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Improving the Failure Behavior of High Strength and High Hardenable Steels Resistance Spot Welds

Aghajani, Hamid Reza | 2019

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 52099 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Pouranvari, Majid
  7. Abstract:
  8. Selecting materials is a key stage for designing vehicles. Today, advanced high-strength steels and stainless steels are attractive by the automotive industry. In this study, the improvement of mechanical properties of the martensitic stainless steels (MSS420 with annealed condition) and advanced high strength martensitic steels (MS1400 with quenched condition), which have a different response to mechanical tests, were investigated. The welding of MSS420 steel was carried out in three phases. In the first phase, the MSS420 double pulse welding with variable second pulse times (0.2s-1.8s) with a constant secondary pulse current of 4 kA and in the second phase a double pulse welding with secondary pulse currents (4 to 9 kA) with a constant second pulse duration of 0.24s, were investigated. In these phases, the aim was creating a weld nugget in the range of 4t0.5 - 5t0.5 and improving mechanical properties via double pulse welding without enlarging weld nugget. It was observed that in the first phase with a suitable second pulse time (1.4s) and in the second phase with a suitable second pulse current (5 kA), the mechanical properties of both tests are improved due to increased toughness due to martensite tempering mechanism. However, pullout failure mode in both tests was not achieved. It is also shown in the second phase that in the range of second pulse currents (6kA-9kA), reaustenitization and reformation of untempered martensite, temper embrittlement and intergranular fracture, as well as remelting of weld nugget, which causes the mechanical properties to be decreased and should be controlled. In the third phase, a pure nickel interlayer was used to improve the weldability of MSS420 steel. It was observed mechanical properties were unprecedentedly improved and pullout failure mode was achieved, caused by formation a tough predominately austenitic microstructure and carbide precipitations in the FZ. In the fourth phase, the MS1400 double pulse welding was investigated in variable secondary currents ranging from 5.5 to 9 kA with a constant second pulse time of 0.24 s. It has been shown that by in-situ tempering (7kA), although the cross-tension properties are improved, the tensile-shear properties are reduced (unlike MSS420). It was also observed that in the range of secondary pulse currents (8.5 and 10kA), the optimum mechanical properties were improved through the grain refining and the formation of a soft zone adjacent to FZ edge. Therefore, by in-situ tempering of MSS420 and MS1400 steels, the response of these two steels under mechanical tests was not similar and this study clearly showed this distinction
  9. Keywords:
  10. Mechanical Test ; Resistance Spot Welding ; Advanced High Strength Steel ; Martensitic Steels ; Martensitic Stainless Steel ; Double Pulse Welding ; Interlayer ; Tempering

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