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The Effect of Electropulsing on the Mechanical Properties of Cold Rolled Low Carbon Steel

Alaghmand Fard, Reza | 2015

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 49751 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Kazeminezhad, Mohsen
  7. Abstract:
  8. In this research, low carbon steel rolled under 16%, 33% and 50% deformation and applied under electric pulses. Microstructure investigated by optical microscopy and scanning electron microscopy in order to observe grain size, precipitation size and distribution and hardness measured by Vicker's hardness test, after each electric pulse treatment. I order to investigate microstructure evolution as well as grain size, precipitates and hardness all the species with different deformation percentages applied under electric pulses with different total electric pulse duration, number of pulses and the off time between two successive pulses. To obtain required information about species with different deformation percentages, the duration of each pulse varied between 0.1, 0.3 and 0.5 second in a constant off time (1 second). Furthermore, species undertake 65-4000 electric pulses in order to obtain 20-400 seconds of total time. By applying electric pulses to the cold rolled steel species it is observed that increasing total applying time to 20 seconds leads to dislocation accumulation on subgrains and increasing subgrain misorientation which causes forming main grain boundaries, hence finer grains obtain. Moreover, due to increasing the precipitation nucleation kinetic which form more nucleation sites, decreasing the precipitation growth kinetic, dislocation recovery which are effective diffusion paths for atoms and vacancies, precipitation size for every deformation percentage occur. Moreover, hardness decreases owe to decreasing grain size and precipitation size. By increasing total applying time to 140 seconds, grain size increases by dislocation recovery and they become coarser due to increasing aging time, increasing the width of precipitation free zones (PFZ) and temperature accumulation that enhance diffusion coefficient therefor hardness decreases. Altering the on-time of each electric pulse affects hardness changes. In longer on-times microstructure carries more current and atoms get more chance to diffuse and precipitate. On the other hand, increasing the on-time of each pulse leads to decreasing thermodynamic barriers which causes obtaining maximum hardness in less numbers of applying pulses. Also, applying electric currents causes temperature increasment and owe to high density of passing current in highly deformed species, temperature will increase. By increasing deformation percentage, the temperature in which the maximum hardness obtains increases. According to the results of this research it can be concluded that cold rolled low carbon steel achieves its finest grain size and precipitation size in 20 seconds of applying electric current, as a result the maximum hardness obtains in this state. Additionally, applying electric current has greater effect on the species with lighter deformation. Furthermore, to obtain the maximum of hardness in shorter times, the on-time should increase from 0.1 to 0.5 seconds. According to what was said, the most optimum state of obtaining finest microstructure and the maximum hardness in the shortest time is applying electric pulses for 20 second with 0.5 second on-time on 16% deformed specimen
  9. Keywords:
  10. Microstructure ; Mechanical Properties ; Low Carbon Steel ; Rolling Process ; Electric Pulse

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