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Effect of Titanium on the Microstructure and Sour Environment Cracking Resistance of API 5L-X70 SAW Weld Metals

Beidokhti, Behrooz | 2009

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 39521 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Kokabi, Amir Hossein; Dolati, Abolghasem
  7. Abstract:
  8. In this research, the effect of addition of titanium to the weld metal of API 5L-X70 steel at two levels of manganese (1.4 and 2.0%) was studied; and the microstructure, mechanical properties, and hydrogen cracking resistance of the weld metals were investigated carefully. The submerged arc welding method was used for preparation of the welds and the metallographic test, hardness test, longitudinal tension test, Charpy V-notch impact test, HIC and SSC evaluation tests and thermal desorption spectroscopy test were done for each weld. The results showed that the addition of titanium to the weld metal increased formation of acicular ferrite; therefore, toughness of the welds was increased simultaneously. By further addition of titanium, the hard phases like bainite and martensite/austenite were formed in the weld and ductility was decreased again. The optimum percentage of titanium in the weld was decreased by increasing the manganese amount of the weld; consequently, the best combination of microstructure and properties in the low-manganese and high-manganese containing welds was obtained in two compositions, i.e. 1.40%Mn-0.08%Ti and 1.92%Mn-0.02%Ti, respectively. Furthermore by the addition of titanium to the weld, the amount of this element in inclusions was increased while the amount of manganese in them was decreased. The HIC and SSC tests revealed that the best performance in sour environments depends on the microstructure and the inclusions distribution. The weld metals containing more than 60% acicular ferrite and good distribution of titanium carbonitrides in the microstructure experienced no cracking after completion of the H2S tests. Also by using thermal desorption analysis, the hydrogen trapping activation energy of Ti(C, N) particles was calculated between 81.4 and 83.1 kJ/mol. Therefore, these particles act as strong hydrogen traps and can delay the formation of hydrogen cracking. In the Ti-free weld metals, formation of microvoids around MnS inclusions caused cracking in the specimens. On the other hand in the specimens containing high percentage of alloying elements, formation of bainite, perlite and martensite/austenite microphases as weak hydrogen traps encouraged catastrophic cracking of the weld metals in sour environments.

  9. Keywords:
  10. Titanium ; Steels ; Sour Gas ; Inclusion ; Weld Metal ; Hydrogen Trap

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