Reducing pinhole porosity: adjusting, the treatment alloy used with ductile iron can impact the incidence of pinhole defects and lower the operating costs of the metalcaster.

Dotson Iron Castings, Mankato, Minn., began to see a rise in the rate of pinhole-related scrap in the production of its engineered ductile iron castings. Fortunately, the association of pinhole defects with specific residual aluminum levels is well documented. So, studies to examine the effects of operating variables on the defect quickly led the company to make a correlation between a rise in residual aluminum and pinhole-related scrap.

The next step was to figure out how to contain the problem. The solution turned out to be straightforward--lower the aluminum content in the treatment alloy, and you will lower the aluminum content in the casting alloy.

That Was Then

In a 1980 study, R.V. Naik described pinhole defects by stating, "Hydrogen pinholes are spherical or pear-shaped gas holes 0.03125 to 0.125 in. in diameter which occur just beneath the surface of iron castings and, therefore, remain undetected until a considerable amount of work has been expended." Multiple authors have recognized the relationship between aluminum level and pinhole defects in ductile iron. But ideas about the mechanism of their formation have varied.

"When the surface tension [of iron] is lowered by the addition or presence of some elements to molten gray and ductile iron, or by these irons contacting moisture in the mold, hydrogen pinhole susceptibility is increased," Naik went on. "Increasing aluminum content in the metal exerts an inverse effect on surface tension and consequent pinhole formation in both gray and ductile iron." In other words, Naik suggested that surface tension drops and then rises in the iron according to aluminum levels. Therefore, a critical range exists for the production of pinhole defects.

Fifteen years later, S. Katz proposed that "precipitation of aluminum and nitrogen dissolves in iron as aluminum nitride particles during cooling of iron in the mold." This, he said, would be followed by a "subsequent reaction of these aluminum nitride particles with iron-oxide-rich material (e.g. slags) to form alumina, nitrogen bubbles, and iron. The nitrogen bubbles which remain in the casting constitute pinhole defects."

These researchers and others agree that aluminum content can contribute to a higher incidence of pinhole defects. As such, lowering aluminum content is a logical route to fewer defects. The approach to solving the problem also can focus on the minimization of moisture in the metal handling process or the addition of agents to raise surface tension.

But when Dotson needed to lower its pinhole defects, it examined the control and adjustment of aluminum content in the treatment alloy (magnesium ferrosilicon, or MgFeSi)--and, in turn, the iron--as a method for reducing the incidence and cost of pinhole defects. And the company saw results.

This Is Now

Dotson was experiencing a significant scrap cost impact from the pinhole defects. Laboratory analysis was done on three representative samples with defects on machined surfaces.

"The pores have spherical form, and the inner surfaces of the pores are covered with a graphite layer," the laboratory reported. "In conjunction with the pores, a graphite free zone around the pores can be observed. The shape and appearance of the pores is very typical to see in connection with gas related defects. The general graphite structure is good in all three samples, and there is nothing irregular to be seen in the chemical composition. Since the nitrogen content is in the normal range (0.055, 0.072 and 0.075%) and the shape of the pores are more spherical, hydrogen is probably causing the observed defect." Figs. 1 and 2 illustrate the observed defect.

[FIGURES 1-2 OMITTED]

Analyses of these castings showed aluminum contents of 0.009, 0.012 and 0.015%. The sources of the aluminum in the ductile iron are varied, so eliminating it was difficult. However, one easily controllable remedy is the level of aluminum in the MgFeSi.

The level of aluminum in a given supply of MgFeSi is usually governed by the alumina content of the quartzite and reducing agents used as raw materials in the alloy smelting process. This level also can be adjusted by the alloy manufacturer by refining the aluminum out of the liquid ferrosilicon alloy before converting it to MgFeSi. Dotson decided to try a treatment alloy with lower aluminum content in an effort to minimize this level and in turn the incidence of defects.

The company used a 6% MgFeSi treatment alloy with an aluminum specification of 0.4-1%. The new alloy used an aluminum specification with a maximum aluminum content of 0.5%. Charge materials, gating, inoculation and molding parameters were not changed. The addition rate of the alloy also was not changed. Typical final iron aluminum levels went from 0.016 to 0.012%.

The change in the design of the treatment alloy ensured that the final iron would contain less aluminum. The result was a severe and nearly immediate drop in the incidence of the defect and the associated scrap costs (Fig. 3). These results suggest a straightforward solution to those producing ductile iron castings with a propensity toward pinhole defects.

[FIGURE 3 OMITTED]

For More Information

"New Source for Gas Defects Found," S. Katz, MODERN CASTING, June 2006, p. 23.

E. Jay Zins, Dotson Iron Castings, Mankato, Minnesota 

V. Popovski, Elkem Metals Inc., Pittsburgh, Pennsylvania