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Microstructural Evolution During Post-Bond Heat Treatment of Brazed Nickel Base Super Alloys

Soltani Abri, Hamid | 2022

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 55825 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Pouranvari, Majid; Kokabi, Amir Hossein
  7. Abstract:
  8. The brazing process is the preferred method for bonding and repairing high temperature resistant superalloys. In this process, filler metals containing MPD elements such as B and Si are used. The presence of these elements causes the formation of brittle and eutectic phases such as borides and silicides in the joint area. These phases reduce the mechanical properties and re-melting temperature of the joint. By increasing the temperature and time of bonding and using the concept of isothermal solidification, the formation of boride and silicide phases can be prevented. In wide gap joints, the time to complete the isothermal solidification is very long, and applying such temperature and time not only causes grain growth and reduction of the mechanical properties of the base metal, but is also not economically viable. In this case, where it is not possible to complete the isothermal solidification, increasing the bonding time does not help to increase the strength of the joint and can even have destructive effects. In this study, the brazing of Inconel 718 alloy with BNi-2 filler metal and 225 um gap joint was investigated. The ASZ zone in this joint includes γ, Ni3B, CrB and Ni-Si-B ternary eutectic phases. γ is the softest, CrB is the hardest and Ni-Si-B ternary eutectic has the lowest melting point temperature phase among ASZ zone phases. By increasing the bonding time, chemical composition of the residual liquid changes, and this changes the microstructure of the ASZ zone. This change is such that it increases the amount and continuity of harmful CrB and Ni-Si-B ternary eutectic phases and decreases the soft γ phase. As a result, in wide gap joints, reducing the holding time at the bonding temperature will improve the microstructure and increase the joint strength. After a short time brazing, 4 heat treatment strategies were done to modify the microstructure. Heat treatment of partial melting and solution at 980 °C caused re-melting of the Ni-Si-B ternary eutectic phase and its decomposition into γ and Ni3B phases. Heat treatment of aging caused the CrB to break and disperse, spheroidization of Ni3B and increasing the strength of γ phases in the ASZ zone. Finally, under the heat treatment of solution and aging, the benefits of both heat treatments were used. In this case, the average hardness of the ASZ region decreased and the shear strength of the bonding increased from 250 MPa to 420 MPa. Resistance to crack growth was also increased by ductile phase toughening
  9. Keywords:
  10. Hard Brazing ; Heat Treatment ; Microstructure Modification ; Nickel-Base Superalloy ; Microstructure Modification ; Microstructure Evolution ; Ductile Phase Toughening

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