Reducing slag-related defects in cast iron.

Analyzing melt operations can identify possible slag and carbon interactions that generate defect-forming CO gas.

Producers of iron castings sometimes are confronted by a rash of blowhole or pinhole defects that take time and money to correct. These peculiar casting failures usually result in scrapped castings or costly rework to bring marginal parts up to customer expectations.

Slag-related defects are caused by a variety of conditions and evolve typically from:

* low permeability or free moisture in molding sands;

* underbaked cores;

* hydrogen released from refractory moisture;

* gassy binder materials;

* blocked vents, rusty chills or chaplets;

* aspiration products caused by turbulent flow of metal into a mold.

Defects and Slag

The singular defect of concern here, however, is the so-called reaction pinhole or blowhole. This defect is formed when a very fluid slag and carbon react during solidification, causing the release of carbon monoxide (CO).

Pinhole and blowhole defects are spherical or elongated voids that are most often found in clusters just beneath the top surface of a casting. Typically, a complex crystalline slag is associated with these holes. In gray iron, a nonmetallic manganese sulfide is present throughout the slag and segregates into the adjacent microstructure. These defects occur in all types of castings with wide ranging section sizes and chemical compositions.

Slag Origins

In a study of pinhole defects in white iron castings|1~, R.W. Heine observed the spontaneous formation of an iron oxide-rich slag in ladles and gating systems as the iron cooled. This iron silicate slag, sometimes containing small amounts of MnO, was found to be intrinsic to white iron at temperatures below 2530-2600F. The slag forms according to one of the two following reactions:

* Si + Fe + O |right arrow~ x FeQ |center dot~ y Si|O.sub.2~

* FeO + Si|O.sub.2~ |right arrow~ x FeO |center dot~ y Si |O.sub.2~

Table 1. Composition of Slag

C(%)   Fe     O      Si     Mg     Mn     Al

24      8     20     17      8     17      5
10      8     25     25     11     17      5

Exact composition of these slags changes, depending on oxidation and temperature conditions, but both of the above reactions proved to be associated with pinholing.

When researching the cause of blowholes that periodically plagued inmold-produced ductile iron castings at General Motors foundry|2~, E. Ryntz, Jr. and others examined slag present at defect sites. At one particular blowhole, researchers found a small area of the hole that had a rough coating of slag.

Analyzing the rough slag in two different places, Table 1 shows its composition. This differed from the smooth area of a second blowhole site, which had only small amounts of slag on its surface. Analysis showed it to be 43% C, 46% Fe and 9% O.

Concerns about unreacted MgFeSi or Mg vapor from the in-mold alloy reacting with the iron were also investigated. The slag interaction with the solidifying metal was determined to have caused the blowhole.

Possible sources for these defects were cupola slag, the refractories used holding or transporting ladles or from slag generated in the ladle as a consequence of oxidation or other chemical reactions.

Cupola slag generally is not a problem. Its viscosity increases as its temperatures falls, and it is easily removed by skimming or trapping it in the gating system.

Some refractories, however, pose additional difficulties, such as the botting clay used to plug cupola tap holes that can be composed of low fusion point materials. These materials create a thin, reactive, liquid dross on the surface of the metal to produce an erosive effect on gating system sands. This can result in dross and gas holes.

Ladle Slags

Ladle slags present the most serious problems and are responsible for the highest percentage of scrap castings. Experience has shown that these slags, products of the chemical reactions mentioned above, can accumulate in ladles and gating systems, and carried into castings.

As a result of a study of gray iron blowholes associated with manganese sulfide segregation, W. Tonks found that as hot metal temperature fell, slag formation in the ladle rose|3~. The study further upheld the premise that as Mn content of a melt increases, heavier amounts of ladle slag are formed. As expected, it validated the well-known reaction between Mn and S in cast iron: Mn + FeS |tautomer~ MnS + Fe.

These tests concluded that low pouring temperatures caused these types of blowholes, and, at a given S level, blowholes proliferated as the Mg content of the iron rose. For ladle slags, it determined that in many foundries with unsolved sporadic blowhole problems, the probability of slag contamination of ladles and their intermittent use (thus, reliquefying such slags) was the root of these defects.

A. Morgan explained the actual mechanics of the slag's role in the formation of gas defects by linking them to the effects of MnS|4~. The basic reaction between oxides in the slag and graphite precipitated during solidification produces the offending CO. The simplified reaction is: FeO + C = CO + Fe.

Carbon's Role

The activity of carbon in liquid iron is insufficient to react with slag--only in extremely fluid conditions can slag be brought into intimate contact with graphite at eutectic temperatures. The MgS dissolves in the FeSi/MgSi slag, lowering its melting temperature, thereby enabling the slag to remain fluid enough to react with graphite at these temperatures and produce CO.

In microstructures surrounding one of these blowholes, a complex silicate slag containing an MgS precipitate can be found with additional MgS segregated in the vicinity of the blowhole.

Heine indicated the defect was initiated by the same process--the evolution of CO as a result of iron oxides from the slag reacting with the graphite at solidification. He concluded that the nucleation of the pinhole by the CO starts at the surface where the slag is located. The slag and pinholes intermix and, due to this interaction and the continual generation of CO, the pinhole grows and extends into the casting.

During the study of ductile iron at General Motors, the three reactions found most pertinent to blowhole formation were:

* a) Si|O.sub.2~ + 2 C |right arrow~ Si + 2 CO

* b) MnO + C |right arrow~ Mn + CO

* c) FeO + C |right arrow~ Fe + CO

In addition, the Richardson Diagram (standard free energy of formation of metal oxides versus temperature), pointed out that each of the three reactions is thermodynamically favorable above: a) 2786F, b) 2588F and c) 1310F. This, however, only addresses the possibility of the reactions occurring, whereas kinetic factors determine the probability of occurrence.

Kinetics

Melt kinetics are controlled by slag temperature and composition, giving the slag the fluidity necessary to wet the graphite. Time, in addition to temperature, is an important factor impacting these reactions. Enough time must be allowed for the fluid slag to make intimate contact with the graphite. This explains why heavy-section castings, having a relatively longer period of time to solidify, are more prone to blowholes.

In addition to casting size and shape and their effect on freezing rate, some other factors can influence the reactions that produce gas holes. Excess sand moisture or undermulling (allowing uneven distribution of moisture throughout the sand) provides the |O.sub.2~ sources that increase slag products.

Other conditions promoting oxidation also increase the pinholing situation, such as accumulation of FeO or FeSi in return sand, slag accumulation in feeders, slow pouring, elevated pouring temperatures, hot spots, aspiration and slag caused by poor gating designs and other factors less defined.

Typically, however, only when slag enters the casting cavity can pinholing occur. For example, one riser feeding several castings that cannot be kept full at all times will furnish the necessary slag for reaction pinhole formation. In rare cases, reaction pinholing can occur without the slag, such as when contaminants like dirt, slag, scale or oxides buildup in excessively moist molding sand.

Avoiding Defects

Some basic guidelines in daily foundry practice can be useful for reducing the possibility of reaction pinholing:

* gating systems should be designed to properly trap slag in feeders or runners;

* operators should pour fast enough to keep the sprue post full, but slow enough to keep turbulence to a minimum;

* sand cleanliness should be maintained;

* new sand additions should be made regularly to decrease the chance of a buildup of foreign matter in the molds;

* moisture content of the molding and core sands should be closely monitored;

* mulling must be thorough enough to ensure equal distribution of water throughout the sand.

References

1. R. Heine, "Observations in Pinhole Defects in White Iron Castings," Modern Casting, pp. 53-57 (Feb. 1958).

2. E. Ryntz, Jr., R. Schroeder, W. Chaput, W. Rassenfoss, "The Formation of Blowholes in Nodular Iron Castings," AFS Transactions, vol 139, pp. 161-164 (1983).

3. W. Tonks, "Surface Blowholes in Gray Irons and Their Association with Manganese Sulphide Segregation," AFS Transactions, vol 64, pp. 551-564 (1956).

4. A. Morgan, "Blowholes from Slag Inclusions," BCIRA Journal, pp. 438-445, vol 10 (July 1962).