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High temperature Alloys
Description of material
GL1 is a Nickel-Iron-Chromium alloy with high resistance to several kinds of corrosion at different temperatures.
Even if GL1 offers a suitable corrosion resistance at room temperature, it is useful to remember that this grade has been designed to warrant good performance at high temperature. The structure and composition of GL1 offers an excellent resistance to several corrosive aggressive media from cryogenic up to high temperature environments. GL1 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, oil production systems and installations in industrial furnaces working even in carburizing atmospheres. In addition, this alloy is used in the automotive industry in high temperature devices/sensors, heat exchangers, evaporators, boilers and in applications involved oxidizing/none oxidizing acids.
GL1 is resistant to several organic chemicals and inorganic compounds, atmospheric corrosion, and sterilizing solutions but offers a feeble resistance at moderate temperature in Sulphur-containing environments. This grade is more resistant to uniform corrosion than super-austenitic grades providing a good resistance to Chloride-induced stress corrosion cracking thanks to its high Nickel content. Nevertheless, in certain high temperature atmosphere environments, a stress relieving annealing heat treatment shall be done in order to warrant the resistance to this kind of corrosion, while a soft annealing is required for good intergranular corrosion resistance if GL1 had to work in some aggressive environments. A solution annealing at higher temperature should be done to obtain better creep performance.
GL1 has a good cold working hardening factor compared to similar other austenitic grade thanks to its high Nickel content. GL1 can be fabricated by cold working operations such as cold drawing and bending, and could be even used for a moderately heavy 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, applying a intermediate annealing if it were necessary increase its resistance to stress corrosion. Cold working doesn’t increase its magnetic permeability as compared to type 316 and similar austenitic grades.
GL1 has the typical machinability of fully austenitic not micro-resulphured structures and some difficulties could happen in drilling, turning, threading and milling processes due to 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. Some improvement could be obtained by dissipating heat, using an appropriate and large amount of cutting fluids 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.
GL1 can be welded by using any one of welding process applied 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 annealing is recommended in the case of moderately high temperature applications of welded material and in order to strongly increase its 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. GL1 require special filler metals to obtain a high corrosion resistance together with high strength and toughness of the weld.
GL1 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, GL1 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 the lower range of hot working temperature is recommended, especially in the 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 avoiding slow cooling. A stabilizing annealing could be necessary in the case of large forgings because these heavy sections could be prone to cause 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 creates the better properties of GL1.