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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austenite – ferrite transformation. The composition of this particle is shown in Table 2 as spectrum 3. There is also cementite formation on the grain boundary as seen in spectrum 4. The large laths are confirmed to be cementite as shown by spectrum 1 and 2, with a higher percentage of carbon of around 6%.

Table 2: Composition results from EDX analysis on a sample 15 mm from the surface. Fig. 4 show the location of the spectrums taken.

C

Al

Si

S

Ti

Mn

Fe

Spectrum 1 Spectrum 2 Spectrum 3 Spectrum 4

6.76 6.24

- -

2.68 2.88 1.17 2.58

- -

- -

- -

90.55 90.88 26.42 90.34

-

1.18

25.9

3.06

42.26

7.08

-

-

-

-

4. Discussion The microstructure of an as-cast slab high silicon slab has been characterized by EDX, optical microscopy and XRF. The ductile to brittle transition temperature of samples taken through the thickness of the slab is also characterized. The samples were fractured along the plane of the columnar cracks and the brittle to ductile transition temperature appeared similar for the 3 locations tested through the thickness of the slab. This transition is likely to occur between around room temperature and 300 ° C, however insufficient tests were completed to determine any differences in upper shelf energies or transition temperatures. The ductility clearly drops below 200 ° C and hence the use of hot charging alloys, maintaining a high temperature is clearly beneficial to prevent a brittle failure. A definitive value for the ductile brittle transition temperature has not been obtained due to the lack of temperatures and the spread of results. Optical microscopy of samples through the thickness of the slab revealed, as expected, smaller grains at the top surface, followed by larger columnar grains extending toward the centre of the slab. The microstructure shows a ferritic steel with large, dispersed platelets of cementite. The EDX analysis showed that large platelets of cementite have also formed along the grain boundaries. There is also strong evidence showing void formation around a second phase particle of MnS. This particles were seen in the region close to the grain boundary. Hot ductility troughs in continuous casting are present due to deformation induced ferrite forming at the grain boundaries. As ferrite at these temperatures is softer than the austenite, strain accumulates in this region. Second phase particles, particularly MnS and AlN, act as sites for void formation, which then link up to form catastrophic failure. The XRF testing shows how the composition of the steel varies through the thickness of the slab. Transverse cracking is often alleviated through controlling the composition. However, if macro segregation occurs, whereby detrimental elements group together – then the sample will become increasingly susceptible to clinking. It is seen that Sulphur has a relatively uniform segregation through the thickness – although this is frequently debated as an element that creates a poor ductility zone in the centre, called centerline segregation (Fujda 2005). Aluminium is of interest as one specimen, in the top quarter of the slab shows no trace of aluminium, whereas the surface sample shows a high percentage of aluminium. This is an element known to form AlN, detrimental to clinking susceptibility. 5. Conclusion The microstructure of an as-cast high silicon slab has been analysed to understand how the microstructure may affect the fracture mechanisms behind clinking. Charpy tests confirmed that the ductility increases around 200 ° C, which validates the use of ‘hot charging’ to alleviate the brittle region of the material. EDX analyses showed void formation around a large second phase particle, MnS, near to the grain boundary. This damage is in line with the material damage one would expect to occur in transverse cracking. XRF analyses showed how the segregation of elements, specifically aluminium is not uniform through the slab. High concentrations of AlN, aswell as MnS, are known to be specifically detrimental to the formation of transverse cracks.