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An Investigation on Multistage Strain Aging Using Rolling Process on Mechanical and Formability of Stainless Steel 304L

Mousavinia, Ali | 2022

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 55297 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Karimi Taheri, Ali; Akbarzadeh Changiz, Abbas
  7. Abstract:
  8. Austenitic stainless steels have extensive and special mechanical properties because of austenite to α΄-martensite transformation due to plastic work. Obtaining a process path that provides maximum strength and flexibility and minimum α΄-martensite content (α΄-martensite increases corrosion current) in these steels is important in the industry. Applying the aging process after applying plastic strain (single-stage aging) results in an increase in strength of up to 200 MPa and an increase in hardness of up to 60 Vickers. However, single-stage aging does not cause significant changes in the amount of α΄-martensite in steel and in the temperature range of 400 to 500 ° C, it causes a slight increase in α΄-martensite. By applying multi-stage aging in optimal temperature and time and the amount of plastic strain, an increase in the desired strength compared to the unaged sample along with an increase in flexibility compared to the single-stage strain aging sample, with a significant decrease in α΄-martensite could be achieved. In this regard, tensile and hardness tests were used to compare the mechanical properties of non-aged samples and single-stage and multi-stage aging samples. In order to validate the process path and compare the amount of α΄-martensite in different samples, frittoscopic tests were carried out at different stages, which shows a decrease in the amount of α΄-martensite from about 36% to about 8% with increasing temperature in four-stage aging. X-ray spectroscopy and metallography were used to ensure the accuracy of frittoscopic data in reducing the amount of α΄-martensite by increasing the multi-stage strain aging temperature. The optimum temperature for multi-stage strain aging was obtained in the temperature range of 300 to 350 ° C and aging time of 70 minutes for each pass. An increase in strength and hardness compared to the non-aged sample due to a higher density of dislocations in multi-stage strain aging and an increase in ductility compared to single-stage strain aging samples due to a significant drop in α΄-martensite due to an increase in the stacking fault energy, at the corresponding aging temperature were among the observed effects of multi-stage strain aging.
  9. Keywords:
  10. Austenite ; Martensite ; Mechanical Properties ; Multistage Strainaging ; Single-Stage Strain Aging ; Rolling Process ; 304 Austenitic Stainless Steel

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