Developing In-the-Mold Treatment for CGI.

As the production of CGI components continues to grow, foundries must utilize the correct solution factor to ensure properly treated castings with consistent Mg concentrations.

The process of treating ductile iron in the mold was introduced to the foundry industry 25 years ago and eliminated the need for any type of external inoculation treatment device (Fig. 1). The process is based on the principle that the desired magnesium (Mg) concentration in the final casting is inversely proportional to the pour rate of the molten metal (which also is the rate at which the molten metal will encounter the nodularizer) and directly proportional to the cross-sectional area of the treatment reaction chamber within the gating system of the mold. This principle, termed the solution factor (SF), is defined as the ratio of the pour rate over the chamber area:

SF (lb/sec/sq in.) = Pour rate (lb/sec) / Reaction chamber area (sq in.)

As shown in this equation, higherSFs (faster pour rates and/or smaller reaction chambers) will yield lower residual Mg. However, critical to the effectiveness of treating in-the-mold was the development of a nodularizer with a consistent dissolution rate that would reliably and predictably introduce a percentage of Mg into the iron that then could be correlated to the SF.

With the development of in-the-mold treatment, three main advantages emerged for producing ductile iron:

1. Each individual pattern can be assigned a separate SF. This allows the metallurgist and/or engineer to introduce higher residual Mg in castings with heavier sections and lower residual Mg can be used for castings with thinner sections.

2. There is no Mg fade. Every mold with the same SF will have the same residual Mg provided that the pour rate and inoculant are consistent.

3. The iron produced is highly and uniformly inoculated.

CGI Treatment

Due to the recent success of several automotive and truck applications (cylinder heads and engine blocks) for compacted graphite iron (CGI) and enhanced CGI, there has been a resurgence in the interest in the material for applications requiring simultaneous mechanical and thermal loading. In application, CGI offers better strength and stiffness than gray iron and better castability, machinability and thermal conductivity than ductile iron. Due to these unique properties, the demand for CCI for automotive, truck and construction applications, according to Stratecasts, Inc., is expected to more than double over the next 10 years, and experience a 7.5% annual growth rate.

But as with ductile iron castings, CCI processing must receive particular attention. Successful production of CCI entails a proper balance of tramp elements with spherodizing elements for the section size being cast in order to obtain the correct compacted graphite morphology. With these measures in control, the advantages of treating in-the-mold still apply, particularly the ability to tailor residual Mg and rare earth elements to suit the section size being cast and the fact that Mg fade is virtually eliminated.

CGI requires far less Mg then ductile iron. Ductile iron components require residual Mg levels that range from 0.025-0.055% depending on the section size and level of tramp elements, while CGI and enhanced CGI components' residual Mg typically runs from 0.012-0.020%. Therefore, in-the-mold nodularizers developed for CGI typically contain less Mg than the standard alloying element.

SF Differences

With a lower Mg requirement for CGI, a higher SF is required to produce quality components regardless of which nodularizer is selected. The original SF relationship (curve), developed for the standard 6% Mg-containing nodularizer, was designed for generating residual Mg in the higher nodular realm of ductile iron components. Typical SF values utilized in production today range from 0.65 to 0.85 lb/sec/sq in. The following equation is used for the original SF for ductile iron:

SF (lb/sec/sq in.) = 1.6 - (20)(%Mg)

In this equation, %Mg is the desired percentage of Mg in the final cast component. If this linear relationship were extrapolated, a SF of 1.6 would yield a Mg concentration of zero and higher SFs would actually generate negative residual Mg concentrations. This is not practical for today's foundrymen manufacturing CGI components since the solution factor required to generate the lower Mg residuals ranges from 1.6-2.1 lb/sec/sq in.

The following equation more accurately predicts the desired Mg concentration as a function of the SF deployed:

SF (lb/sec/sq in.) = [Ln(%Mg) + 2.53]/(-0.89)

For this equation, CGI appropriate treatment should be utilized in the 0.010-0.020% Mg range. Both of these equations are plotted in Fig. 2.

The two equations yield similar results in the 0.7-1.0 SF or 0.045-0.03% Mg range. In order to obtain higher Mg concentrations in the cast components, the new formula requires slightly lower SFs compared to the original study. The principal divergence between the equations begins around 0.03% Mg and increases with decreasing Mg content. The new formula predicts higher (and more accurate) SFs to attain the lower Mg concentrations required for CGI components. (If the standard 6% inoculant were to be used, the curve would be even more divergent in the lower Mg range and require even higher SFs for the same Mg content.).

The SF equation and the general shape of the new curve also can be utilized for late mold inoculation of conventionally treated ductile iron. In this case, the Mg treatment occurs by one of many alternate treatment methods, and then a supplemental inoculant is added to the mold in order to enhance the metallurgical quality of the iron (improve nodule count, suppress carbides, etc.). Numerous products exist to accomplish this objective, and all have different compositions, forms and dissolution rates. The addition levels, however, are even less than what is used in processing CGI in-the-mold and the SF employed for this application varies widely (between 2 to 4 lb/sec/sq in.) depending on the product. It is essential that similar curves be developed for each of the products to enable foundrymen to estimate the quantity of inoculant and the level of inoculation expected.