Abstract
The microstructure of two centrifugally cast ductile iron pipes with a diameter of DN100 and wall thickness of 6 mm was studied. After casting, the pipes undergo annealing and finishing with anticorrosion coatings. Tensile tests were performed using longitudinal samples, cut in an appropriate manner from the pipe wall. Depending on the graphite morphology and the concentration area of casting defects, tensile samples cut from the same ductile iron pipe may show significant scatter of mechanical parameters. Thus, when tensile samples are machined from pipe ready for installation, the results of mechanical tests must be carefully interpreted. Particular attention should be paid to the porosity, present in the cross-section of the strained samples. The defective material has insufficient strength and very low elongation, i.e. tensile properties of the individual sample could be well below the required minimum of 420 MPa and 10 % in the standard EN 545.

Key words
ductile iron, water pipes, centrifugal casting, tensile properties, internal porosity, chunky graphite

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