Iron inoculation: an overview of methods.

This article provides iron foundrymen with a review of the current techniques - and some of their considerations - to ensure the consistent, cost-effective inoculation of molten iron.

Inoculation has become one of the major metallurgical operations in the production of quality iron castings. Generally, inoculation refers to the practice of adding different alloys, mostly silicon (Si)-based, to promote graphitization during solidification, increase cell count and reduce dendrite size. In gray iron, inoculation promotes small, uniformly dispersed Type A graphite flakes and minimizes chill by inhibiting the formation of primary iron carbides. In ductile iron, inoculation increases the number of nodules as well as prevents the formation of iron carbides. Proper inoculation practice results in reduced shrinkage, improved fluidity, the reduction of residual stresses and better machinability.

Inoculation may occur in the ladle or while the molten iron is poured into the mold. In ductile iron, inoculation may be performed during or after magnesium (Mg) treatment. Figure 1 groups the inoculation methods and techniques that are most used in current cast iron melting practice.

Inoculation tends to fade, with more than half of its effect lost in the first 5-7 min. In high-volume production when automatic pouring and holding furnaces are utilized, therefore, post- or late-inoculation typically follows ladle inoculation. For low-volume production, ladle inoculation is usually the first and final inoculation step. Sometimes, it is followed by late in-mold inoculation. Meanwhile, for gray iron, late inoculation in the mold also may be the only step in inoculation procedure.

Among the factors affecting an iron's nucleation potential is the oxygen ([O.sub.2]) content in the molten iron, which is why preconditioning or X of irons is sometimes necessary for the production of consistent quality castings. Soluble [O.sub.2] usually is accompanied in iron by suspended silica particles or other complex oxides. The deoxidization of irons using Mg, aluminum (Al) or calcium (Ca) will increase the nucleation potential of an iron by providing oxides, sulfides or silicate particles that provide heterogeneous nuclei for the graphite formation. For this purpose, some ductile iron foundries are adding silicon carbide as a preconditioning agent to the charge at a rate of 0.2-1.5%. For gray iron, silicon carbide also is used as a preconditioning agent at a typical addition charge rate of 1-2%. Preconditioning with silicon carbide has shown a tendency to increase the effect of most inoculants in gray or ductile iron and reduce fade.

The sections that follow examine inoculation techniques, alloys and equipment.

LADLE INOCULATION

Ladle inoculation is the simplest and most flexible method to inoculate iron. In order to ensure proper inoculant dissolution and to utilize the stirring effect, inoculants are typically added to the metal stream once the ladle becomes 25% full. Inoculant should not be added to the bottom of an empty ladle prior to tapping due to the risk of the inoculant being encapsulated in liquid slag and/or its oxidation.

There are three major methods of inoculant addition to the ladle [ILLUSTRATION FOR FIGURE 2 OMITTED]:

* gravity feeding into the stream;

* air-assisted injection of fine particles into the stream;

* wire inoculation technique. In this case, while the iron is tapped from the furnace into the pouring ladle, the wire [(containing foundry or inoculation grade 75% ferrosilicon (FeSi75)] is injected into the molten metal.

Gray and ductile irons are usually inoculated with inoculation grade FeSi75, which contains increased amounts of Ca and Al. The need for greater chill control in thin sections initiated the development of special ladle inoculants containing barium (Ba), bismuth (Bi), strontium (Sr), rare earth metals and other chemical elements. These effectively reduce the occurrence of iron carbides in light sections of castings poured with a low carbon equivalent. Reduced inoculation fade also is reported from the use of some proprietary inoculants, particularly those containing combinations of Ba, Bi and rare earths.

For gray irons, the inoculant additions are at the rate of 0.1-0.4% (typically 0.2-0.3%). High inoculant additions are ineffective because of possible dissolution problems, ladle accumulations, filters plugging, dross defects and melt over-inoculation risks. In ductile iron production, the inoculant addition rate depends upon the desired metallic matrix and casting section size. Because the undercooling of ductile iron is greater than gray iron, higher inoculant additions are recommended at the rate of 0.2-0.3% for pearlitic grades, 0.3-0.5% for ferritic grades and 0.4-0.6% for thin-wall castings.

LATE INOCULATION

Late or post-inoculation involves additions to the metal stream while the iron is being poured into the mold or directly into the mold cavity. Late inoculation mostly is applicable to foundries utilizing automatic or mechanical pouring systems. The use of these pouring systems implies that treated iron is stored in holding furnaces at high temperatures for long periods of time, reducing any previous inoculation effect. In some cases, however, late inoculation (in-stream and in-mold) may be used in conjunction with manual pouring.

Advantages of late inoculation include virtual elimination of fading and significant reduction of inoculant addition rates. Improper late inoculation, however, can cause some serious problems, such as the possibility of undissolved inoculant in the casting and nonuniform inoculant distribution. Also, monitoring and controlling the inoculation effect are hampered.

As shown in Fig. 1, late inoculation techniques may be categorized into three groups. The first involves the addition of powdered inoculant (gravity fed or air-blown) into the metal stream during pouring into the mold. The second group utilizes a cored wire inoculant in the basin of a pressurized induction holding furnace. The third technique is in-mold inoculation, which is performed in the gating system by placing inoculant into the pouring basin of the mold or in the sprue/runner system.

In-Stream Inoculation

This method requires a fine-sized material with proper chemical analysis and low oxygen content. Figure 3 presents a schematic drawing of a typical automatic in-stream inoculation utilizing compressed air as the inoculant carrier. Finely sized inoculant from the hopper reaches the dispensing unit and then, prompted by the molding line "push" sequence, is injected by dry compressed air into the stream when the metal is poured into the mold. Sensors are used to stop the inoculant flow.

When manual pouring is used, the inoculant also may be gravity fed into the metal stream as it enters the mold. In this case, the inoculation system is activated by a sensor-operating unit dispensing inoculant from the hopper as the pouring ladle tilts. The feed tube position must be synchronized with any lateral or vertical ladle movement.

As discussed, in-stream inoculation usually is used where automated pouring is used on automatic molding lines. Inoculant addition in in-stream inoculation units is measured either by flow rate (grams/sec) or by fixed mass (grams/mold). Both methods are usually calibrated to a target weight percent addition of 0.050.25%, typically 0.15-0.20%. Conventionally, fine-granular (20x100 mesh) FeSi75 is used as an inoculant, but some foundries prefer to use a special in-stream sized 0.2-0.7 mm (30x70 mesh) inoculant with higher manganese (Mn) content (3-5%). These foundries claim that due to the absence of finer sized dust particles, the inoculant is less abrasive to dispenser components than a resized FeSi75, and the higher Mn and zirconium contents assist inoculant dissolution. Due to improved solution characteristics in iron, trace inoculating elements are distributed more uniformly and reliably. It also is common to use rare earth-containing inoculants, for instance FeSi75 with up to 2% rare earths, at the addition rate of 0.1-0.3%.

Air-assisted inoculant injection is sometimes inefficient due to inevitable losses of inoculant, when the metal stream does not catch it.

Wire Inoculation

Wire inoculation [ILLUSTRATION FOR FIGURE 4 OMITTED] is another common method of late inoculation. The essence of this method is that FeSi75-containing cored wire, 5-10 mm in diameter, is injected into the molten iron located in the pouring siphon of the pressurized induction holding furnace just prior to pouring the mold. The injection angle may vary from 75 [degrees] to 90 [degrees] . based on ductile iron experience, some foundries recommend the use of double wire injection and maintain the temperature of molten iron in the holding furnace at 2555-2595F (1402-1424C), with the typical inoculant addition varying from 0.05-0.1%. The low additions are usually due to the addition being one part of the inoculation program.

A potential problem of adding inoculant to the liquid iron directly at the autopouring spout or siphon is that it may aggravate an already significant problem of dross buildup, particularly in the areas with poor mixing.

In-Mold Inoculation

Mold or in-mold inoculation involves the placement of the inoculation alloy directly into the gating system, such as in a pouring basin, in the sprue well area or in suitable chambers in the running system. Inoculants used for this method may be in the form of crushed material, fine granular material bonded into pellets, or as precast slugs or blocks. As is the case of any late inoculation, alloy dissolution rate is an important factor. The precast and bonded alloys are designed to dissolve at a controlled rate throughout the entire pouring cycle.

Figure 5 (left) illustrates the in-mold inoculation technique utilizing the inoculant block placed in the sprue well area. As the metal enters the mold cavity, inoculation material gradually dissolves in the stream, resulting in an effective inoculation process.

Figure 5 (right) shows inoculation in the reaction chamber located in the runner. The metal enters the mold through the sprue, flows over the inoculant placed in the reaction chamber and, as it is being inoculated, fills the mold cavity. Typically, crushed inoculant alloy of 20-70 mesh in size is used for this application, and the recommended addition rate varies from 0.05-0.1%.

For efficient in-mold inoculation, the reaction chamber design must permit a regular iron flow over the alloy to facilitate its gradual dissolution and minimize the amount of undissolved residues that could reach the mold cavity. To prevent undissolved inoculant particles from entering the casting, recommended practice involves combining in-mold inoculation methods with effective in-mold filtration - cloth or ceramic filters.

A recent development is an in-filter inoculation method as an alternative to the in-mold inoculation technique. Essentially, the inoculation alloy in the shape of fine granules, crushed particles or tablet is placed in a small cavity in the middle of a ceramic filter as is shown in Fig. 6. During pouring, the metal flow dissolves the inoculant, and the filter prevents undissolved alloy from reaching the mold cavity.

Another possible solution of in-mold inoculation is the positioning of an inoculant block into the pouring basin of the mold. In the design shown at the top in Fig. 7, the block is anchored in the base of pouring basin and is dissolved during pouring. The slag trap of the pouring basin prevents the penetration of undissolved inoculant into the mold cavity. In the case of high amounts of iron poured per mold, a stopper may be used. The stopper delays the mold filling, so that inoculant in the pouring basin has enough time to dissolve and be distributed evenly in the iron.

At bottom in Fig. 7 is an illustration of the late inoculation technique, which employs an inoculant floating in the pouring basin. The cylindrical or cubic block of inoculant is placed into the pouring basin before the mold is poured. Partial dissolution of inoculant ensures the effective late inoculation of iron. The block size must be such that even after partial dissolution, it would be greater than the diameter of the spme. This, along with slag trap, prevents the block from falling into the spme and mold cavity.

The selection of a proper inoculation technique is particularly important in thin-wall ductile iron casting where parts must be free of carbides and slag inclusions. For this purpose, this three-step method is recommended:

* the iron is preconditioned with silicon carbide;

* inoculant and Mg-containing masteralloy or pure Mg is added to the ladle;

* post inoculation that may include two subsequent operations: in-stream inoculation and/or in-mold inoculation utilizing some of previously described methods.

Method Selection

There is no inoculation method that is universally suitable for all types of cast iron production. Each method has it own advantages and limitations. The choice of the inoculation method depends upon the technological peculiarities of processes/operations at the given foundry, but final selection will be justified based on reliability of the technique(s), performance level of the material and value to the overall casting production. Following are some observations on the various methods and their application:

* Ladle inoculation technology is characterized by a fairly high inoculant addition and may be utilized in both low- and high-volume casting production. The major problem of this technique - inoculation fading - can only be partially addressed by the use of special inoculants containing elements such as Ba, Bi, Ca, Sr and rare earths that slow fade.

* Air-assisted injection of inoculant and wire inoculation techniques virtually eliminate inoculant fading and its negative impact on solidification structure and properties of iron. These methods are efficient only in high-volume production in conjunction with automated molding and pouring systems. Air-assisted injection of inoculant is sometimes inefficient due to inevitable losses of inoculant, when the metal stream does not catch it, and a portion of inoculant may be wasted and mixed with a mold material. One of the potential problems of adding inoculant to the liquid iron directly to the autopour spout or siphon is that it may aggravate an already significant problem of dross build-up, particularly in the areas with poor mixing.

* For manual pouring, the gravity feeding of inoculant into the stream prior to entering the mold may be a good solution. Generally, this technique is cost-effective due to the low level of inoculant required in a single treatment.

* The in-mold inoculation technique, employing placement of the inoculation alloy in the form of pre-made blocks, inserts or crushed inoculants directly into the gating system, is also is an effective post-inoculation method that eliminates inoculant fading and may be used successfully in low- and high-volume production. To prevent undissolved inoculant particles from entering the casting, an inoculant-filter technique is recommended.

Michael Barstow, U.S. Pipe & Foundry Co., Birmingham, Alabama, and Ken Copi, Exolon-ESK Co., Savannah, Georgia, also contributed to this article.