Browsing by Author "Van Rooyen, Melody"

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    Creep damage assessment of ex-service 12% Cr power plant steel using digital image correlation and quantitative microstructural evaluation
    (MDPI, 2019) Van Rooyen, Melody; Becker, Thorsten; Westraadt, Johan; Marx, Geneveve
    ENGLISH ABSTRACT: The lifetime of steam pipelines in long-term operation in coal-fired power plants are limited due to material damage that resulted from creep exposure. In the present study, the authors comparatively assess the damage of ex-service 12% Cr piping steel with varying degrees of exposure while using accelerated creep tests that employ digital image correlation (DIC) as well as microstructural investigation that is based on electron microscopy. The DIC technique, which allows multiple creep curves to be measured at temperatures ranging from 550–600 °C from a single specimen, revealed higher Zener–Hollomon parameters for a high damage material with a high void density when compared to a material with lower damage and lower void density. Both of the material states showed similar hardness values, subgrain sizes, and boundary character, despite the difference in void densities. Slightly higher inter-particle spacing of MX precipitates results in a lower threshold stress of 79 MPa for the high damage steel when compared to 97 MPa for the low damage material. Besides large Laves phase particles (>0.2 µm) that are found in the higher damaged materials that result in solid solution depletion, the most prominent microstructural damage indicator was a lower density of M₂₃C₆ precipitates. Therefore, the observations indicate that the Zener–Hollomon parameter and M₂₃C₆ particles are good damage assessment indicators between the most extreme damage states and they predict a lower damage level for a medium void density material.
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    Heat treatment of TI-6AL-4V produced by laserCUSING
    (SAIIE, 2015) Becker, Thorsten; Van Rooyen, Melody; Dimitrov, Dimitri
    LaserCUSING® is a selective laser melting (SLM) process that is capable of manufacturing parts by melting powder with heat input from a laser beam. LaserCUSING demonstrates potential for producing the intricate geometries specifically required for biomedical implants and aerospace applications. One main limitation to this form of rapid prototyping is the lack of published studies on the material performance of the resulting material. Studies of the material’s performance are often complicated by dependence on several factors, including starting powder properties, laser parameters, and post-processing heat treatments. This study aims to investigate the mechanical properties of LaserCUSING-produced Ti-6Al-4V and its performance relative to the conventional wrought counterpart. A combination of conventional and LaserCUSING-tailored heat treatments is performed. The resulting microstructures are studied and linked to the properties obtained from hardness tests. The findings highlight that LaserCused Ti-6Al-4V is competitive with traditional materials, provided that optimal parameters are chosen and parts are subject to tailored post-processing. In the as-built condition, LaserCused Ti-6Al-4V displays superior strength and hardness as a result of a martensitic microstructure, and a poorer performance in ductility. However, the material performance can be improved using tailored heat treatments. Careful consideration must be given to suitable post-processing before application in critical components in the aerospace or biomedical industry can occur.
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    Thermal power plant steel creep deformation measurement using digital image correlation
    (Stellenbosch : Stellenbosch University, 2020-12) Van Rooyen, Melody; Becker, Thorsten Hermann ; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH ABSTRACT: Following the rapid expansion of populations and industries with a concomitant rise in the demand for electricity in South Africa, increased focus has been directed to the structural health monitoring of critical components of aging thermal power plants. Long-term operation at high temperatures and loads encountered by plant components results in the degradation of material properties through creep exposure. Assessing creep damage using conventional creep testing poses several challenges in terms of large sampled material requirements for manufacturing standard specimen geometries. This is often not possible for service-retrieved material due to the limited availability for destructive sample removal. This dissertation documents a novel experimental technique that employs digital image correlation (DIC) to characterise the creep damage of service-exposed power plant steel through creep deformation measurement across nonuniform temperature and stress fields. Through the full-field property of DIC, multiple properties are measured from single specimens ofX20CrMoV12-1 (X20) piping steel supplied by Eskom in virgin and various stages of service exposure. Initial development of the technique involves a stereo-DIC setup adapted for high temperature deformation measurement.A temperature profile ranging from 550–600°C is applied using resistive heating from a Gleeble thermo mechanical simulator and measured using infrared imaging.Near-uniform stress profiles result from appropriate specimen geometry and region of interest selection. Several temperature-dependent elastic moduli and Poisson’s ratios are measured from single specimens of virgin X20. Creating an extension to accelerated creep tests(shorter-term tests conducted at higher stresses and temperatures than encountered in service)used on virgin and various ex-service X20 states is explored.Creep curves at several temperatures are successfully realised from single specimens and are useful for identifying differences in creep resistance between ex-service levels in line with traditional damage classification through surface cavity density replication. Micro structural quantification using electron microscopy shows subgrain and M23C6precipitate growth as well as Laves phase precipitation in higher damage states of X20. For middle damage classes, it is highlighted that several complementary damage assessment methods are necessary to identify differences in material deterioration. Zener-Hollomon parameters calculated from DIC-measured strains over the temperature profiles serve as suitable damage indicators. Single camera DIC measurement of creep curves across spatially varying stress profiles is achieved with a waisted specimen design in a traditional creep testing setup for medium-term tests. Virgin X20 creep curves at 140–150MPaare used to calibrate baseline parameters for a continuum damage mechanics model. Subsequently, the model is optimised for subgrain and precipitate parameters using the corresponding ex-service X20 creep responses. These micro structural-based parameters serve as comparative damage sensors that agree with traditional cavity and hardness-based methods. This dissertation reveals that a wealth of creep data can be extracted from fewer X20 specimens, which is highly beneficial in characterising material integrity in a supplementary manner to existing methods.Expansion to small samples through small punch creep testing is also initiated. Although beyond the scope of this work, future aspirations are that these approaches will be integrated into life management philosophies to better guide inspection and maintenance strategies.