PSI - Issue 13

Guy Khosla et al. / Procedia Structural Integrity 13 (2018) 1447–1452 Guy Khosla/ Structural Integrity Procedia 00 (2018) 000–000

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This paper aims to characterize the microstructure of a ferritic high silicon clinking susceptible steel. The steel is in the as-cast condition, where the steel is in the condition susceptible to clink cracking. Charpy tests are conducted to understand brittle to ductile transition temperature. This helps to determine whether the hot charging temperature is effective at keeping the steel outside the brittle temperature range. The composition through the thickness of the sample is analysed by Energy Dispersive X-Ray Spectroscopy (EDX) and X-Ray Flourescence (XRF) testing. XRF gives the bulk composition of samples at various locations through the thickness. EDX testing is able to give the composition of grain boundary segregates and precipitates. This can help us determine if there are certain locations that give compositions susceptible to low ductility troughs as discussed. 2. Materials and Methods A thin slab cast, 70 mm thick, of non-grain oriented silicon steel has been used for this study. The slab has been cast and hot-connected through a tunnel furnace. Charpy samples have been taken through the thickness of the slab. Samples were notched to give crack propagation corresponding to the through the thickness direction of the slab as clink cracks propagate. Charpy specimens were tested at 5 temperatures: -80°C, 20°C, 200°C, 300°C, and 400°C. The samples at -80°C were cooled using methanol and liquid nitrogen, and samples at 200°C, 300°C and 400°C were heated and soaked at that temperature for 2 hours before testing. For each temperature one sample had a thermocouple attached which evaluated the temperature after the Charpy was removed from the furnace and tested – and this was ensured to be at the test temperature +/-5 ° C. After Charpy testing, the samples were cut away from the fracture face, polished and etched with 5% nital. These samples were then observed under an optical microscope and analysed with the EDX to determine the composition of second phase particles inside the matrix and around the grain boundaries. Three fractured Charpy samples are cut into ͳͲ ൈ ͳͲ ൈ ͳͲ mm blocks to analyse the bulk composition at locations through the thickness using XRF. Samples were taken at 10 mm intervals from the top surface. 3. Results 3.1. Charpy Results Fig. 2 shows the Charpy test results from 10 mm, 30 mm and 50 mm deep in the 70 mm thick slab. This ferritic steel shows a ductile to brittle transition occurring in the region between room temperature and 300 ° C for samples taken in all locations. There is insufficient upper shelf data to analayse any differences in Charpy energy on the upper shelf between locations in the slab.

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Fig. 2: Charpy results from specimens taken from location 1, 2 and 3 from an as-cast high silicon slab

3.2. Optical Microscopy Charpy samples were cut 10mm away from the notched surface, polished down to 1μm and then etched with 5% nital to view the grain structure that has been fractured. Figures of the plane of the fracture face are shown in Fig. 3, fracture occurs along the direction of the columnar grains for all samples. The microstructure is dispersed bainite, laths of cementite form often in a random orientation.