Inoculation Material Improves Graphite Formation in Ductile Iron. (Technology in Progress).

Elkem ASA

Kristiansand

Inside This Story:

* An inoculant containing cerium (Ce), calcium (Ca), sulfur (S) and oxygen (O) was developed to create more graphite nodules within a casting.

* The combination of Ca, Ce, S and O improves ductile iron machinability and reduces shrinkage tendency, while establishing a more consistent microstructure in thin-walled castings.

* Four case studies noted nodule count and shrinkage-related defect improvements in ductile iron castings when using the (Ca, Ce, S, O)-containing inoculant.

In conventional ductile iron production, the availability of sulfide and oxide nucleation sites is determined by the purity of the base metal and its additives, holding times and temperatures, and metallurgical treatment processes and additives.

Traditionally, commercial inoculants have been FeSi alloys containing metallic additives such as calcium (Ca), barium (Ba), strontium (Sr), manganese (Mn) and zirconium (Zr). The purpose of those reactive elements is to combine with S and O in the iron and form potent heterogeneous nucleation sites for graphite. However, because of the restricted availability of S and O in the iron, the metallic inoculant additives will reach a performance limit. The number of potent nucleation sites that can be formed after treatment will restrict the inoculants' effectiveness.

To increase effectiveness, a new inoculation material utilizing Ce, Ca, S and O (Table 1) has been introduced. This article describes the (Ca, Ce, S, O)-containing inoculant that has proven successful in making ductile iron more machinable with a more homogeneous microstructure and less shrinkage tendency.

The combination of Ca-Ce-S-O

This inoculant contains levels of Ca and Ce that are adjusted to minimize chill formation and neutralize subversive trace elements that prevent the formation of nodular graphite in the iron. In addition, the inoculant contains small and controlled amounts of S and O available for reaction with the Ca and Ce during introduction into molten iron.

The primary objective of the (Ca, Ce, S, O)-containing inoculant is to introduce controlled concentrations of nonmetallic elements such as S and O with the metallic elements. From balanced and controlled inclusion engineering, this deliberately produces a greater density of nucleation sites for graphite from a reaction taking place between the highly reactive metallic elements (Ca and Ce) and the nonmetallic elements (S and O) of the inoculant. S benefits graphite nucleation and O plays a vital part in the inoculation process.

The combined use and performance of both elements through post-inoculation is designed to reap the benefits from Ca, Ce, S and O simultaneously in the graphite nucleation process. Ca is used as the primary reactive element in inoculation and has proven crucial for eutectic graphite nucleation. The balanced Ce content neutralizes subversive elements to produce stable intermetallic compounds and also has a strong affinity for S and O, resulting in the formation of highly stable Ce oxides, sulfides and oxysulfides. These Ce compounds result in improved nucleation throughout the entire solidification range to benefit the inoculation process.

These additional nucleation sites then will perform in parallel to the traditional sites formed during reactions between nodularizer, inoculant and the base metal.

The inoculant's composition is designed to enhance graphite nucleation conditions in ductile irons in addition to chill and shrinkage reduction. The inoculation concept is novel in the sense that it also introduces nonmetallic powders with the FeSi alloy to give it special characteristics (Fig. 1).

The (Ca, Ce, S, O)-containing inoculant forms extra nucleation sites in ductile iron in addition to those initially generated by magnesium (Mg) treatments. This can be advantageous when producing the higher ductility and impact resistant grades of ferritic iron.

The inoculant is especially potent in ductile irons with low S content and in irons treated with Mg in a converter or wire injection process.

Creating More Nodules

Cast iron is inoculated to:

* improve machinability by avoiding the formation of hard carbides;

* promote the formation of graphite and ferrite;

* reduce the tendency of alloying and trace elements to segregate;

* reduce shrinkage during solidification;

* increase ductility;

* create more homogenous structures and properties in different sections of complex castings.

The (Ca, Ce, S, O)-containing inoculant was found to improve upon most of these properties because it increased the graphite nodule count in the ductile iron castings.

The (Ca, Ce, S, O)-containing inoculant has proven to be highly successful in providing fresh nucleation sites to ductile irons of long holding times where the base iron or Mg-treated iron have been held for prolonged times before the addition of the post inoculant. Such long hold times are well known to reduce the overall nucleation capabilities of the iron prior to inoculation, resulting in so-called "dead" iron.

Strong nucleation effect and high nodule count are prerequisites to maximize the ferrite content when producing as-cast ferritic grades of ductile iron.

The (Ca, Ce, S, O)-containing inoculant results in a bi-modal or skewed size distribution of nodules that aid in counteracting shrink and forming more ferrite by acting as effective carbon sinks in the segregated areas enriched in pearlite promoting elements. This indirectly improves the machinability of ductile iron.

Case Studies

Case Study 1--The foundry in this case study uses electric induction melting and a tundish ladle process for preparing ductile iron. It experienced problems with carbides and excess shrinkage in complex castings with thin sections. An instream post inoculation addition of (Zr, Mn, Ca)-containing inoculant normally was used.

When the (Ca, Ce, S, O)-containing inoculant was directly compared as an instream addition, results showed the new inoculant eliminated carbides in thin sections. The extension of microporosity was small enough to avoid rough inner surfaces after the drilling of critical knob sections on the complex castings.

Examination of a 2-3 mm section of a ductile iron casting revealed that the (Zr, Mn, Ca)-containing inoculant gave 315 nodules/sq mm and the (Ca, Ce, S, O)-containing inoculant gave 602 nodules/sq mm.

After the foundry switched to the new inoculant, it experienced significant drops in rejections and improvements in casting quality. The present addition rate of the (Ca, Ce, S, O)-containing inoculant also is reduced by 25%.

Case Study 2--Another foundry, which used induction melting and a sandwich treatment operation, tested inoculants for its automatic pouring and instream inoculation.

Nodule counts were compared in plate sections of 5, 10, 20 and 40 mm thicknesses. The new inoculant had more nodules/sq mm than a Sr-bearing inoculant (Fig, 2). Using the (Ca, Ce, S, O)-containing inoculant, the 40-mm section recorded a higher number density of 340/sq mm than the 5-mm section of 312/sq mm. This suggests a second type of nucleation event later in the cooling process, giving maximum benefits in heavier sections of a casting.

In a crossbar casting, the (Ca, Ce, S, O)-containing inoculant gave a bimodal nodule size distribution and numerous smaller nodules that were precipitated late during solidification. The shrinkage tendency can be directly related to nodule formation and the rate of graphite growth through the entire solidification sequence.

The introduction of Ce in combination with S and O in the (Ca, Ce, S, O)-containing inoculant also possibly produced a second type of site activated during solidification.

After converting to the (Ca, Ce, S, 0)-containing inoculant, the foundry reduced shrinkage problems and kept inoculant addition to a minimum.

Case Study 3--This foundry produced heavy section ductile iron castings using induction melting and tundish treatment. Heavy section casting problems included graphite flotation, segregation, shrinkage and relatively poor nodularity. The (Ga, Ce, S, O)-containing inoculant and a Ba-inoculant were tested as ladle additions.

Results showed that the (Ca, Ce, S, O)-containing inoculant produced 357 nodules/sq mm and the Ba-inoculant 187/sq mm. In the casting's microstructure, the new inoculant showed a wider spread of nodules sizes with the characteristic bi-modal distribution minimizing shrinkage.

The (Ca, Ce, S, O)-containing inoculant formed smaller nodules and a general 10% improvement of nodularity. The new inoculant also reduces intercellular pearlite from 25% to 13%. The interconnected network of pearlite at 25% (and higher) is broken down into minor fragments of pearlite in a predominantly ferritic matrix.

As a result, the foundry produced castings with better tensile and impact properties with homogenous nodule distribution. The new inoculant also was credited with improving tool life by 50% during machining.

Case Study 4--A foundry using electric induction melting and sandwich Mg treatment produced castings with shrinkage problems with "massive" cavities in critical sections.

The (Ca, Ce, S, 0)-containing inoculant was tested on a special pattern using a cube piece attached to a large riser, The cube and riser were sliced in half and evaluated for porosity and distribution of cavities. Very small porosity was found inside the casting (Fig. 3). The foundry converted to the new inoculant to solve its shrinkage problem.

This article was adapted from a paper (01-078) presented at the 2001 Casting Congress and is available through AFS Publications at 800/537-4237 or the AFSE-Store at www.afsinc.org.

For a free copy of this article circle No. 341 on the Reader Action Card.

[Figure 2 omitted]

Table 1

Specifications and Typical Composition of the (Ca, Ce, S, O)-Containing
Inoculant

                % Silicon  % Calcium  % Cerium  % Aluminum  % Sulfur

Specifications    70-76    0.75-1.25   1.5-2    0.75-1.25    max. 1
Typical            73          1        1.75        1        trace

                % Oxygen

Specifications   max. 1
Typical          trace