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The Effect of Heat Treatment on Microsructure, Hot Tensile Properties and Creep Bihavior of Nickel-based Superalloy CM88Y

Gorji, Morteza | 2015

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 48047 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Asgari, Sirous
  7. Abstract:
  8. The Ni-base superalloy CM88Y has good hot corrosion resistance, high stability and strength at elevated temperature; accordingly, the alloy is used in manufacturing of gas turbine hot components. In most applications, heat treatment is required in order to achieve the desired mechanical properties and corrosion resistance of the working conditions; often three stages of heat treatment is designed for precipitation hardening nickel-base superalloy. In general, the effective parameters of heat treatment cycles includes temperature, time, heating rate and the cooling rate. In this study, the effect of cooling rate from partial solution temperature (second stage of heat treatment) on the microstructure, hot tensile properties and creep behavior of superalloy CM88Y were investigated. For this purpose, six different cooling rates between 1.7℃/min and 125℃/min were applied on homogenized CM88Y specimens after partial solution at 1050℃ for 4.5 h. Then, microstructures of the specimens were studied using optical and scanning electron microscopy. It was found out that with increasing cooling rate the particle size of primary and secondary γ΄ decreases. Also, nucleation of secondary γ΄ decreases by reducing cooling rate; hence, at the rate of 125℃/min the nucleation is maximized, while at 1.7 ℃ / min cooling rate the secondary γ΄ was not observed. The primary γ΄ particle shape varied from spherical to cubic by reducing cooling rate.
    To verify the effect of microstructure changes on mechanical properties, tensile test at 900℃ and stress rupture test at 900℃/274 MPa condition have been done. Therefore, the tensile test results showed that the yield strength and the tensile strength at 900℃ are promoted by increasing the cooling rate. Furthermore, the stress rupture curves analysis showed that by increasing the cooling rate from partial solution temperature, minimum creep rate is decreased and stress rupture life is improved
  9. Keywords:
  10. Heat Treatment ; Scanning Electron Microscopy (SEM) ; Hot Tension ; Nickel-Base Superalloy ; Microstructure ; Creep Behavior ; Stress Ruptur

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