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Austenitic Stainless Steel
Description of material
AIP is a low Carbon, Chromium and high Nickel austenitic stainless steel with good general corrosion resistance as well as good intergranular corrosion resistance after welding processes. This grade has been designed in order to offer a good cold deforming and cold head ability thanks to an exact chemical balance of elements such as Carbon and Nitrogen, with a high Nickel content compared to standard type 304L.
AIP is suitable for the fabrication of many products such as flanges, valves, bolting, screws, pumps shafts, food /beverages industry equipment, storage tanks, pharmaceutical devices, many organic chemicals and parts working in mild to medium corrosive environments.
Argon Oxygen Decarburization
AIP is resistant to fresh water, many organic chemicals and inorganic compounds, atmospheric corrosion, rural applications and sterilizing solutions where the chloride content 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, AIP suffers from stress corrosion cracking about forty /fifty degrees (C°) above room temperature and above certain levels of stress and halogen concentrations. Very strain hardened structures increase the risk of stress corrosion cracking. Its intergranular corrosion resistance is very 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.
AIP offers a little bit higher cold working hardening factor compared to Copper alloy austenitic grades. These grades, such as AISRUH and AISRU, whose chemical balance provides a higher cold deformability, are the right choice if high cold heading performance were required and where the high Copper content would not be a jeopardy in certain applications. AIP has good polish-ability or mirror finishing capacity, but it should be pointed out that these properties improve with a fine grain structure, harder than a fully annealed one. In the case of cold heading for screws and bolts production, AIP is usually supplied in different conditions such as: (1) annealed+ pickled (2) annealed + pickled and special mill coating (3) annealed + cold drawn with some kind of mill coating and ready to be entered in the header (4) annealed +cold drawn + protective atmosphere annealed + mill coated ready for cold heading with or without sizing operations before entering in the header. It should be noted that, as with all hot rolled wire rod surfaces, conditions (1) and (2) offer a higher surface roughness compared to other ones.
AIP has a poor machinability due to a low Sulfur content but, however, a little bit better than Copper grades such as AISRUH and AISRU. The best performance is obtained when employing the correct machining parameters while using multi - spindle and automatic screw machines. However, 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 roughness of machined parts improves in the case of a harder structure than the fully annealed one.
AIP has normally a special chemical composition which helps to avoid solidification cracks in the fused-zone of autogenous welds due to a suitable Ferrite balance. 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 high energy autogenous welds. In the case of filler metal welding, a filler with a matching composition of AIP or over alloyed is recommended to maintain weld steel properties. Neither preheating nor post welding heat treatment is required. Nevertheless, in the case of aggressive environments, a post welding heat treatment is suggested.
AIP 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, AIP 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.