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GL5- High temperature Alloys

Steel data sheets

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Valbruna Grade

GL5

Steel type

High temperature Alloys

Description of material

GL5 is a Nickel - Iron - Chromium - Aluminum alloy with high resistance in several corrosive media in high temperature environments.

Applications

This grade has been designed to warrant high performance in applications requiring high resistance to high temperatures in oxidation, carburizing and carbo-nitriding environments. The structure and composition of GL5 offers an excellent resistance to several corrosive aggressive media from cryogenic up to high temperature environments. GL5 is suitable for the fabrication of many products such as installations in industrial furnaces working in different atmospheres, in petro-chemical processes, in the automotive industry in high temperature devices/sensors, heat exchangers, heating elements, furnaces and muffles, chains, burners, evaporators, incinerators and in applications involved high temperature oxidizing environments.

Corrosion resistance

GL5 has a very good resistance to oxidation at high temperature thanks to the presence of Aluminum and the high content of Chromium that generates a protective oxide layer able to reduce/avoid oxide spalling, but offers a feeble resistance at moderate/high temperature in Sulphur - containing environments. GL5 has good creep strength in the annealed condition but a high temperature solution annealing produces a better creep performance.

Cold working

GL5 has a higher cold working hardening factor than other austenitic grades and heat resisting steel GL1, but it can still be fabricated by cold processing operations such as cold drawing and bending. This grade can only be used for a moderate amount of cold heading, because its chemical balance doesn’t 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. In the case of a fatigue resisting structure, an annealing at a lower temperature than of solution annealing should be chosen to avoid the risk of an increase in grain size. Cold working doesn’t increase its magnetic permeability as compared to austenitic grades.

Machinability

GL5 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 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. 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. Moreover, a little increasing of machinability and roughness of machined parts could be improved by a harder structure obtained by a cold drawing process.

Weldability

GL5 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 avoiding the loss of Aluminum 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 high temperature applications of the welded structure. 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. GL5 require special filler metals for obtain a high corrosion resistance together with high strength and toughness of the weld.

Hot working

GL5 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 should be fast cooled in air or water avoiding slow cooling.

Designations

Commercial name Alloy 601
International Designation NiCr23Fe
W.N. 2.4826
UNS N06601
BS NA49
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