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Shirmohammadi, Davood | 2016

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 50868 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Movahedi, Mojtaba
  7. Abstract:
  8. Martensitic stainless steels with high strength, good corrosion resistance and formability in the annealed condition are good choices for automotive applications, but they suffer from low weldability due to high hardenability and formation of brittle martensite without tempered structure in the weld metal and heat affected zone (HAZ). In other word, martensitic stainless steels have the worst weldability among all kinds of stainless steels. On the other hand, resistance spot welding is the main method for joining sheets in the automotive industry. Therefore, in the present work, microstructure and mechanical properties of the resistance spot welds of AISI 420 martensitic stainless steel in the as-weld condition and after in-situ post-weld heat treatment (PWHT) were investigated. For this purpose, effects of welding and in-situ PWHT variables including welding current (7-14 kA), cooling time between welding and PWHT (10-99 cycle) and PWHT time at the current of 4 kA (in 5-99 cycle) on the microstructure and mechanical properties of the weld zone were studied. The results showed that weld nugget after welding contains mainly martensite along with some delta ferrite. Also after welding, HAZ includes tempered martensite (far away from the nugget) and martensite with carbides (near the nugget). Fracture mode in the as-weld samples was interfacial with brittle fracture features. The results of the mechanical tests showed that fracture force as well as nugget diameter and depth increases by increasing welding current to 13 kA. However, increasing welding current to more than the optimized value leads to extended expulsion and electrode indentation which consequently reduces weld strength. After in-situ PWHT, the martensite in the welding nugget is tempered gradually from the nugget perimeter towards the center of it. Development of tempering in the nugget changes brittle and interfacial fracture to partial interfacial and soft fracture which in turn increases the weld strength. Optimal variables for PWHT at the current of 4 kA are cooling time of 99 cycle and PWHT time of 99 cycle. In this situation, tensile shear force and fracture energy increase about 100% and 300%, respectively, in comparison to as-weld condition
  9. Keywords:
  10. Resistance Spot Welding ; Weld Metal ; Post Weld Heat Treatment Cracking (PWHT) ; Heat-Affected Zone ; Martensitic Steels ; Stainless Steel

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