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Austenitic Stainless Steel
Description of material
APM/DE2 is a low-carbon austenitic stainless steel with more Nickel and Molybdenum than type 316L series steels. It offers good general, pitting, and crevice corrosion resistance as well as good intergranular corrosion resistance after welding processes and in all applications where an increased requirement of corrosion resistance were demanded.
APM/DE2 is suitable for the fabrication of many products such as flanges, valves, bolting, pumps shafts, food /beverages industry equipment , heat exchangers, storage tanks, pharmaceutical equipment, paper production equipment and parts working in medium corrosive environments where the type 304L /316L steels don’t offer sufficient corrosion resistance. APMLDE2 is also used in drinking water plants and wastewater processes.
APM/DE2 is resistant to fresh water, several organic chemical and inorganic compounds, atmospheric corrosion, marine environments, rural applications, and many products used in chemical processing and sterilizing solutions. In sea water, this grade is more resistant to pitting than type 304/304L and 316L. However, pitting and crevice corrosion may occur in environments if the chloride and halogens concentrations, pH and temperature are at determinate levels. As with other standard austenitic grades, APM/DE2 suffers from stress corrosion cracking about fifty degrees (C°) above room temperature and above certain levels of stress and halogen concentrations. Strain hardened structures increase the risk of stress corrosion cracking. 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.
APM/DE2 is readily fabricated by cold working operations such as cold drawing and bending and can be used for cold heading thanks to its higher Nickel and low Carbon contents. Its structure, after cold deformation, is less hard than AISL and APML.
Austenitic grades are different from Ferritic and Alloy steels and require more rigid and powerful machines in addition to the correct choice of tools, coating and cutting fluids. The Austenite structure is prone to transform into α’Martensite caused by strain hardening of the tool on the surface of the work piece. Even if APM/DE2 has a hardening factor lower than AISL / APML, 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. APM/DE2 has special chemical balance for a suitable machining, by having a micro re-sulphured structure. In the case of a non-micro re-sulphured structure, required for particular use of a product, its chip-ability is strongly decreased.
APM/DE2 has a special chemical composition which should help to avoid solidification cracks in the fused-zone of autogenous welds. Nevertheless, high energy density autogenous welds require an evaluation of the Creq/Nieq ratio because a higher Ni content may result in a change in solidification mode from primary ferrite to primary austenite. This could increase solidification cracking susceptibility. This kind of welding require a particular care and technics in the case of a fully austenitic structure. . APM/DE2 can be welded without PWHT due to its low carbon content which avoids the precipitation of Cr-Carbide on the grain boundaries. However, in the case of aggressive environments or of a risk of stress corrosion, a PWHT should be considered. In the case of filler metal welding, a filler with a matching composition of APM/DE2 or over-alloyed is recommended to maintain weld steel properties. In solid state joining such as Friction Welding, APM/DE2 provides a quality bond line.
No preheating is required for this grade. In Primary hot transformation processes, a high temperature homogenization of large ingots and dynamic recrystallization parameters should be rightly evaluated. In case of open die forging of large ingots and shapes, APM/DE2 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 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 to make ultrasonic testing easier to detect small indications as required by several International Norms. Small forgings can be cooled rapidly in air or water.