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Austenitic Stainless Steel
Description of material
APMLN is a low-Carbon, Nickel, Nitrogen and Molybdenum austenitic stainless steel with good general, pitting and crevice corrosion resistance, as well as good intergranular corrosion resistance after welding processes. This grade offers good low temperature toughness and higher strength than the typical 316/316L grades.
APMLN is suitable for the fabrication of products such as flanges, valves, screws, bolting, pumps shafts, medical applications, pharmaceutical, many products used in chemical processes, paper industries, food /beverages industry equipment , storage tanks, parts working in the mild to medium corrosive environments and pressure vessels products. For Marine Propellers, see MARINOX 16.
APMLN is resistant to fresh water, several organic chemicals and inorganic compounds, atmospheric corrosion, marine environments, rural applications and sterilizing solutions. In marine environments, this grade is slightly more resistant to pitting and crevice corrosion than type 316/316L steels. However, pitting and crevice corrosion may occur in environments if the chloride concentrations, pH and temperature are at determinate levels. As with other standard austenitic grades, APMLN suffers from stress corrosion cracking about thirty /forty 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.
APMLN 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) mainly due to its high Nitrogen content. Even if it’s high Nickel content could reduce this hardening, when compared to AISLN, this could still result in a certain amount of 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 APMLN is not micro - resulphured and this strongly reduces its chip breaking ability.
APMLN 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 risk of stress corrosion, a PWHT should be considered. In the case of filler metal welding, a filler with a matching composition of APMLN or over-alloyed fillers are recommended to maintain weld steel properties. Neither preheating nor post welding heat treatment is required. Nevertheless, the Cr/Ni equivalent balance of the supplied product should be well evaluated to avoid the risk of solidification cracks in the fused-zone of high energy autogenous welds.
APMLN 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, APMLN 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 should be cooled rapidly in air or water.