Vpliv temperature in časa žarjenja na kinetiko aluminiziranja in mikrostrukturo zlitine FeCrAl

Krajnik, Tina (avtor), Karpe, Blaž (komentor)

Zlitine FeCrAl (Kanthal ®) so visokotemperaturno obstojne zlitine železa, kroma in aluminija, z odlično oksidacijsko odpornostjo pri povišanih temperaturah. V večini primerov se uporabljajo za električne grelne in konstrukcijske elemente, obstojne pri povišanih temperaturah (do 1400 °C). Oksidacijska odpornost teh zlitin je posledica nastanka termodinamično stabilnega in za kisik nepropustnega aluminijevega oksida (Al2O3) na površini, ki jih ščiti pred nadaljnjo oksidacijo. Kljub odlični oksidacijski odpornosti, pa je lahko življenjska doba elementov izdelanih iz teh zlitin omejena, če so pogosto izpostavljeni termičnemu cikliranju pri temperaturah nad 1000 °C. Razlog za to je pokanje oksidne plasti, zaradi hitrih temperaturnih sprememb in različnih raztezkov oksidne plasti ter kovinske osnove ter nadaljnja oksidacija aluminija, zaradi konstantne tvorbe nove oksidacijske plasti. Vse dokler je v podpovršinski plasti zlitine dovolj velika koncentracija aluminija, bo razpoko hitro zapolnila nova plast aluminijevega oksida. Ko pa se enkrat aluminij v podpovršinski plasti porabi in se njegova koncentracija zniža pod kritično vrednost (< 3 mas.%), zaščitna plast aluminijevega oksida ne bo več nastala, kar vodi do oksidacije železa in ostalih legirnih elementov in s tem postopnega propada grelnega oz. konstrukcijskega elementa. Namen magistrskega dela je bil povečanje koncentracije aluminija v površinski plasti FeCrAl zlitine (® Kanthal AF) s čemer bi se podaljšala sposobnost obnavljanja oksidne plasti in posledično življenjska doba elementov izdelanih iz te zlitine. Z namenom povečanja koncentracije aluminija v podpovršinski plasti smo zlitino obdelali s postopkoma aluminiziranja v zasipu. Postopek aluminiziranja je potekal v cevni peči, v kateri smo žarili vzorce zakopane v mešanici prahov sestave 1 mas.% Al, 1 mas.% NH4Cl in 98 mas.% Al2O3 pri različnih časih in temperaturah. Po aluminiziranju je sledila visokotemperaturna oksidacija aluminiziranih ter za primerjavo tudi nealuminiziranih vzorcev. Oksidacija je potekala pri 1200 °C v zračni atmosferi. Aluminizirane in oksidirane vzorce smo analizirali z vrstičnim elektronskim mikroskopom (SEM). Preiskovali smo stanje površine, globino aluminiziranja in strukturo oksidnega površinskega sloja. Ugotavljamo, da so temperatura in čas žarjenja ter sestava zasipa ključnega pomena do katere globine bo difundiral aluminij v osnovni material med procesom aluminiziranja v zasipu in katere faze se bodo tvorile na površini. Naloga je potekala v okviru projekta za podaljšanje življenjske dobe žarilnih svečk dizelskih motorjev.

• Jezik: Slovenski jezik
• Ključne besede: FeCrAl zlitine, aluminiziranje, visokotemperaturna oksidacija, žarilne svečke.
• Vrsta gradiva: Magistrsko delo/naloga
• Organizacija: NTF - Naravoslovnotehniška fakulteta
• Leto izida: 2019
• PID: 20.500.12556/RUL-113271
• COBISS.SI-ID: 1851231
• Datum objave v RUL: 18.12.2019

Sekundarni jezik

• Jezik: Angleški jezik
• Naslov: Influence of annealing temperature and time on aluminization kinetics and microstructure of FeCrAl alloy

Izvleček

FeCrAl (Kanthal ®) alloys are high-temperature resistant alloys of iron, chromium, and aluminum, with excellent oxidation resistance at elevated temperatures. In most cases are used for electrical heating and structural elements resistant to elevated temperatures (up to 1400 °C). The oxidation resistance of these alloys is related to the formation of thermodynamically stable and oxygen-impermeable alumina (Al2O3) on the surface, which inhibits further oxidation of the material. Despite their excellent oxidation resistance, the life span of the elements made of these alloys is limited if they are subjected to thermal cycling at temperatures above 1000 °C. The reason for this is cracking of the alumina scale, due to different temperature expansion compared to the alloy matrix, and the consumption of the aluminium in the subsurface area of alloy matrix, due to the constant re-formation of the alumina scale. Once the aluminium content in the subsurface area is reduced beneath critical concentration (<3 wt.%), the alloy can no longer reform the continuous alumina scale, resulting in oxidation of iron and alloying elements and thus gradual destruction of the heating or structural element. The purpose of the master's thesis was to increase the concentration of aluminum in the surface layer of FeCrAl alloy (® Kanthal AF), which would extend the ability to recover the aluminium oxide layer and consequently the life span of the elements made of this alloy. In order to increase the concentration of aluminum in the subsurface layer, the alloy was treated by the pack aluminization process. The pack aluminization process was carried out in the tube furnace in which samples were annealed, buried in a powder mixture of 1 wt.% Al, 1 wt.% NH4Cl and 98 wt.% Al2O3 at various times and temperatures. The aluminization was followed by high-temperature oxidation of the aluminized and for comparison also non-aluminized samples. The oxidation was carried out at 1200 °C in an air atmosphere. The aluminized and oxidized samples were analyzed by a scanning electron microscope (SEM). The surface condition, the diffusion depth of the aluminium and the structure of the oxide coating were investigated. We find out that annealing temperature, annealing time and the composition of the powder mixture are crucial to the depth at which aluminum will diffuse into the base material during the pack aluminization process and which phases will form on the surface. The experiments were part of a project to extend the life span of diesel engine glow plugs.

• Ključne besede: FeCrAl alloys, pack aluminization, high-temperature oxidation, glow plugs.