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Mold inoculation of cast iron using pressed blocks.

Pressed and sintered blocks offer a new alternative for mold inoculation of cast iron.

The benefits of late inoculation of gray and ductile iron castings are well known to foundrymen as are the techniques developed to achieve them. Early methods inserted a few grams of loose inoculant into the sprue bottom or used a pressed inoculant bonded with wax or sodium silicate. As the need for improved casting quality grew and the use of automatic pouring increased, foundrymen looked for more potent and consistent methods of inoculation. This led to positioning inside the molds of solid cast inoculant blocks or adding granular inoculant to the metal stream during mold pouring.

Both cast blocks and stream inoculation have grown in popularity during the past few years. A new development called Tenbloc|R~ combines the advantages of these while providing additional benefits. This new inoculant is a pressed and sintered block that forms a metallurgical bond between the individual inoculating particles. Unlike the earlier pressed blocks that were bonded together with an organic or inorganic binder, the new blocks use no chemical binder.

Pressed, Sintered Block

The new inoculant delivery system uses a pressed and sintered block developed specifically for cast iron mold inoculation. The blocks are produced from prealloyed blends of silicon-based materials graded and mixed prior to pressing, sintering and heat treating. Typical samples are shown in Fig. 1.

The pressed inoculant is produced in regular cylinder forms of 0.80-3.00 in. (20.5-76.2 cm) in diameter and with a weight range of 10-300 g. A recent development are blocks with a taper at one end, as illustrated in Fig. 2. The weight and surface-to-volume ratios can be adjusted to specific foundry requirements. An addition rate of 0.08-0.12% is sufficient to provide the necessary nucleating characteristics. The complexity of the running system and the total metal poured are factors determining the number of blocks used per mold.

Inoculation Practice

The inoculation of cast iron is generally carried out by adding controlled additions of ferrosilicon-base inoculants in the ladle during metal transfer. This usually involves the addition of 0.2-0.8% of inoculant of controlled size and can involve special chemistry to reduce inoculant fade. Ductile iron inoculation has shown that nodularity fade occurs with time as dissolved magnesium vaporizes. Fade also happens even though expensive complex ladle inoculants are used to retard it.

To counteract this tendency to fade, foundrymen increasingly are resorting to more sophisticated techniques--now widespread--that postpone ladle inoculation and secondary additions to the last possible moment. Metal stream inoculation and mold inoculation basically distinguish where the inoculant addition is made. Generally, mold inoculation now uses cast or pressed blocks inserted into the molds.

Mold Inoculation Benefits

Mold inoculation is gaining favor for a variety of reasons. Among these are:

* A more homogeneous matrix microstructure is developed between differing casting section thicknesses resulting in optimum combinations of mechanical properties (hardness, strength, ductility, toughness, etc.).

* No free cementite phase is present down to 0.25 in. sections (even down to 0.125 in. in certain applications). This allows the foundry to use relatively higher levels of manganese and chromium in the iron to achieve carbide-free structure.

* The higher nodule count minimizes grain boundary segregation of carbide-forming elements and reduces carbon flotation.

* Graphite shape is improved to a more spheroidal form under marginal situations.

* Microporosity (shrink) is minimized under certain situations.

* Control of silicon content is important in product applications requiring high toughness. Mold inoculation allows this without the risk of carbide formation at low silicon levels.

* Maximum as-cast ferrite minimizes the need for heat treatment and also improves machinability.

Pressed Block Features

Pressed block, being porous, has a relatively poor thermal conductivity which results in the block heating up rapidly and progressively. This produces an almost instantaneous dissolution of the localized areas of the block as soon as the first iron comes into contact with it. The aspect ratio (size, weight) of the block is controlled so that the total reaction time is satisfactory for the class of work/castings being produced.

Produced to a precise shape and size, block weight is closely controlled resulting in minimal slag while reducing oxidized blow holes, cold lap and shrinkage defects. There is also no mismatch. The pressed compact is fine grained and of uniform density through the cross section. Because there is no chemical segregation, a homogeneous nucleation reaction in iron results. The cast block, however, exhibits some chemical segregation during solidification, which can lead to a nonuniform reaction and inconsistent inoculation results.

The pressed block can be produced to almost any chemical composition and more accurate dimensions to facilitate positioning in the mold. The regularity in size and shape enables coresetting machines to set the blocks in place. Also, the dissolution of the pressed block is less sensitive to metal temperature and the gating arrangement lowering, the risk of getting undissolved inoculant particles into the casting.

Application Considerations

As a rule, pressed blocks should be placed in the mold system so that the initial hot metal impinges upon them to promote the initial reaction. This is required so that a uniform degree of nucleation throughout the mold cavity is achieved. The hot metal should pass over or around the block uniformly to promote a steady dissolution of the block throughout the pouring cycle.

Experience indicates that the placement of pressed block under the downsprue in a print with 25% of the block within the molding media gives the best results. Some foundries have found equal success when placing the block above a ceramic filter situated in the downsprue. Others anchor the block within the pouring bush. The common factor, whatever the method, is that the block is held below the metal either by anchoring methods (i.e., in a print) or by the ferrostatic pressure of the metal.

The metal-controlling factors that affect the inoculant dissolution are mold temperature and flow rate. Metal temperature is normally in the range 2450-2825F, depending upon the type and section thickness of the casting being produced. The block reactivity is, as expected, faster at the higher temperature and lower at reduced temperatures, all other factors being equal. Flow rate controls the solubility rate on which the principle of mold inoculation depends. Experience indicates that the normal choke areas of the gating system can be applied successfully.

Pressed blocks are now widely used in gray and ductile iron castings produced by a variety of treatment processes. The standard grade is the most used, but the zirconium grade has been found effective in eliminating carbides in thin sections. The three available grades contain magnesium to supplement the residual magnesium required to produce the desired structure (nodularity) in ductile iron. It is sometimes possible to pour iron with a slightly lower residual magnesium without risking low nodularity, particularly where a precise control of magnesium and silicon is required to avoid microshrinkage. Magnesium in pressed blocks has no adverse effect in gray iron applications.

Figure 5 illustrates the microstructure comparison with and without pressed block in a class 35 gray iron casting with a 0.187 in. wall thickness. Without mold inoculation, the structure is fully carbidic. The use of pressed block inoculation helped eliminate the heat treatment otherwise required to anneal the carbides in the non-inoculated castings.

Photomicrographic comparison shown in Fig. 6 illustrates the microstructure of a ductile iron exhaust manifold made with and without mold inoculant. The significant increase in nodule count and ferrite content is evident using the pressed block inoculant. The iron was treated in a tundish ladle and then post-inoculated with 0.40% foundry grade 75% FeSi in the ladle.

Figure 7 illustrates what a pressed block inoculant can do under extreme conditions. In this heat, the iron was treated with pure magnesium by wire injection. No post-inoculation was added in the ladle except what was added inside the mold with a pressed block. The iron structure is very brittle after magnesium treatment, but becomes fully ductile after the dissolution of the block to produce a very homogenous microstructure. Several foundries that use pure magnesium for the nodularization of iron are using pressed blocks for their inoculation requirements.

Figure 8 compares casting photomicrographs produced from pressed block and solid cast inoculant in a high-production foundry. Nodule count is at least 10% higher with the pressed block and the nodules are much more uniform. The amount of ferrite is also about half.

Pressed and sintered blocks offer a novel and well-proven method of inoculation of gray and ductile irons castings. Different chemical analyses, size and weight ranges available to precisely match individual requirements make the pressed blocks an attractive and costeffective inoculating material. The development of tapered pressed blocks is expected to increase its application range to highly automated production lines.
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Article Details
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Author:Mohla, P.P.
Publication:Modern Casting
Date:Apr 1, 1993
Words:1462
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