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Austenitic Stainless Steel
Description of material
AISLF is a low Carbon Cr –Ni austenitic stainless steel with good general corrosion resistance as well as good intergranular resistance corrosion after welding processes. This grade has been designed in order to offer a good hot plasticity (forge-ability) thanks to an exact chemical balance of elements such as Carbon, Nitrogen and Nickel.
AISLF is suitable for the fabrication of many products such as flanges, valves, bolting, pump shafts, food /beverages industry equipment, storage tanks, many organic chemicals and parts working in mild to medium corrosive environments.
Argon Oxygen Decarburization
AISLF is resistant to fresh water, many organic chemicals and inorganic compounds, atmospheric corrosion, rural applications and sterilizing solutions where the chloride is low. Pitting and crevice corrosion may occur in chloride environments if concentration, pH and temperature are at determinate levels. As with other standard austenitic grades, AISLF suffers from stress corrosion cracking about forty degrees (C°) above the room temperature and above certain levels of stress and halogen concentration. Very strain hardened structures increase the risk of stress corrosion cracking, but its intergranular corrosion resistance is good. 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.
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 machined 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. AISLF has a chemical balance for a suitable machining thanks to its micro re- sulphured structure.
AISLF has a special chemical composition which helps to avoid solidification cracks in the fused-zone of autogenous welds due to a suitable Ferrite balance. In the case of filler metal welding, the typical ER for type 304 group steels could be used. AISLF can be welded without PWHT, such as annealing and fast cooling, due to its low carbon content which avoids the precipitation of Cr-Carbide on the grain boundaries. However, in the case of aggressive environments, a PWHT should be considered.
AISLF is specifically designed for hot working 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, AISLF 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 rates 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.