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An Investigation Into the Aging Kinetics of a Heat Resistant Stainless Steel

Jalilvand, Sepideh | 2015

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 46807 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Karimi Taheri, Ali
  7. Abstract:
  8. The austenitic heat resistant stainless steels have been widely used in petrochemical industry as furnace tubes which are exposed to extremely high temperatures. In this study, the quench aging process of a HP modified heat resistant stainless steel is assessed and the kinetics of aging is evaluated by the JMAK model. The microstructure of steel in as cast and aged conditions was characterized by optical microscope and scanning electron microscope equipped with EDS. The effects of aging time and temperature on the microstructure, tensile propertices, shear strength, hardness, and ductility of the aged steel were determinded. The results of tensile and shear punch tests show a linear correlation with coefficient of 1.93 between the shear and tensile stress. As cast microstructure consisted of an austenitic matrix and a network of two types of primary carbides identified as niobium carbide and chromium carbide. Aging process proceeded with phenomena of precipitation of secondary choromium carbides and partial transformation of niobium carbide to a brittle phase with Ni-Nb silicide characteristics. Fractography observations revealed that the fracture surfaces are characteristic of a quasi-brittle fracture process. Furthermore, the final fracture was an interdendritic boundary phenomenon. By using the JMAK equation, a relationship was derived to estimate the hardness of the steel at a given time and temperature. Activation energy for phase transformation was determined as 210 kJ.mol-1 which is close to the activation energy for Cr diffusion in austenite, indicating that diffusion of Cr is the major process in the precipitation phenomenon of the steel. It was found that aging at 700C for 96 hours promoted about 26% increase in the hardness around 11% in tensile strength, and reduction in ductility of about 50%
  9. Keywords:
  10. Mechanical Properties ; Heat Resistant Steel ; Shear Punch Test ; Stainless Steel ; Quench Aging Kinetics

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