CAST IRON a widely used

material

What it is ?

Cast iron is one of the oldest ferrous metals used in constructionand outdoor ornament.  It is primarily composed of iron (Fe),carbon (C) and silicon (Si), but may also contain traces of sulphur(S), manganese (Mn) and phosphorus (P).  It has a relatively highcarbon content of 2% to 5%.  It is hard, brittle, nonmalleable(i.e. it cannot be bent, stretched or hammered into shape) and morefusible than steel.  Its structure is crystalline and relativelybrittle and weak in tension.  Cast-iron members fracture underexcessive tensile loading with little prior distortion.  Cast ironis, however, very good in compression.  The composition of castiron and the method of manufacture are critical in determining itscharacteristics.

How it is made

Cast iron is made when pig iron is re-melted in small cupola furnaces (similar to the blast furnace in design and operation) and poured into molds to make castings. Cast Iron is generally defined as an alloy of Iron with greater than 2% Carbon, and usually with more than 0.1% Silicon.

A brief History

Cast iron has its earliest origins in China between 700 and 800 B.C.

The use of this newly discovered form of iron varied from simple tools to a complex chain suspension bridge erected approximately 56 A.D.

Cast iron was not produced in mass quantity until fourteenth centaury A.D.

In 1325 A.D. water driven bellows were introduced which allowed for a greater draft to be fed to the pit, thus increasing temperatures.

The next significant development in cast iron was the first use of coke in 1730 by an English founder named Darby.

This leads to invention of James Watt’s first steam engine in 1794

In 1810, Swedish chemist Bergelius, and German physicist Stromeyer discovered that by adding Silicon to the furnace, along with scrap and pig iron, consistently stronger cast iron is produced.

In 1885 Turner added ferrosilicon to white iron to produce stronger gray iron castings.

In the later 20th century the major use of cast irons consisted of pipes, thermal containment units, and certain machine or building entities which were necessary to absorb continuous vibrations.

Tyes of Cast Iron

White iron Gray Iron Ductile Iron Malleable Iron

White Iron: large amount of carbide phases in the form of flakes, surrounded by a matrix of either Pearlite or Martensite. The result of metastable solidification. Has a white crystalline fracture surface because fracture occurs along the iron carbide plates. Considerable strength, insignificant ductility.

Gray Iron: Graphite flakes surrounded by a matrix of either Pearlite or -a Ferrite. Exhibits gray fracture surface due to fracture occurring along Graphite plates. The product of a stable solidification. Considerable strength, insignificant ductility.

Ductile (Nodular) Iron: Graphite nodules surrounded by a matrix of either -a Ferrite, Bainite, or Austenite. Exhibits substantial ductility in its as cast form.

Malleable Iron: cast as White Iron, then malleabilized, or heat treated, to impart ductility. Consists of tempered Graphite in an -a Ferrite or Pearlite matrix.

Microstructures

White Cast Iron White Cast Iron

Ductile (Nodular) Iron Malleable Iron

Sub Classifications Chilled Iron: White Iron that has been produced by

quenching through the solidification temperature range.

Mottled Iron: Solidifying at a rate with extremes between those for chilled and gray irons, thus exhibiting micro-structural and metallurgical characteristics of both.

Compacted Graphite Cast Iron: consists of a microstructure similar to that of Gray Iron, except that the Graphite cells are coarser and more rounded. Namely, it consists of a microstructure having both characteristics of Gray and Ductile Irons.

High-Alloy Graphitic Irons: produced with microstructures consisting of both flake and nodule structures. Mainly utilized for applications requiring a combination of high strength and corrosion resistance.

General characteristics of white cast iron

White Cast Irons contain Chromium to prevent formation of Graphite upon solidification and to ensure stability of the carbide phase. Usually, Nickel, Molybdenum, and/or Copper are alloyed to prevent to the formation of Pearlite when a matrix of Martensite is desired.

Fall into three major groups: Nickel Chromium White Irons: containing 3-5%Ni, 1-

4%Cr. Identified by the name Ni-Hard 1-4 The chromium-molybdenum irons (high chromium

irons): 11-23%Cr, 3%Mo, and sometimes additionally alloyed w/ Ni or Cu.

25-28%Cr White Irons: contain other alloying additions of Molybdenum and/or Nickel up to 1.5%

General Characteristics of Gray Cast Irons

Gray Cast Irons contain silicon, in addition to carbon, as a primary alloy. Amounts of manganese are also added to yield the desired microstructure. Generally the graphite exists in the form of flakes, which are surrounded by an a-ferrite or Pearlite matrix. Most Gray Irons are hypoeutectic, meaning they have carbon equivalence (C.E.) of less than 4.3.

Gray cast irons are comparatively weak and brittle in tension due to its microstructure; the graphite flakes have tips which serve as points of stress concentration. Strength and ductility are much higher under compression loads.

Mechanical Properties of Gray Cast Iron

Graphite morphology and matrix characteristics affect the physical and mechanical properties of gray cast iron. Large graphite flakes produce good dampening capacity, dimensional stability, resistance to thermal shock and ease of machining. While on the other hand, small flakes result in higher tensile strength, high modulus of elasticity, resistance to crazing and smooth machined surfaces.

Mechanical Properties can also be controlled through heat treatment of the gray cast iron. For example, as quenched gray cast iron is brittle. If tempering is accomplished after quenching, the strength and toughness can be improved, but hardness decreases. The tensile strength after tempering can be from 35-45% greater than the as-cast strength and the toughness can approach the as-cast level.

Composition The chemical analysis of gray iron can be broken into three main

categories;

The main elements: These are Carbon, Silicon, and Iron. Gray cast irons typically contain 3.0-3.5% carbon, with silicon levels varying from 1.8-2.4%.

The minor elements: Phosphorus and the two related elements, manganese and sulfur. Phosphorus is found in all gray irons, although rarely added intentionally, it does increase the fluidity of iron to some extent. High levels promotes shrinkage porosity, while very low levels can increase metal penetration into a mold. Thus, most castings are produced with 0.02-0.10% P. Sulfur plays a significant role in nucleation of graphite in gray iron, with optimum benefit at 0.05-0.12% sulfur levels. It is also important to note that sulfur levels need to be balanced with manganese to promote formation of manganese sulfides.

The trace elements: Many other elements are utilized in limited amounts to affect the nature and properties of gray iron. Although some are not intentional, they do have a measurable affect on the gray cast iron. Some promote Pearlite, such as tin, while others compact graphite and increase strength, such as nitrogen.

General Characteristics of Ductile Irons

As a liquid, Ductile Iron has a high fluidity, excellent castability, but high surface tension. Thus, sands and molding equipment must provide molds of high density and good heat transfer.

Solidification of Ductile Cast Iron usually occurs with no appreciable shrinkage or expansion due to the expansion of the graphite nodules counteracting the shrinkage of the Iron matrix. Thus, risers (reservoirs in mold that feed molten metal into the cavity to compensate for a decrease in volume) are rarely used.

Require less compensation for shrinkage. (Designers compensate for shrinkage by casting molds that are larger than necessary.)

Most Ductile Irons used as cast. Heat treating (except for austempering) decreases fatigue properties. Example: Holding at the subcritical temperature (705˚C) for ≈ 4 hours improves fatigue resistance. While heating above 790˚C followed by either an air or oil quench, or ferritizing by heating to 900˚C and slow cooling reduces fatigue strength and fatigue resistance in most warm environments.

Austempered Ductile Iron has been considered for most applications in recent years due to its combination of desirable properties. A matrix of Bainitic Ferrite and stabilized Austenite with Graphite nodules embedded. Applications include: Gears, wear resistant parts, High-fatigue strength applications, High-impact strength applications, automotive crankshafts, Chain sprockets, Refrigeration compressor crankshafts, Universal joints, Chain links, and Dolly wheels.

Mechanical Properties Effects of microstructure on properties: Graphite nodules cause the Iron to be both strong and ductile (more so

than gray Cast Iron). Approaches the characteristics of Steel (Tensile strengths between 400 and 480 MPa Ductility between 15 and 20%. The smaller and more numerous the nodules the higher the tensile properties of the material. If some minor alterations are made to the microstructure compacted graphite Iron may be achieved, namely, between 5 and 20% of the graphite is of the shpeoridial form, the rest being of a compacted blunt, vermicular form. This material has better thermal transfer characteristics and fewer tendencies for porosity.

The % nodularity, a measure of the amount of graphite in the nodular form, corresponds with the characteristics of this material. %nodularity is measured by resonant frequency or ultrasonic velocity measurements.

Mechanical Properties

ASTM system for designating various Ductile Iron grades incorporates numbers indicating Tensile Strength in ksi, yield strength in ksi, and elongation in percent. For example A 536 grade 80-60-03 = (80 ksi|552 MPa) min tensile strength, (60ksi|414MPa) min yield strength, 3 % elongation.

Ductile Iron Applications Used for a variety of applications, specifically those requiring

strength and toughness along with good machinability and low cost. Casting, rather than mechanical fabrication (such as welding), allows the user to optimize the properties of the material, combine several castings into a desired configuration, and realize the economic advantages inherent in casting.

Microstructure is consistent; machinability is low due to casting forming the desired shape; porosity is predictable and remains in the thermal center.

Ductile Iron can be austempered to high tensile strength, fatigue strength, toughness, and wear resistance. Lower density

Ductile Iron seems to work in applications where theories suggest it should not.

Ductile Iron shipments exceeded 4 million in 95. Cast Iron pipe make up to 44% of those shipments. 29% used for automobiles and light trucks (economic

advantages and high reliability) Other important applications are: Papermaking machinery;

Farm equipment; Construction machinery and equipment; Power transmission components (gears); Oilfield equipment.

Typical uses

Cast iron is used in a wide variety of structural and decorativeapplications, because it is relatively inexpensive, durable andeasily cast into a variety of shapes.  Most of the typical usesinclude:

    -  historic markers and plaques    -  hardware:  hinges, latches    -  columns, balusters    -  stairs    -  structural connectors in buildings and monuments    -  decorative features    -  fences    -  tools and utensils    -  ordnance    -  stoves and firebacks    -  piping.