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High temperature Alloys
Description of material
AN3 is a Nickel-Iron-Chromium alloy enhanced by a Silicon addition that enables it to offer good oxidation and scale resistance.
Even if AN3 offers a suitable corrosion resistance at room temperature, it is useful to remember that this grade has been designed to warrant good performance at high temperatures. The structure and composition of AN3 offers an excellent resistance to carburizing and oxidizing environments even in the case of continuous variations of both oxidizing and carburizing in hot service atmospheres. AN3 is suitable for the fabrication of many products such as flanges, food processing equipment, bolting, chains, hooks, conveyor chains, parts working in corrosive environments such as chemical processing, oil production systems, heat exchangers and installations in industrial furnaces working in different atmospheres.
AN3 has been particularly designed to supply good resistance when submitted to cyclic conditions of cooling and heating in oxidation, carburization and nitriding environments. Another important characteristic is its capacity to generate a strong adherent scale layer formed during exposure to high temperatures enabling it to protect the surface of parts in service, avoiding oxide spalling. Thanks to its high Nickel content, this grade avoids, or least delays, the formation of sigma phase, but offers a feeble resistance at medium/high in Sulphur-containing environments.
AN3 has a better cold working hardening factor compared to similar to other austenitic grade thanks to its high Nickel content. It can fabricated by cold working operations such as cold drawing and bending, and could even be used for a moderate amount of heavy cold heading, because its chemical balance allows it to obtain a soft strain hardened structure after cold deformation. In any case, cold processes shall be carried out in the annealed condition, avoiding high levels of cold working, applying a intermediate annealing if were necessary if a suitable creep-strength were a requirement. Cold working doesn’t increase its magnetic permeability as compared to type 316 and similar austenitic grades.
AN3 has a poor machinability due to its high Nickel content and low Sulphur content if compared to typical Austenitic grades. The best performance is obtained when employing the correct machining parameters while using multi - spindle and automatic screw machines and it requires more rigid and powerful machines in addition to the correct choice of tools, coatings and cutting fluids. Some improvement could be obtained by dissipating heat, using an appropriate and large amount of cutting fluids and tools with a correct edge geometry. Moreover, a little increasing of machinability and roughness of machined parts could be improved by a harder structure obtained by a cold drawing process.
AN3 can be welded by using any one of welding process employed with typical austenitic grades but requires some different welding process evaluations when compared to these ones. Correct welding practices such as right heat inputs, inert shielding gas and cleanliness before/after welding must be followed to obtain best results in terms of corrosion resistance. In the case of high energy autogenous welding processes, there could be some risk of hot cracking in the fused zone. No preheating or post welding heat treatment are normally required but AN3 requires an adequate inert shielded gas protection and special filler metals to obtain a high corrosion resistance together high strength and toughness of welding. The weld discoloration should be removed by acid pickling or, at least, by mechanical pickling (shot blasting) if were impossible to perform the first one.
AN3 has a good hot plasticity and is suitable for processing by hot extrusion or by upsetting with electric resistance heating. This grade can be hot headed but it’s important to point out that its forging temperature range is less wide than that of typical austenitic stainless steels. In any case, overheating must always be avoided. The choice of hot working temperature and process parameters must always evaluate both the strain rate and the consequent increasing of temperature that is reached after hot deformation. High strain rates and temperatures at the top of the range during the hot forming process, could generate structural loss of cohesion or internal bursts. Good rules impose that 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, AN3 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. Suitable strain in terms of section reduction at lower range of hot working temperature is recommended especially in case of open – die forging. This practice is suggested in order to obtain a fine grain structure which is very important for mechanical, fatigue and corrosion resistance properties and make it easier for ultrasonic testing to detect small indications as required by several International Norms. It’s important to point out that the final temperature and amount of reduction of forging section influence the structure after annealing and solution annealing in terms of grain size. Forgings can be cooled rapidly in air or forced air, especially in the case of large forgings in order to avoid some carbide precipitation on the grain boundaries. Nevertheless, an annealing is recommended after forging to warrant a good structure and a maximum corrosion resistance.
|Commercial name||Alloy DS|
|International Designation||X8NiCrSi38 - 18|