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Austenitic Stainless Steel
Description of material
AISH is a Carbon-Cr-Ni austenitic stainless steel with a good general corrosion resistance at both low and high temperatures, as well as high temperature strength above 500°C. This grade has been designed in order to offer a good hot plasticity thanks to an exact chemical balance of elements such as Carbon, Nitrogen and Nickel. AISH is particularly suitable for heavy forging processes and applications where high temperature corrosion may occur.
AISH is suitable for the fabrication of many products such as flanges, fittings, valves, bolting, pump shafts and many organic chemicals and parts working in mild to medium corrosive environments. In addition, this grade is widely used in heat exchangers and pressure vessels, chemical and petrochemical and petroleum refining plant.
Argon Oxygen Decarburization
AISH 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, AISH 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 may occur if the temperature range and slow cooling of the environment is able cause carbide precipitation at grain boundaries. In reducing atmospheres, AISH doesn’t offer a suitable high temperature corrosion resistance but has a good resistance in oxidizing environments thanks to the formation of protective oxide layer in oxidizing environments. 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.
AISH is readily fabricated by cold working operations such as cold drawing and bending, but should only be used for a moderate 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. In any case, the structure of AISH is slightly micro-resulphured and this offers a little advantage in chip breaking ability.
Due to its Carbon content, optimized to improve high temperature strength and creep resistance, a PWHT should be applied in order to avoid intergranular corrosion caused by carbide precipitation on grain boundaries in the 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. In the case of filler metal welding, a filler with a matching composition of AISH is recommended to maintain weld steel properties.
AISH 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, AISH 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.