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High temperature Alloys
Description of material
AN1 is a Nickel-Iron-Chromium alloy with additions of Aluminum and Titanium. A well-controlled chemical balance and heat treatment enable it to resist many high temperature atmospheres and to provide good creep-rupture properties.
This grade has been designed to have a very good performance in applications requiring high resistance to high temperatures in oxidation, carburizing and carbo-nitriding environments. Also, this grade has good resistance to creep and rupture in applications where these characteristics are required. AN1 is suitable for the fabrication of many products, such as installations in industrial furnaces working in different atmospheres, in petro-chemical processes, heat exchangers, heating elements, furnace and muffles, chains, burners, flanges and in power generation industries.
AN1 has a very good resistance to oxidation at high temperature thanks to the presence of Aluminum and a high Chromium content that generates a protective oxide layer able to reduce/avoid oxide spalling, but could offer a feeble resistance at certain temperatures in some Sulphur-containing environments. AN1 has good creep rupture which is improved by a high temperature solution annealing as compared to simple annealing. These properties are mainly due to the formation of a large grain size, associated with a structure where Carbon, Aluminum+Titanium offer their beneficial effect in terms of strength and resistance to creep and rupture at high temperatures. In this regard, the use of Alloys 800H and 800HT is suggested when higher and better performances are required (Alloy 800H: C=0,05%-0,10% / Alloy 800HT: C=0,06%-0,1% and Al+Ti= 0,85%-1,2%). Moreover, the high Nickel and Chromium contents allow to increase respectively the resistance to nitriding and to oxidation.
AN1 has a lower cold working hardening factor compared to similar other austenitic grade thanks to its high Nickel content. AN1 can fabricated by cold working operations such as cold drawing and bending, and could be used even for a moderate amount of cold heading, because its chemical balance allows it to obtain a soft strain hardened structure after cold deformation. In any case, cold processes should be carried out in the annealed condition, avoiding high levels of cold working and applying an intermediate annealing if it were necessary for an improvement of its stress corrosion resistance. It’s important to evaluate the effect and the total amount of strain of cold working on recrystallization in order to consider the structural behavior to enable to warrant the intended service of parts. Cold working doesn’t increase its magnetic permeability as compared to type 316 and similar austenitic grades.
AN1 has a poor machinability due to its high Nickel content and low Sulphur content if compared to typical Austenitic grades. The best performance is obtained when employing the correct machining parameters while using multi - spindle and automatic screw machines and it requires more rigid and powerful machines, in addition to the correct choice of tools, coatings and cutting fluids. This grade can be machined in the annealed or in the solution treated condition. Nevertheless, annealing results in a better machinability and better surface roughness thank to a fine grain structure. Some improvement could be obtained by dissipating heat using an appropriate and large amount of cutting fluid 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.
AN1 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. No preheating or post welding are normally required but AN1 requires an adequate inert shielded gas protection and special filler metals to obtain a high corrosion resistance together with high strength and toughness of the weld. Weld discoloration should be removed by acid pickling or, at least, by mechanical pickling (shot blasting) if were impossible to perform the first one.
AN1 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, AN1 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 the 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 especially in the case of large forgings, in order to avoid some carbide precipitation on the grain boundaries. This situation could cause intergranular corrosion in certain aggressive environments.