Three-dimensional characterisation of micropores and graphite nodules in ductile cast iron


Powerful non-destructive inspection techniques such as micro radiography and X-ray based multi-scale computed tomography (XCT) including synchrotron X-ray computer tomography (SCT) are capable to reveal the heterogeneities in the microstructure of ductile cast iron samples. Both graphite particles and micropores show a strong absorption contrast with respect to the iron matrix owing to the difference in X-ray absorption. The quantitative analysis of size distribution and nodularity which is usually performed by destructive optical metallography is essentially confirmed by three-dimensional tomography. An interdendritic three dimensional extension of feeding pores is revealed by XCT in the central main bearing of a nodular cast iron crankshaft. The orientation of those pores is continued by equally aligned graphite nodules. Microporosity is also revealed in between regions of homogeneous graphite distribution and is clearly discriminated by SCT. Performing in-situ tensile tests during synchrotron tomography show the micropore distribution in the gauge length and its role in crack formation and propagation in miniature tensile samples. Both micropores and graphite particles are the weakest points of the casting under tensile stress. Micropores < 150 ┬Ám in width and below 1 vol. % do not degrade the strength of the casting. Cracks form at the micropore with the largest extension perpendicular to the tensile direction and propagate along pores in regions where the local porosity is > 1 vol. %.
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