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Austenitic Stainless Steel
Description of material
AIMR is a high Manganese and low Nickel austenitic stainless steel whose Nitrogen content provides a higher yield and tensile strength compared to type 304. This grade reaches a high strength after cold working, without a significant increase of magnetic permeability, which is also true after a strong cold working reduction.
AIMR is suitable for the fabrication of many products such as screws, bolting, springs, cold drawn wire and parts working in mild to medium corrosive environments.
Argon Oxygen Decarburization
AIMR has the same general corrosion resistance of type 304. Nevertheless, pitting and crevice corrosion may occur in chloride environments if concentration, pH and temperature are at determinate levels. AIMR suffers from stress corrosion cracking above certain levels of temperature and stress. Very strain hardened structures increase the risk of this form of corrosion, mainly in aggressive mineral acid and hot alkaline solutions. In addition, this grade suffers from intergranular corrosion if heated or slow cooled between 450°C and 900°C. 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.
AIMR is readily fabricated by cold working operations such as cold drawing and bending, but should only be used for a minimal amount of cold heading, because its chemical balance does not allow it 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 and Carbon contents. This could result in a rapid die wear.
AIMR 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. This grade requires more rigid and powerful machines in addition to the correct choice of tools, coatings and, above all, cutting fluids with good cooling and lubricating properties. The structure of AIMR is not micro - resulphured and this is a disadvantage in chip breaking.
Due to its Carbon content, a PWHT shall be always 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 due to a solidification mode from primary ferrite to primary austenite. In the case of filler metal welding, a filler with a matching composition of AIMR is recommended to maintain weld steel properties. Attention must be paid on high energy autogenous welding processes such as LBW and, particularly, for the EBW-HV process, because of the high risk of outgassing.
AIMR has a good hot plasticity and is suitable for processing by hot extrusion or by upsetting with electric resistance heating; it offers a very good hot workability and is usually supplied as billets, blooms, or ingots. No preheating is required. However, overheating must always be 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 temperature at the top end of the range during the extrusion and forging process, could generate internal bursts. Small forgings can be cooled rapidly in air or water. An annealing after forging should be performed for maximum corrosion resistance in order to avoid carbide precipitation on the grain boundaries.