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Corrosion Resistant Alloys
Description of material
AVC276 is a low-Carbon Nickel alloy with Tungsten and a very high Molybdenum content. It offers a very high resistance to pitting and crevice corrosion in the presence of Chlorides together with a very good resistance to general corrosion and to stress corrosion cracking (SCC).
AVC276 is widely used in many chemical process in the presence of moderate oxidizing and reducing media. Therefore, is suitable for the fabrication of many products such as flanges, valves, bolting, pump shafts, offshore plant, heat exchangers, storage tanks, paper and pulping bleach equipment, oil & gas production, oil field applications in sour environments, rural applications, many products used in chemical processes, sterilizing solutions, and parts working in corrosive environments where type 316 grades and AN4 do not provide a sufficient corrosion resistance. In particular, this grade, offering a very good resistance in sea water, is widely used in naval applications.
AVC276 is probably the best grade that is resistant to every kind of chlorine environment, and to many organic chemicals and inorganic compounds, atmospheric corrosion and marine environments. This grade is also resistant to uniform corrosion providing a very high resistance to Chloride-induced stress corrosion cracking even in sour environments and an outstanding pitting corrosion resistance. All these properties against pitting and crevice are obtained by Chromium together with high Molybdenum and Tungsten contents while the high Nickel provides the resistance against the SCC. However, it should be considered that a strain hardened structure generated by cold working could jeopardize its SCC resistance. AVC276 has good high temperature strength but it should be considered there is some risk of the formation of precipitates. The same attention should be done about its intergranular corrosion resistance in the case of very slow cooling despite its low Carbon content. It should be noted that AVC276, as for every kind of stainless steel, surfaces should be free of contaminant and scale, heat tint and surface finishing for optimum resistance to corrosion.
AVC276 can be fabricated by cold working operations such as cold drawing and bending, but should not be used over a certain amount of cold heading, because its chemical balance does not allow 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 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.
AVC276 has a poor machinability when compared to austenitic grades mainly due to its high alloyed composition that causes a significant strain hardening of the austenitic structure when the tool works on the surface of the piece. 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. AVC276 does not have the low hardening factor of 316 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 with suitable chip breaker. This is particularly important when using multi - spindle and automatic screw machines.
AVC276 has a good weldability and could be used as welded in several applications even if some risk of intergranular corrosion may exist in the case of slow cooling and despite its low Carbon content. Therefore, it should be annealed for optimum corrosion resistance especially in the case of aggressive environments. This alloy requires some different welding process evaluations when compared to standard austenitic grades due to its highly alloyed chemical balance. Correct welding practices such as low heat inputs are able to reduce the risk of hot-cracking. Low temperature inter-pass, special filler metals or matching fillers, inert shielding gas and cleanliness before/after welding must be followed to obtain best results in terms of corrosion resistance. The weld discoloration should be removed by acid pickling or, at least, by mechanical pickling (shot blasting or grinding) if it were impossible to perform the first one.
AVC276 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 is lower than that of typical austenitic stainless steels. Because this alloy has hot high hardness, 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 and a heavy scale formation as well. 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 case of open die forging of large ingots and shapes, AVC276 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: 20-25%) 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 easier to detect small indications as required by several International Norms. Forgings must be cooled rapidly in air or water, avoiding slow cooling but an annealing after hot forging is strongly suggested in order to obtain the best corrosion resistance.
|Commercial name||Alloy C276|