nitrogen content but also its Carbon content as well. This could result in a rapid die wear.
AISN has the typical machinability of austenitic structures strengthened by Nitrogen and some difficulties could happen in turning, threading and milling processes due to their capacity to cold work harden. Austenitic grades are different from Ferritic and Alloy steels and require more rigid and powerful machines in addition to the correct choice of tools, coatings and cutting fluids. The Austenite structure is prone to transform in to α’Martensite caused by strain hardening of the tool on the surface of the work piece. The knowledge of this behavior must be correctly considered when a piece requires two or several cutting steps to be finished. The layer of α’Martensite is very hard and, if the subsequent turning or milling processes work on this hardened layer, a rapid tool wear could happen. The tool must work under this layer. Even if the structure of AISN is micro - resulphured which offers a little advantage in chip breaking ability, the strain hardening effect due to Nitrogen strongly influences these kinds of operations.
Due to its Carbon content, a PWHT should be applied in order to avoid intergranular corrosion caused by carbide precipitation on grain boundaries in HAZ. Moreover, the Cr/Ni equivalent balance of the supplied product should be evaluated to avoid the risk of solidification cracks in the fused-zone of autogenous welds. In the case of filler metal welding, a filler with a matching composition of AISN is recommended to maintain weld steel properties.
AISN offers a very good hot workability and is usually supplied as billets, blooms, or ingots. No preheating is required. In Primary hot transformation processes, a high temperature homogenization of large ingots and dynamic recrystallization parameters should be rightly evaluated. In the case of open die forging of large ingots and shapes, AISN offers a good hot plasticity if a suitable soaking and a right temperature are applied. In Secondary hot transformation processes, such as extrusion, rolling or close die forging, temperatures, strain and strain rate should be well considered because they influence the properties of the austenitic structure. Suitable strain in terms of section reduction ( for instance: 15-30%) at a lower range of hot working temperature is recommended in order to obtain a fine grain austenitic structure which is very important for mechanical , fatigue and corrosion resistance properties and makes it easier for ultrasonic testing to detect small indications as required by several International Norms. Small forgings can be cooled rapidly in air or water. It should be noted that AISN has a hot hardness higher than typical 304/304L series steels.