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Austenitic Stainless Steel
Description of material
AISC is a Niobium stabilized austenitic stainless steel with both good general corrosion resistance and very good intergranular corrosion resistance, after welding processes or subjected to a slow cooling rate in service.
AISC is suitable for the fabrication of many products such as flanges, valves, bolting, pump shafts, food /beverages industry equipment , high temperature chemical processes, rings , storage tanks, many organic chemicals, steam power plant, furnace manifolds, heat temperature devices and parts working in the 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
AISC is resistant to fresh water, several organic chemicals and inorganic compounds, atmospheric corrosion, rural applications and sterilizing solutions where the chloride level is very 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, AISC suffers from stress corrosion cracking about forty/fifty 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. When annealed and fast quenched, AISC offers the best resistance corrosion. 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.
AISC is readily fabricated by cold working processes such as cold drawing and bending, but should only be used for a moderate amount of cold heading and cold up setting.
Machinists should know that 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. AISC isn’t generally supplied with a micro re - sulphured structure and, therefore, doesn’t offer a good chip-ability.
AISC has a chemical composition with a controlled balance of Niobium content, whose carbides do not precipitate on the grain boundaries after slow cooling or after welding. No preheating or post welding are normally necessary. However, an annealing after welding should be done if the weld works in very aggressive environments and post welding stabilization in the case of high temperature service. In the case of autogenous welding process, it could be some risk of hot cracking in the fused zone due to a solidification mode from primary ferrite to primary austenite. This could be avoided only if the heat of supplied product has a chemical balance that allows a primary ferrite solidification mode or, alternatively, by a right filler metal and accurate welding procedures involving heat inputs and joint geometry.
AISC has a good hot plasticity and is suitable for processing by hot extrusion or by hot upsetting with electric resistance heating. However, overheating must be always avoided. The choice of hot working temperature and process parameters must always evaluate the strain rate and the consequent increasing of temperature that is reached after hot deformation. High strain rates and temperatures at the top end of the range during the extrusion and forging process, could generate internal bursts. Small forgings should be cooled rapidly in air or water quenched. However, the best corrosion resistance is obtained by annealing followed by water quenching.