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Welding Metallurgy and Weldability of Martensitic Advanced High Strength Steel and Duplex Stainless Steels during Resistance Spot Welding

Sobhani, Saeed | 2017

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 50093 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Pouranvari, Majid
  7. Abstract:
  8. Duplex stainless steels and advanced high strength steels are both known as new materials in automobile industry applications. Process of resistance spot welding is the main process in welding of auto-body structures. Hence investigating the welding metallurgy and weldability of similar and dissimilar joints of duplex stainless steels and advanced high strength steel are crucial. The purpose of this study is to analyse the relationship between welding process and geometric characteristic of weld nugget, phase transformations of Weld nugget and heat affected zone, mechanical properties (i.e force bearing capacity and energy absorption capability) and failure mode (interfatial or pullout). in this research, to study mechanical properties, tensile-shear and microhardness tests were used. In addition, optical and scanning electron microscopes were utilized in order to carry out structural study of the joints. In the fusion zone of similar joint of martensitic steel, martensite was mainly formed because the cooling rate of process was higher than critical cooling rate. The HAZ of martensitic steel has three distinct zones. In the similar joint of duplex stainless steel, phase balance in the HAZ and FZ was lost as a consequence of high cooling rate of RSW process. The microstructure of FZ in dissimilar joint of martensitic and duplex steels, by considering the WRC 1992 diagram, includes martensite and ferrite. In this study, structural diagrams were used to predict the fusion zone's structure of DSS-MS joint. The results of these diagrams were accordant to the experiental result. Failure mode of the investigated joints transforms from IF to PF as the current rises. Critical weld nugget's diameter of the joints was compared to the theoritical equations recommended by other researchers.changes in the mechanical properties of the joints (including failure force, elongation and failure energy) in relation to current's changes were surveyed as well. As the current increased, first, mechanical properties improved but after that, due to expulsion and electrode indentation the mechanical properties deteriorated. In the pullout failure mode in all joints, failure started from the location which had the lowest hardness level. In a dissimilar RSW weldment, others factor such as strain hadening exponent and electrode indenation depth effect pullout failure location. In this study, macrosegregation occurred at 7 and 8 KA in the MS-DSS joint. This phenomenal took place because of the differences between chemical composition and melting point temperature of the two mentioned base metals. By increasing the heat input which is the result of current's increament, the macrosegregation was eliminated
  9. Keywords:
  10. Duplex Stainless Steel ; Resistance Spot Welding ; Advanced High Strength Steel ; Heat-Affected Zone ; Macrosegregation ; Weld Metal

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