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Influence of Rejuvenation Treatment on Microstructure and Creep Properties of Damaged IN738LC Superalloy

Hosseini, Shahabeddin | 2012

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 43633 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Nategh, Saeed; Ekrami, Ali Akbar
  7. Abstract:
  8. The goal of the present study is to investigate the effect of rejuvenation treatment on microstructure and creep properties of damaged IN738LC superalloy. Virgin alloy was damaged by interrupted creep tests at 926°C and 170 MPa for 100, 200 and 250 hours. The results obtained showed that damaging the virgin alloy by interrupted creep tests resulted in microstructural deterioration (consisting of coarsening of ' precipitates, formation of continuous films of ' on grain boundary regions and formation of transition zone around MC carbides) and, consequently, degradation of creep properties. Rejuvenation treatment was carried out with the purpose of healing the damaged alloy and restoration of microstructure and creep properties to those of virgin alloy.Rejuvenation treatment consists of heat treatment and or combination of heat treatment and hot isostatic pressing. After damaging the virgin alloy, the type of damage and its effect on creep properties was investigated by microstructural studies and creep test. During solution stage, dissolution of coarsened ' precipitates, grain boundary films and transition zone around primary MC carbides was observed. The precipitates that were not dissolved entirely and those formed during cooling after solution treatment were named remnant and cooling precipitates, respectively. Solution treatment at higher temperature resulted in formation of lower volume fraction and smaller size of remnant precipitates, further dissolution of grain boundary films and disappearance of transition zone around primary MC carbides. In addition, higher volume fraction of cooling precipitates was observed after solution treatment at higher temperatures. The results showed that slower cooling after solution treatment led to formation of coarser ' precipitates with higher volume fraction. Also, aging treatment resulted in increase in volume fraction of ' precipitates. In order to investigate the effect of rejuvenation treatment on creep properties of damaged alloy, the damaged alloy was creep tested after different rejuvenation treatments. The results showed that creep properties of damaged alloy will improve if rejuvenation treatment results in formation of fine cooling and or secondary precipitates in proximity to primary and or remnant ones. Furthermore, application of HIP treatment resulted in improvement of creep properties of damaged alloy and gained creep properties near those of virgin alloy. Application of HIP treatment on the alloy damaged via interrupted creep test until middle stages of creep life had more effect in increasing life duration
  9. Keywords:
  10. Microstructure ; Creep Properties ; Inconel 738 Nickel Based Superalloy ; Rejuvenation Treatment ; Hot Isostatic Pressing (HIP)Treatment

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