Abstract The study presents the influence of pouring delay time on the decay of spheroidisation effect and magnesium content in the alloy, as well as the influence of magnesium content on the shape and number of graphite precipitations in bar samples. For one particular set of production conditions, it was observed that 17 min after completing the spheroidisation and modification process, the magnesium content had decreased from 0.07% to 0.04%. This resulted in a decrease in the graphite precipitation shape index S s from 0.081 to 0.067, as well as a decrease in the average number of graphite precipitations N A from 568 to 305 mm −2 . Dependencies have been worked out to describe the influence of time passed after the end of spheroidisation and modification on the amount of magnesium in the ductile iron, as well as the influence of magnesium content on the graphite shape index S s and number of graphite precipitations N A . It has been found that a reduction in magnesium content of approximately 0.027% corresponds to a reduction in graphite precipitation shape index S s of about 0.014, as well as a reduction in the average number of graphite precipitations N A of 255 mm −2 . On the basis of the longitudinal ultrasonic wave velocity measurements c L , as well as the measurement of the graphite shape index S s and the number of graphite precipitations N A in the analyzed ductile iron, dependencies have been worked out: c L = 3218.4 S s + 5407.6, R = 0.99, c L = 0.1887 N A + 5567.6, R = 0.99, where: S s is the graphite shape index, N A is the number of graphite precipitations, mm −2 , c L is the ultrasonic velocity, m/s. Changes in the shape and number of graphite precipitations are accompanied by a decrease in the average longitudinal ultrasound wave velocity c L of 45 m/s. This resulted in establishing nomograms to evaluate the graphite shape index and the number of graphite precipitation from a measured ultrasonic wave velocity in control samples. Obtained dependencies are only valid in the conditions of the specific foundry, for which they have been developed. The applied methodology allowed the achievement of dependencies with high correlation coefficient values, which demonstrates its suitability for industrial use.