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Austenitic Stainless Steel
Description of material
AISTH is a Titanium stabilized austenitic stainless steel with good general corrosion resistance, as well as very good intergranular corrosion resistance after welding processes or when subjected to a slow cooling rate in service. In addition, it has good corrosion resistance at both low and high temperatures and has a high temperature strength above 500°C.
AISTH is suitable for the fabrication of many products such as flanges, fittings, valves, bolting, pump shafts, parts working in mild to medium corrosive environments or in contact with many organic chemicals. In addition, this grade is widely used in heat exchangers and pressure vessels, chemical and petrochemical/petroleum refining plant, furnace equipment and energy industries plants.
Argon Oxygen Decarburization
AISTH 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, AISTH suffers from stress corrosion cracking about 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. In reducing atmospheres, AISTH doesn’t offer a suitable high temperature corrosion resistance but has a good resistance in oxidizing environments thanks to a formation of a protective oxide layer. In the case of high temperature service, a stabilizing heat treatment should be carried out. 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.
AISTH 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 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 AISTH is not micro - resulphured like grades such as AISH and AISH2 and this strongly reduces its chip breaking ability.
Titanium carbides do not precipitate on the grain boundaries after slow cooling following 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 a post welding stabilization should be done in the case of high temperature service. AISTH has a special chemical composition which helps to avoid 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 AISTH is recommended to maintain weld steel properties.
AISTH 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, AISTH 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.
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