Survey on ductile iron practice in U.S.

Survey on Ductile Iron Practice in U.S.

Every known ductile cast iron producer in the U.S. was sent a survey form in order to determine process trends in producing ductile iron. It was the third survey in this series, with the previous two conducted in 1957 and 1978.

Seventy-seven foundries completed the questionnaire and returned it to the committee for compilation. Although this was only about a 50% response rate, the committee believes that the available data is sufficient to reflect actual trends and practices. Table 1 details the complete survey results.

In every subcategory the results are displayed as a simple percentage of the total responses in that particular tonnage group, with the split being made at more or less than 200 tons a weeks. When the percentages exceed 100% in any group, it is indicated that some foundries had more than one response in that group (e.g., multiple types of melting units, more than one treatment ladle size, etc).

When the percentages are substantially less than 100%, it is indicated that several foundries in the group do not employ that particular process step (e.g., desulfurization, holding, preconditioning) or that there was confusion as to the nature of the survey question.

It is difficult to design one survey form to serve such diverse ductile iron processes that are used in the U.S. Pressure pipe producers with high melt rates, low nodulizing treatment temperatures and pouring temperatures of about 2400F were asked to respond to the same questions as foundries producing 0.200 in. thick, chill prone, green sand castings from small, coreless induction furnaces.

These types of survey responses tend to spread the data out, but as will be seen when reviewing the survey results, trends and differences between small and large foundries will become apparent. In an effort to improve the questionaire, several areas were added or expanded upon in 1988. Where there were no 1978 responses for comparison, "na" (not applicable) is shown on the data sheet.

Melting and Holding

In 1957, 65% of the respondents used cupolas as primary melters. This dropped to 27% in 1978, with major growth being in induction melting. The 1988 data shows that foundries producing fewer than 200 tons a week of ductile iron continued to convert to coreless induction furnace melting--now being used by 84% of the respondents.

Larger foundries showed a growing preference for acid slag cupola melting, while the use of direct arc and channel furnaces as primary melting units declined. Almost 100% of the larger foundries duplexed, with channel furnaces as the favorite holder. The use of forehearths was predominant only in pressure pipe foundries.

Only 36% of the smaller foundries reported duplexing, indicating a great deal of direct tapping from the coreless furnace to the nodulizing ladle. Carbon rod furnaces were the most frequently named "other" holding media.

Base Iron Information

Smaller foundries were much more in accord in 1988 regarding the base carbon range of 3.6-3.9%, perhaps reflecting improved controls possible with the growth in induction melting. With 90% of the smaller foundries now reporting base sulfur levels of less than 0.025%, improvements in carbon raiser and raw material selection are reflected.

The wide disparity in sulfur levels in large foundries is due to the relatively large number of cupolas used. About a 50/50 split between cupolas and electric furnaces in larger foundries is indicated. As a result of improved desulfurization techniques and more efficient nodulizing methods, magnesium-ferrosilicon alloy inputs have been reduced, causing the noticeable trend to higher base-silicon levels.

Desulfurization

Only 22% of the smaller foundries responding indicated the use of desulfurization, as compared to about 60% of the larger foundries, once again reflecting the electric furnace/cupola difference. Smaller foundries favored the batch porous plug mixing technique, with cupola foundries reporting rapid growth in continuous porous plug mixing.

Although calcium carbide was the overwhelming favored reagent, some larger foundries indicated that they are experimenting with various alternative materials. (For a more in-depth review of desulfurization details, see the AFS Committee 5-L desulfurization survey published in the 1986 Transactions.)

Preconditioning

Survey responses may indicate that there was some difference in perception of the term "preconditioning" from foundry to foundry. The term generally has related to small additions of various materials to the base melt with an aim to improve inoculation effect or response to the nodulizing treatment.

The magnitude of some of the graphite and silicon carbide additions cited in the responses also may indicate that the normal carbon and silicon additions made to the melting furnace for base-chemistry control were considered as being preconditioning additions.

Nodulizing

The biggest change has been the growing use of the tundish ladle and, to a lesser degree, the flow-through process, with these two techniques now being used by more than half of the responding foundries. The growth has come at the expense of the open ladle, plunging, porous plug and sandwich processes.

Ferrosilicon has become more popular as a cover material in larger foundries. "Other" cover materials mentioned included iron shot and remelt. Additional nodulizing processes mentioned included the Fischer converter.

Magnesium ferrosilicon alloys containing 5-6% magnesium have continued to grow in usage, primarily at the expense of 9% magnesium ferrosilicon. The low response concerning rare earth content and type of rare earth source being used would seem to indicate that there is still a lot of confusion in this area. Pure magnesium is now being used in 12% of the responding large foundries, primarily in the pressure pipe segment.

Responses to the question on the addition of a nodulizing alloy show a trend toward reducing additions due to improved techniques now being used. Larger foundries show the added benefit of lower treatment temperatures permissible when using the covered, sealed ladles. Smaller foundries showed an upward trend in treating temperature, but this is probably due to improved control attainable through the growth in electric furnace melting. Concurrently, actual percent magnesium inputs generally are being reduced.

Post-Inoculation

A surprising 42% of responding larger foundries use no post-inoculation. This data is influenced by users of the inmold process as well as pressure pipe producers. Smaller foundries tend to use larger inoculant additions, but there is wide disparity as to specific amounts added.

Almost half of the larger foundries now employ automatic metal stream inoculation but it is not clear, based on survey results, if this group comprised a large segment of the 42% that do not use post-inoculation.

Nearly half of the fewer than 200 tons per week foundries use some type of supplementary inoculation, compared to almost 100% of the large foundries. "Other" post-inoculating materials mentioned included calcium bearing 50% ferrosilicon, 75% ferrosilicon with 3-4% aluminum and 63% ferrosilicon containing calcium and barium. More than half of the larger foundries reported using 75% ferrosilicon with calcium levels greater than 0.8% and aluminum levels of 1.0-1.25%.

Final Chemistry

Final carbon levels have trended downward, again probably as a result of the improved controls attained with the growth of electric melting. Median silicon levels do not appear to have changed much, although the data indicates less disparity in the final results. Lack of response with respect to final iron rare earth analysis may indicate the difficulty of obtaining reliable analyses, in addition to ongoing confusion in this area.

Pouring Temperature

The survey data shows an increased concurrence that first iron is being poured in the 2550-2560F range, somewhat higher than shown in the 1978 survey. Of the large foundries, the 8% of respondents who indicated low pouring temperatures were comprised of pressure pipe foundries.

Types of Ductile Iron

Of those foundries responding, 32% of the smaller ones indicated some production of austempered ductile iron, compared with half that number of responses from larger foundries. All ductile Ni-Resist responses were from foundries pouring fewer than 200 tons a week. The split between ferritic and pearlitic grades being cast showed littled marked change from the 1978 data.

Quality Control

More than 90% of reporting foundries check micro-lugs for nodularity--most of these on every ladle but some on every other ladle or twice an hour. Chemical analysis is used by about 90% of respondents--some on every ladle and some as frequently as twice a day.

Other traditional quality checks included test bars for mechanical properties and hardness testing. Mentioned at about the same frequency was the use of ultrasonics, but only when necessary and usually applied to safety parts. Pouring temperature control, chill wedges and sonic testing were all listed by a lesser number of foundries.

Other, perhaps more sophisticated, tools included timers for fade limit control, computerized alloy weighing systems, controlled shakeout for matrix structure control and the use of SPC charts.

Table : Table 1. Detailed Analysis of Survey on U.S. Ductile Iron Casting Practices