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Austenitic Stainless Steel
Description of material
APMLNU is a low-carbon austenitic stainless steel with a little higher Nickel and Molybdenum than the standard T316L grade offering a good general, pitting, crevice and intergranular corrosion resistance also after welding process and in all applications where an increased requirement of corrosion resistance were demanded. This grade, processed with a very low Cobalt content, is widely used in Nuclear plants and is substantially similar to APMDE/2 .
APMLNU 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 processes, many organic chemicals and parts working in medium corrosive environments where T304L /T316L grades don’t offer sufficient corrosion resistance. APMLNU is also used in drinking water plants and wastewater processes. Important information: Customer should inform Acciaierie Valbruna in case this grade were used for nuclear or special applications as well as for any requirements referring to specific Norms or Procedures.
APMLNU is resistant to fresh water, several organic chemicals and inorganic compounds, atmospheric corrosion, and is used in paper production equipment, marine environments, rural applications, many products used in Chemical processes and sterilizing solutions. In sea water, this grade is more resistant to pitting than TP 304/304L. 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, APMLNU 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.
APMLNU is easily produced by cold working operations such as cold drawing and bending and is suitable for cold heading thanks to its higher Nickel and low Carbon content. Its structure, after cold deformation process, is less hard than 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, coatings 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 APMLNU has a hardening factor lower than APML, the awareness of this behavior must be properly 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. Due to a very light micro re-sulphured structure, APMLNU should have a special chemical balance to satisfy a good performance in machining. In the case where a none micro re-sulphured structure were required for special applications, its chip-ability will be strongly decreased.
APMLNU 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 the solidification cracking susceptibility. This kind of welding requires a particular care and skill-know-how in the case of a fully austenitic structure. APMLNU 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 the risk of stress corrosion, a PWHT is suggested. In the case of filler metal welding, a filler with a matching composition of APMLNU or over-alloyed is recommended to maintain weld steel properties. In solid state joining such as Friction Welding, APMLNU provides a good 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 the case of open die forging of large ingots and shapes, APMLNU 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 the 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 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.
|RCCM-M MATERIAL REF.||Z2CND17-12 / Z2CND18-12 / Z6CND17-12 / Z5CND17-12|
|AISI||316 / 316L|
|W.N.||1.4401 / 1.4404|
|UNS||S31600 / S31603|
|EN||X5CrNiMo17-12-2 / X2CrNiMo17-12-2|