Practical inoculation principles.

Inoculation, as defined by the AFS Metalcasting Dictionary, is "the addition to molten metal of substances designed to form nuclei for crystallization." That is, inoculation is an attempt to control the final microstructure of a metal casting.

In the solidification of cast iron, the cooling metal changes from having a random distribution of constituents to a structure with an ordered arrangement. The undercooling of the iron, however, presents an energy barrier to this structural organization. The addition of an inoculant--either as the metal is poured into a ladle or directly into the mold--is a means of controlling undercooling and reducing this barrier.

Why Inoculate?

In gray iron, inoculation promotes the formation of type A graphite, improves machinability, reduces chill, eliminates carbides, refines the microstructure and, under certain conditions, increases tensile strength.

For ductile iron, inoculant addition ensures good machinability, eliminates carbides, increases nodule counts, reduces chill and refines the microstructure. Inoculation is more critical in ductile iron because it has more influence on the final, desired microstructure.

Ineffective Inoculation

There is a tendency in gray and ductile foundries to blame inoculation if the finished iron lacks the desired properties. What seems to be ineffective inoculation, however, may be the result of any number of factors. Some of these include:

* Iron pouring temperature--low temperatures can promote carbide formation;

* Casting section size--thin sections can solidify too rapidly if not enough inoculant is used, forming carbides, while the longer solidification time for thick sections can result in inoculant fade;

* Finning--chill can occur at the fin/casting contact point;

* High Mg content (or combined Mg + Ce)--magnesium is a carbide stabilizer and can promote carbide formation;

* Low sulfur content--can deteriorate inoculant performance;

* Unsuitable inoculant--size, amount, if it's not potent enough, and addition technique;

* Long holding times--iron stored overnight is not as responsive to inoculation.

With the knowledge of these factors, the inoculation process can be treated accordingly. Attention to detail is the guiding principle.

Important Issues

Important issues relating to inoculation that arise daily in the production environment include:

How do you increase nodule count?--Nodule count can be increased with more potent inoculation. Late inoculant additions, such as stream or mold inoculation, will give higher nodule counts than adding it at the ladle. Higher nodule count can lead to a better casting with improved mechanical properties.

Should late inoculation be adopted?--Additions to the pouring stream or mold require lower addition rates and allow less opportunity for inoculant fade. Areas of concern with stream inoculation include reliably hitting the iron stream, angle of impingement and control of feed rate. Mold inoculation concerns are the possibility of undissolved inoculant, spalling and improper size. Pre-treatment at the furnace or ladle is thought to couple with late inoculation in a cumulative effect.

Does inoculation increase tensile strength?--In low-carbon equivalent irons, inoculation can increase tensile strength slightly. More effective alternatives are CE adjustment or alloying with Cu, Mo, Cr, Ni or Sn.

How do you address surface flake graphite in ductile iron?--Not necessarily an inoculation problem, surface flake graphite can be caused by high sulfur and excess moisture in the molding sand. This can reduce the effective Mg in the iron and cause the graphite to form as flakes at the casting surface. This flake surface can give erroneous hardness readings if enough metal is not removed before testing.

How do you obtain better inoculant mixing?--Because inoculants tend to float and gather with slag in an open ladle, it is best to add them to the stream when the ladle is being filled.

How do you know that a part hasn't been inoculated?--Typical methods include hardness and file testing, which are costly and time-consuming. Employing both ladle and stream inoculation or having two feeds in the stream process can reduce this uncertainty.

Ensuring Consistency

Foundrymen employing inoculation should look at control of the final product through the entire production process. Some of those variables to be controlled or monitored are:

* charge materials;

* iron source (electric melt, cupola);

* target iron chemistry;

* thermal analysis data;

* pouring temperature and practices;

* section size.

Inoculation should not be thought of as isolated from the rest of the process. Carbides or other structural deficiencies can be caused at any point in the operation, not just in this last step prior to pouring. All variables that affect the final product must be monitored.