Ductile iron

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Ductile iron, also known as ductile cast iron, nodular cast iron, spheroidal graphite iron, spheroidal graphite cast iron [1] and SG iron, is a type of graphite-rich cast iron discovered in 1943 by Keith Millis. [2] While most varieties of cast iron are weak in tension and brittle, ductile iron has much more impact and fatigue resistance, due to its nodular graphite inclusions.

Contents

On October 25, 1949, Keith Dwight Millis, Albert Paul Gagnebin and Norman Boden Pilling received US patent 2,485,760 on a cast ferrous alloy for ductile iron production via magnesium treatment. [3] Augustus F. Meehan was awarded a patent in January 1931 for inoculating iron with calcium silicide to produce ductile iron subsequently licensed as Meehanite, still produced in 2017.

Metallurgy

Ductile iron microstructure at 100x magnification, showing carbon islanding effect around nodules. Ductile Iron.png
Ductile iron microstructure at 100× magnification, showing carbon islanding effect around nodules.
Another micrograph showing the carbon islanding effect, with nodules surrounded by areas depleted of carbon Gusseisen mit Kugelgraphit.jpg
Another micrograph showing the carbon islanding effect, with nodules surrounded by areas depleted of carbon

Ductile iron is not a single material but part of a group of materials which can be produced with a wide range of properties through control of their microstructure. The common defining characteristic of this group of materials is the shape of the graphite. In ductile irons, graphite is in the form of nodules rather than flakes as in grey iron. Whereas sharp graphite flakes create stress concentration points within the metal matrix, rounded nodules inhibit the creation of cracks, thus providing the enhanced ductility that gives the alloy its name. [5] Nodule formation is achieved by adding nodulizing elements, most commonly magnesium (magnesium boils at 1100 °C and iron melts at 1500 °C) and, less often now, cerium (usually in the form of mischmetal). [6] Tellurium has also been used. Yttrium, often a component of mischmetal, has also been studied as a possible nodulizer.

Austempered ductile iron (ADI; i.e., austenite tempered [7] ) was discovered in the 1950s but was commercialized and achieved success only some years later. In ADI, the metallurgical structure is manipulated through a sophisticated heat treating process.

Composition

Mass fraction (%) for ferritic ductile iron castings [8]
Fe C Si Ni Mn Mg Cr P Cu
Balance3.0–3.71.2–2.31.00.250.070.070.030.1

Other ductile iron compositions often have a small amount of sulfur as well.

Elements such as copper or tin may be added to increase tensile and yield strength while simultaneously reducing ductility. Improved corrosion resistance can be achieved by replacing 15–30% of the iron in the alloy with varying amounts of nickel, copper, or chromium.

Silicon as a graphite formation element can be partially replaced by aluminum to provide better oxidation protection. [9]

Applications

Much of the annual production of ductile iron is in the form of ductile iron pipe, used for water and sewer lines. It competes with polymeric materials such as PVC, HDPE, LDPE and polypropylene, which are all much lighter than steel or ductile iron; being softer and weaker, these require protection from physical damage.

Ductile iron is specifically useful in many automotive components, where strength must surpass that of aluminum but steel is not necessarily required. Other major industrial applications include off-highway diesel trucks, class 8 trucks, agricultural tractors, and oil well pumps. In the wind power industry nodular cast iron is used for hubs and structural parts like machine frames. Nodular cast iron is suitable for large and complex shapes and high (fatigue) loads.

SG iron is used in many grand piano harps (the iron plates to which high-tension piano strings are attached).

See also

Related Research Articles

Cast iron

Cast iron is a group of iron-carbon alloys with a carbon content more than 2%. Its usefulness derives from its relatively low melting temperature. The alloy constituents affect its colour when fractured: white cast iron has carbide impurities which allow cracks to pass straight through, grey cast iron has graphite flakes which deflect a passing crack and initiate countless new cracks as the material breaks, and ductile cast iron has spherical graphite "nodules" which stop the crack from further progressing.

Austenite Metallic, non-magnetic allotrope of iron or a solid solution of iron, with an alloying element

Austenite, also known as gamma-phase iron (γ-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron, with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 1000 K (727 °C); other alloys of steel have different eutectoid temperatures. The austenite allotrope is named after Sir William Chandler Roberts-Austen (1843–1902); it exists at room temperature in some stainless steels due to the presence of nickel stabilizing the austenite at lower temperatures.

Keith Dwight Millis was an American metallurgical engineer and inventor of ductile iron.

Tempering (metallurgy) Process of heat treating used to increase toughness of iron-based alloys

Tempering is a process of heat treating, which is used to increase the toughness of iron-based alloys. Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to some temperature below the critical point for a certain period of time, then allowing it to cool in still air. The exact temperature determines the amount of hardness removed, and depends on both the specific composition of the alloy and on the desired properties in the finished product. For instance, very hard tools are often tempered at low temperatures, while springs are tempered at much higher temperatures.

Zamak brand of zinc alloys

Zamak is a family of alloys with a base metal of zinc and alloying elements of aluminium, magnesium, and copper.

Gray iron

Gray iron, or grey cast iron, is a type of cast iron that has a graphitic microstructure. It is named after the gray color of the fracture it forms, which is due to the presence of graphite. It is the most common cast iron and the most widely used cast material based on weight.

Malleable iron cast as white iron, the structure being a metastable carbide in a pearlitic matrix. Through an annealing heat treatment, the brittle structure as first cast is transformed into the malleable form

Malleable iron is cast as white iron, the structure being a metastable carbide in a pearlitic matrix. Through an annealing heat treatment, the brittle structure as first cast is transformed into the malleable form. Carbon agglomerates into small roughly spherical aggregates of graphite leaving a matrix of ferrite or pearlite according to the exact heat treatment used. Three basic types of malleable iron are recognized within the casting industry: blackheart malleable iron, whiteheart malleable iron and pearlitic malleable iron.

Ferrosilicon

Ferrosilicon is an alloy of iron and silicon with a typical silicon content by weight of 15–90%. It contains a high proportion of iron silicides.

Austempering

Austempering is heat treatment that is applied to ferrous metals, most notably steel and ductile iron. In steel it produces a bainite microstructure whereas in cast irons it produces a structure of acicular ferrite and high carbon, stabilized austenite known as ausferrite. It is primarily used to improve mechanical properties or reduce / eliminate distortion. Austempering is defined by both the process and the resultant microstructure. Typical austempering process parameters applied to an unsuitable material will not result in the formation of bainite or ausferrite and thus the final product will not be called austempered. Both microstructures may also be produced via other methods. For example, they may be produced as-cast or air cooled with the proper alloy content. These materials are also not referred to as austempered.

2219 aluminium alloy is an alloy in the wrought aluminium-copper family. It can be heat-treated to produce tempers with higher strength but lower ductility. The aluminium-copper alloys have high strength, but are generally less corrosion resistant and harder to weld than other types of aluminium alloys. To compensate for the lower corrosion resistance, 2219 aluminium can be clad in a commercially pure alloy such as 1050 or painted. This alloy is commonly formed by both extrusion and forging, but is not used in casting.

5154 aluminium alloy is an alloy in the wrought aluminium-magnesium family. As an aluminium-magnesium alloy, it combines moderate-to-high strength with excellent weldability. 5154 aluminium is commonly used in welded structures such as pressure vessels and ships. As a wrought alloy, it can be formed by rolling, extrusion, and forging, but not casting. It can be cold worked to produce tempers with a higher strength but a lower ductility. It is generally not clad.

5454 aluminium alloy is an alloy in the wrought aluminium-magnesium family. It is closely related to 5154 aluminium alloy. As an aluminium-magnesium alloy, it combines moderate-to-high strength with excellent weldability. Like 5154, 5454 aluminium is commonly used in welded structures such as pressure vessels and ships. As a wrought alloy, it can be formed by rolling, extrusion, and forging, but not casting. It can be cold worked to produce tempers with a higher strength but a lower ductility. It is generally not clad.

5456 aluminium alloy is an alloy in the wrought aluminium-magnesium family. While it is closely related to 5356 aluminium alloy, it is used in structural applications, like most other aluminium-magnesium alloys, and not as filler for welding. As a wrought alloy, it can be formed by rolling, extrusion, and forging, but not casting. It can be cold worked to produce tempers with a higher strength but a lower ductility. It is susceptible to exfoliation corrosion when held at temperatures above 65 °C (150 °F) for extended periods of time.

5754 aluminium alloy is an alloy in the wrought aluminium -magnesium family. It is closely related to the alloys 5154 and 5454. Of the three 5x54 alloys, 5754 is the least alloyed, but only by a small amount. It is used in similar applications. As a wrought alloy, it can be formed by rolling, extrusion, and forging, but not casting. It can be cold worked to produce tempers with a higher strength but a lower ductility.

6005A aluminium alloy is an alloy in the wrought aluminium-magnesium-silicon family. It is closely related, but not identical, to 6005 aluminium alloy. Between those two alloys, 6005A is more heavily alloyed, but the difference does not make a marked impact on material properties. It can be formed by extrusion, forging or rolling, but as a wrought alloy it is not used in casting. It cannot be work hardened, but is commonly heat treated to produce tempers with a higher strength at the expense of ductility.

6262 aluminium alloy is an alloy in the wrought aluminium-magnesium-silicon family. It is related to 6162 aluminium alloy, but sees much more widespread use. It is notably distinct from 6162, and most other aluminium alloys, in that it contains lead in its alloy composition. It is typically formed by extrusion, forging, or rolling, but as a wrought alloy it is not used in casting. It can also be clad, but that is not common practice with this alloy. It cannot be work hardened, but is commonly heat treated to produce tempers with a higher strength but lower ductility.

The 6463 aluminium alloy is an aluminum alloy in the wrought aluminium-magnesium-silicon family. It is related to 6063 aluminium alloy, but unlike 6063 it is generally not formed using any processes other than extrusion. It cannot be work hardened, but is commonly heat treated to produce tempers with a higher strength but lower ductility. Like 6063, it is often used in architectural applications.

Austempered Ductile Iron alloy

Austempered Ductile Iron (ADI) is a form of ductile iron that enjoys high strength and ductility as a result of its microstructure controlled through heat treatment. While conventional ductile iron was discovered in 1943 and the austempering process had been around since the 1930s, the combination of the two technologies was not commercialized until the 1970s.

Ductile Iron with special properties is a form of malleable cast iron which possess a complex of high mechanical properties: strength and plasticity, crack and contact fatigue resistance, self-lubrication at friction and ability to dampen the dynamic loading. The following specific feature can be seen in the "strength vs plasticity" diagram: plasticity increases with the increase in strength. Such properties can be explained by the balanced chemical composition with an optimal ratio of three modifying elements: Mo-Ni-Cu and are assured by the corresponding microstructure which is obtained by the respective heat treatment.

References

  1. Smith & Hashemi 2006 , p. 432.
  2. "Modern Casting, Inc". Archived from the original on 2004-12-14. Retrieved 2005-01-01.
  3. USpatent 2485760,Keith Millis,"Cast Ferrous Alloy",issued 1949-10-25
  4. Yaqub, Ejaz; Arshad, Rizwan (2009). "ME-140 Workshop Technology - Slide 25" (images). Air University. Retrieved 2011-10-30.
  5. http://www.ductile.org/didata/Section2/2intro.htm
  6. Gillespie, LaRoux K. (1988), Troubleshooting manufacturing processes (4th ed.), SME, p. 4, ISBN   978-0-87263-326-1.
  7. "ADI the Material". ADI Treatments Ltd. Archived from the original on 2010-10-26. Retrieved 2010-01-24.
  8. ASTM International. A874/A874M-98(2018)e1 Standard Specification for Ferritic Ductile Iron Castings Suitable for Low-Temperature Service. West Conshohocken, PA; ASTM International, 2018. doi: https://doi-org.ezpxy-web-p-u01.wpi.edu/10.1520/A0874_A0874M-98R18E01
  9. Aluminum ADI

Bibliography