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Corrosion Resistant Alloys
Description of material
AV925 is an age hardening Nickel - Iron - Chromium alloy with the addition of Molybdenum and Copper as well as Titanium and Aluminum that provide its strengthening by an ageing heat treatment that enables it to generate a precipitation of Gamma Prime Phase Ni3(Al,Ti).
The structure and composition of AV925 offers a high strength together with corrosion resistance properties in both oxidizing and reducing environments. These characteristics provide a good resistance to different kinds of corrosion such as pitting, crevice, intergranular and as well as stress corrosion cracking. AV925 is widely used in sour gas applications due to its resistance to SSC caused by Hydrogen sulfide, in petrochemical industries, in crude oil and natural gas components and subsea and deep-water offshore oil production systems and several marine applications. It is suitable for the fabrication of many products such as flanges, valves, bolting, pump shafts, chains, fittings and parts working in corrosive environments typical of chemical processing.
AV925 is resistant to several organic chemicals and inorganic compounds, atmospheric corrosion and marine environments. In sea water, this grade is resistant to uniform corrosion providing a high resistance to Chloride-induced stress corrosion cracking and an outstanding pitting and intergranular corrosion resistance. In sour gas applications, this alloy offers excellent resistance to Hydrogen embrittlement typical of Hydrogen Sulfide environments. All these properties against pitting and crevice are obtained by the Chromium, Molybdenum and Coppers content while the high Nickel provides the resistance against the SCC. The high mechanical properties can only be reached by an ageing heat treatment. It should be noted that AV925, as for every kind of stainless steel, surfaces should be free of contaminant and scale, heat tint for optimum resistance to corrosion.
AV925 can fabricated by cold working operations such as cold drawing and bending, but should not be used for a large amount of cold heading, because its chemical balance does not allow 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 an intermediate annealing if necessary. Obviously, high cold deformation in the aged condition should be avoided. However, after cold working, this grade should be solution annealed before the aging hardening depending on final use. Cold working doesn’t increase its magnetic permeability as compared to type 316 and similar steels.
AV925 has the typical machinability of strengthened austenitic structures not micro-resulfured and some difficulties could happen in drilling, turning, threading and milling processes due to its capacity to 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. AV925 has a high hardening factor 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 with a suitable chip breaker. This is particularly important when using multi - spindle and automatic screw machines. This alloy can be machined in the solution treatment or aged condition but it recommended to machine in the solution treatment condition and to make finishing operations after aging hardening. A correct choice of both cutting fluids and a right dimension of chip breakers helps to reduce the typical machining difficulties of this alloy.
AV925 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. Therefore, AV925 require special filler metals to obtain a high corrosion resistance together with high strength and toughness of the weld. No preheating or post welding heat treatment are normally necessary. The weld discoloration should be removed by acid pickling or, at least, by mechanical pickling (shot blasting or grinding) if were impossible to perform the first one.
AV925 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, AV925 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. For close die forging, at least 15% cold be acceptable. This practice is suggested in order to obtain 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, avoiding slow cooling but a solution annealing should be performed before aging for the best results in terms of corrosion resistance and uniform mechanical properties.
|Commercial name||Alloy 925|