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.