Effect of short-term overheating on mechanical properties and microstructure of centrifugal heat-resistant casting alloy - HP40 Nb

[M. S. Timotijević]

VII, 191 listova

Normalni projektovani radni vek cevi za reformere definisan je ARI standardom i iznosi 100 000 sati. Stvarni radni vek cevi zavisi od mnogih mehanizama oštećenja koji se javljaju tokom eksploatacije, može biti duži ili kraći od projektovanog radnog veka. Kratkotrajno pregrevanje može da nastane posle nekoliko sati rada i obično se javlja kada je cev ostala bez radnog fluida. Pregrevanje može da dovede do mikrostrukturnih promena, pada mehaničkih osobina i na kraju do loma cevi. Ova doktorska disertacija bazirana je na istraživanju uticaja kratkotrajnog pregrevanja na mehaničke osobine i razvoj mikrostrukture, sadržaja primarnih i sekundarnih karbida, mehanizama oštećenja cevi sa aspekta pregrevanja cevi za reformere (peći za pirolizu). Za ispitivanje su upotrebljeni uzorci segmenta cevi novog (HP40-0,63%Nb) i starog materijala (HP40-1,5%Nb) nakon više od 100 000 sati rada. Analize su sprovedene na temperaturama pregrevanja od 950, 1050 i 1150 °C, jer ovi materijali imaju široku primenu u petrohemijskoj industriji, posebno u uslovima dugotrajne izloženosti temperaturama između 850 i 1150 °C. Istraživanje je obuhvatilo analizu hemijskog sastava, metalografska i fraktografska ispitivanja, rendgensku difrakcionu analizu, ispitivanje zateznih osobina i tvrdoće. Dobijeni rezultati pokazali su da temperatura i trajanje pregrevanja značajno utiču na mehaničke karakteristike 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 na 950 i 1050 °C smanjeno za 12%, a na 1150 °C povećano za 35 %. Mikrostrukturna analiza je pokazala da viša temperatura dovodi do smanjenja količine sekundarnih karbida i izmene morfologije primarnih karbida, koji na 1150 °C dobijaju globularni oblik. Kod starog materijala primećen je napredak degradacije. Vrednosti izduženja su se smanjile za 44% na 950 i 1050 °C, ali su se povećale za 67 % na 1150 °C. Zatezna čvrstoća je u proseku porasla za 12,7 %, dok je napon tečenja blago opao (za 1,6 %) u odnosu na stanje pre pregrevanja. Na temperaturi od 1150 °C došlo je do potpunog rastvaranja sekundarnih karbida u mikrostrukturi. Na kraju, primenjene su ANOVA i regresiona analiza radi optimizacije parametara pregrevanja. Dobijene su maksimalne i minimalne vrednosti mehaničkih osobina, poput zatezne čvrstoće, napona tečenja, izduženja i tvrdoće po Vikersu. Ovakvim pristupom potvrđeni su rezultati eksperimentalnih istraživanja i dobijene smernice za dalje primene u industrijskoj praksi.

The normal designed service life of reformer tubes is defined by the API standard and amounts to 100.000 hours. The actual service life of the tubes depends on numerous damage mechanisms occurring 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 is devoid of working fluid. Overheating can lead to microstructural changes, a decline in mechanical properties, and ultimately, tube failure. This doctoral dissertation is based on the study of the effects of short-term overheating on the mechanical properties, microstructural evolution, primary and secondary carbide content, and damage mechanisms of reformer tubes (pyrolysis furnace tubes) from the perspective of overheating. 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 these materials are widely used in the petrochemical industry, particularly under long-term exposure to temperatures ranging from 850 to 1150 °C. The study included chemical composition analysis, metallographic and fractographic examinations, X-ray diffraction analysis, and testing of tensile properties and hardness. The obtained results demonstrated that both temperature and overheating duration have a significant impact on the mechanical characteristics and structure of the material. In the case of the new material, the highest yield strength was observed at 950 °C, while the lowest was noted at 1150 °C. The tensile strength increased by an average of 20 %, whereas elongation decreased by 12 % after overheating at 950 °C and 1050 °C. However, elongation improved by 35 % at 1150 °C. Microstructural analysis showed that higher temperatures resulted in a reduction of secondary carbide content and altered the morphology of primary carbides, 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 an average of 12.7 %, while the yield strength experienced a slight decrease of 1.6 % compared to the pre-overheating condition. At 1150 °C, secondary carbides were completely dissolved within the microstructure. Finally, ANOVA and regression analysis were applied to optimize overheating parameters. The study identified the maximum and minimum values of mechanical properties, including tensile strength, yield strength, elongation, and Vickers hardness. This optimization process validated the experimental findings and provided guidelines for further industrial applications.

HP40 legura, mehaničke osobine, mikrostruktura, primarni karbidi, sekundarni karbidi, pregrevanje, ANOVA, regresiona analiza.

HP40 alloy, mechanical properties, microstructure, primary carbides, secondary carbides, overheating, ANOVA, regression analysis.

620.1:66(043.3)

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