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Corrosion Resistant Alloys
Description of material
AN4 is multi-rule super austenitic steel with Nickel-Chromium, Molybdenum and Copper with high resistance to several, different, kinds of corrosion.
The structure and composition of AN4 offers an excellent resistance to several corrosive aggressive media. AN4 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 pulp and paper chemical processing, handling acid, pharmaceutical and medical devices, oil production systems, petrochemical industries and several marine and seawater applications.
AN4 has better corrosion resistance performances than the most popular austenitic steels. It is resistant to chemical environments such as sulphuric acid within certain concentrations, a number of organic chemicals and inorganic compounds, atmospheric corrosion, marine and seawater applications, and sterilizing solutions. In sea water, this grade is more resistant to uniform corrosion providing a very high resistance to Chloride-induced stress corrosion cracking as well as outstanding pitting, crevice and intergranular corrosion resistance. This is due to respectively, to its low carbon content, a particular chemical balance of Chromium and Molybdenum and a high Nickel content.
AN4 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 final use. Cold working doesn’t increase its magnetic permeability as much, compared to type 316 and similar steels.
AN4 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. AN4 has a somewhat 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 dissipating heat, using an appropriate and large amount of cutting fluids and tools with a correct edge geometry.
AN4 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 the 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. Moreover, even if AN4 is less prone than VAL 4529 to cause significant variations in composition of some elements in high energy autogenous welding, the knowledge of this behavior should be well evaluated, unless post-welding full annealing and quenching has been planned. No preheating or post welding heat treatment are normally necessary but an annealing is recommended in case of a weld structure under constraint in order to improve/avoid stress 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. AN4 requires high alloyed filler metals or Ni-alloy to restore, or improve, the composition, allowing welded parts to be used in the as-welded condition.
AN4 has a good hot plasticity and is suitable for processing by hot extrusion or by upsetting with electric resistance heating. 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, AN4 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 (for instance: 20-30 %) 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. Forgings can be cooled rapidly in air or water.