Browsing by Subject "Stainless steel"

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    Ageing of chromium(III)-bearing slag and its relation to the atmospheric oxidation of solid chromium(III)-oxide in the presence of calcium oxide
    (Elsevier, 2003) Pillay, K; von Blottnitz, H B; Petersen, J
    Slag arising in ferrochromium and stainless steel production is known to contain residual levels of trivalent chromium. As the chromium is normally bound in the slag matrix in various silicate or spinel phases, and hence not easily mobilised, utilisation or controlled disposal of such slag is generally considered unproblematic. Experimental test work with a number of slag materials indicates, however, that very gradual oxidation of trivalent to hexavalent chromium does occur when the slag is exposed to atmospheric oxygen, rendering a quantifiable but small portion of chromium in this much more mobile and toxic form. Mechanisms and rates of the oxidation reaction were investigated in a number of long-term studies using both original slag materials and artificial mixes of chromium and calcium oxides. Powders of these materials, some of them rolled into balls, were left to age under different conditions for periods of up to 12 months. In the slag samples, which contained between 1 and 3 wt.% chromium, 1000–10 000 μg Cr(VI) were found per gram of chromium within 6–9 months of exposure to an ambient atmosphere. The rate of the oxidation reaction decreased exponentially, and the reaction could generally be said to have ceased within 12 months. In mixtures of calcium and chromium oxides the oxidation reaction is presumed to occur at the boundaries between chromium oxide and calcium oxide phases through diffusion of oxygen along the grain boundaries and of Cr3+ across the boundaries, resulting in the formation of calcium chromate. In the slags, where calcium and chromium oxide can form a solid solution, the oxidation is likely to occur at the exposed surface of grains containing this solution.
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    Quantification of fatigue damage in AISI 316L stainless steel using X-Ray powder diffraction (XRD)
    (2025) Ramasimong, Duduzile; Knutsen, Robert; Westraadt, Johan
    In an electricity generation utility, there exists an enduring insistence on improving the efficiency, performance, and reliability of mechanical components with respect to economic challenges. Thus, the comprehension of materials and their behaviour in a typical operating environment is key to enabling and aiding capabilities in component life assessments. Fatigue damage is among the major issues in engineering, because it increases with the number of applied loading cycles in a cumulative manner and can lead to fracture and failure of the considered part. This study systematically investigates the microstructural changes in a material due to cyclic stressing using a non-destructive examination Power X-ray diffraction (PXRD). In industry, the evaluation of material deformation post cycling loading is often performed using microscopic techniques such as transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). However, each of these techniques have their own limitations. The PXRD technique is advantageous when applied to crystalline materials, as it is sensitive to changes in the crystalline structure of the metal, thus providing a foundation for the development of a capability to enable the early detection of fatigue damage with the aim of improving asset management. The microstructural baseline undertaken in the study involves the calculation of dislocation density in the material. The evolution of dislocation density in steels is an important aspect of the mechanical response. It could potentially be used as a fingerprint to relate the material state to the life-consumption fraction in materials subject to fatigue conditions. To mimic the fatigue damage mechanism, cyclic loading of material specimen is performed in a laboratory. In view of the exploratory nature of the present study, a stable single-phase austenitic microstructure, namely AISI316L stainless steel, was selected for the relative ease of characterizing dislocation development and comparison with PXRD analysis. The initial step included plotting an S-N curve for the AISI316L material in the material annealed state following fatigue testing in the INSTRON machine. A specific stress level was selected to ensure sufficient data retrieval prior to point of failure for the material specimen. Thereafter candidate specimen for fatigue including as received and annealed specimens formed part of the material states used in the study for microstructure analysis using three evaluation techniques. The EBSD results mostly show good qualitative agreement with the PXRD analysis. TEM analysis was used to qualitatively visualise the individual dislocations but is very time-consuming to perform quantitatively and the results are subject to large scatter. Validation of the PXRD techniques was performed using qualitative and quantitative TEM dislocation density results and semi-qualitative EBSD results. This study presents a consistent approach to determine the dislocation density using a benchtop laboratory based PXRD. The calculated dislocation density results analysed from the vast volumes of experimental data collected throughout the study highlighted important aspects to be undertaken. These include sample preparation, control of instrumentational parameters and the correct selection of instrument modelling standard. The Williamson-Hall plot and the whole pattern fitting methods are the two evaluation techniques utilised for size and micro-strain broadening of PXRD peaks. The results have proven to be repeatable owing to the systematic manner in the number of samples used per specimen as well as the stringent control of parameters used.
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    Revisiting the effect of capture heterogeneity on survival estimates in capture-mark-recapture studies: does it matter?
    (Public Library of Science, 2013) Abadi, Fitsum; Botha, Andre; Altwegg, Res
    Recently developed capture-mark-recapture methods allow us to account for capture heterogeneity among individuals in the form of discrete mixtures and continuous individual random effects. In this article, we used simulations and two case studies to evaluate the effectiveness of continuously distributed individual random effects at removing potential bias due to capture heterogeneity, and to evaluate in what situation the added complexity of these models is justified. Simulations and case studies showed that ignoring individual capture heterogeneity generally led to a small negative bias in survival estimates and that individual random effects effectively removed this bias. As expected, accounting for capture heterogeneity also led to slightly less precise survival estimates. Our case studies also showed that accounting for capture heterogeneity increased in importance towards the end of study. Though ignoring capture heterogeneity led to a small bias in survival estimates, such bias may greatly impact management decisions. We advocate reducing potential heterogeneity at the sampling design stage. Where this is insufficient, we recommend modelling individual capture heterogeneity in situations such as when a large proportion of the individuals has a low detection probability (e.g. in the presence of floaters) and situations where the most recent survival estimates are of great interest (e.g. in applied conservation).