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Deformation mechanism map and microstructural behaviour of austenitic stainless steel incoloy 800H tube during uniaxial creep
(2018) Beardsley, Aaron Luke
Incoloy 800H is an austenitic stainless steel alloy with nominal composition Fe-32Ni-21Cr developed by the Special Metals Corporation in the 1950s. The alloy was developed for the purpose of creating a corrosion- and creep-resistant metal which was cheaper than its nickel-based superalloy counterparts. It has a solid substitution strengthened single-phase Fe-Ni-Cr matrix, with additional strengthening from Ti(C,N) and M23C6 precipitates. As a result of this, it has since been used in various applications. In particular, it has been used in the petrochemical industry for methane reformer exit tubes. These tubes are created from 800H billets by a pilgering process, which involves several multiaxial, pseudo-periodic deformation steps in order to reduce the wall thickness to the desired level. Following the pilgering process, the tubes are solution annealed to recrystallize the material and increase the average grain size to the American Society for Testing and Materials (ASTM) grain size number 5 or coarser. In the field, the pigtail tubes can operate at temperatures in excess of 900 °C and stresses up to 10 MPa. They have a target service life of 20 years.
To quantify the creep performance of a material, researchers devised creep deformation mechanism maps (CDMMs). These maps plot isominimum strain-rate lines as a function of normalised stress and homologous temperature. During creep deformation, there are several competing creep mechanisms, such as the diffusion-controlled climb of edge dislocations, or the diffusion of vacancies through grain boundaries. CDMMs present regions on the map where each mechanism is the dominant mechanism, which is defined as the mechanism which contributes the largest amount to the total minimum strain-rate. Despite being used extensively in creep-based applications, there is a significant lack of knowledge about the creep performance capabilities of 800H. To date, a CDMM for 800H has not been created. This means that the industry uses the alloy without any knowledge of the true failure mechanism during service, or the limits to which they can maximise the temperature and stress during service. There is also significant lack of knowledge about how the microstructure, in particular the average grain size, affects the creep performance. Filling these voids of knowledge will be the primary objective of this research.
For this research, 800H was received in an as-pilgered condition. This was useful as it meant that there was more control over the microstructure, as each batch of material could be heat-treated for different average grain sizes by solution annealing at different temperatures. In this research, separate batches of creep test samples with mean grain diameters of 109 μm, 180 μm and 248 μm were achieved by solution annealing for 2 hours at 1100 °C, 1150 °C and 1200 °C respectively. With sixteen samples from each heat-treatment, and an addition ten samples of unknown heat-treatment, a total of fifty-eight samples were creep tested. These tests were performed at temperatures ranging from 750 - 1020 °C and stresses ranging from 15 - 105 MPa. Of the fifty-eight tests, twenty-four were tested until rupture, and the remainder were terminated after the minimum strain-rate had been passed in order to save time. Test times ranged between 10 - 2000 hours. By fitting data to a hyperbolic-sine power-law model, minimum strain-rates were reliably extracted from the raw creep test data. Minimum strain-rates varied from approximately 1.6 x 10-6 - 2.0 x 10-10 s-1. These testing conditions covered a comprehensive region of the CDMM.
The primary creep mechanisms of interest are power-law creep, which can be divided into high-temperature and low-temperature submechanisms, Coble creep, and Nabarro-Herring creep. By summing the constitutive equations for these models, the total minimum strain-rate as a function of temperature, stress and average grain size can be modelled. Using a computational optimisation algorithm called the genetic algorithm (GA), the material constants for the model were numerically fitted to the experimental data, a novelty for this application. The solutions were then assessed using a training and testing technique. Once the material constants were optimised, the CDMM was created for 800H. This map was then expanded to three-dimensions by showing a series of maps with different average grain sizes. Several novel CDMM concepts were explored, such as adding colour to visually represent the contribution of each mechanism to the total minimum strain-rate, converting the axes to absolute units, and truncating the axes of the figure to show more detail in the more important region of the map. Other creep models, such as a hyperbolic-sine model, and a model based on the Larson-Miller and Monkman-Grant equations were also explored and compared to the traditional model.
After finalising the CDMM for 800H, it was determined that these maps alone are not sufficient for practical applications. The maps show the mean minimum strain-rate based on the fit of the model to the experimental data. Although error for the fitting process was minimised by using a GA, the error still exists and is not represented on any maps that exist today. Therefore, it was decided to create a map which showed the level of certainty for the minimum strain-rate as a function of the position on the CDMM. Using several statistical analyses of the data to quantify aspects such as the variability of data tested at the same temperature and stress, and the extrapolability of the data, an error contour map was created. This novel concept is a first approximation and will require further development before it can be implemented.
The final section of work focused on microstructural aspects during creep deformation of 800H. Using several different microscopy techniques, in particular, energy-dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD), the microstructure of creep deformed 800H was explored. This work focused on the accumulation of damage as a function of creep strain, and the determination of the creep mechanism involved during the deformation. Key microstructural aspects that were assessed include the formation of subgrain boundaries, precipitation and void formation. The gathered information was compared to a 20-year service 800H pigtail tube, which was used to determine the level of strain in the observed microstructure.
Overall, this work provides a significant contribution towards the understanding of the creep performance characteristics of 800H. This includes novel contributions to the fields of creep model optimisation, statistical analysis of creep data, and microstructural characterisation, all of which can be potentially extrapolated to related materials. Specifically, this work acts as a foundation for the understanding of creep deformation of 800H, which others can build upon. With this work, industries should be able to more reliably predict the creep deformation performance of 800H.
Debt Finance and Economic Activity in the Euro-Area: Evidence on Asymmetric and Maturity Effects
(2023) Guender, Alfred; Das, K; Donald, L
This paper presents a model of alternative sources of credit – bank vs. bond finance - to
examine the credit substitution hypothesis. Our framework produces testable hypotheses
about the behaviour of price- and quantity-based information variables. Examining data from
ten Euro-area countries, we find that a credit spread outperforms a finance mix as a predictor
of economic activity in both time series and pooled data regressions. There are clear signs of
asymmetric and maturity effects in the data. Positive changes in the credit spread predict
decreases in economic activity while negative changes bear no informative content. The
asymmetric effect is exceptionally strong in pooled data and is present in short-term, long-term, and total credit spreads. In country-specific time-series regressions the asymmetric
signalling property is strongest for the long-term credit spread. By contrast, we find no
substantive evidence that changes in a quantity-based finance mix have robust predictive
power.
Trait-habitat associations explain novel bird assemblages mixing native and alien species across New Zealand landscapes
(Wiley, 2022) Barnagaud, JY; Brockerhoff, Eckehard G.; Mossion, R; Dufour, P; Pavoine, S; Deconchat, M; Barbaro, L; Thomassen H
Aim: Species introductions have reshaped island faunas for the last 200 years, often threatening native biodiversity. Approximately equal numbers of native and alien species currently co-occur in the New Zealand avifauna, but they show distinct habitat use. Antagonistic interactions, habitat affinities and legacies of introduction history may concur to explain their segregation along habitat gradients. To investigate these processes, we explored how habitat, ecological traits and introduction history relate with the current composition of bird assemblages. Location: New Zealand. Taxon: Birds. Methods: We analysed 917 bird point counts spread along habitat and elevation gradients in the Canterbury region, South Island and related 10 ecological traits to landscape composition using a three-table ordination method known as “RLQ analysis”, accounting for spatial autocorrelation and phylogeny. We tested whether alien species’ positions in the RLQ were related to proxies of introduction history. Results: Eighteen endemic, 11 native and 19 alien species were distributed along a gradient from forest to open-habitat assemblages, in relation to foraging mode, nesting site and body size. A second gradient segregated species between native and exotic forests according to territoriality, sedentarity and diet. Traits accounted for the separation of native and alien bird species in forests, but not in open habitats. Phylogenetic signals emerged from the separation of native and alien species by forest type, and spatial structures suggested a landscape-level, rather than regional or local determinism. These correlations were independent of introduction history, although open-habitat assemblages tended to host alien species introduced later in time. Main conclusions: Habitat type and resource availability explain the spatial partitioning of New Zealand bird assemblages between native and alien species more consistently than competitive exclusion. We conclude that trait-mediated ecological differences among species have likely played a predominant role in species’ segregation among landscapes, while maintaining endemic bird assemblages in native forests.
Application of travelling behaviour models for thermal responses in large compartment fires
(Elsevier BV, 2022) Nan, Z; Khan, Aatif Ali; Jiang, L; Chen, S; Usmani, A
Large open-plan compartment fires in modern buildings may exhibit a local burning region travelling across the floor plan as a ‘travelling fire’. This phenomenon has been found in the forensic investigations of fire accidents and in the large compartment fire tests. The fire impact in a large compartment is spatially non-uniform and time-variant, which can cause severe local damage to structural components. Advanced from the previous models assuming constant travelling, the natural fire model established in this paper comprises time-variant and test-based travelling behaviour models and localised fire models of various modes. It is demonstrated with the fast-spread Veselí fire test and the slow-spread Malveira fire test. A generic structural model is set up within OpenSees for fire to examine the thermal impact on structural members under various travelling fire scenarios of different travelling parameters, fire travelling directions, and beam sizes. Locally much higher thermal responses are represented after introducing behaviour models while adopting the same design fire load. Based on the work in this paper, a library of design fire models can be potentially enabled to examine the fire safety performance of structures regarding the realistic fire load and fire impact aiming for discovering unknown worse fire scenarios.
Facade Fire Hazards of Bench-Scale Aluminum Composite Panel with Flame-Retardant Core
(Springer Science and Business Media LLC, 2023) Khan, Aatif Ali; Lin S; Huang X; Usmani, A
Façade fires in tall buildings are currently occurring more than once a month globally that are responsible for many casualties and billions of dollars in losses. In particular, the tragic Grenfell Tower fire in London with more than 70 fatalities raised the profile of façade fire hazard. This work used well-controlled irradiation up to 60 kW/m2 to re-assess the fire hazard of typical flame-retardant aluminum composite panels (ACPs) with a dimension of 10 cm × 10 cm × 0.5 cm. We found that the vertically oriented ACPs with the “non-combustible” A2-grade and “limited-combustible” B-grade cores could still be ignited above 35 kW/m2 and 25 kW/m2, after the front aluminum layer peeled off. The peak heat release rate per unit area of these ACPs could be higher than common materials like timber and PVC. Moreover, compared to the B-core panel, the A2-core panel showed a greater fire hazard in terms of a shorter ignition delay time, a higher possibility of the core peel-off, and a longer flaming duration under current test size and fixing condition. Because the ACP is a complex system, its fire hazard is not simply controlled by the core material. The structural failure of ACP in fire, including peel-off, bending, softening and cracking, may further increase the fire hazard depending on the scale effect, boundary and fixing conditions. This research improves our understanding of the systematic fire behaviors of façade panels and helps rethink the fire risk and test methods of the building façade. Graphical Abstract: [Figure not available: see fulltext.].