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Austenitic Stainless Steel
Description of material
AISTF is a Titanium stabilized austenitic stainless steel with a good general corrosion resistance as well as very 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, Titanium and Nickel.
AISTF 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. This grade is chosen in the case of welding processes and in applications where intermittent heating up to 850 C° can happen.
Argon Oxygen Decarburization
AISTF is resistant to fresh water, several 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, AISTF suffers from stress corrosion cracking about forty degrees (C°) above room temperature and with certain levels of stress and halogen concentration. Strain hardened structures increase the risk of stress corrosion cracking. In the case of high temperature service, a stabilizing heat treatment should be carried out to improve the intergranular corrosion resistance. 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 into 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. AISTF has a chemical balance for a suitable machining thanks to its micro-resulphured structure.
AISTF has a special chemical composition which helps to avoid solidification cracks in the fused-zone of autogenous welds due to a special Ferrite balance. In case of filler metal welding, E /ER 347 group could be used. PWHT is generally not necessary. Stabilization heat treatment should be carried out in case of high temperature applications.
AISTF 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, AISTF 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 austenitic structure. Suitable strain in terms of section reduction ( for instance: 20-30%) at a lower range of hot working temperatures is recommended in order to obtain a fine grain austenitic structure which is very important for mechanical , fatigue and corrosions resistance properties and makes it easier for ultrasonic testing easier to detect small indications as required by several International Norms. Small forgings can be cooled rapidly in air or water.