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Corrosion Resistant Alloys
Description of material
AN2 is a Nickel - Iron - Chromium alloy stabilized by Titanium with additions of Molybdenum and Copper and has high resistance to several different kinds of corrosion.
The structure and composition of AN2 offers an excellent resistance to several corrosive aggressive media from cryogenic up to moderately high temperature environments. AN2 is suitable for the fabrication of many products, such as flanges, valves, bolting, pump shafts, chains, fittings, food/beverages industry equipment, parts working in corrosive environments such as chemical processing, handling acid, subsea and deep-water offshore, oil production systems and several marine applications. In addition, this alloy is used in heat exchangers, evaporators, boilers and in applications involved oxidizing/not oxidizing acids.
AN2 is resistant to several organic chemicals and inorganic compounds, atmospheric corrosion, marine environments, and sterilizing solutions. In sea water, this grade is more resistant to uniform corrosion than super-austenitic grades providing a very high resistance to Chloride-induced stress corrosion cracking and outstanding pitting, crevice and intergranular corrosion resistance. These properties warrant its application in both organic and mineral acids and several alkali environments. In addition, AN2 offers good performance up to 450°-500° C.
AN2 can 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 hardened structure after cold deformation as is typical of standard 300 austenitic groups. In any case, cold processes should be carried out in the annealed condition, avoiding high levels of cold working, applying an intermediate annealing if necessary. However, after cold working, this grade should be annealed depending on the final use. Cold working doesn’t increase its magnetic permeability as compared to type 316 and similar steels.
AN2 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 its low 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. AN2 has a little 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 dissipating heat using an appropriate and large amount of cutting fluids and tools with a correct edge geometry.
AN2 can be welded 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 necessary. But a stabilizing anneal is recommended in the case of moderately high temperature applications of welded parts and in order to strongly improve intergranular corrosion resistance. 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. AN2 require special filler metals to obtain a high corrosion resistance together with high strength and toughness of the weld.
AN2 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 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, AN2 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 closed die forging, temperatures, strain and strain rate should be well considered. Suitable strain in terms of section reduction (for instance: 20-30%) at the 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 to make it easier for ultrasonic testing to detect small indications as required by several International Norms. Forgings can be cooled rapidly in air or water avoiding slow cooling. A stabilizing anneal could be necessary in the case of large forgings because these heavy section could be prone to create a precipitation of carbide in the grain boundaries, especially if the Carbon content of some heats were not very low . This heat treatment restores the corrosion resistance and ensures the better properties of AN2.