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Investigation of Microstructure and Fracture Behavior of Similar and Dissimilar Dual Phase Steels and Martensitic Steel Spot Resistance Welding

Tamizi, Moein | 2016

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 48953 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Pouranvari, Majid; Movahhedi, Mojtaba
  7. Abstract:
  8. The new generation of advanced high-strength steels use in the automotive industry cause resistance spot welding study and their failure characteristics during static and dynamic loading are important. In this study, the relationship among microstructure, failure mode and mechanical properties are investigate during similar and dissimilar DP980 and MS1400 jointing advanced high-strength steels resistance spot welding. To search for micro and macro structural changes, the optical microscope, stereographic microscope and scanning electron microscope were used. Micro-hardness test, tensile shear test and Cross tension test to evaluate the mechanical properties were conducted. As well as tensile shear test during several steps stopped for fractography and tracking of crack growth path by optical microscope and scanning electron microscope. The results show that in the sub critical heat affected zone (SCHAZ) microstructure with submicron particles of iron carbide (Fe3C) leads to softening and a sharp decreasing hardness. The softening percent in the sub critical heat-affected zone of Dual phase steel and martensitic steel were 67% and 50%, respectively. Weld button hardening as result of martensite with a small amount of bainite and widmanstätten ferrite microstructure. Fracture mode changing with increasing weld nugget size during tensile shear and cross tension test was investigated. Full interfacial (FIF) and pull out (PO) fracture for similar and dissimilar joining was observed in tensile shear test while partial thickness-partial pull out (PT-PO) mode also fail in dissimilar joining. There is no occur pull out fracture for all joining in accordance with 5√t and 4√t common weld nugget size design standards. Failure modes change from FIF to the PIF and then to the PT-PP and finally to the PO, with heat input increasing in tensile shear test for similar and dissimilar DP980 and MS1400 joining. By increasing the diameter of the weld nugget, peak load and failure energy increase in the low heat input (7 to 10 kA). Due to sharply softening phenomenon in high heat input (>12kA), mechanical properties has been remained almost constant. Failure energy for similar DP980 joining was 20% more than dissimilar and similar MS1400 joining. The presence of the soft SCHAZ between the hard martensitic weld nugget and BM zone promotes strain localization in MS1400, lead to limiting mechanical property for dissimilar joint
  9. Keywords:
  10. Fracture Mode ; Softening ; Resistance Spot Welding ; High Strength Steels ; Ferrite-Martensite Dual Phase Steels ; Advanced High Strength Steel ; Tensile Strength Test ; Cross Tension Test

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