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Low-temperature interstitial hardening of 15-5 precipitation hardening martensitic stainless steel

Zangiabadi, Amirali
http://orcid.org/0000-0003-3586-0687
http://rave.ohiolink.edu/etdc/view?acc_num=case1480769348244855

Abstract Details

2016, Doctor of Philosophy, Case Western Reserve University, Materials Science and Engineering.
Surface engineering is a relatively new branch of science and technology. Low-temperature (= 723K) interstitial hardening via carburization and nitridation are effective ways to enhance engineering performance of stainless steels surfaces and developed in the past 20 years. At these para-equilibrium processing temperatures, the substitutional elements in the steels are effectively immobilized, thereby suppressing carbide or nitride formation. The surface hardness, fatigue resistance, and corrosion resistance are significantly enhanced due to the resulting “colossal” interstitial super-saturations achieved during such para-equilibrium interstitial hardening. The studies on 15-5PH precipitation hardening martensitic stainless steels resulted in unusual phenomena, following para-equilibrium nitridation. Firstly, isothermal martensite-to-austenite phase transformation has been observed after low-temperature nitridation in the martensite phase. The transformation occurs in the near-surface regions of the alloy, in which the nitrogen concentration reaches more than 15%at. These observations are consistent with the notion that nitrogen is a strong austenite stabilizer and substitutional diffusion is effectively frozen at the processing temperature. Our microstructural observations and diffraction analyses provide conclusive evidence for the martensitic nature of this phase transformation. The second response of this alloy (similar to the other alloys, e.g. 13-8 PH, 17-7 PH and 2205) is an anomaly in the martensite (or ferrite) lattice, which can be attributed to the enormous tetragonality, approaching c/a = 1.12. Due to the distortion of both phases at the interface, it is sometimes hard to differentiate one from another in their DPs (diffraction patterns). The phenomenological crystallographic theory of the martensite-to-austenite phase transformation has been applied. The theory indicates that the martensitic phase transformation necessitates the closed-packed planes (i.e. {111}¿) of the newly-formed austenite phase undergo shearing. The microstructural studies confirm this internal shearing of the austenite phase. It further appears that the martensitic austenite observed in this work deviates from cubic symmetry. Finally, this study shows that high concentrations of nitrogen interstitials cannot be realized in marten- site or ferrite even under “nitrogen paraequilibrium” conditions, because of the formation of martensitic austenite.
Arthur Heuer, PhD (Advisor)
Frank Ernst, PhD (Advisor)
Sunniva Collins, PhD (Committee Member)
Matthew Willard, PhD (Committee Member)
Farrel Martin, PhD (Committee Member)
189 p.
Engineering
Low temperature; nitridation; carburization; Martensite; Stainless Steel; 15-5; habit plane

Recommended Citations

Citations

  • Zangiabadi, A. (2016). Low-temperature interstitial hardening of 15-5 precipitation hardening martensitic stainless steel [Doctoral dissertation, Case Western Reserve University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=case1480769348244855

    APA Style (7th edition)

  • Zangiabadi, Amirali. Low-temperature interstitial hardening of 15-5 precipitation hardening martensitic stainless steel . 2016. Case Western Reserve University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=case1480769348244855.

    MLA Style (8th edition)

  • Zangiabadi, Amirali. "Low-temperature interstitial hardening of 15-5 precipitation hardening martensitic stainless steel ." Doctoral dissertation, Case Western Reserve University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=case1480769348244855

    Chicago Manual of Style (17th edition)

case1480769348244855
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This open access ETD is published by Case Western Reserve University School of Graduate Studies and OhioLINK.