Visokotemperaturna oksidacija z aluminizacijo v zasipu ali s sol-gel postopkom obdelanih FeCrAl zlitin : magistrsko delo

Brguljan, Nana (author), Karpe, Blaž (mentor)

Zlitine železa, kroma in aluminija, poznane pod tržnim imenom Kanthal (A1, APM, AF, D, E), se uporabljajo za elektrouporovne grelce in konstrukcijske elemente, izpostavljene visokim temperaturam. Njihova odpornost proti visokotemperaturni oksidaciji je povezana z nastankom aluminijevega oksidnega filma na površini zlitine, ki je praktično neprepusten za kisik in zato ščiti zlitino pred nadaljnjo oksidacijo. Kljub dobri odpornosti proti visokotemperaturni oksidaciji pa je lahko njihova življenjska doba omejena, če so izpostavljene hitrim cikličnim visokotemperaturnim obremenitvam. Ker pri izdelavi teh zlitin ne moremo poljubno povečevati deleža aluminija, saj pri deležu Al nad 7,5 mas.% zaradi vse večje krhkosti teh zlitin ne moremo več valjati v trak ali vleči v žico, je bil glavni cilj naše raziskave ugotoviti, ali je mogoče povečati koncentracijo aluminija samo v podpovršinski plasti izdelka. S tem bi podaljšali sposobnost obnavljanja oksidne plasti in posledično življenjsko dobo izdelka. V ta namen smo zlitine modificirali s postopkoma aluminizacije v zasipu (ang. pack aluminization) in potapljanja v sol-gela Al2O3 in SiO2. Pri klasični aluminizaciji v zasipu se na površini zlitin (jekla, nikljeve superzlitine) tvorijo aluminidne plasti, ki so odporne na nekatere agresivne atmosfere, pri potapljanju v sol-gele pa se na površini tvorijo oksidne plasti že pri razmeroma nizkih temperaturah (< 200 °C) in bodisi ščitijo material pred nadaljnjo oksidacijo ali spreminjajo fizikalne lastnosti površine. Ker se je v predhodnih raziskavah izkazalo, da nastanek aluminidnih faz na površini negativno vpliva na življenjsko dobo visokotemperaturno ciklično obremenjenih zlitin FeCrAl, smo aluminizacijo v zasipu prilagodili tako, da se je v podpovršinski plasti z Al obogatila le trdna raztopina ?-FeCrAl. Ugotavljamo, da je to mogoče, če se kot vir aluminija v zasipu uporablja predzlitina AlFe ter prilagodi temperaturni režim toplotne obdelave. V delu so opisani vpliv deleža predzlitine AlFe v zasipu in vpliv temperaturnega režima žarjenja na debelino modificirane plasti in koncentracijski gradient aluminija v njej ter njun vpliv na spremembo električne upornosti zlitine. V drugem delu naloge smo raziskovali vpliv deleža plastifikatorja T4 (2,4,6,8-tetrametil-2,4,6,8-tetrakis[2-(dietoksimetilsilil)etil]ciklotetrasiloksana), temperature sušenja in tehnoloških parametrov na tvorbo oksidne plasti, s potopno metodo nanesenih sol-gelov Al2O3 in SiO2. Opisana sta sinteza sol-gelov ter postopek nanašanja s potopno metodo. V vseh primerih se je sicer tvorila tanka oksidna plast, ki pa je bila vedno vsaj delno razpokana. Opisani so problemi pri nanašanju sol-gelov ter predlagane izboljšave.

• Language: Slovenian
• Keywords: zlitine FeCrAl, aluminizacija v zasipu, sol-gel, oksidne prevleke, meritve električne upornosti
• 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: N. Brguljan
• Year: 2023
• Number of pages: 65 f.
• PID: 20.500.12556/RUL-153372
• UDC: 669
• COBISS.SI-ID: 180658179
• Publication date in RUL: 23.12.2023

Secondary language

• Language: English
• Title: High temperature oxidation of FeCrAl alloys treated with pack aluminization or sol gel process : master's thesis

Abstract

Alloys of iron, chromium, and aluminum, known under the trade name Kanthal (A1, APM, AF, D, E), are used for electric resistance heaters and structural elements which are usually exposed to high temperatures. Their resistance to high-temperature oxidation is linked to the formation of an aluminum oxide film on the alloy's surface, which is almost impermeable to oxygen and protects the alloy from further oxidation. Despite their good resistance to high-temperature oxidation, their lifespan can be limited when exposed to rapid high temperature cycling. Because we cannot indefinitely increase the mass percentage of aluminum, due to the increasing brittleness of these alloys when the aluminum percentage exceeds 7.5 mass%, they can no longer be rolled into strips or drawn into wires. The primary goal of our research was to investigate whether it is possible to increase the aluminum concentration only in the subsurface layer of the product. This could extend the ability to regenerate the oxide layer and consequently prolong the product's lifespan. For this purpose, we modified the surface of the alloy using pack aluminization and dipping into sol-gels of Al2O3 and SiO2. In conventional pack aluminization, aluminum-rich layers resistant to certain aggressive atmospheres forms on the alloy's surface (of steel, nickel superalloys). While dipping into sol-gels results in oxide layers forming on the surface at relatively low temperatures (< 200 °C). This layer is either protecting the material from further oxidation or altering the surface's physical properties. Previous research indicated that the formation of aluminum-rich phases on the surface negatively affects the lifespan of cyclically loaded FeCrAl alloys at high temperatures. Therefore, we adjusted the pack aluminization to enrich only the subsurface layer with aluminum by using AlFe as the aluminum source in the pack and adapting the heat treatment temperature regime. The work describes the influence of the percentage of AlFe in the pack and the influence of the heat treatment regime on the thickness of the modified layer, aluminum concentration gradient, and their impact on changes in the alloy's electrical resistance. In the second part of the study, we explored the influence of the T4 plasticizer (2,4,6,8-tetramethyl-2,4,6,8-tetrakis[2-(diethoxymethylsilyl)ethyl]cyclo-tetrasiloxane), drying temperature, and technological parameters on the formation of oxide layers by the dipping sol-gel method using sol-gels of Al2O3 and SiO2. The synthesis of sol-gels and the immersion application process are described. In all cases, a thin oxide layer was formed, albeit always partially cracked. Challenges in the application of sol-gels are highlighted, along with suggested improvements.

• Keywords: alloys FeCrAl, pack aluminization, sol-gel, oxide coatings, measurements of electrical resistance