Thesis (M.S.)--Brigham Young University. Dept. of Mechanical Engineering, 2004. / Includes bibliographical references (p. 27-40).

Thesis (Ph. D.)--Brigham Young University. Department of Mechanical Engineering, 2004. / Includes bibliographical references (p. 49-50).

Cold-formed stainless steel sections have been increasingly used in architectural and structural applications. Yet the high price of stainless steel limits the application to construction projects. The lean duplex stainless steel (EN 1.4162) offers an opportunity for stainless steels to be used more widely due to its competitive in price, good mechanical properties and corrosion resistance. The lean duplex stainless steel is a relatively new material, and research on this material is limited. Currently, the lean duplex stainless steel is not covered in any design specification, and no design rules are available for such material. Therefore, the behaviour and design of cold-formed lean duplex stainless steel members are investigated in this study. The investigation focused on columns, beams and beam-columns of square and rectangular hollow sections. Both experimental and numerical investigations were performed and reported. Design rules for cold-form lean duplex stainless steel members are proposed. The experimental investigation included material tests, column tests, beam tests and beam-column tests at room and elevated temperatures ranged from 24 – 900 °C. The test specimens were cold-rolled from flat strips. The test program consists of two square hollow sections and four rectangular hollow sections. Coupon specimens were extracted from each hollow section, and their material properties were obtained from tensile coupon tests at room temperature and elevated temperatures. In this study, a modified design rule was proposed to predict the cold-formed lean duplex stainless steel material properties at elevated temperatures. The local and overall geometric imperfections were measured. A total of 38 column tests were conducted. The effective column length ranged from 75 to 1660 mm in order to obtain a column curve for each test series. The test program for beams included 10 pure bending tests, and the bending capacities of the specimens were determined. A total of 37 beam-column specimens were compressed between pinned ends at different eccentricities in order to obtain an interactive curve for each series of test. Numerical investigation on columns, beams and beam-columns at room temperature as well as elevated temperatures are also presented. Accurate finite element models were developed and verified against the experimental results for columns, beams and beam-columns at room temperature. The structural members at elevated temperatures were simulated by replacing the material properties with those obtained at elevated temperatures. Extensive parametric studies were carried out, including 150 columns, 126 beams and 150 beam-columns at room temperature, as well as 180 columns, 125 beams and 195 beam-columns at elevated temperatures. Column, beam and beam-column strengths obtained from the experimental and numerical investigations as well as available data were compared with the design strengths calculated using American, Australian/New Zealand, European specifications for stainless steel structures of duplex material, since lean duplex material is not covered by these specifications. In addition, direct strength method for carbon steel and stainless steel as well as continuous strength method for stainless steel were assessed for cold-formed lean duplex stainless steel. Modified design rules were proposed. The reliability of the current and modified design rules was evaluated using reliability analysis. / published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy

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The following work divides into two parts: a: a study of the effect of stress on the inter-granular stress assisted corrosion attack of sensitised 20% Cr, 25% Ni, 0.7% Nb in HNO₃ environments. This problem was suggested by the C.E.G.B. and relates to the potential corrosion problems of AGR fuel cladding during storage after use. The aim of this work was therefore to determine how metallurgical condition, test potential and mechanical test variables affect corrosion behaviour. Low frequency cyclic loading offers a way to investigate the stress corrosion of systems at realistic stress levels and strain rates found in practice. b: an investigation into the effect of a low frequency cyclic stress on the process of stress assisted corrosion. The aim of this work was to gain information on the effect of stress cycling on the process of stress assisted corrosion attack. Tensile specimens were subjected to static loads both alone and with superimposed low frequency (10^-⁴ to 10^-² Hz) saw-tooth stress cycles. Cycling was carried out potentiostatically in HNO₃ environments, at below yield stress levels and ambient temperatures. Different frequencies, cyclic amplitudes and levels of background tensile stress were used. Irrespective of loading conditions the optimum potential for accelerated stress assisted corrosion attack was found to be -200mV (SCE). The results of tests showed that test potential, cycle frequency, cycle amplitude and level of background stress strongly affect rates of attack. Grain boundary penetration rates were found to increase as frequency decreased and as peak stress and stress amplitude increased. Different kinetics of penetration were seen for cyclic and static loading. Increase of penetration depth with time for cyclic loading experiments was found to vary with (time)^0.5 whilst that for static loading experiments increases linearly with time. A number of reasons are discussed to explain the difference in observations between cyclic and static penetration rates. Such reasons included the difficulty of ion transport down narrow paths, blunting of the penetration front, the possibility of local strain induced martensite transformation leading to hydrogen embrittlement and plastic strain enhanced dissolution resulting during cyclic loading. The anomalous effects observed during cyclic loading (such as "strain softening") were examined for tensile specimens cycled under a range of mechanical conditions. It was found that the extent of plastic strain increased for higher stress and larger cycle amplitudes. The process of thermal sensitisation of 20 wt% Cr, 25 wt% Ni, 0.7 wt% Nb stainless steel in three different material starting conditions (bar, "reworked bar" and tube) was investigated. Both Cr depletion and impurity segregation are discussed as mechanisms of sensitisaton. An attempt was made to correlate response in chemical and electrochemical tests with both microanalytical (STEM/EDX) observations on the shape of Cr depletion profiles and with analytical modelling. The collector plate model was found to describe AEM measured Cr depletion profiles well.

WhisperGenTM heater head components are currently machined from the austenitic stainless steel Sandvik 253MA. The inner surface of the heater heads is subjected to the working gas of the engine, which is a high-pressure, high-temperature nitrogen-based environment with carbonaceous impurities. As a result of this exposure, a scale forms during operation and eventually spalls. This spalled scale causes abrasive damage to piston seals and guides, which leads to pressure loss and eventual failure of the engine. The aim of the present work was to compare the performance of the austenitic alloy 253MA with two alternative alloys, Incoloy 800H and AISI 310, thereby enabling a material recommendation. A literature review provided information about many general aspects of high temperature corrosion in similar alloys. However, little was found about the application of these alloys in environments similar to those experienced by a WhisperGenTM heater head. Therefore, laboratory experiments were conducted to indicate the relative performance characteristics of the three potential alloys (253MA, Incoloy 800H and AISI 310). To overcome the difficulties with testing at high temperatures and pressures, Thermo-Calc™ was used to calculate gas mixtures at 1 bar that approximated the chemical potentials of carbon and nitrogen in the working gas at 24 bar. Comparisons of the different materials were made via weight loss/gain measurements and metallographic analysis, which included optical microscopy, scanning electron microscopy, X-ray mapping and electron back scatter diffraction (EBSD). The laboratory test sample results were also compared with results from heater heads of the same materials that were run in an actual WhisperGenTM engine. The experimental results taken in total indicate that 253MA is the least suitable alloy for the heater head application because it exhibited poor spalling performance, internal oxidation and formation of a large amount of Cr23C6. AISI 310 was shown in all cases to develop the detrimental sigma phase, although this alloy was the least susceptible to internal oxidation. Incoloy 800H was the most resistant alloy to all forms of degradation and is thus recommended for the heater head application.