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Study of Microstructure and Mechanical Properties of Heat Affected Zone with Gleeble Simulator for Tandem Submerged Arc Welding of X80 HSLA Steel

Moeinifar, Sadegh | 2010

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 40897 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Kokabi, Amir Hossein; Madaah Hosseini, Hamid Reza
  7. Abstract:
  8. The X80 high-strength low-alloy microalloyed steel was procured as a hot rolled plate with accelerated cooling. The four-wire tandem submerged arc welding process, with different heat input, was used to generate a welded microstructure. The effects of real and Gleeble simulated double pass thermal cycles on the properties of the heated and reheated coarse grained heat affected zones (CGHAZ) in X80 microalloyed pipeline steel has been investigated. The thermal simulated process for heated region involved heating the X80 steel specimens to the peak temperature of 1400 °C, with different cooling rates. The influence of the second peak temperature during real and simulated welding hass been studied on properties of the subcritically (S), intercritically (IC) and supercritically (SC) reheated CGHAZ regions. This involved heating to a first peak temperature (TP1) of 1400 °C, then reheating to different second peak temperatures (TP2) of 700, 800 and 900 °C with a constant cooling rate. The microstructure of the real and simulated reheated heated and reheated regions was investigated using an optical microscope, field emission scanning electron microscope (FESEM) and transmission emission electron microscope (TEM). The martensite/austenite (M/A) constituent appeared in the microstructure of the heat affected zone region for all the specimens along the prior-austenite grain boundaries and between bainitic ferrite laths. The size and area fraction of (M/A) constituents were obtained by a combination of FESEM and image analysis software. The blocky-like and stringer M/A morphology were observed in the CGHAZ region. In addition, misorientation was characterized by electron back-scatter diffraction analysis. It is clear that the intercritically thermal cycles have a significant effect on morphology of M/A constituents. The hardness values were not different for the thermal simulated CGHAZ cycles with different prior-austenite grain size. The fractional area of the M/A particles was the main factor in increasing the hardness values in the real and simulated CGHAZ region. The hardness values of real simulated reheated CGHAZ increased with increasing heat input due to higher fraction of M/A constituent Toughness of the real and simulated heated and reheated regions were assessed using Charpy impact testing at 0 and -50 °C. The M/A constituent’s size, such as mean diameter and length, are important factors influencing Charpy impact properties of thermally simulated intercritically reheated heat affected zones. The intercritical reheated thermal cycles with lower heat input value showed higher Charpy impact absorbed energy due to a decrease in the prior-austenite grain and M/A particle size. Brittle particles, such as martensite/austenite constituent along the grain boundaries, can make an easy path for crack propagation. Similar crack initiation sites and growth mechanism were investigated for specimens welded with different heat input values. The SC CGHAZ region showed higher absorbed energy with lower hardness. Design of multipass weld with less IC CGHAZ regions can result in a higher toughness property
  9. Keywords:
  10. Thermal Modeling ; Martensite ; Grain Size ; Thermal Cycle ; Austenite ; Heat-Affected Zone ; Tandem Submerged Arc Welding ; Microalloys Steels

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