Doctor of Philosophy, Case Western Reserve University, 2017, Materials Science and Engineering

Increased use of second-generation duplex stainless steels is expected as demands for tougher, more economical, and corrosion resistant alloys increase. A novel gas-phase carburizing and nitriding procedure operating in the temperature range of 325 °C to 440 °C was utilized with the intent of improving both the tribological and electrochemical performance of the 2205 duplex alloy (22Cr–5Ni–3Mo–0.15N) under so-called paraequilibrium conditions. A suite of state-of-the-art microchemical and structural characterization tools were employed following each treatment, and performance of the treated alloys assessed by chloride-based cyclic polarization and nano-indentation hardness measurements. Particular emphasis was placed on understanding the response of the ferritic phase, which has been a source of speculation and confusion since the development of such treatments. CALPHAD-based thermodynamic modeling was also used to predict phase stability in the 2205 system subjected to gas-phase paraequilibrium nitridation or carburization. Analysis of the interstitially-hardened layer in the austenitic phase of 2205 provides results consistent with similar surface hardening treatments of single-phase austenitic stainless steels: a colossally supersaturated and precipitate-free hardened layer of expanded austenite is formed. The interstitial concentration, case depth, and concomitant mechanical properties can be tailored through control of the temperature, duration, and chemical activity with the gas-phase process. Spatially-resolved chemical and structural analysis within the d-ferrite of 2205 revealed two competitive transformation behaviors following nitridation, while carburization led to only one response. For both carburization and nitridation, carbon or nitrogen supersaturations in ferrite on the order of 20 at.% and 25 at.%, respectively, were observed—greater than 10^6 times the equilibrium concentration at room temperature, yet remarkably with unmeasurable expansion or d (open full item for complete abstract)

Committee: Arthur Heuer Prof. (Advisor); Frank Ernst Prof. (Committee Member); Matthew Willard Prof. (Committee Member); Farrel Martin Prof. (Committee Member) Subjects: Materials Science

Master of Sciences, Case Western Reserve University, 2014, 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-e (open full item for complete abstract)

Committee: Gary Michal Prof. (Advisor); Arthur Heuer Prof. (Committee Chair); Frank Ernst Prof. (Committee Member); James McGuffin-Cawley Prof. (Committee Member) Subjects: Materials Science

Master of Sciences, Case Western Reserve University, 2012, Materials Science and Engineering

CALPHAD based interstitial solid solution thermodynamic modeling was used to determine carbon solubility in the presence of select carbide phases and the eutectoid temperatures of 15-5 PH and 17-7 PH stainless steels under paraequilibrium conditions. Predictions using CALPHAD parameters from different sources in the literature were compared. Resulting values for the eutectoid temperature in 15-5 PH stainless steel varied nominally 100K depending upon the choice of Cr-Fe-C CALPHAD parameters employed. By converting from a dilute solution to a CALPHAD model, Cu-C interaction parameters were derived and applied to the numerical prediction. CALPHAD based thermodynamic modeling also was used to predict the effects of Cr and Ni on the solubility in Fe-based bcc and fcc matrices of graphite and paraequilibrium M3C, M7C3, and M23C6 carbides. The solubility of graphite increases with increasing Cr, and decreases with increasing Ni contents in both bcc and fcc matrices. For the Fe-Ni-C system, CALPHAD modeling of compositions up to 40 wt. pct. Ni found that the formation of paraequilibrium conbides increases the solubility of carbon relative to graphite for both bcc and fcc matrices at 700K. The Swagelok low-temperature carburization process was applied to 15-5 PH stainless steel over a temperature range from 380 to 450C. Through this process, hardened cases 8-12 µm thick were produced having a carbon content of 8-10 at. pct. as determined using Auger electron spectroscopy (AES). Near surface microstructures were examined using optical microscopy and scanning electron microscopy (SEM). Microhardness testing measured case hardness values of 950-1100 HV which are much higher than the core hardness values of approximately 500 HV.

Committee: Gary M. Michal PhD (Committee Chair); Arthur H. Heuer Phd (Committee Member); Frank Ernst Phd (Committee Member); Mark R. DeGuire Phd (Committee Member) Subjects: Materials Science

Master of Engineering, Case Western Reserve University, 2012, Materials Science and Engineering

Low temperature paraequilibrium carburization is an effective method to enhance surface hardness and corrosion resistance in austenitic stainless steels, provided that equilibrium carbide formation is suppressed [1]. In this study, 17-7 precipitation-hardening stainless steel (PH SS) was subjected to paraequilibrium carburization at 350 and 380 °C. Metallographic images taken on etched cross sections of carburized samples and the hardness depth profile are consistent with the results found in carburized austenitic stainless steels – a hardened, etch-resistant surface layer can be formed. Besides, experimental results from X-ray Diffraction (XRD), magnetic force microscopy (MFM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) of carburized 17-7 PH SS have shown a carburization-induced transformation form martensite to austenite during paraequilibrium carburization, which results from the known ability of carbon to act as an austenite stabilizer. Although EBSD results reveal that δ-ferrite remains the same phase after paraequilibrium carburization, a higher carbon concentration than the calculated value using the CALPHAD method in δ-ferrite was measured. In this work, the carburization-induced phase transformation and the effect of low temperature carburization on 17-7 PH SS have been investigated by scanning electron microscopy (SEM), X-ray energy dispersive spectrometry (XEDS), Auger electron spectroscopy (AES), XRD, EBSD and TEM. The goal of this study is to identify the structural evolution and transformation mechanism in carburized 17-7 PH SS in order to understand the relationship between carburization and the martensitic transformation.

Committee: Arthur Heuer PhD (Advisor); Gary Michal PhD (Committee Member); Frank Ernst PhD (Committee Member) Subjects: Materials Science

Master of Sciences (Engineering), Case Western Reserve University, 2009, Materials Science and Engineering

Ferritic stainless steels with a high chromium content were case hardened at 350 to 420° C using a carburization technique developed by the Swagelok company. This carburization treatment applied to body-centered cubic ferritic stainless steels has resulted in a case only a few microns thick in contrast to the 20 to 40 micron thick casesproduced in face-centered cubic austenitic stainless steels carburized at 450 to 470° C. This situation has been investigated using CALPHAD based thermodynamic simulation techniques to explore the possible formation of carbides having the same composition of metal elements as their ferritic matrix, i.e. paraequilibrium carbides. Experimental work has included X-ray Diraction (XRD), hardness measurements, metallographic analysis using Scanning Electron Microscopy (SEM), Auger Electron Spectroscopy, X-ray Photoelectron Spectroscopy (XPS), and Transmission Electron Microscopy (TEM). Both the simulation and the experimental work indicate that paraequilibrium carbides are forming a layer in the near surface region that creates a barrier which greatly restricts carbon diffusion into the ferrite phase during the carburization treatments.

Committee: Gary Michal PhD (Advisor); Frank Ernst PhD (Committee Member); Arthur Heuer PhD (Committee Member); John Lewandowski PhD (Committee Member) Subjects: Engineering; Materials Science; Metallurgy

Master of Sciences (Engineering), Case Western Reserve University, 2010, Materials Science and Engineering

Historically it has been very difficult to harden the surface of stainless steels without the loss of corrosion resistance. A novel low temperature carburization process developed by Swagelok Co. which greatly improves the wear resistance of stainless steel has accomplished this feat. This project has studied the friction and wear behavior of low temperature carburized AISI 316L; 1.) Sliding against hard materials, 2.) Sliding under a range of contact stresses and sliding speeds, and 3.) The relationship of wear resistance to the properties of the surface hardened region. It was found that low temperature carburization provided an immense improvement in wear resistance when sliding against hard materials, and this improvement is maintained under stresses and sliding speeds that are too severe for non-treated stainless steels. Furthermore, the improvement in wear resistance was found to be heavily dependent on the hardness and thickness of the case hardened region.

Committee: Arthur Heuer PhD (Advisor); Gary Michal PhD (Committee Member); Frank Ernst PhD (Committee Member); Gerhard Welsch PhD (Committee Member) Subjects: Materials Science

Master of Sciences (Engineering), Case Western Reserve University, 2008, Materials Science and Engineering

Advances in low temperature carburization technology have provided an impetus to examine the solubility of interstitials in stainless steels and nickel based superalloys under paraequilibrium conditions. The solubilities of interstitials in such alloys under paraequilibrium conditions were calculated employing the CALPHAD formalism for solution thermodynamics. The basis of CALPHAD multi-sublattice modeling was reviewed and its application to interstitial solid solutions was developed. The accuracy of such modeling is directly dependent upon the values of the CALPHAD parameters used. An extensive evaluation of the fcc chromium parameters was performed, because the currently accepted CALPHAD parameters in the steels and Fe-based alloys database and used with Thermo-Calc software were found to be insufficiently exothermic. The solubilities of carbon and nitrogen under paraequilibrium conditions in several alloys were calculated. The reliability of the revised fcc chromium parameters and the possibility of spinodal decomposition in the fcc metal-carbon solid solutions were discussed.

Committee: Gary Michal (Advisor) Subjects: Engineering, Materials Science