Aluminiziranje zlitine FeCrAl za električne uporovne grelne elemente : magistrsko delo

Ferčak, Žan (author), Karpe, Blaž (mentor)

Zlitine FeCrAl, s komercialnim imenom Kanthal®, so visokotemperaturno obstojne zlitine z visoko električno upornostjo in odlično oksidacijsko odpornostjo, zato se jih uporablja za uporovno grelne in konstrukcijske elemente, ki so izpostavljeni visokim temperaturam (do 1400 °C). Visokotemperaturna oksidacijska odpornost je posledica nastanka termodinamično stabilne plasti aluminijevega oksida (Al2O3) na površini, ki ovira prehod kisika v notranjost materiala in ščiti material pred nadaljnjo oksidacijo. Vendar pa je življenjska doba teh elementov omejena, če so izpostavljeni cikličnim toplotnim obremenitvam pri temperaturah nad 1000 °C. Težava je v tem, da zaradi relativno hitrih temperaturnih sprememb in različnih temperaturnih raztezkov oksidne plasti ter osnovne zlitine prihaja do pokanja oksidne plasti. Dokler je v podpovršinski plasti dovolj visoka koncentracija aluminija v trdni raztopini ?-Fe(CrAl), bo nastalo razpoko hitro zapolnila nova plast aluminijevega oksida. Aluminij v trdni raztopini se zaradi tega porablja in ko se njegova koncentracija v podpovršinskem področju zniža pod kritično vrednost (? 3 mas.%), zaščitna plast aluminijevega oksida preneha nastajati. To povzroči oksidacijo železa in drugih legirnih elementov ter postopen propad grelnega oziroma konstrukcijskega elementa. Ker pri izdelavi teh zlitin ne moremo poljubno povečevati deleža aluminija, saj jih pri deležu Al >7,5 mas.% zaradi vse večje krhkosti ne moremo več valjati v trak ali vleči v žico, je bil glavni cilj naše raziskave ugotoviti ali je možno povečati koncentracijo aluminija v podpovršinski plasti izdelka iz zlitine Kanthal AF, ne da bi se nam na površini tvorila aluminidna plast iz intermetalnih faz AlxFey. S tem bi se podaljšala sposobnost obnavljanja oksidne plasti in življenjska doba izdelka. Obogatitev podpovršinskega področja z aluminijem smo izvajali s postopkom aluminiziranja v zasipu. Postopek aluminiziranja v zasipu je potekal v cevni peči, v kateri smo vzorce, zakopane v mešanici prahov aluminija Al ali predzlitine FeAl, aluminijevega oksida Al2O3 in aktivatorja (halogenidne soli: AlCl3 ali NH4Cl), žarili pri različnih temperaturah in časih. Obdelane vzorce smo analizirali z vrstičnim elektronskim mikroskopom in preiskovali stanje površine, globino aluminiziranja in strukturo površinskega sloja. Ugotavljamo, da imajo vir aluminija ter delež aktivatorja in aluminija ključno vlogo pri hitrosti depozicije aluminija na površino izdelka, le ta pa na to, katere faze se bodo tvorile na površini in do katere globine bo difundiral aluminij po določenem času žarjenja pri določeni temperaturi. V kolikor želimo samo obogatiti podpovršinsko področje zlitine Kanthal AF z aluminijem, ne da bi se nam tvorila aluminidna plast na površini, je priporočljivo uporabljati predzlitino FeAl kot vir aluminija v zasipu.

• Language: Slovenian
• Keywords: FeCrAl zlitine, aluminiziranje v zasipu, zaščitne oksidne plasti, aluminidne plasti, difuzija v trdnem
• Work type: Master's thesis/paper
• Typology: 2.09 - Master's Thesis
• Organization: NTF - Faculty of Natural Sciences and Engineering
• Place of publishing: Ljubljana
• Publisher: [Ž. Ferčak]
• Year: 2021
• Number of pages: XIX, 61 f.
• PID: 20.500.12556/RUL-134186
• UDC: 669
• COBISS.SI-ID: 92258051
• Publication date in RUL: 28.12.2021

FERČAK, Žan, 2021, Aluminiziranje zlitine FeCrAl za električne uporovne grelne elemente : magistrsko delo [online]. Master’s thesis. Ljubljana : Ž. Ferčak. [Accessed 27 March 2025]. Retrieved from: https://repozitorij.uni-lj.si/IzpisGradiva.php?lang=eng&id=134186

Secondary language

• Language: English
• Title: Aluminizing FeCrAl alloy for electrical resistance heating elements : master's thesis

Abstract

FeCrAl alloys, trade name Kanthal®, are temperature resistant alloys with high electrical resistance and excellent oxidation resistance and are used for resistance heaters and components for high temperature applications (up to 1400 ° C). The oxidation resistance is related to the formation of a thermodynamically stable layer of aluminum oxide (alumina, Al2O3), which hinders the diffusion of oxygen into the interior of the material and protects the material from further oxidation. However, the lifetime of these elements is limited when the elements are subjected to cyclic thermal loads at temperatures above 1000 °C. The problem is that the alumina oxide layer cracks due to the relatively rapid temperature changes and the difference in the coefficient of thermal expansion of the oxide layer and the base alloy. As long as there is a sufficiently high concentration of aluminum in the solid solution α-Fe(CrAl) in the underlying layer, the resulting crack will be quickly filled with a new layer of alumina. Aluminum in solid solution is thus consumed, and when its concentration in the subsurface falls below a critical value (⡈ 3 mas. %), the protective layer of alumina no longer forms. This leads to the oxidation of iron and other alloying elements and the gradual deterioration of the heating or structural element. Since we cannot arbitrarily increase the aluminum content (max. 7.5 mas. %) in the production of these alloys, since they can no longer be rolled into strips or drawn into wires due to the loss of ductility, the main objective of our research was to determine whether it is possible to increase the aluminum concentration in the subsurface layer of the alloy product without the formation of an aluminide layer of intermetallic AlxFey phases on the surface. This would increase the ability to renew the oxide layer and extend the life of the product. The enrichment of the subsurface area with aluminum was carried out by the pack aluminizing process. The main objective of this master thesis was to increase the concentration of aluminum in the subsurface layer of α-Fe(CrAl) solid solution in order to improve the ability to form oxide layers during the lifetime of the element made of this alloy. For this purpose, the surface of the studied FeCrAl alloy samples was treated with a process called pack aluminization. Pack aluminization took place in a tube furnace where the samples were buried in a mixture of pure aluminum powder or FeAl master alloys, alumina powder and activator (halide salts: AlCl3 or NH4Cl). Treated samples were analysed with a scanning electron microscope and the surface texture, the depth of aluminization and the structure of the surface layer were studied. We find that the aluminum source and the ratio of activator to aluminum in the pack play a key role in the rate of aluminum deposition on the workpiece surface, while the deposition rate is the key factor that determines which phases form on the surface and to what depth the aluminum diffuses after a given annealing time at a given temperature. If we want to enrich only the subsurface area of the alloy Kanthal AF with viii aluminum without forming an aluminide layer on the surface, it is recommended to use the master alloy FeAl as a source of aluminum in the pack.

• Keywords: FeCrAl alloys, pack aluminization, protective scale, aluminide layer, solid solution enrichment with Al, diffusion in solids