Naslov (srp)

Uticaj kratkotrajnog pregrevanja na mehaničke osobine i mikrostrukturu centrifugalno livene legure otpornena toplotu - HP40 Nb

Autor

Timotijević, Milica, 1993-

Doprinosi

Erić Cekić, Olivera, 1966-
Rajnović, Dragan, 1975-
Đurđev, Mića, 1988-
Janjatović, Petar, 1991-

Opis (srp)

Normalni projektovani radni vek cevi za reformere definisan je ARI standardom iiznosi 100 000 sati. Stvarni radni vek cevi zavisi od mnogih mehanizama oštećenjakoji se javljaju tokom eksploatacije, može biti duži ili kraći od projektovanog radnogveka. Kratkotrajno pregrevanje može da nastane posle nekoliko sati rada i obično sejavlja kada je cev ostala bez radnog fluida. Pregrevanje može da dovede domikrostrukturnih promena, pada mehaničkih osobina i na kraju do loma cevi.Ova doktorska disertacija bazirana je na istraživanju uticaja kratkotrajnogpregrevanja na mehaničke osobine i razvoj mikrostrukture, sadržaja primarnih isekundarnih karbida, mehanizama oštećenja cevi sa aspekta pregrevanja cevi zareformere (peći za pirolizu). Za ispitivanje su upotrebljeni uzorci segmenta cevinovog (HP40-0,63%Nb) i starog materijala (HP40-1,5%Nb) nakon više od 100 000 satirada. Analize su sprovedene na temperaturama pregrevanja od 950, 1050 i 1150 °C, jerovi materijali imaju široku primenu u petrohemijskoj industriji, posebno u uslovimadugotrajne izloženosti temperaturama između 850 i 1150 °C. Istraživanje jeobuhvatilo analizu hemijskog sastava, metalografska i fraktografska ispitivanja,rendgensku difrakcionu analizu, ispitivanje zateznih osobina i tvrdoće. Dobijenirezultati pokazali su da temperatura i trajanje pregrevanja značajno utiču na mehaničkekarakteristike i strukturu materijala.Kod novog materijala, najveći napon tečenja zabeležen je na 950 °C, a najniži na 1150 °C.Zatezna čvrstoća se povećala u proseku za 20 %, dok je izduženje nakon pregrevanja na950 i 1050 °C smanjeno za 12%, a na 1150 °C povećano za 35 %. Mikrostrukturna analizaje pokazala da viša temperatura dovodi do smanjenja količine sekundarnih karbida iizmene morfologije primarnih karbida, koji na 1150 °C dobijaju globularni oblik.Kod starog materijala primećen je napredak degradacije. Vrednosti izduženja su sesmanjile za 44% na 950 i 1050 °C, ali su se povećale za 67 % na 1150 °C. Zatezna čvrstoćaje u proseku porasla za 12,7 %, dok je napon tečenja blago opao (za 1,6 %) u odnosu nastanje pre pregrevanja. Na temperaturi od 1150 °C došlo je do potpunog rastvaranjasekundarnih karbida u mikrostrukturi.Na kraju, primenjene su ANOVA i regresiona analiza radi optimizacije parametarapregrevanja. Dobijene su maksimalne i minimalne vrednosti mehaničkih osobina,poput zatezne čvrstoće, napona tečenja, izduženja i tvrdoće po Vikersu. Ovakvimpristupom potvrđeni su rezultati eksperimentalnih istraživanja i dobijene smerniceza dalje primene u industrijskoj praksi.

Opis (srp)

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Opis (eng)

The normal designed service life of reformer tubes is defined by the API standard and amountsto 100.000 hours. The actual service life of the tubes depends on numerous damage mechanismsoccurring during operation and may be either longer or shorter than the designed life. Shortterm overheating can occur after a few hours of operation and typically arises when a tube isdevoid of working fluid. Overheating can lead to microstructural changes, a decline inmechanical properties, and ultimately, tube failure.This doctoral dissertation is based on the study of the effects of short-term overheating on themechanical properties, microstructural evolution, primary and secondary carbide content, anddamage mechanisms of reformer tubes (pyrolysis furnace tubes) from the perspective ofoverheating. The research was conducted on tube segment samples of both new (HP40-0.63%Nb) and used material (HP40-1.5%Nb) after more than 100.000 hours of operation.The analyses were performed at overheating temperatures of 950, 1050, and 1150 °C, as thesematerials are widely used in the petrochemical industry, particularly under long-term exposureto temperatures ranging from 850 to 1150 °C. The study included chemical compositionanalysis, metallographic and fractographic examinations, X-ray diffraction analysis, and testingof tensile properties and hardness. The obtained results demonstrated that both temperature andoverheating duration have a significant impact on the mechanical characteristics and structureof the material.In the case of the new material, the highest yield strength was observed at 950 °C, while thelowest was noted at 1150 °C. The tensile strength increased by an average of 20 %, whereaselongation decreased by 12 % after overheating at 950 °C and 1050 °C. However, elongationimproved by 35 % at 1150 °C. Microstructural analysis showed that higher temperaturesresulted in a reduction of secondary carbide content and altered the morphology of primarycarbides, which took on a globular shape at 1150 °C.For the aged material, signs of degradation were evident. Elongation values decreased by 44%at both 950 °C and 1050 °C, but increased by 67 % at 1150 °C. The tensile strength rose by anaverage of 12.7 %, while the yield strength experienced a slight decrease of 1.6 % compared tothe pre-overheating condition. At 1150 °C, secondary carbides were completely dissolvedwithin the microstructure.Finally, ANOVA and regression analysis were applied to optimize overheating parameters. Thestudy identified the maximum and minimum values of mechanical properties, including tensilestrength, yield strength, elongation, and Vickers hardness. This optimization process validatedthe experimental findings and provided guidelines for further industrial applications.

Jezik

srpski

Datum

2025

Licenca

Ovo delo je licencirano pod uslovima licence
Creative Commons CC BY-NC-ND 3.0 AT - Creative Commons Autorstvo - Nekomercijalno - Bez prerada 3.0 Austria License.

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