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The Effect of Martensite Volume Fraction on Toughness of Triple-Phase Steels

Zare, Ahmad | 2010

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 41093 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Ekrami, Ali Akbar
  7. Abstract:
  8. AISI 4340 steel bars were austenitized at 900°C for 1 hour followed by heating at 740°C (ferrite and austenite region) for 100 min and quenching into a salt bath at 300°C for different times followed by quenching into water to obtain triple phase microstructures with 34 Vol.% ferrite and various martensite (or bainite) contents. Presence of three phases in adjacent of each other confirmed by metallographic analysis and TEM technique. Volume fraction of different phases was measured by image analyser. The results of optical microscopy showed that by increasing VM, morphology of martensite varies from uniform distribution of small spherical network particles to large blocky islands with non-uniform distribution. Tensile, impact and hardness tests showed that increasing VM up to 21.4% leads to increase in yield and ultimate tensile strengths and hardness, but reduction in uniform and total elongation, tensile fracture energy and impact energy. Charpy impact test results at different temperatures show that, DBTT of DP steel is less than TP steel with 8.5 Vol.% martensite by 8 degree centigrade. Fracture surfaces of tensile and impact specimens were examined by SEM and stereo microscopes confirmed decrease in toughness of TP steels with increasing VM. The observed decrease in toughness in different terms can be attributed to high density of dislocations at interphase boundaries and ferrite grains adjacent to bainite and martensite, in triple phase microstructures. Also the tensile flow behavior of all samples was explained, using Holloman equation. Results showed that in both DP and TP steels, work hardening takes place in two stages and Holloman equation parameters (K and n) in both stages increased with increasing VM
  9. Keywords:
  10. Martensite Volume Fraction ; Tensile Properties ; Triple Phase Steel ; Impact Energy

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