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Thermodynamics of Paraequilibrium Carburization and Nitridation of Stainless Steels

Dalton, John Christian
http://rave.ohiolink.edu/etdc/view?acc_num=case1386586585

Abstract Details

2014, Master of Sciences, Case Western Reserve University, Materials Science and Engineering.
Owing to its thermodynamic control and conformal nature, gas-phase nitriding and carburizing of steels has for over a century been a popular method for increasing the hardness, wear-, and fatigue-resistance of ferrous components. Only in recent years (~3 decades) have such processes been successfully applied to stainless steels, under so-called paraequilibrium conditions whereby a truly colossal (>10^5 times the equilibrium concentration) supersaturation of interstitials can be achieved, thus imparting extraordinary improvements in the mechanical and electrochemical performance of the alloy. Such novel thermochemical techniques are relatively low-cost, industrially viable, and represent one of few value-added processes of a material that results in all gain and no loss. The thermodynamic origins of this metastable supersaturation, in particular carbon in austenitic 316, have been previously discussed within the context of the CALPHAD based multi-sublattice model for solid solutions. It is the present e ort of this work to review the complete thermodynamic database as applied to stainless steels, and develop a generalized approach for modeling metastable paraequilibria in both fcc austenitic and bcc ferritic (or martensitic) alloys upon low-temperature gas-phase nitriding or carburizing. A flexible and user-friendly program was developed to allow for predictions of the paraequilibrium carbon and nitrogen solubility in model binary to senary systems exposed to some carbon/nitrogen-rich ambient, as well as to model the solubility of carbon or nitrogen when a paraequilibrium, i.e. partitionless, carbide or nitride has formed. Changes in the paraequilibrium eutectoid temperature as a function of alloy content were also determined to predict the feasibility of an isothermal nitrogen- or carbon-induced ferrite (or martensite) to austenite (a (a’) ¿ ¿) phase transformation. Such thermodynamic calculations can be used in optimizing interstitial hardening treatments of pre-existing alloys, as well as aid future alloy development. Sound thermodynamic descriptions further provide the basis needed for modeling the kinetics behind paraequilibrium surface treatments, investigations of which are still in their infancy. Lastly, previous sensitivity analyses on published interaction parameters (acquired from high-temperature data) within the chromium-carbon system have led to some parameter revision for better agreement with experiment in the low-temperature regimes of interest. The routine developed and used for the analysis contained herein is readily amenable to parameter optimization and expansion as the thermodynamic database becomes more complete, something which is not presently attainable in commercial thermodynamic platforms such as Thermo-Calc® Software (Stockholm, Sweden).
Gary Michal, Prof. (Advisor)
Arthur Heuer, Prof. (Committee Chair)
Frank Ernst, Prof. (Committee Member)
James McGuffin-Cawley, Prof. (Committee Member)
141 p.
Materials Science
stainless steel, carburizing, nitriding, carbides, nitrides, CALPHAD, thermodynamics, phase diagrams, modeling, paraequilibrium, expanded austenite, S-phase

Recommended Citations

Citations

  • Dalton, J. C. (2014). Thermodynamics of Paraequilibrium Carburization and Nitridation of Stainless Steels [Master's thesis, Case Western Reserve University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=case1386586585

    APA Style (7th edition)

  • Dalton, John. Thermodynamics of Paraequilibrium Carburization and Nitridation of Stainless Steels. 2014. Case Western Reserve University, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=case1386586585.

    MLA Style (8th edition)

  • Dalton, John. "Thermodynamics of Paraequilibrium Carburization and Nitridation of Stainless Steels." Master's thesis, Case Western Reserve University, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=case1386586585

    Chicago Manual of Style (17th edition)

case1386586585
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This open access ETD is published by Case Western Reserve University School of Graduate Studies and OhioLINK.