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The Evaluation of Microstructure and Mechanical Properties of a HSHR Stainless Steel Strengthen by Nano-Precipitates

Attarian, Mitra | 2016

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  1. Type of Document: Ph.D. Dissertation
  2. Language: English
  3. Document No: 49196 (57)
  4. University: Sharif University of Technology, International Campus, Kish Island
  5. Department: Science and Engineering
  6. Advisor(s): Karimi Taheri, Ali; Davami, Parviz
  7. Abstract:
  8. In this research, a computational method based on thermodynamic and physical metallurgy coupled by the d-electron approach was used to design the composition of a high strength and heat resistant (HSHR) stainless steel. Then, three series of the heat resistant 25Cr-35Ni-Nb (HP-Nb) stainless steels with tungsten, nitrogen, and micro alloying elements additions were cast. In these alloys a Nb/(C+ 6/7 N) ratio of ≈ 2.4 to 3.7 was chosen and they were directionally solidified under two cooling rates of 23 Ks-1 and 6 Ks-1. The effects of the alloy chemical composition and solidification cooling rate on the macrostructure, microstructure, microsegregation, and inclusions in the alloys were studied. Also, the solidification sequences and the solidification path were analyzed both theoretically and experimentally. It was found that the degree of negative segregation of Ni and positive segregation of Nb are increased by tungsten addition. The partition coefficient of Nb was determined as 0.24, 0.31, and 0.52 for the alloy with tungsten and nitrogen, alloy with tungsten, and the alloy without tungsten and nitrogen, respectively. The addition of nitrogen plus tungsten significantly increased the eutectic temperature and thus refined the dendrites and altered the morphology of M7C3 eutectic carbide. At short time creep aging, nitrogen addition decreased the M7C3 carbide fragmentation, increased the secondary M23C6 precipitation and the Nano-sized Nb(C, N) densities, and impeded the formation of Cr-rich M23C6 carbide on the primary Nb(C, N) carbide / matrix interface. These phenomena enhanced the creep resistance of the steel. The apparent activation energy and creep exponent of the steel containing both tungsten and nitrogen were determined as 210±30 kJmol-1 and 5.2, respectively, indicating that the grain boundary region plays a significant role in controlling the creep deformation of the steel
  9. Keywords:
  10. Stainless Steel ; Heat Resistant Steel ; Creep ; Directional Solidification ; Nanoprecipitation ; High Strength Steels

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