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Corrosion Resistant Alloys
Description of material
AV20 is a Niobium stabilized alloy with additions of Nickel, Chromium, Molybdenum and Copper with excellent resistance in acid aggressive environments.
The structure and composition of AV20 offers an excellent resistance to corrosive environments typical of the processing and production of acids. AV20 is widely used in chemical industries, in pharmaceutical, plastic and food processes and is suitable for the fabrication of many product such as flanges, valves, bolting, pump shafts, chains, fittings, tanks, handling acid, pickling plants, oil and gasoline production systems, petrochemical industries and several marine applications.
AV20 has a better corrosion resistance than the most popular 316 austenitic steels. It is very resistant to chemical environments such as sulphuric and phosphoric acid within certain concentrations, temperatures and contaminations, several organic chemicals and inorganic compounds, atmospheric corrosion, marine and seawater applications, and sterilizing solutions. It offers a good resistance against intergranular corrosion due to Niobium stabilization and stress corrosion cracking thanks to its high Nickel content. The Molybdenum ensures very good pitting and crevice corrosion resistance. However, evaluations should be done in the case of environments with a high chloride content and considered in designing. 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.
AV20 can fabricated by cold working operations such as cold drawing and bending and can be used for a moderate of cold heading, because its chemical balance allows it to obtain an acceptable soft structure after cold deformation. In any case, cold processes shall be carried out in the annealed condition, avoiding high levels of cold working applying an intermediate annealing if necessary. After cold working, this grade should be soft or stabilized annealed depending on final use.
AV20 has the typical machinability of fully austenitic not micro - resulphured austenitic structures and some difficulties could happen in drilling, turning, threading and milling processes due to its capacity to cold work harden and lowest chip-ability . Operators should know that this grade requires more rigid and powerful machines, in addition to the correct choice of tools, coating carbides and cutting fluids. AV20 has a little bit higher hardening factor than 300 austenitic grades and the knowledge of this behavior must be correctly considered when a piece requires two or several cutting steps to be finished. The cold worked layer caused by the cutting tool is 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. Some improvement could be obtained by a dissipation 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.
AV20 can be welded by using any one of welding process employed with typical austenitic grades. 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 risk of hot cracking in the fused zone, mainly if the weld were under stress. No preheating or post welding are normally necessary but a stabilizing annealing is recommended in case of the weld structure were under constraint and elevated temperature service but AV20 requires an adequate inert shielded gas protection and special filler metals for obtain a high resistance corrosion together high strength and toughness of welding. The weld discoloration should be removed by acid pickling or, at least, by mechanical pickling (shot blasting or grinding) if were impossible.
AV20 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 be always 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, AV20 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 influences the structure after annealing and solution annealing in terms of grain size. Forgings can be cooled rapidly in air or forced air.
|Commercial name||Alloy 20|