Figure 1 shows that maximal elongation occurs for an oxygen activity of 72 ppb. For lower oxygen activities, elongation, ferrite content and nodularity decrease. Lower oxygen activity results from the initial higher magnesium content in the iron. (Unless otherwise stated, magnesium content always refers to the residual magnesium content in the iron.) In the previous research8
maximal elongation in
ferritic ductile iron was obtained for residual magnesium content between 0.024 and 0.042 percent. The harmful ef- fect of too much magnesium has been noticed by others before. The references that follow pertain to normal sized wall thickness (about 25 mm, 1 in.) as in the present case. Ellis and Donoho17
found a maximum in the elongation
determined free magnesium and found at about 0.025 percent (free Mg) that maximal values for elongation and nodularity were obtained. Up to the maximal content of free magnesium examined by Itofuji (about 0.045 per- cent) elongation and nodularity remain constant. Sokoljuk et al.20
finds highest nodule
count in 30 mm wall thickness at 0.045 percent magne- sium. Nodularity decreases at higher magnesium content however most pronounced in thick walls (75 mm).
The previous references show that with increasing magne- sium content, elongation goes to a maximum and then de- creases. For nodularity, results are contradictory. The fact that optimal magnesium content is higher than in the case of the previous research carried out8
that magnesium is determined in the Y-block and not in a separate test immediately after magnesium treatment as mentioned by Ellis and Donoho.
The close relation which exists in ductile iron between oxy- gen activity and various mechanical properties becomes more clear when plotting data as a function of oxygen ac- tivity. At about 72 ppb, the iron has optimal properties, the highest strength in combination with maximal elongation. For the first time, the graphite transition in ductile iron could
International Journal of Metalcasting/Spring 10
Figure 1. Change of various properties during holding of melt 060295-S1. Oxygen activity (at 1420ºC), ferrite percentage, tensile strength, proof strength are shown in red and pertain to the left scales. Nodularity, elongation and Brinell Hardness are shown in blue (right hand scales).8
27
of standard keel blocks between 0.065 and 0.090 percent magnesium in as cast ductile irons. At higher magnesium levels elongation drops. At excessive magnesium levels (over 0.14 percent) spiky graphite forms. Ellis and Donoho recognize that by sampling some time after the magnesium treatment, substantially lower magnesium contents are found to give maximal properties. This is the major reason why the present authors determined the chemical analysis on the Y-blocks and not on separately chilled samples. It would be quite difficult to take a chilled sample simulta- neously with the pouring of the Y-blocks. Sofue et al.18 also notice a maximum in elongation at 0.060 percent mag- nesium. However, nodularity keeps increasing up to more than 0.12 percent magnesium. They attribute the drop of elongation to an increased amount of nonmetallic inclu- sions containing magnesium. They advise to spheroidize the graphite with the least possible amount of magnesium. Itofuji19
be examined using the oxygen activity sensor during pro- duction itself, and not afterwards by examination of graphite structures in castings. The optimal value of the oxygen activ- ity was previously listed as 100 ppb.8
The difference with the
present value of 72 ppb results from a further optimization of the measuring equipment. Older equipment used the oxygen activity sensor mounted on a pneumatic vibrating lance. The new lance, which is electrically vibrated, gives a higher and constant frequency during measurement.
optimal). Both pictures are intended for illustration and have been taken at magnification 100x while image analy- sis is the average of 10 pictures taken at magnification 200x. Higher magnification lowers nodularity by image analysis because surface disturbances of nodules become
Nodularity also shows a pronounced maximum at the same oxygen activity (Figure 2). Two examples of the graphite structure from Fig. 2 (060295 S1) are shown in Fig. 3 (Mg too high, aO aO
very low) and in Fig. 4 (Mg and
mention that for magnesium levels higher than 0.050 percent, the plastic deformation ability at the ferrite grain boundaries decreases. Hasse21
, may be due to the fact
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