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Ductile iron: one of the century's metallurgical triumphs.

Good graphite nodules make quality ductile iron. A variety of nodulizing processes is available to fit most foundry operations.

British and American metallurgists arrived on the patent office doorstep at about the same time in 1948 with their processes for manufacturing a form of cast iron called ductile iron. A marvelous product that could be made using most of the available equipment and technology of the gray iron foundry, it had some special physical and economic advantages that set it apart from gray and malleable irons, and forged or cast steel. Ductile iron has become one of the most important metallurgical developments of the late 20th century. In the first 25 years from its inception, production zoomed from zero to an annual total of 3.2 million tons in the U.S. alone, and it has continued to build market share percentages in today's leaner metalcasting market.

Ductile iron is a relatively inexpensive material for many applications when compared with metals of equal utility. It is ideally suited to the manufacture of a variety of cast metal parts, ranging from thin-sectioned to large castings. it is essentially gray iron in composition and castability, but exhibits much of the high strength and ductility found in steel castings. Its use has grown to the point that it now nearly equals the total U.S. production of gray iron.

Graphite Content

The difference between gray iron and ductile iron lies in the orientation of the graphite structure of each metal. Gray iron's graphite content is present in an overlapping network of variably sized graphite flakes. The graphite in ductile iron occurs as distinct spheroidal graphite nodules. Magnesium is universally used to transform the flake graphite into nodular form.

When a gray iron casting is stressed to its fracture point, a crack will tend to follow its randomly oriented, contiguous graphite flakes, rapidly propagating the fracture to failure. Though it offers excellent compressive strength and damping qualities, gray iron has little ductility.

In ductile iron, the graphite spheres, appearing as small islands of graphite surrounded by the base iron matrix, tend to isolate stress cracks, giving the material the added toughness, strength and shock resistance that permits some stress-induced deformation without rupture.

Nodulizing Elements

Forcing the graphite flakes that are characteristic of gray iron to agglomerate into spheres, Qr nodules, to form ductile iron as the molten metal solidifies is the challenge for the metallurgist. Nodulizing is possible most economically in low sulfur-base irons (less than 0.03% and for some processes less than 0.01%). Generally, the higher the sulfur content, the more nodulizing agent required-first, to desulfurize the metal bath, and second, to leave enough residual nodulizing material for the formation of spherical graphite.

In the search for nodulizing elements, a broad variety of materials was tested. Of those successful in forming nodular graphite, many were too costly or inefficient for production applications. The nodulizing agents judged most effective were magnesium and cerium.

In the British patented system, the nodulizing process is activated by adding Ce to the metal bath to cause the graphite to precipitate into equally distributed graphite nodules. The American process uses Mg alloys to accomplish the same transformation.

Either Ce or Mg in precise amounts, often together with other rare earth elements and calcium, are added to the heat at the optimum time to cause the graphite nodules to form and remain in spheroidal shape during solidification.

As with any new technology, early ductile iron production had as many failures as successes. It took time for foundries to learn the intricacies involved in producing ductile iron (base metal preparation, nodulizing, inoculation and quality control). Research and expanding foundry operating skills continue to add to the economic advantages of ductile iron and growing numbers of foundries are now producing it.

Magnesium is the nodulizer of choice in the U.S. It is an effective desulfurizer, deoxidizer and nodulizer, and less expensive than Ce.

Traces of lead, antimony, bismuth, aluminum, titanium and arsenic are often present in the melt and can interfere with the graphite nodule formation. However, small amounts of Ce combined with the Mg treatment alloy neutralize the undesirable effects of these elements. Because of their presence, Ce remains an integral part of the nodulizing treatment process. Cerium is also used with Mg to offset Mg fade and supplement the material's nodulizing effect.

As noted, Mg is an excellent deoxidizer and desulfurizer. Tests have indicated that a base iron melt treated with Mg can reduce the iron's oxygen content from 0.0135% to 0.003%. It has also been found that one pound of Mg can remove 1-112 lb of sulfur.

Magnesium's content in molten iron decreases with increasing temperatures and the length of time the heat is held. Sufficient Mg must be retained to maintain quality nodules down to the last metal poured from a treated ladle. Excellent nodules can be obtained with as little as 0.018% Mg residual.

The normal working minimum Mg content necessary to prevent flake graphite formation should be in the 0.02-0.06% range; with Ce and other rare earths and/or calcium added, the Mg level can be reduced further.

Among the rare earth elements, Ce was the original element used to bring about graphite nodule formation in hypereutectic cast irons. Misch metal, a combination of Ce and other rare earth elements (lanthanum, praseodymium, neodymium, samarium and yttrium), was also used as a nodulizer.

Cerium enjoys the advantage over Mg in its high vapor point. Cerium has a VP of 4362F (3405C) compared to Mg's 2021 F (1105C). Its ability to form stable oxides and sulfides minimizes fading and increases the time a heat can be held without graphite nodule deterioration-a weak point with Mg.

Dross contamination and cracking of the surfaces of ductile iron castings are leading causes of scrap in some highly stressed castings applications. The dross problem is caused by the formation of magnesium silicates, oxides and sulfides resulting from the Mg treatment. Replacing up to half the Mg with Ce significantly reduces or eliminates dross. Another advantage of using Ce in ductile iron is its ability to control the adverse effects of Ti, Pb, As and Sb.

Nodulizing Methods

A variety of treatment methods exists for introducing the master Mg alloy to molten gray iron in a manner that extends the recovery time of the Mg and facilitates the formation of graphite nodules. All involve the introduction of Mg alone or in combination with Ce or other rare earth metals or compounds. The key to the success of each method is the extent to which it economically brings about the formation of graphite nodules by the addition of Mg and other elements.

At molten iron temperatures, Mg and its alloys react violently, creating flare, smoke and fumes. Each of the nodulizing processes seeks to control the metal's intense pyrotechnics. Many systems are patented and enjoy limited application; some are more efficient than others. Individual foundry melting practices and volume will influence the nodulizing technique best suited to its requirements.

Following are brief descriptions of some of the most commonly used nodulizing systems. In view of all other nodulizing processes available, it is sufficient to say that no single nodulizing system is satisfactory for all foundries producing ductile iron. It remains an individual foundry's decision as to which system meets its economic and manufacturing needs.
 * Open Ladle or Pour Over Method-An
 early, relatively simple process
 that is still widely used, it requires a
 deep, preheated ladle that contains in
 its bottom a specific weight of Mg
 alloy measured according to the
 amount of iron to be treated. The base
 iron is poured into the ladle as quickly
 as possible to prevent the nodulizing
 alloy from floating to the top and
 burning, and also to obtain the longest
 Mg recovery time. Open ladle
 nodulizing requires that the slag be
 skimmed off immediately. A stream
 inoculant may be added if the melt is
 transferred from the treatment vessel
 to a pouring ladle.
 * Sandwich Method-An offshoot of the
 pour over system, this method reportedly
 yields a higher Mg recovery rate
 than the earlier system. It involves
 placing a specific amount of Mg alloy
 in a depression-a specially designed
 pocket or sump-in the ladle bottom.
 The alloy is then covered (sandwiched)
 with steel punchings or trimmings.
 The sizes of the treatment alloy granules
 (suggested 1x8-in. mesh) and
 the metal cover pieces are important.
 The metal used to cover the nodulizing
 alloy should be sized just large
 enough to retard the treatment process)
 to allow the reaction of the
 nodulizing alloy to progress at the
 most effective rate as molten metal is
 poured into the ladle. The advantages
 of the sandwich method include improved
 recovery of Mg, shorter treatment
 time, operational simplicity and
 less slag.
 * Covered Tundish Ladle Method-The
 foregoing two nodulizing systems
 result in relatively violent Mg/molten metal
 reactions because they are open to an
 unlimited supply of oxygen. To reduce
 that violence and improve the nodulizing
 effect of the Mg treatment, the covered
 tundish ladle was developed.
 The ladle is fitted with a cover that
 limits the amount of available oxygen
 for burning the Mg and improves the
 adverse environmental aspects of the
 nodulizing process. A port in the ladle
 cover allows the insertion of the treatment
 alloy into the ladle. Cover vents
 relieve the treatment reaction pressure.
 This process is similar to the
 sandwich process except for the
 presence of the cover.
 Covered ladle nodulizing offers more
 consistent Mg recovery and eliminates
 reaction flare and most fumes. it also
 reduces metal bath temperature loss,
 metal splashing, carbon loss and slag
 formation.
 * Porous Plug Process-Much of the Mg
 added to a molten iron bath vaporizes
 or is burned because of its low vaporization
 temperature. Much of what is
 left is consumed in Mg's potent desulfurization
 reaction, leaving a small
 fraction of the material to nodulize the
 iron. This often leads to the addition of
 more treatment alloy than the amount
 theoretically required for the nodulizing
 process.
 * To maximize the utility of the costly Mg
 treating materials and allow for the
 precise addition of the nodulizing
 agent, desulfurization prior to adding
 the Mg alloy is desirable. It can be
 accomplished by bubbling nitrogen
 through a porous refractory plug(s) in
 a furnace or ladle bottom directly into
 the molten metal to create a strong
 stirring, mixing action. A desulfurizer
 such as calcium carbide can be added
 to the swirling melt before the addition
 of the Mg nodulizer and thoroughly
 blended into the molten metal to reduce
 the sulfur to an acceptable level.
 Then the nodulizing alloy can be
 added and similarly mixed into the
 iron mass to complete the nodulizing
 process in the most efficient and effective
 manner.
 The porous plug process can also be
 used for recarburization and inoculation.
 Porous refractory materials permeable
 to gas but impervious to liquid
 metal are available for a wide variety
 of furnace and ladle sizes.
 * Plunging Method-This is an effective
 method of introducing nodulizing
 materials into molten metal. It uses a
 vented bell-shaped plunger that contains
 the Mg nodulizing alloy. The
 alloy is packed into a can, wrapped in
 metal foil or in some way secured in
 the graphite or refractory bell. The bell
 is inserted (plunged) at 12-15 in./sec
 into the molten metal to a position near
 the bottom of the ladle for the time
 required to nodulize the metal, usually
 until the ladle stops vibrating. The
 system results in less Mg flare and
 smoke than the open ladle method
 because less oxygen is available to
 support combustion.
 With this nodulizing method, the ladle
 and the bell must be preheated to
 about 2200F (1200C). The refractory
 plunger is subject to severe operating
 conditions of thermal shock, impact
 and erosion. Slag tends to stick to the
 bell, plugging the holes necessary for
 the Mg vapors to escape.
 * In-mold Process-This patented
 method combines the nodulizing and
 casting processes by placing an exact
 weight of the nodulizing material
 into a preformed chamber or receptacle
 in the mold where it forms part of
 the pressurized gating system.
 Nodulizing occurs in the chamber as
 the base iron flows over the treatment
 alloy. It eliminates smoke and reaction
 fumes because the sand mold absorbs
 the reaction byproducts. Magnesium
 recovery rates are reportedly excellent.
 The process reportedly eliminates the post inoculation
 requirement.
 * In-stream or Flow-through Process
- In this patented process, the treatment
 unit consists of a covered pouring
 basin and separate alloy reaction
 and expansion chambers. Useful for
 heats of up to 3000 lb, iron is poured
 into the pouring basin, flows into a
 chamber containing the nodulizing
 alloy and into an expansion chamber
 before emptying into a ladle. The dimensions
 of the chamber ports are
 critical to retard the metal flow and
 facilitate the maximum magnesium
 recovery. Wide variations in sulfur
 content cannot be tolerated, but Mg
 recoveries of 60-70% are reported.
 This method is sensitive to production
 techniques. Flashbacks can occur if
 pouring is inconsistent, and premature
 alloy ignition can result if the unit is not
 flushed clean of all slag and alloy
 buildup inside the alloy reaction
 chamber.
 * Converter Process-This licensed
 process uses pure Mg placed in a
 separate chamber above the liquid
 metal while the nodulizing ladle is in a
 horizontal position. When the filled
 vessel is rotated to a vertical position,
 the metal engulfs the alloy chamber to
 complete the nodulization reaction.
 The most cost-effective nodulizing
 treatment system for the production of
 quality ductile iron depends on many
 factors, including equipment and Mg
 alloy costs. But paramount with them
 all is the strict adherence to the specific
 process and metallurgical factors
 that determine the manufacture
 of quality ductile iron.
 A sampling of typical ductile iron
 nodulizing alloys might include:
 * Nickel-base Alloys-85% Ni, 15% Mg;
 50% Ni, 30% Si, 15% Mg; 95% Ni, 5%
 Mg; 60% Ni, 35% Fe, 4% Mg. These
 alloys are used principally in open ladle
 treatment and provide up to 80% or
 more Mg recovery, and low treatment
 reaction.
 * Silicon-base Alloys-3%,5% and 9%
 Mg, plus 45% Si, 1 % Ca, 1 % Al, and
 the balance Fe. They are used respectively
 in open and tundish ladles;
 in the plunging and sandwich processes;
 and for low sulfur irons requiring
 high Mg recovery and low fugitive
 emissions.
 * Rare Earth Additions-Alloyed with
 Mg-Fe additives at various levels
 (0.1- 0.3% rare earth); as Misch metal (100%
 rare earth with 50% Ce, 30% La and
 the balance other rare earths); or rare
 earth/silicon alloys;
 * Metallic Magnesium-Pure Mg reacts
 violently in combination with molten iron
 and is usually encapsulated with a refractory
 to restrict the reaction during
 treatment. Only a portion of the Mg is
 exposed to start the reaction.
COPYRIGHT 1991 American Foundry Society, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Author:Bex, Tom
Publication:Modern Casting
Date:Feb 1, 1991
Words:2446
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