Monday Morning Iron - Phase I

The primary work was done by M. Popescu from Materials Technology Laboratory (CANMET), Ottawa, Ontario, Canada and the results presented to the public at the AFS Convention in Kansas City, May 2002. A complete report is available from AFS and is copyrighted by them.

The project was commissioned by the AFS 5L committee on Molten Metal Processing to answer a number of questions regarding the holding of ductile base (prior to treatment) iron on the "Monday morning effect" and the effect of this on the nucleation potential and possible promotion of carbides and shrinkage. Note that experiments were also run on grey iron, but not reported upon in this summary.

Heats of ductile iron were produced using 10% pig, 40% steel, returns and trim additions of C & Si. Originally the heats were melted and held in a 220 pound induction

Crucible, but the C & Si losses were high so a second set of experiments were done in a larger furnace of 550 pounds capacity utilizing a synthetic slag cover to minimize losses and variation. Holding was done at three temperature levels; 2456^oF, 2565^oF, and 2709^oF. The chemical composition was to be held consistent, but his proved impossible over the holding time of up to 10 hours, so some trimming was done at the end of the holding periods in one data set to normalize the results. 

All test samples were made in CO₂ bonded silica sand molds. Pouring temperatures of the test molds were relatively low (2460^oF typical maximum) even from the heats with the highest holding temperatures, since no heating up was allowed from the holding temperature.

The conclusions drawn were as follows:

1. The higher temperature of holding contributed to changes in chemistry at all three levels of holding temperature. The changes depended upon the holding temperature and the synthetic slag protection. At higher temperatures carbon decreased and silicon increased slightly. At lower temperature both elements decreased slowly. There wasn't much difference in chemistry at the two lower temperatures.
2. All trimming performed on the base irons had re-nucleation effects that were picked up in the mg treated final results. Test samples taken at the end of the holding period, from the base iron and the final iron, both before and after trimming, show that the nucleation potential of the base iron affects the final nodule counts. The project did show that ductile base iron having a good nucleation potential generated higher nodule counts in the final structures. They surmised that a systematic control of the base iron microstructure and chilling tendency could predict the metal quality before the magnesium treatment, but said that more investigative work should be done in this area.
3. Examination of final microstructures of test samples with similar carbon equivalents indicated the holding time at lower temperatures (2456 & 2565) increased the nodule counts (some had trimming additions), while holding at high temperature (2709) did not seem to have any effect on the nodule count.
4. While the holding time did not seem to affect the matrix in the 1 inch samples, the microstructural study indicated that holding at lower temperatures encouraged more ferrite formation in 6 & 9 mm diameter samples. However these also had higher nodule counts.
5. The 5 % MgFeSi plus 1% TRE used for treatment and the 75% FeSi used for inoculation had strong nucleation effect, which seems to make the ductile iron less susceptible to the holding time effect compared to grey irons.

DIS comments about this topic and conclusions are:

1. Holding temperature was relatively low for two out of three temperatures studied. Holding at the lowest possible temperature for your operation is always best as less alloy losses occur and nucleation is changed less. Holding time after superheat should also be kept as short as possible before pouring.
2. Trimming additions had a re-nucleation effect. So it is possible to return the base iron to normal nucleation levels. This is true even as wedge values go to very high numbers. Using small additions of carbon, silicon, silicon carbide and pig iron can reduce the base chill value.
3. Checking base iron chill values so that there is a known base value is important. This can be done with chill wedges or with thermal analysis techniques. 
4. Checking chill values when there is additional holding time due to breakdowns, etc, can tell you what needs to be done to re-nucleate the melt to avoid low nodule counts, carbides and sporadic shrinkage.
5. Starting with a good charge and keeping a good nucleation value carries through to the final iron and keeps high nodule counts.