Producers look to optimize process.

Gathering to learn ways to better optimize the process of making ductile iron castings, 152 foundry officials met in Southern Pines, North Carolina October 2325, 1996, for the Fall Meeting of the Ductile Iron Society. In addition to nine technical presentations, the meeting also featured a tour of nearby Foundry Service Co., a 250-man green sand foundry shipping 2.5 million lb of ductile iron a month.

Tips for Better Machinability

Noting that ductile iron producers are under attack from forgers and steel producers for castings' "poor" machinability, P.H. Mani shared how Wagner Castings Co. has improved its casting machinability. Besides careful raw material selection, he noted that chemistry and metallurgy are critical considerations for casting machinability. Among his recommendations were:

* The variation in CE between chemistry and metallurgy should be less than [+ or -]0.02. Individual values of C and Si are important, not the CE as a whole number.

* A properly deoxidized and nucleated melt prior to treatment is critical. The best material for deoxidation is SiC (at least 4 lb of SiC for 1000 lb steel). If it doesn't go into solution, however, it will greatly hinder machinability.

* Crushed electrodes are a good source of crystalline graphite for nucleating base iron.

* The chill wedge test is quick and reliable. Don't tap unless chill is good.

* Check slag basicity. The more basic (drier), the easier it is to slag. Deslagging before tapping is a must.

* If base iron is held, float a chunk of graphite on the metal surface to minimize chill loss.

For treatment, he offered these additional tips for foundries:

* Allow time for reaction products to rise to top.

* SiC in the treatment ladle has potential of not going into solution, and can hurt casting.

* Pure Mg treatment produces a basic slag, and thus cleaner treated iron.

* Keep the Al content of MgFeSi as low as possible when using the sandwich method. MgFeSi with high Ca (2%) is desirable - the treatment reaction is quieter and Mg recovery improved.

* Thorough deslagging after treatment is critical.

* Wagner prefers an inoculant low in Al and high in Ca. Inclusions containing Ca-compounds are softer than alumina inclusions (less abrasion on tools).

* Refractories indirectly affect machinability - a minimum of 65-70% alumina refractory is desirable.

* Indiscriminate stream inoculation can affect machinability. High Al inoculants and those with rare earths (REs) adversely affect machinability. Likewise, solid inserts, if not used with great care, affect machinability.

* While filters reduce inclusions, they'll hurt machinability if they disintegrate during pouring.

* Time of pouring to shakeout is extremely important. Castings shaken out too fast have inferior machinability. For Disa molding, there should be 1 min of cooling time for 1 lb of mold weight. A practical check is to use a magnet. Above the Curie point, the iron is nonmagnetic.

Rare Earths in Ductile Iron

Citation's Al Alagarsamy and McMaster Univ.'s S.V. Subramanian tackled the controversial topic of REs in ductile iron. REs refer to the Lanthanide series of elements (Ce, La, Pr and Nd).

As part of a literature review, they explained that Mg promotes good nodularity (can produce 100% by itself), can tolerate excessive additions better than REs and is economical and universally used. REs provide increased recovery during treatment, slow fading due to stable sulfides (which also act as nuclei for graphite) and neutralize subversive elements. RE limitations are that they promote carbides more than Mg, aggravate graphite floatation, cause exploded graphite and can't ensure good nodularity in heavier sections by themselves.

Research indicated that by balancing REs and their compounds, foundrymen can nucleate graphite until the end, require minimal or no risering in rigid molds, extend processing time, control carbides, reduce chunk graphite in heavy sections and increase nodule count.

According to the research, the least amount of Mg and an optimum amount of REs (which depends on base iron sulfur level, residual Mg, when they're added and total subversive elements) will yield metallurgically superior ductile iron.

Extend Refractory Life

Dave Williams, Allied Mineral Products, Inc., discussed how silica, mullite, spinel-forming alumina, magnesia and silica carbide/carbon furnace refractories are uniquely affected by ductile iron.

To improve refractory life, he suggested that ductile iron foundries: lower operating temperature at metal tapping; minimize galvanized autobody scrap use (zinc penetration in the lining); allow for greater dwell time in forehearth or ladle during desulfurization to collect more slag; submerge Mg during treatment to minimize formation of insoluble MgO; follow proper cold restart procedures when the lining has experienced thermal shock; upgrade SiC level in cupola refractory if there's a severe change in slag chemistry; make late additions to the iron with high-yield ferroalloys or direct wire injection; add SiC to the melt to reduce FeO and MnO (which cause chemical erosion of refractory); and modify sinter chemistry to include more Si by adjusting chemistry or by using a gray iron.