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Microstructure-mechanical Properties Relation of Duplex Steel Friction Stir Welds

Ansari, Reza | 2017

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 50591 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Movahedi, Mojtaba; Pooranvari, Majid
  7. Abstract:
  8. Fusion welding of Duplex Stainless Steel (DSS) and dissimilar welding of This alloy and other steels have a lot of problems such as losing phase balance and formation of secondary phases i.e. Sigma Phase. Therefore, Friction Stir Welding (FSW) was used as a substitute for traditional fusion welding techniques of DSS. The present study investigates the relationship between the process variables and microstructural and the mechanical properties of frictional welding of similar and dissimiliar FSW of DSS to carbon steel in order to obtain the joint with the highest mechanical strength. In this study, similar welding of S2304 DSSs and dissimilar welding of S2304 and St12 using FSW were Investigated. The base sheets with the thickness of 10 and 15 cm/min and rotational speeds of 1000, 1200 and 1500 rpm. In order to study mictostructure and phase formation, optical microscopy (OM), Scanning Electron Microscope (SEM) and Energy-Dispersive X-Ray Spectroscopy (EDS) and X-ray Diffraction (XRD) analyses were used. Mechanical properties were studied using microhardness, Transverser Tensile and Shear Punch tests. In similar welding of S2304, it was observed that varition in the process parameters did not have a considerable effect on the grain size. Also, in contrast to fusion welding, variation in the welding parameters had no significant effect on the Austenite/Ferrite ratio. Microstructural study of stir zone showd that from RS to AS, due to an increase in the strain rate, grain size decreased from 10 µm to 5 µm and hardness increased. Tensile strength of the welds increased %16 for the sample with highest Heat input (HI) and %40 for a sample with lowest HI due to grain size refining in SZ. Fracture mostly occurred in RS, which was the weakest part of the weld. In Dissimilar welds, existence of martensite structure in SZ was approved using EDS and microhardness tests. Martensite was formed due to diffusion of atoms between two base alloys. Fracture in transverse tensile samples of dissimilar welds occurred in carbon steel base metal. Shear punch test showed that strength of dissimilar welds was %16 more than DSS base metal and twice carbon steel Base metal. Putting the DSS in AS increased the joint strength up to %70 and decreased hardness due to a decrease in volume fraction of martensite
  9. Keywords:
  10. Carbon Steels ; Duplex Stainless Steel ; Friction Stir Welding ; Microstructure ; Mechanical Properties ; Similiar Welding ; Dissimiliar Welding

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