Thesis (M.Sc.(Applied Science: Metallurgy))--University of Pretoria, 2009. / Abstract in English. Includes bibliographical references.

M.Phil. / Stainless steel is considered to be the most appropriate material for the fabrication of plant and equipment in the food and beverage industry. The apparent ease by which the surface finish of the material can be kept hygienically clean is a key factor in favour of stainless steel over other materials and those with applied surfaces, as is its resistance to corrosion. The manufacture of stainless steel and subsequent processes to fabricate the finished product into plant or equipment are well researched and documented. The relevance of a so called clean surface during manufacture and fabrication is documented in international standards to ensure that the material is protected against corrosion to prevent failures during the life cyle of the product. The importance of cleanability of specific surface finishes on stainless steel and comparative materials has been researched in small scale experimental set-ups. The results indicate that the rougher surfaces make cleaning more difficult and that the cleaning processes have a significant impact on the final cleanliness of the surface. No research has been documented on the effect of the operational environment in a brewery on the passive oxide layer of the stainless steel equipment surface. The possible breakdown of the passive layer on the surface is generally known to cause corrosion, that in turn causes failures in the fabricated equipment. The critical importance of maintaining strict hygienic standards in food processing plants has been the focus of international standard bodies to reduce the incidents of foodborne diseases. It has therefore become critical to understand how clean is the surface, and how the surface can be measured in an operational environment by using effective and reliable non-destructive testing procedures. A brewery operational environment review of stainless steel equipment was carried out to assess the impact on the surface of the equipment after 10 years. The results obtained from this review were used to design the experimental set-up. The test vessel is a fermenter that forms part of a training brewery that produces beer using standard processes. The internal surfaces of the fermenter were prepared with 4 different finishes (2B milled, 120 and 240 mechanically polished and electropolished). These are finishes that are used in the food and beverage industry. The fermentation process carried out in the vessel created a standard soil that was then cleaned off by the standard Cleaning-In-Place (CIP) process using caustic, acid and sterilant regimes. The experiments were repeated to assess the results of the comparative cleanability on the different surfaces and the possible changes occuring on the surfaces during the fermentation and cleaning cycles. The method used to check for cleanability is based on ATP Bioluminescence that detects minute traces of organic material that indicate the level of hygiene in the vessel. The methods used to check the surface roughness include standard Two-Dimensional Profilometry directly on the metal surfaces and Three-Dimensional Microscopy on replicated samples. Visual appraisal of cleanability was also done at each step of the process. All these tests were carried out on the surfaces before use and after each fermentation and CIP cycle. The results indicate that all surfaces are equally clean in areas where the CIP chemicals impinge directly on the surface at the top of the vessel. As the chemicals flow down the side of the vessel and reach the bottom cone, the levels of hygiene reduce. The surface that achieved the best level of hygiene is the electropolished surface finish, even at the lower section of the vessel. The mechanically polished surface (240 Grit) started to pit after the second cycle. Both the lower cleanability of surfaces at the bottom of vessel, and the roughening caused by pitting, have been observed during the operational review. The results also indicate that further work can be done to optimize the CIP processes to achieve effective cleaning at the lowest cost, and that the surface breakdown can be assessed and analysed using the replicating sample method with microscopy to determine the extent of change over the life cycle of the equiment.

Dissertation (Ph.D.)--University of Toledo, 2009. / Typescript. "Submitted as partial fulfillment of the requirements for the Doctor of Philosophy in Engineering." Bibliography: leaves 234-240.

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Super duplex stainless steels (SDSSs) with lower nickel content are cost effective substitutes for higher alloyed austenitics and nickel alloys in demanding environments such as oil and gas production tubulars and pipelines due to their excellent corrosion resistance and high strength. The overall properties of SDSS are derived from its dual microstructure of equal ferrite and austenite, higher alloying additions of chromium, molybdenum, nitrogen and tungsten and its thermomechanical history. Higher alloying renders SDSS prone to secondary phase precipitation such as sigma phase during improper welding operations or fabrication, affecting the materials properties. Reports suggest that tungsten additions in SDSS delays sigma phase precipitation, hence the development of tungsten based SDSS such as UNS S39274. However, secondary phases cannot be entirely avoided in SDSS. Secondary phase evolution in DSS and the mechanical properties/corrosion behaviour of SDSS has been studied extensively yet their interaction is still not clear. In-service failures of SDSS components have identified gaps in the understanding of the link between secondary phase evolution and material properties, thus limiting the safe and efficient use of SDSS. The work presented in this thesis explored and quantified experimentally the role of aging on secondary phase evolution and ensuing effects on the mechanical properties, corrosion behaviour and toughness of UNS S39274 SDSS. The results revealed that chi phase precipitation occurred preferentially before the sigma phase, although chi phase was metastable in the studied alloy. Numerical modelling established that the measured concentration of the precipitated sigma phase follows the prediction by the Johnson-Mehl-Avrami kinetic model. The time-temperature -transformation was computed using experimental data, the results were compared with theoretical predictions. Results established that increase in both sigma and chi phase led to significant drop in the uniform strain and enhancement of the modulus, hardness and yield and tensile strengths. We note that the sigma phase was attacked by corrosion in comparison to other grades of 25Cr SDSS where the sigma phase remains inert to corrosion attack. Pitting corrosion resistance was influenced more by sigma phase than the chi phase composition. Chi phase was more damaging on the toughness than sigma phase. Another key finding is that the corrosion behaviour and fracture behaviour is more sensitive to lower secondary phase volume fraction than the tensile properties.

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An investigation has been undertaken to examine the effect of microstructure on the mechanical properties of FV520B, a precipitation hardenable martensitic stainless steel. This high performance grade of stainless steel was heat treated to three commercially available material specifications, namely the peak hardened, standard and softened overaged conditions. These three precipitation hardened conditions were found to exhibit a range of tensile properties. In order to determine the role of the microstructure, a full materials characterisation programme was performed. The investigative techniques used to characterize the microstructures, were Transmission Electron Microscopy (TEM); Analytical Scanning Electron Microscopy (ASEM); optical microscopy; dilatometry and X-Ray Diffraction (XRD). The microstructural phases and features identified were measured and quantified wherever possible. The effect of the material microstructure and environment on the fatigue properties of FV520B have been investigated. Fatigue tests were performed under uniaxial loading conditions at a stress ratio R (omin/omax) of -1. The tests were undertaken using highly polished specimens to determine the fatigue strength of the three precipitation hardened conditions. The test conditions employed were air and a corrosive 3.5% sodium chloride environment, at pH2 and ambient temperature. The role of the microstructure and the effectiveness of the tensile strengthening mechanisms on the fatigue and corrosion fatigue strength have been discussed. Using SEM, the fatigue crack nucleation mechanisms prevalent within the three microstructures in air and the chloride environment have also been identified. For the peak hardened material, nonmetallic inclusions dominated the fatigue crack nucleation process in air and chloride environments. For the softened overaged condition, multiple site nucleation due to slip band cracking was the prevalent mechanism especially at higher nominal stress amplitudes. The tolerance of this high strength material to small defects at higher stress levels and the actual size of the critical microstructural defects initiating failure have also been highlighted. The microstructure has been shown to strongly influence the processes of fatigue crack nucleation, Stages I and II crack propagation and the concept of the microstructure acting as barriers and providing resistance to crack growth have been discussed. The effectiveness and the size of these microstructural barriers to crack growth have been considered. This discussion has led to the proposal of a model that facilitates flow stress and fatigue lifetime predictions as a function of the quantity of a key microstructural phase. The key microstructural phase, namely reverted austenite affected both the tensile and fatigue properties of FV520B as a function of the heat treatment. The standard overaged material was found to exhibit the greatest resistance to fatigue crack propagation.

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