Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

Pits have been observed to initiate at certain inclusions in stainless steel, which affect the composition and properties of the passive film in that region. In this thesis, the VG MA500 high resolution scanning Auger microscope associated with Link EDX is used to investigate the surface chemistry of inclusion groups in stainless steel 316 relative to the bulk composition, before and after exposure to sodium chloride solution. The work demonstrates that careful control of experimental conditions allows the progress of initiation of pits to be observed systematically. The role of inclusions as initiators of pitting corrosion has been studied from the very inception of attack. Detailed observation have been taken from separate manganese sulphide as well as mixture of manganese sulphide and multi-element oxide at various pH values of solutions to trace the behaviour of these inclusions during exposures. It is suggested from this investigation that corrosive attack starts on MnS inclusion proper. The dissolution of MnS takes place in sodium chloride solution followed by the acidification in the microarea and dissolution of metal in the region adjacent to the inclusion. The mechanism for the crevice corrosion is of importance after the microcrevice forms. The observations showed that sulphur precipitated on the area surrounding the inclusion group in acidic sodium chloride solution. This is in contrast to the finding in prior studies by electron probe or EDX analyses in which sulphur precipitation was only observed on the inclusion. Because pitting normally occurs on surfacesthat are initially exposed to air before being immersed in the corrosive solution, an understanding of the change of surface film associated with the change of medium is very important. XPS was used in this work to study the air formed and aqueous films, the transformation when the specimens were immersed in solutions. The study of the energy-loss structure of photoelectron peaks has lead to a better understanding of these various films. It has been used to study in greater detail the composition and properties of the films formed on Fe-Cr alloys during oxidation, vacuum annealing, and to find the relationship between the depth distribution of the components in the films and energy loss background of photoelectron peaks. Three features are of particular importance: the background loss tail height, the baseline slope and the post-peak loss structure. These features have been related quantitatively to the relative amounts of the element and the surrounding matrix in the layers which contribute the greatest signal at any given take-off angle. The present work illustrates that the additional depth information available from closer inspection of the inelastic loss background feature may aid an understanding of the change from the air-formed film to the aqueous film as well its effect on the initiation of pits in stainless steel 316. The combination of AES and EDX with high spatial resolution gave an opportunity to re-investigate the initiation of pits at the site of inclusion on stainless steel by offering both bulk and surface information. The use of AES and XPS associated with the energy loss structure of photoelectron peaks facilitates a comprehensive understanding for the whole surface of various films and the local areas which are involved in the formation of pits.

A study of the constitution, transformation and precipitation effects in ternary Fe-Cr-Ni alloys and quaternary Fe-Cr-Ni-X alloys containing Mo, Nb, Ti, and Si was carried out. A systematic approach was adopted so that the microstructural effects observed as a result of ageing the selected iron base ternary alloys could be directly compared to the quaternary alloys. A series of ageing curves were plotted for the six ternary alloys over the temperature range 400°-900°C and for the ten quaternary alloys in the temperature range 650°-850°C. Optical and electron metallography were used to study the transformation and precipitation of intermetallic phases and carbides in the aged microstructures. The transformation of delta ferrite to sigma phase in a duplex (gamma+delta) ternary alloy was studied. The transformation was preceded by the precipitation of a cellular structure which formed on the delta/gamma grain boundaries and consisted of M23C6 and new austenite. A complex transformation product, which resembled a region of imiscibility, then formed at the delta/(gamma + M23C6) interface, this product consumed the delta ferrite grain, eventually transforming to sigma phase and new austenite. The alpha' phase, more commonly referred to as 475°C embrittlement, was also observed precipitating in the delta ferrite grains in two of the iron based ternary alloys in the temperature region 400°-500°C. Overageing of the alpha' precipitates after 1000 hours at 500°G was accompanied by the precipitation of a rod like austenite. The elements 2%Mo, 1%Nb, 1/4%Ti and 1% and 2%Si were added totwo Fe-Ni-Cr base alloys. One alloy was an austenitic (20Cr,23Ni) and the other was a transformable alloy (18Cr, 7Ni). The quaternary element additions had no effect on the constitution of the austenitic ternary alloy. However the addition of Mo to the 18Cr, 7Ni ternary alloy caused the transformation of delta ferrite to sigma phase over ageing temperature range 650°-850°C. The addition of Si moved the constitution of the ternary alloy further into the gamma+delta phase field, a small amount of sigma phase was observed in the 2%Si quaternary alloy on ageing at 650°C for 1000 hours. Irradiation damage studies were performed using High Voltage Electron Microscope and Variable Energy Cyclotron. The effect ofthe quaternary alloying additions Mo, Si, and Nb on the voidswelling behaviour of a 20Cr, 23Ni alloy were studied using 46MeV Ni6 ions in the Variable Energy Cyclotron, irradiating to a total dose of 10dpa at 550°C. All additions reduced void swelling, the largest reduction was observed in the Si containing allov. The Insert A Thermal ageing showed the presence of M23C6 the amount of which increased with increasing ageing temperature. No delta ferrite was observed in these alloys.

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M.Ing. / Please refer to full text to view abstract

M.Ing. (Civil Engineering) / The results of a study presented on the behaviour of cold-formed stainless steel beams subjected to shear taking into account elastic shear buckling, inelastic shear buckling and shear yielding, as well as an investigation into the interaction relationship between shear and bending are presented. From this investigation the results obtained show good relation to the theory. The local shear buckling stress was experimentally determined and it was found that for unreinforced beam webs the shear buckling coefficient is that of an infinitely long plate, namely k=5,34. A good agreement between the experimental ultimate shear strength and the predicted ultimate shear strength was found. Stainless steel beams comprising of lipped channels were manufactured and tested to failure. The types of stainless steels used in this investigation were Types 304, 316, 430 and Type 3CR12 corrosion resiting steel, a modified Type 409 stainless steel. The stress-strain relationship for stainless steels differs from that of carbon steel in that stainless steel is a gradual yielding material. It was concluded in this investigation that Gerard's plasticity reduction factor, Gs/G o, should be used as a plasticity reduction factor in calculations concerning shear. It was found that present design criteria are adequate.

High frequency corrosion fatigue crack propagation behavior of AISI 316L stainless steel was studied in 1M NaCl and 1M NaCl + 0.01M Na₂S₂O₃, under various anodically and cathodically polarized potentials, and dessicated air at 22°C and 1 atmosphere pressure. Constant load amplitude fracture mechanics techniques employing single edge notch specimens were used to assess the fatigue crack growth rate in the various environments. Unique specimen preparation procedures were developed which allowed near-threshold behaviors to be studied under gradually rising crack tip stress intensity conditions. Polarization studies showed that the presence of thiosulphate catalyzed the dissolution of stainless steel in low pH solutions(pH~1), due to reduction of thiosulphate species to H₂S, but had no effect in the near neutral solutions. Fatigue tests conducted in the neutral NaCl + Na₂S₂O₃ solution at cathodic potentials showed that the presence of thiosulphate had an insignificant effect. This indicated that high frequency fatigue produces efficient exchange of bulk solution with the crack tip environment, which prevented the lowering of pH in the crack by hydrolysis effects and prevented reduction of thiosulphate to H₂S. Fatigue crack retardation phenomena were very pronounced in the near-threshold regions in most of the fatigue tests. The cause of this retardation was attributed mainly to the surface-roughness- induced crack closure effect, which reduced the effective crack tip cyclic stress intensity ΔKth to a lower level. This closure effect only predominated in the near-threshold region where significant Mode II loading was present. The influence of various imposed anodic and cathodic potentials was found to be consistent with the surface-roughness-induced crack closure effects. The observed crack growth accelerating effect of high anodic potentials was attributed to the corresponding high removal rate of surface roughness in the wake of the crack, which kept the effective cyclic stress intensity level near the applied values. Crack fractography was studied by scanning electron microscopy. It showed that the fractography generally consisted of three regions; a crystallographic cleavage-like near-threshold region, a feathery and fibrous transition region, and a striated region. Using an etch pitting technique, it was determined that the crack plane and crack propagation directions in the near-threshold region were mainly those of {111} <110>, {110} <112>, {110} <001>, and other higher indexed planes. These crack orientations were effected by the activation of a single slip system or the alternate activation of two intersecting slip systems. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

D.Ing. / In mechanical design and analysis the mechanical properties of the material used are crucial to achieve effective design or analysis. In designing structures that are susceptible to dynamic loading different mechanical properties of the material may be needed than those used for quasi-static situations. Usually when one refers to the dynamic properties of a metal one refers to the notch toughness of the material. That is the resistance of the material to crack propagation under dynamic loading. Another less well known dynamic property of a metal is strain rate sensitivity. This implies that mechanical properties like yield strength, tensile strength and rupture strain varies according to strain rate. Typical applications where these properties are of use are in impact situations such as vehicle collisions and cold and hot working of metals in the manufacturing industry. The mechanical properties of certain metallic components or structures may change when the component or structure are subjected to dynamic loading that causes permanent deformation. The purpose of this investigation is to investigate the strain rate sensitive behaviour of certain stainless steels. The steels investigated are AISI Types 304, 316 and 430 stainless steels, 3CR12 corrosion resisting steel (a proprietary alloy also known as Type 1.4003) and mild steel which acts as a reference. The strain rate sensitivity of the above mentioned steels are investigated experimentally at room temperature for strain rates between 10' to approximately 100 s -1 . The steels are all tested in as delivered sheet form and testing is conducted in both rolling directions. The testing at the medium strain rates necessitated the design and construction of a dynamic tensile tester, the design of which, is also presented. The implementation of strain rate sensitive material properties into structural design and analysis are investigated and a constitutive model is proposed. The implementation of the proposed constitutive model into numerical methods analysis tools such as the finite element method is discussed and presented. The practical implementation of the proposed constitutive model is illustrated by numerically analysing the problem of a clamped beam struck transversely by a mass and comparing this with available experimental data. The validity of a typical constant velocity tensile test that is used to determine strain rate sensitive material properties is also investigated numerically to place the experimental results obtained into perspective. All the steels tested are found to be strain rate sensitive. Their behaviour is satisfactorily described by the constitutive model presented. No general trend regarding strain rate sensitivity is found when the results of the two rolling directions are compared. The importance of including strain rate sensitivity into structural design and analysis is illustrated by the analysis of the clamped beam struck transversely by a mass. The numerical results compare well with the available experimental data. It transpires from the numerical analysis of a typical constant velocity tensile test that it is difficult to obtain a constant strain rate throughout the gauge length of a typical test specimen. It also shows that there exists an optimum specimen geometry where the strain rate variation in the gauge length is at a minimum.

A variation of the double cantilever beam specimen has been calibrated and used to study the propagation of stress corrosion cracks as a function of stress intensity in 316 and 310 stainless steels, and a TRIP steel exposed to hot aqueous magnesium chloride solutions. The effects of cold work, temperature and applied potential on both the fractography and cracking rates have been examined. The effects of cold work and crack path on crack branching were also investigated. Bpth stress Intensity dependent (Region I) and stress intensity independent (Region II) cracking were observed. Region II having apparent activation energies from 15.1 kcal/g.mole to 18.1 kcal/g.mole. The crack velocities of 25% cold rolled 316 were found to be independent of applied potential over a range of more than 50mV in Region I and 75mV in Region II. In the same material the crack path changed from solely transgranular at low stress intensities and noble potentials to more than 80% intergranular at high stress intensities and active potentials. The topography of the transgranular fracture was similar to that observed by others except in the case of the TRIP steel where nodular features were observed. These observations have been discussed with respect to mechanisms involving the following (i) electrochemical dissolution, (ii) absorption of hydrogen and (iii) adsorption of a damaging species. Of these, an adsorption assisted process is most compatible with the observations. Qualitatively the adsorbed species are envisioned as modifying the behaviour of the surface atoms at the crack tip. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate