Metallurgical Properties of Cast Irons~The worlds best element~Engineering 45SRJC, Dec. 2003Emanuel PerezAlex Stameroff

Cast IronsCast 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. Exception: Silicon and other alloying elements may considerably change the maximum solubility of Carbon in Austenite. Thus certain alloys can solidify with a eutectic structure having less than 2% Carbon.

A Brief History of Cast Irons Cast iron has its earliest origins in China between 700 and 800 B.C.

Until this period ancient furnaces could not reach sufficiently high temperatures.

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.

A Brief History of Cast IronsIn 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.

Coke could be used more efficiently than coal, thus lowering the cost and time necessary to yield a final product.

A Brief History of Cast IronsThis discovery, with stronger and more efficient cast iron alloys being produced, led to larger scale production of cast irons with varying properties.

Due to this revolution, better casts were available for more versatile roles, such as James Watt's first steam engine, constructed 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.

A Brief History of Cast IronsIn 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.

Classifications of Cast 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.Microstructure of White Cast IronMicrostructure of Gray Cast Iron

Classifications of Cast IronDuctile (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.Microstructure of Ductile IronMicrostructure of 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.

Cast Iron Alloying System Most common alloying element of Cast Irons is Silicon for various reasons, which include the manipulation of temperatures required to achieve desired microstructures.

example: Increases the stability of solidification of Graphite phases. Decreases the stability of the solidification of Fe3C. Eutectic and eutectoid temperatures change from single values to temperature ranges. Eutectic and eutectoid compositions are affected. %C + 1/3%Si = 4.3 Carbon Equivalent CE: a measure of the equivalency of Carbon coupled with other alloying elements to that of just Carbon. An easier basis for classifying the properties of a multialloy material. %C + 1/3%Si = CE [1] (hypoeutectic) < CE = 4.3 < (hypereutectic) With the addition of Phosphorus CE = %C + 1/3(%P + %Si) [2]

Solidification of phases in Cast Irons, Nucleation & Growth of Graphite Graphite structure is a factored crystal bounded by low index planes. Growth occurs along (00) & (000) planes (direction A & C). Unstable growth occurs along direction A, giving the Graphite microstructure rough, poorly defined, edges in certain areas. When grown from the solidification of liquid Iron Carbon alloys, Graphite takes on a layer structure. Among each layer covalent chemical bonds with strengths between (4.19E5 - 5E5)J/mol exist. Between layers weaker bonds exist on the order of (4.19E3 - 8.37E3)J/mol.The structure of the Graphite depends on the chemical composition, the ratio of temperature gradient to growth rate, and the cooling rate. Such structures are: Flake or Plate Graphite (a) Compacted vermicular Graphite (b) Coral Graphite (c) Spheroidal Graphite (d)

Nucleation of Flake and Spheroidal Graphite A wide variety of compounds and certain metals have been claimed to serve as either inoculants or nuclei for Flake Graphite growth. (Silicon dioxide, silicates, sulfides, boron nitride, carbides, sodium, potassium, calcium, ect...)Two methods of growth. The nucleation of Flake Graphite Iron occurs mainly on silicon dioxide particles.Salt like carbides containing the ion Carbon are used as inoculants. Such carbides include NaHC2 & KHC2 from Group I, CaC2, SrC2, BaC2 from group II, and YC2 & LaC2 from group III.

Cooling Curve Analysis The solidification of a sample is represented mathematically by the relationship. dQ/dt = VCp dT/dt [3] where V is the volume of the sample, is the density, Cp is the heat capacity, and dT/dt is the cooling rate of the liquid. dT/dt is the slope of the cooling curve before solidification begins. When the liquid cools below the liquidus temperature, crystals nucleate and begin to grow. The rate is thus reexpressed as:dQ/dt = (VCp + Hd/dT)dT/dt [4]where H is the heat of solidification and d/dT is the volume fraction of solid formed at a changing temperature.

General Characteristics of White Cast Irons 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%

Nickel Chromium Produced for more than 50 years, effective materials for crushing and grinding in industry. Consists of Martensite matrix, with Nickel alloyed at 3-5% in order to suppress transformation of Austenite to Pearlite. Chromium usually included between 1.4-4% to ensure Carbon phase solidifies to Carbide, not Graphite. (Counteracts the Graphitizing effect of Ni)Abrasion resistance (usually desired property of this material) increases with Carbon content, but toughness decreases. Various grades class I Type A abrasion resistant; class I type B toughnessApplications: Because of low cost, used primarily in mining applications as ball mill liners and grinding balls. Class I, type A (max abrasion resistance) ash pipes, slurry pumps, roll heads, Muller tires, augers, coke crusher segments, and brick molds. Class I type B (more strength and exerting moderate impact) crusher plates, crusher concaves, and pulverizer pegs. Class I, type C (designed for single purpose of grinding aggregate) sand cast and chill cast Class I, type D (higher strength and toughness) pump volutes handling abrasive slurries and coal pulverizers.

Table of White Cast Iron Information

High Chromium White Irons Excellent abrasion resistance. Provide the best combination of toughness and abrasion resistance among White Cast Irons. The tradeoff is between wear resistance and toughness. Two types: the hard, discontinuous, X7C3 eutectic Carbides present in the microstructure the softer, more continuous, X3C present in Irons containing less Chromium. Usually produced by hypoeutectic compositions.For Abrasion Resistance: 11-23%Cr, 3.5%Mo. Usually supplied as cast with an Austenitic or Austenitic-Martensitic matrix, or heat treated with a martensitic matrix. Considered the hardest of all grades of White Cast Iron. For Corrosion Resistance: 26-28%Cr 1.6-2.0%C. Provide the max Cr content in the matrix for room temperature. Fully austenitic matrix provides the best resistance to corrosion in this alloy, but with a decrease in abrasion resistance. For High-Temperature Service: Often used for high temperature applications because of castability and cost. Alloyed with between 12-39% Cr at temperatures up to 1040C for scaling resistance Cr causes the formation of a Cr rich oxide film at high temperatures. Fall into three categories depending on the Matrix: Martensitic irons alloyed with 12-28%Cr; Ferritic Irons alloyed w/ 30-34%Cr; Austenitic irons that contain 15 - 30% Cr with 10-15%Ni to stabilize the Austenite phase. Applications include recuperator tubes, breaker bars and trays in sinter furnaces, other furnace parts, glass bottle molds, and valve seats for combustion engines.

General Characteristics of Gray Cast IronsGray 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 IronGraphite 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 (705C) for 4 hours improves fatigue resistance. While heating above 790C followed by either an air or oil quench, or ferritizing by heating to 900C 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 Effect of composition on properties: Carbon: During solidification, Carbon precipitates to Graphite, which offsets shrinkage. Amount necessary to achieve this: %C + 1/7%Si 3.9% Also, Carbon contents greater than this amount decrease fatigue strength. Silicon: Graphitizing agent. Increasing amount of Silicon also increases amount of Ferrite. Increases strength and hardness of this Ferrite and reduces impact resistance. Also, provides high temperature oxidation resistance. For applications of high temperature, such as turbocharger housings, Silicon contents of between 3.75% and 4.25% are required. Manganese: Acts as a Pearlite stabilizer and increases strength, but decreases ductility and machinability. Nickel: Increases strength by promoting formation of fine Pearlite. Increases hardenibility. Copper: Used to form Pearlite upon solidification with high strength and good toughness and machinability. Molybdenum: Used to stabilize structures at high temperatures.

Mechanical PropertiesEffects 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.

Table of Values for Ductile Cast Irons

Heat Treatment Stress Relieving: 540 - 595C reduces warping and distortion during subsequent machining. Annealing: Full feritizing Annealing used to remove carbides and stabilized Pearlite, heat to 900C, holding long enough to dissolve carbides, then cooling at a rate of 85C/h to 705C, and still air cooling to room temperature. Also improves low temperature fracture resistance but reduces fatigue strength. Normalizing, Quenching, and Tempering: heated to 900C, held for 3 hours (allowing 1h/25mm or 1h/in of cross section to reach that temperature). The air blasted or oil quenched. Followed by tempering between 540 - 675C. Used to achieve grade 100-70-03. Hardnesss as high as 255HB, increase short time fatigue strength, decreases fatigue life. Increases tensile and yield strengths at the expense of ductility. Autempering: Used to achieve Austempered Ductile Iron, requires two stages. Stage 1: heating to and holding at 900C. Stage 2: quenching and isothermally holding at the required austetempering temperature, usually in a salt bath. austempering temperatures shown in figure below.

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.

Malleable Irons Type of Iron with Graphite in the form of irregularly shaped nodules. Produced by first casting the Iron as a White Iron, then heat treating to transform the Carbide into Graphite. Categorized into 3 categories: Ferritic, Pearlitic, and Martensitic Malleable Cast Iron. Two types of Ferritic: Blackheart (top) and Whiteheart (not produced in US) Posses considerable ductility and toughness due to the combination of nodular graphite and low carbon matrix. Often a choice between Malleable and Ductile Iron for some applications, decided by economic constraints. Solidification of white Iron throughout a section is essential in producing Malleable Iron, thus, applications of large cross section are usually Ductile Irons. Usually capable in section thickness between 1.5-100mm (1/16 4 in) and 30g (1oz) - 180 kg (400lb) Preferred in the following applications: This section casting; parts that are to be pieced, coined, or cold worked; parts requiring max machinability; parts that must retain good impact resistance at low temp; parts requiring wear resistance.

Microstructure Consists of rosette nodules of graphite. Derived by control annealing of a cast of white Iron. During annealing cycle, Carbon, whether in Carbide form or as a micro constituent in Pearlite, is converted to a form of Graphite known as temper Carbon. opt Iron 1st stage 3.5h at 940C(1720), Irons w/ lower Si count or less opt nodule counts require approx 20h. Ferritic: stage 2 opt cooled to between 740 & 760(1360-1400) in 1-6h. Then cooled at a rate of 3-11/h(5-20/h) ; Carbon dissolved in Austenite is converted to Graphite and deposited on existing particles of tempered Carbon. ferritic malleable Irons require a two stage annealing cycle. first: converts primary carbides to temper Carbon. Second: converts Carbon dissolved in Austenite at the first-stage annealing temperature to temper Carbon and Ferrite.Consists of temper Carbon in a matrix of Ferrite. Contain a slight amount of controlled Iron. Pearlitic: 1st stage identical to that of Ferrite. Casting is slowly cooled to approx 870C. When the combined Carbon content of the Austenite is reduced to about .75% the castings are air cooled. Usually air blasted to avoid the formation of ferrite around the temper Carbon particles. Then, the castings are tempered to specs. Nodule Count: Low nodule count mechanical properties reduced from opt. and second stage annealing time is unnecessarily long. Excessive nodule leads to low hardenibility and nonuniform tempering. 80-150 nodules/mm of a photomicrograph magnified at 100x.

Ferritic Malleable Irons Microstructure consists of temper Carbon in Matrix of Ferrite. No Flake and no combined Carbon. Usually contain between 2.00-2.70% Graphite. Si and Mg reduce the elongation and increase the strength of the Ferrite.Mechanical properties: tensile prop: Lower than Pearlitic and Martensitic.fatigue limitmod of Elasticity: approx 170GPa(25E6) in tension, 150-170GPa(22E6 25E6)compression, 65-75GPa Tor(9.5E6 11E6)Fracture ToughnessCorrosion resistance: corrosion resistance increased by addition of Cu; such as chain links. Can be galvanized to provide additional protection.

Pearlitic & Martensitic Can be produced with a wide variety of properties, depending on heat treatment, alloying, and melting practices. Lower strength produced by air cooling and casting after first stage anneal, stronger liquid quench after first stage anneal. mechanical properties: vary in a substantially linear relationship with Birnell Hardness. > 207 HB properties of air quenched & tempered pearlitic = oil-quenched and tempered martensitic. This is due to the fact that both treatments yield a coarsening of matrix carbides and partial second stage graphitization. Oil quenched pearlitic has finer Pearlite, thus having more strength and being harder, as high as 321HB. Air quenched 255HB. tensile propertiesCompressive strength seldom determined because failure almost never occurs in the manner. Compressive yield strengths slightly higher than tensile due to decreased influence of graphite nodules and delayed onset of plastic deformation. Shear and Torsional Strength: 80% of tensile strength for ferritic, 70-90%for pearlitic. Modulus Elasticity in Tension pearlitic 176-193GPa(25.5E6 28.0Eg psi)Fracture Toughnessfatigue limits: tempered pearlitic 40-50% of tensile strength, temp martensitic 35-40% Excellent wear resistance due to structure.

Applications Essential parts in trains, frames, suspensions, and wheels. differential cases, bearing caps, steering-gear housings, spring hangers, universal-joint yokes, automatic transmission parts, rocker arms, disc brake calipers, wheel hubs, ect...a) Driveline yokes, b) Connecting rods, c) Diesel pumps, d) Steering-gear housing.

Sample Analysis and IdentificationPossible to identify and classify a sample of unknown cast iron.

Identification can be accomplished through the use of many known mechanical, chemical and structural properties of the cast iron alloys.

As part of our research, we attempted to examine and identify separate cast iron samples of unknown composition.

Sample Analysis and IdentificationWe obtained two samples of what was initially perceived to be cast iron. The first being a cooking pot, and the second a 90 pipe fitting.

Both samples were cut, mounted and polished in a bakelite round, for microstructure examination.

Followed by a Rockwell hardness test of each specimen.

Sample Analysis and IdentificationThe microstructure analysis was accomplished after polishing and etching (utilizing a 3% nitric acid solution in ethanol) to reveal the grains.

Initially it was believed that both samples were cast irons, and although the 90 pipe fitting appears to be cast iron, the microstructure of the pot handle yielded a different conclusion.

Microstructure AnalysisThe two micrographs are shown above, the 90 pipe fitting on the left, and the cooking pot handle on the right.

The Rockwell hardness was also measured for both samples, yielding a hardness of 157 HRB for the fitting, and 120 HRB for the pot handle.

Microstructure AnalysisUpon analysis of the microstructures, it became apparent that the cooking pot does not conform to any common form of cast iron.

Although the handle fractured in a manner very similar to that of cast iron, the microstructure shows little resemblance to those of cast iron.

Although further analysis would be required, one possibility is a cast steel material. This would account for the similarity of its fracture to that of cast iron.

Microstructure AnalysisThe 90 fitting shows indications of a dark graphite surrounded by lighter colored matrix. The graphite appears to be in the form of dark rosettes, while the matrix is a lighter color.

Upon comparison to the known structure of different types of cast irons, it can be seen that the microstructure of malleable iron most closely matches our sample.

To the right is a side-by-side comparison, with our sample on the top, and a known Malleable microstructure on the bottom.

Pipe Sample (150X mag)Malleable Iron (150X mag)

ConclusionCast Iron and historical significance

Used throughout history since its discovery, stepping stone to the development of modern technology (First Steam Engine)

Types of Cast Irons and microstructure

Grays, Whites, Ductile, Malleable

Applications

Automotive, Industrial, Household, Aeronautical, & Construction

Sample Analysis and Comparison for identification and Classification

Capability to analyze and compare microstructures to determine Cast Iron Type. And Hardness measurements to identify heat treatment,

ReferencesC.F. Walton, Gray and Ductile Iron Castings Handbook, Iron Founders Society, 1971, p 193C.F. Walton and T.J. Opar, Iron Castings Handbook, Iron Casting Society, 1981, p 235 & p 297-321Chrome-Moly White Cast Irons, Publication M 630, AMAX, Inc., 1986Davis, J.R., Cast Irons, ASM International, Materials Park, OH, 1996G. Lambert, Ed., Typical Microstructures of Cast Metals, 2nd ed., The Institute of British Foundrymen, 1996, p 47H.T. Angus, Physical and Engineering Properties of Cast Iron, BCIRA, 1960, p 21http://www.msm.cam.ac.uk/doitpoms/miclib/index.phphttp://www.matweb.com/search/GetSubcat.asphttp://www.wynndanzur.com/glossary.htm