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The Effect of Martensite Morphology on Mechanical Properties of TLP Bonded Dual Phase Steels

Fazaeli, Abolfazl | 2015

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 47729 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Ekrami, Ali Akbar; Kokabi, Amir Hossein
  7. Abstract:
  8. In the present study the production of ferrite-martensite dual-phase (DP) Steel was investigated during the bonding process by transient liquid phase method. The effect of martensite morphologies on mechanical properties of bonding zone were also studied. To make the bonding process and DP steel production heat treatment cycle simultaneously, the step in which the isothermal solidification completed, bonding process, was done simultaneously with austenitising of the steel. Homogenizing of the bond zone, was also done with DP steel making at the intercritical temperature. The St52 steel was utilized for producing DP steel and for bonding process, iron base interlayer with melting point of 1170°C and 40μm thickness was used. Bonding process was done at 1200°C under pressure of 0.5MPa at different time. The bonding time in which isothermal solidification completed was chosen as the optimum time of bonding. Microstructural investigation indicated that isothermal solidification was completed in 40 minutes. Chemical composition of different region in the joint zone before completion of isothermal solidification, illustrated that at the bonding temperature, the isothermal solidification area consists of γ phases, and by decreasing temperature, the residual melt transforms to γ and M2B due to the eutectic transformation. After finding the optimum bonding time in mentioned temperature, various heat treatment cycles were tested to produce DP steels with different martensite morphology but with the same volume fraction of martensite. The holding time of samples at the inter-critical temperature of 735°C to achieve ferrite-continous martensite, ferrite-fibrous martensite, and ferrite-blocky martensite, with about 40% martensite, were 60, 75 and 90 minutes respectively. By combination of isothermal solidification time and the required time for production of 40% martensite with different morphologies of martensite, simultaneous joining and producing of this type of steels was done. Based on the chemical composition of bonding zone, it was clear that for all variant morphology of martensite, the DP steel making condition, temperature and time, is not sufficient for making a homogenous chemical composition in the bonding zone. The inhomogeneity of chemical composition lead to the formation a completely martensite microstructure in the joint line, and a mainly ferrite microstructure in the diffusion affected zone. With conducting the shear strength tests, it was observed that for all three type of produced DP steels, the fracture was propagated in the diffusion affected zone. Generally based on quantitative results of mechanical tests and fracture surface analysis, it was observed that the fibrous martensite, as compared to the other two morphologies, provides better mechanical properties in DP steels, and then continuous network and blocky martensite are in the second and the third position
  9. Keywords:
  10. Transient Liquid Phase (TLP) ; Martensite ; Ferrite-Martensite Dual Phase Steels ; Dual-Phase Steels ; Ductile Fracture Criteria ; Inter-Critical Temperature ; Diffusion Affected Zone

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