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LOW-TEMPERATURE GAS-PHASE CARBURIZING AND NITRIDING OF 17-7 PH STAINLESS STEEL

Wang, Danqi
http://rave.ohiolink.edu/etdc/view?acc_num=case1386165240

Abstract Details

2014, Doctor of Philosophy, Case Western Reserve University, Materials Science and Engineering.
Low-temperature carburization and low-temperature nitridation were successfully applied on 17-7 PH stainless steel and significantly improved the surface hardness. Via an isothermal martensite-to-austenite phase transformation, carbon- and nitrogen-supersaturated expanded austenite was achieved. Delta ferrite grains with astonishing amount (greater than 18 at.%) of carbon (nitrogen) were observed after carburization (nitridation). The interstitially-supersaturated ferrite shows a uniform contrast, i. e. no diffraction contrast from extended structural defects under transmission electron microscopy. Plates with uniform contrast were observed in ferrite grains near the interface between the carburized layer and bulk material after carburization. These plates are enriched in carbon but do not form any carbide. A model based on segregation of carbon interstitials to dislocation cores is proposed, the featureless appearance being ascribed to strain field overlap of a massive dislocation network. As the carbon-dislocation binding energy is higher than that of carbon to iron in cementite, carbon atoms are able to segregate to the dislocations cores. With an extremely high dislocation density (1013/cm2), ferrite can take up to several wt. pct. carbon without any phase transformation. Similar mechanism holds for featureless nitrogen-supersaturated ferrite. But after nitriding at high temperature (713 K), rocksalt-structured nitrides MN1-x (M being Fe, Cr, Ni and Al) were observed in a Bain orientation relationship with respect to ferrite matrix. Low nitriding temperatures (623 K and 653 K) prefer M2N1-x plate formation in ferrite. Continuing nitriding tends to dissolve M2N1-x, forming featureless grain with MN1-x. In spite of processing condition designed to eliminate long-range diffusion of substitutional solute atoms, decomposition of austenite into ferrite and nitride was observed to occur during low-temperature nitridation realized by enhanced diffusion along the austenite–ferrite interphase interface. NiAl formed in ferrite during both carburization and nitridation, but lost ordering afterwards in carbon- and nitrogen- supersaturated ferrite grains.
Arthur Heuer (Advisor)
Engineering; Materials Science; Metallurgy
Stainless steel, carburization, nitridation, transmission electron microscopy, atom probe tomography, dislocation

Recommended Citations

Citations

  • Wang, D. (2014). LOW-TEMPERATURE GAS-PHASE CARBURIZING AND NITRIDING OF 17-7 PH STAINLESS STEEL [Doctoral dissertation, Case Western Reserve University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=case1386165240

    APA Style (7th edition)

  • Wang, Danqi. LOW-TEMPERATURE GAS-PHASE CARBURIZING AND NITRIDING OF 17-7 PH STAINLESS STEEL. 2014. Case Western Reserve University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=case1386165240.

    MLA Style (8th edition)

  • Wang, Danqi. "LOW-TEMPERATURE GAS-PHASE CARBURIZING AND NITRIDING OF 17-7 PH STAINLESS STEEL." Doctoral dissertation, Case Western Reserve University, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=case1386165240

    Chicago Manual of Style (17th edition)

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© 2014, all rights reserved.
This open access ETD is published by Case Western Reserve University School of Graduate Studies and OhioLINK.