Improved Ductile Iron Casting Quality
Through Optimized Coating Technology

Introduction
Although it is possible to produce castings without the use of a refractory mold or core coating, the optimum application of a suitable coating can dramatically improve casting surface finish and overall component quality. Aside from enhancing casting surface appearance, the utilization of refractory mold and core coatings can often result in a reduction or elimination of a number of casting defects, such as :

• metal penetration
• sand burn-in
• mold or core erosion
• gas defects
• metal – mold reactions
• sand expansion defects, e.g. veining
• metallurgical defects

With the growing need for higher quality casting finish, more complex iron casting designs, lower overall process costs, and increased productivity, the requirement for higher performance coating technology is becoming increasingly important.

This paper outlines certain coating technology fundamentals and illustrates through example how advanced coating technology can help improve ductile iron casting quality.

Coating Application
A fundamental objective when using a refractory coating is to apply a uniform coating layer free from surface imperfections - such as runs or drips - which could later replicate on the final cast surface. The dry refractory layer needs to be of sufficient thickness to prevent any detrimental interaction between the molten metal and the mold or core substrate during pouring.

Obtaining an even, consistent coating layer application is dependant upon the application method utilized and the coating chemistry - it is important that coating properties are designed to suit the application method selected. Though brushing or spraying methods can obtain good coating application, both these techniques are operator dependent and consequently prone to inconsistency. It is widely acknowledged that either dipping or flowcoating techniques should be used for reproducible coated cores and molds.

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Different application techniques demand quite different flow behavior for optimum application results and it is important for coating rheology to be adjusted by manipulation of the coating gel components.

For example, pseudo plastic rheology is highly desirable when dipping complex cores. This type of flow behavior ensures that coating viscosity decreases rapidly as the core is immersed in the coating, thereby ensuring complete coverage of the core surface. As the core is removed from the coating, the coating viscosity regains it’s original level quickly to ensure the coating does not flow to form runs and drips, i.e. the deposited coating effectively “gels” on the core surface.

Conversely when a large mold is flowcoated, the coating needs to be flowable for a longer period to enable the coating to flow easily over the entire mold face, producing an even layer of coating, and allowing time for coating excess to run freely from the mold surface.

In addition to flow behavior, the speed at which the carrier liquid penetrates into the core or mold surface during application is also critical to controlling layer thickness and uniformity. This effect - known as “matt time” among other terms - is adjusted primarily through the chemistry and addition level of the surfactants incorporated in the coating.

Refractory Coating Layer
Casting surface finish and quality imparted by a coating is directly related to the dry coating layer deposited onto the core or mold substrate and the layer chemistry.

The refractory blend must be thermally stable at the temperature of the alloy being cast but by careful selection of type and grading, other characteristics can be imparted to the coating. Some examples are :-

• High Insulation – a highly insulating coating layer can delay the thermal expansion of a silica sand substrate long enough to prevent sand expansion defects such as veining.
• Lustrous Carbon – inclusion of lustrous carbon forming agents improve the surface finish of most iron castings.
• Metallurgical Modifiers – the inclusion of active components in the coating can initiate beneficial reactions at the casting surface e.g. localized grain refinement or the elimination of flake reversion (see later case-study).

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Coating Process Control
The main goal in production is to achieve a consistently applied dry layer thickness on each and every coated core or mold. This thickness will have been pre-determined through controlled trials to give optimum casting quality. Coating application control is ideally achieved in two steps - firstly control of the dilute, ready-to-use coating prior to application and secondly, subsequent measurement of the applied layer thickness on the core or mold surface.

In practice, dry coating layer thickness is often difficult to measure accurately in a production situation. However for any given coating, dry layer thickness can be correlated to wet layer thickness, which can be measured easily with a wet thickness gauge. (Figure 1).

Figure 1 - Measuring the wet thickness layer of a coating

Figure 2 - Measurement of viscosity with a Flow Cup

Figure 3 - Measurement of viscosity with a Baume Stick

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In turn, for a given coating, wet film thickness can be accurately correlated to both flow cup viscosity (Fig 2) and baume (Fig 3) measurements - both these tests can be performed easily and quickly on line and have been found to be adequate in-process control tools.

Case Study:  Influence of Coating Chemistry on the Rim-Zone Structure of a Ductile Iron Casting¹

Background
The occurrence of structural anomalies in the casting rim-zone in the production of ductile iron castings with larger metal cross-sections is not uncommon. Degeneration of the desired spheroidal graphite structure can occur due to reactions at the metal-mold interface, which can result in an adverse effect on the resulting component mechanical properties. Under cyclic or dynamic loading, as in the machine or automotive industry, such structural imperfections can lead to catastrophic failure.

The factors influencing the graphite structure are many2, however the appearance of irregular graphite development has been observed notably when using silica sand molds that contain sulfur.

Studies3,4,5 have established that molding sands containing sulfur (i.e. reclaimed sand bonded with furan or phenol binders catalysed by sulfur bearing catalysts ), can be prone to sulfur pick-up in the cast rim-zone, resulting in the presence or promotion of flake graphite.

Experimental Procedure
For the tests, a U-shaped test casting was used with dimensions 7” x 8” x 5” approx. and weighing 55 lbs. Wall thickness in the area of the core was around 2 inches. The casting was simulated to determine solidification times and in-mold temperatures during the casting process.

Molds were produced in reclaimed sand with known sulfur content (0.1%), and a furan binder catalysed with PTSA was used.

Coatings with four different combinations of refractory filler materials were tested, with an applied dry layer thickness of 0.2mm. The properties of the coating refractory filler materials can be seen in Table 1.

Table 1 : Properties of Coating Refractories Investigated.

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Casting Results
Metallurgical structure was checked using micrographs of samples taken from an identified “hot spot” location of the casting. The molds that were coated with Coating A (Figure 4a.) and Coating B (Figure 4b.), showed defective development of the graphite structure in the casting rim zone, mainly in the form of flake graphite.

With Coating C (Figure 4c.) no flake graphite was observed but the nodule structure is significantly disturbed. In the rim-zone many relatively small nodules can be seen compared to the other coatings. This effect can be classified as inoculation by the coating.

Best nodule structure results were obtained with Coating D (Figure 4d.).

Figure 4 - Casting structures using Coatings A,B,C & D

Further Investigation
To further assess the effect of coating chemistry, samples of residual coating were taken from the surface of the casting at the hot spot area location, and the casting was also machined to 0.5mm and 1.0mm respectively. Coating and metal samples were then tested for sulfur level (see Figures 5 and 6)

Figure 5 - Sulfur % content of Coating samples before and after pouring

Figure 6 - Sulfur % content of casting at different sections

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In contrast to Coatings A, B,&C, the sulfur content of Coating D showed a dramatic increase of approximately twenty times its initial value after pouring (Figure 5), while the rim-zone metal section from the casting produced using Coating D showed a greatly reduced sulfur level compared to the effect of other coatings (Figure 6).

These results provide strong evidence that the superior graphite structure observed at the rim-zone when using Coating D is a direct result of the coatings inherent ability to effectively block sulfur migration from the mold sand through to the metal skin.

To further assess the effectiveness of Coating D, sulfur level within the molding sand was manipulated to higher levels by varying the PTSA catalyst and reclaim sand processing parameters. Samples of mold sand, coating and metal were then assessed for sulfur content from the hot spot location of the casting (Figures 7, 8 and 9.).

Figure 7 - Sulfur % content of furan bonded sand.

Figure 8 - Sulfur % content of Coating D

Figure 9 - Sulfur % level of Casting

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Final Results
After casting, the sulfur content of the molding sand was recorded at between 0.014 and 0.030% (Figure 7), indicating that in all cases the sulfur in this area of the mold was almost completely burnt out.

The sulfur content in Coating D after casting was approximately three times the initial values in the molding sand for the trial M1 (S=0.1%) and M2 (S=0.15%). However, when the sulfur content of the molding sand was increased to 0.20% (M3) and above, only about twice the sulfur level was measured in the coating (Figure 8).

Sulfur analysis of metal sections taken at a depth of 0.5mm and 1.0mm from the cast surface (Figure 9) showed that sulfur level in these areas was kept below 0.05% when sulfur content was 0.15% or less within the molding sand. The sulfur level at the casting rim-zone increased steadily as the molding sand sulfur level increased above 0.20%.

No significant graphite flake reversion was observed with a molding sand sulfur content of up to 0.20%, when using Coating D at the nominal 0.2mm dry layer thickness. At higher sulfur levels in the mold or core sand the ability of the coating to absorb sulfur in the coating layer is progressively less effective and rim-zone graphite structure degeneration more likely.

It is anticipated that, at molding sand sulfur levels above 0.20%, greater resistance to graphite flake reversion would be achieved through the application of a slightly heavier layer of Coating D.

Summary
Aside from enhancing casting surface finish quality, refractory mold and core coatings can be used to prevent many different casting defects. In all situations a carefully selected refractory combination and uniform, consistent coating application behavior is vital for quality casting finish and integrity.

Through close attention to the process requirements of the modern foundry, refractory coatings have been developed which suppress or eliminate totally costly defects such as veining and localised metal penetration.

As the case study outlines, inherent casting metallurgical properties can also be enhanced. The application of a suitable refractory coating at a nominal 0.20mm thickness can help prevent graphite reversion at the rim-zone of the casting when the molding sand contains up to 0.2% sulfur content.

Acknowledgements:
T.Birch, Foseco Europe.

References:

[1] Sluis J.R. : Giesserei 84 (1997) No.6, p. 9-13
[2] Reifferscheidt K. : Dr. – Ing.-Dissertation 1991.
[3] Barton R. : Giesserei 65 (1978) No.11, p. 294-301.
[4] Bauer W. : Giesserei – Rundschau 28 (1981) No.10, p.11-19
[5] Karsay S.J.; Martin F. : Giesserei-Praxis (1981) No.12, p. 218-224.

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