Survey on Ductile Iron Practice
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J. M. Csonka - Hickman, Williams & Company, Brook Park, Ohio
E. C. Muratore - Rio Tinto Iron & Titanium America, Rosemont, Illinois
J. E. Woods - Hickman, Williams & Company (retired)
Copyright 2002 American Foundry Society
Reprinted with permission of American Foundry Society

ABSTRACT
The AFS Molten Metal Processing Committee (5-L) sent out a survey (see Appendix) to North American ductile iron producers, which requested information on each foundry's ductile iron practice. The information requested included the areas of melting, base iron chemistry, nodulization, post inoculation, quality control and the types of ductile iron that were being produced at each shop. The results contained are for the fourth of a series that originally started in 1957 and were then followed up in 1978, 1988, and 1998. The foundry responses were broken down into two groups, those producing more than 200 tons of ductile iron per week and those producing less than 200 tons of ductile iron per week. The response data should provide insights into historical process trends.

DISCUSSION OF RESULTS
Surveys were sent out to 508 foundries in the United States, Canada and Mexico. The committee received 196 responses, which would have put the return rate at 38.5%. However, 9 of these responses had little or no information and were deleted from the data pools. This brought the actual responses used to 187 and gave a 36.8% return figure. The committee felt this was an exceptional rate of return for a survey this large. In the 1988 survey, there were 77 returns, but those 77 responses corresponded to about a 50% return rate. As in the past, the results of each sub-category are reported as a percentage of the total return for that group. Also, as in the past surveys, the results were split with foundries producing more than 200 tons of ductile iron per week and less that 200 tons of ductile iron per week. There were 74 responses for foundries producing more than 200 tons, and 113 responses for foundries producing less than 200 tons per week. Foundries that included more than one response per category rationalize the group totals that exceed 100%. This would include foundries that have multiple types of melting groups, treatment methods, treatment sizes and others. Group totals that are less than 100% are due to foundries that did not answer the given question. This would include foundries that do not use certain processes or foundries that were confused by what the survey was asking.

Please note that in the text the terms "foundries that produce more than 200 tons of ductile iron per week" have been replaced with "large foundries". Likewise, "foundries that produce less than 200 tons of ductile iron per week" have been replaced with "small foundries". This is done only to make the text easier to read and is not to be taken as an indication of the size of the foundries that responded to the survey.

Designing a survey that can cover such a diverse process as ductile iron production is a difficult task. While the pipe shops use certain practices, these can differ greatly from the high volume ductile iron producer's practices and even more from the shops that are only occasionally producing a few ductile iron castings. These diverse practices yield trends that vary greatly from each other, but the trends within each group give very good indications of where each group is headed. The survey has been changed every time it has been sent out. This survey is entirely new as the edition from 1988 could not be located and therefore could not be replicated. To ensure that the future surveys are comparable, the survey that was used for this study is included with this report. Responses listed as "not applicable" correspond to survey questions that were not asked in this, or previous, surveys. The data from this and the past surveys are listed in the master data table (Table 1.)

MELTING AND HOLDING
The 1957 survey revealed that primary melting was conducted in a cupola for 65% of those responding to the survey. Cupola melting has since changed dramatically for both responding groups. For the groups producing more than 200 tons of ductile iron per week cupola melting dropped to 39% in 1978, increased to 61% in 1988 and again dropped back to 30% in 1998 (Figures 1 and 2). Coreless induction melting has been on the reverse curve as it went to 45% in 1978, dropped to 38% in 1988 and then rebounded to 62% in 1998. The foundries that are producing less than 200 tons of ductile iron per week have not changed nearly as much in the last 10 years. Cupola melting went from 25% in 1978, dropped to 4% and then 8% in 1988 and 1998 respectively. Coreless induction melting remained at the 84% level for both 1988 and 1998, increasing from 44% in 1978. Holding iron was reduced across the board for both production rated groupings going from 92% to 64% for the greater than 200 ton group and 36% to 13% for the other. While the channel holders and coreless induction holders remained about the same for the greater than 200 tons per week grouping over the last 10 years the arc furnaces and fore hearth holders both decreased. For the less than 200 tons per week group channel holding furnaces took the biggest drop in the last 10 years decreasing from 24% to 7%.

Figure 1. Melting.

Figure 2. Holding.

BASE IRON INFORMATION
For both size groups, the carbon levels remained very stable with the 3.60% to 3.90% level being the most favored. The larger foundries showed a dramatic shift downward in sulfur levels from 54% greater than 0.036% in 1988 to only 26% in 1998, and correspondingly an increase in the less that 0.015% group (27% in 1988 to 42% in 1998). This makes perfect sense as the number of larger foundries melting in cupolas has dropped by 22% in the last 10 years. The smaller foundry  drop to favor the less than 0.015% grouping. Although the melting furnace types for the smaller foundries did not change, the lower sulfur base iron levels suggest higher quality melting materials are being used. This is also reflected in the silicon levels for both foundry groups moving upward from the less than 1.30% level up into the 1.30% to 1.60% grouping.

DESULFURIZATION
For the smaller foundry group, 8% use cupolas and 8% show some type of Desulfurization. For the larger group the number of foundries that desulfurize is down to 34% from 55%, again reflecting the drop in cupola melting shops. Batch desulfurization has declined in all groups and the only real increase is in the two larger tonnage groupings of the continuous desulfurization, 20 to 30 tons per hour and greater that 30 tons per hour. For the larger foundries, the use of calcium carbide declined from 54% in 1988 to 18% in 1998 while the use of CaO/CaF2 showed the only increase from 0% to 12% for the last 10-year survey.

PRECONDITIONING
Across the board, preconditioning appears to have taken a severe blow in the last 10 years (Figure 3). For the larger foundry group 78% show no preconditioning is used while 85% of the smaller foundries likewise show no preconditioning. Using graphite as a preconditioner has taken the largest drop for both groups but the number of foundries using ferrosilicon and silicon carbide have also dropped significantly. There is some concern that the survey may have confused some of the respondents in this section.

Figure 3. Preconditioning.

NODULIZING
Using tundish ladles has made the largest jump of all treating practices and now accounts for 50% of all large foundry treatments and 42% of all small foundries (Figure 4). The sandwich practice has taken the brunt of the blow in the larger foundries being down 16% in the last 10 years and the open ladle practice has dropped 24% in the smaller foundries. The smaller foundry grouping has shown increases in two areas with one being a 7% increase in the use of sandwich practice and a 5% increase in the flow through method to account for the large drop off in the open ladle practice. The larger foundries have shown an 8% drop in using the flow through method. Interestingly, in-mold treatment in the larger production group has taken a 5% drop in the last 10 years after taking a 3% drop in the 1988 survey. Magnesium containing cored wire is running a very small 3% of all foundries with pure magnesium treatment running at 7% in the larger foundries and 2% in the smaller group.

The MgFeSi use of the both groups has remained relatively unchanged in the last 10 years with the majority of users in the 5% to 6% grouping. Both groups show a 1% usage of 9% MgFeSi with the smaller foundries dropping in this category from 12% in 1988. The 3% MgFeSi group has taken a 7% drop in the larger foundries but remained unchanged in the smaller group. The only MgFeSi category that gained in both groups is the 6% to 7% MgFeSi which has taken some of the 5% to 6% users. The rare earth issue is undoubtedly still unclear in the minds of some as 27% of the larger foundry users and 39% of the smaller foundries claim not to use any rare earths in their MgFeSi at all. For those who do acknowledge using rare earths, the low cerium rare earths seen to be in the majority of both groups.

Figure 4. Treatment method.

Nickel magnesium alloys have shown a severe reduction in the last 10 years with only 1% of the smaller foundries showing any use at all. Magnesium ingot or bar has taken a slight drop with 7% of the larger foundries still using this product, mostly in pure magnesium converters.

With more tundish ladles being used, the addition level of the MgFeSi alloy has dropped off with 46% of the larger foundries running in the 1.0% to 1.5% range. This has been brought about by the focus on improving production practices and lower treatment temperatures that are obtainable when using tundish ladles. The smaller foundries are using fewer tundish ladles, running higher treatment temperatures and therefore are still running higher percentages, 1.6% to 2.0% of MgFeSi usage.

POST INOCULATION
Most all of the foundries surveyed indicated that they used post-inoculation this time as opposed to 42% of the larger foundries that indicated no post inoculation in the 1988 survey (Figure 5). At that time, it was felt that the lack of post inoculation was due to foundries using the in-mold process or pressure pipe producers. In-mold magnesium treatment has dropped off since the last survey so that may account for some of the difference, however, it is felt that there may have been some confusion on this subject during the last survey that was issued.

Figure 5. Supplemental inoculation.
Both foundry groups showed more than 50% of the post inoculation was occurring in the pouring ladle. A large number of the bigger foundries, 31%, are using in-stream inoculation. This figure is down from 46% in 1988. The smaller foundries in-stream usage has increased from 2% in 1988 to 14% in 1998. Inserts still have a steady fraction of the post inoculation practices with 18% of the larger foundries and 14% of the smaller foundries using inserts.

The most prominent post inoculant being used is a foundry grade 75% with greater than 0.8% calcium and 1.0% to 1.25% aluminum. This inoculant is being used in 50% of the larger foundries and 35% of the smaller foundries and these figures are very close to the last survey numbers. As in the past, other inoculants are 50% FeSi, 75% FeSi with 2.0% barium, 75% FeSi with 3.0% aluminum, 65% FeSi with calcium and barium and 75% FeSi with lower than 0.8% calcium and lower than 1.0% aluminum.

FINAL CHEMISTRY
Final carbon and silicon levels have changed very little when compared to the last survey issued in 1988. The larger foundries tend to run 3.65% to 3.84% carbon and 2.45% to 2.64% silicon while the smaller foundries tend toward carbon levels of 3.40% to 3.64% and silicon levels of 2.45% to 2.64%. Both groups are showing strong preferences for final magnesium levels of 0.030% to 0.040%.

TYPES OF DUCTILE IRON PRODUCED
The 1988 survey asked what percentage of each foundries work was ferritic, pearlitic, ADI or ni-resist. The current survey requested each foundry report the types of ductile iron that they produced. It is no surprise that both pearlitic and ferritic grades are the most common types of ductile iron that are produced. Pearlitic castings were reported as being produced by 85% of the larger foundries and 78% of the smaller foundries. The percentages of foundries that produce ferritic castings are 82% and 87% for the larger and smaller foundry groups respectively. Austempered ductile iron (ADI) castings are being produced by 9% of the larger and 6% of the smaller foundries, while SiMo iron was listed for 7% of the foundries that are producing more than 200 tons per week. Only 2% of the smaller foundries are currently making SiMo castings. Likewise, 2% of these foundries are also making ni-resist. None of the larger foundries currently responded to making any ni-resist castings.

POURING TEMPERATURE
Note here that the numbers for the 1998 survey exceed 100% by a large amount. This may be because the last survey asked the foundries to report temperature ranges and this current survey asked for temperatures only. The ranges that were previously established overlap by 50 degree segments and thus the higher than 100% responses. The two ranges that are preferred for both larger and smaller foundry groups are 2500F to 2600F and 2550F to 2650F for the first iron poured. These are very much in line with the last survey. However, there is a trend toward pouring at the 2400F to 2500F and 2550F to 2650F ranges for both groups of foundries when comparing with the 1988 survey.

QUALITY CONTROL
More than 70% of each foundry group used either a separately cast coupon or a flow off to check the nodularity as their primary source of nodularity control. The frequency that these tests were performed was not asked for the primary source. Some of the other methods to check the iron were 100% final chemistry check, 100% UT testing and 100% sonic testing. Using a cooling curve analysis was used in only 1% of the larger foundry group while 4% of the smaller foundries used this method for the primary quality control check of their iron. Some other methods mentioned were ring testing, ball impression, bend testing and cutting up a casting.

APPENDIX - Survey on Ductile Iron Practices - in pdf format.

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