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The Effect of Ultra-rapid Annealing on Microstructure and Mechanical Properties of Severely Deformed Low-carbon Steel

Mostafaei, Mohammad Ali | 2016

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 48279 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Kazeminezhad, Mohsen
  7. Abstract:
  8. Severe plastic deformation (SPD) can refine microstructure of steel and increase its strain energy. Ultra-rapid annealing (URA) influences on the temperature and kinetics of recrystallization and transformation, and causes to improve the mechanical properties of steel. In this research, low-carbon steel with the average grain size of 80 μm was subjected to SPD using 2 passes of constrained groove pressing (CGP) method and its grains size is reduced to 30 μm. Ultra-rapid annealing with the reference heating rate of 200 oC/s was performed on SPDed steel up to subcritical and intercritical temperature ranges. Other heating rates of 75 oC/s to 1800 oC/s were performed for further examinations. Also, conventional annealing in the furnace for comparing with URA were done. Finally, according to the results and also other researches, schematic models for analyses and prediction of behavior of steel during URA are proposed. URA causes to increase the start temperature of recrystallization and leads to interaction between recrystallization and ferrite to austenite transformation. Because of occurrence of strong interaction, URA by 200 oC/s up to 730 oC leads to grain refinement of steel to 7.5 μm. In this condition, austenite is formed finely and dispersed massively around the recrystallized ferrite grains, and prevents grain growth during URA up to higher temperatures. Non-SPDed sample after mentioned condition of URA, although shows grain refinement, however it is undergone abnormal grain growth during URA up to higher temperatures due to occurrence of weaker interaction and lower volume fraction of austenite during URA in comparison with those of SPDed sample. By using proposed schematics models, it can be seen that imposing strain causes to shift the interaction zone to higher heating rates. This means that more deformed steel should be URAed with higher heating rates in order to undergo the strong interaction and subsequent grain refinement. URA of SPDed steel by 600 oC/s up to 730 oC causes to form two-size ferrite grains (2 and 10 μm) and leads to increase the hardness, strength and ductility of steel about 67%, 80% and 7%, respectively, with respect to those of as-received steel, and 16%, 44% and 24%, respectively, with respect to those of SPDed sample. This shows the benefit of combining URA and SPD for improving the mechanical properties of low-carbon steel. Intercritically URAed samples show higher hardness, strength and ductility in comparison with those of samples URAed in subcritical temperature range. In order to show the effect of starting microstructure, two different microstructures were subjected to SPD followed by URA; steel contains carbides (as-received sample) and steel contains pearlite (austenitized sample). Results show that the presence of carbides increases the stored strain energy after SPD and provides preferred sites for recrystallization and austenite formation during URA. This causes to increase the intensity of interaction of SPDed steel during URA. Austenitized sample is not undergone grain refinement after URA, since the presence of pearlite causes to localize the austenite transformation and prevent the occurrence of strong interaction. However, URA can improve the mechanical properties of SPDed austenitized sample more than for SPDed as-received one. Determination of recrystallization fraction for URAed and conventionally annealed low-carbon steel also shows the increasing of recrystallization start temperature and its kinetics during URA, which confirms the proposed schematic models and presented explanations for the effect of URA on microstructure
  9. Keywords:
  10. Recrystallization ; Mechanical Properties ; Severe Plastic Deformation ; Microstructure ; Phase Transformation ; Ultra-Rapid Annealing

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