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Austenitic Stainless Steel
Description of material
AU188 is a Cr–Ni austenitic stainless steel with a higher Carbon than the popular 304L. It has a good general corrosion resistance at both low and high temperatures.
AU188 is suitable for the fabrication of many products such as flanges, fittings, valves, bolting, pump shafts, molds, and cables and in contact with many organic chemicals and parts working in mild to medium corrosive environments. In addition, AU188 is widely used in spring fabricating and where a higher strength after cold deforming is required.
Argon Oxygen Decarburization
AU188 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, AU188 suffers from stress corrosion cracking about forty/fifty degrees (C°) above room temperature and above certain levels of stress and halogen concentration. Very strain hardened structures increase the risk of stress corrosion cracking, but intergranular corrosion resistance may occur if the temperature range and slow cooling of the environment is able cause a rapid carbide precipitation at the grain boundaries. 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.
AU188 is readily fabricated by cold working operations such as cold drawing and bending, but should only be used for a minor 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). This could result in a rapid die wear.
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. The structure of AU188 is not micro-resulphured like grades such as AISL and this strongly reduces its chip breaking ability.
Due to its Carbon content, a PWHT must be applied in order to avoid intergranular corrosion caused by carbide precipitation on grain boundaries in the HAZ. AU188 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. However, the Cr/Ni equivalent balance of the supplied product should be evaluated to avoid the risk of solidification cracks in the fused-zone of very high energy autogenous welds. In the case of filler metal welding, filler ER 308 is recommended to maintain weld steel properties.
AU188 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, AU188 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. Forgings can be cooled rapidly in air or water but for a suitable corrosion resistance they shall be annealed.