Iron Casting Properties

Iron castings are produced by a variety of molding methods and are available with a wide range of properties. Cast iron is a generic term that designates a family of metals. The different metals within this family are separated further into the following classifications:

Grey iron – Flake graphite provides gray iron with unique properties (such as excellent machinability) at hardness levels that produce superior wear-resistant characteristics, the ability to resist galling and excellent vibration damping. Grey iron is sometimes spelled gray iron.

Strength and hardness are sensitive to section thickness in gray iron castings. In thin sections, the material can be hard and difficult to machine. In heavy sections, its strength is reduced significantly. Because the modulus of elasticity for gray iron is higher in compression than it is in tension, the use of standard structural formulas results in a conservative design.

Ductile iron – An unusual combination of properties is obtained in ductile cast  iron because the graphite occurs as spheroids rather than as flakes. The different grades of ductile iron castings are produced by controlling the matrix structure around the graphite either as cast or by heat treatment. Only minor compositional differences (to promote the desired matrix microstructure) exist among the regular grades. Alloy additions may be made to assist in controlling the matrix structure as-cast or to provide response to heat treatment.

The high-strength grades can be quenched and tempered by austempering. Austempered ductile iron (ADI) provides twice the strength of conventional ductile iron at a given level of ductility. ADI can have strength in excess of 230 ksi (1,586 MPa); however, its modulus is 20% lower than steel with a comparable strength.

Ductile iron has the ability to be used as-cast and without heat treatments or other further refining. It has a tensile strength comparable to many steel alloys and a modulus of elasticity between that of gray iron and steel. As its name implies, it has a high degree of ductility. It can be cast in a wide range of casting sizes and section thickness; however, thinner sections may require annealing to obtain high ductility. Alloy additions may be needed to obtain the higher-strength grades in heavy sections.

Malleable ironIn malleable iron, the graphite occurs as irregularly shaped nodules called temper carbon because it is formed in the solid state during heat treatment. The iron is cast as a white iron of a suitable chemical composition to respond to the malleabilizing heat treatment.

Grey Iron Compressor Body

Grey Iron Compressor Body

Grey Iron Compressor Head

Grey Iron Compressor Head

Ductile Iron Link Pin

Ductile Iron Link Pin

Ductile Iron Pillow Block

Ductile Iron Pillow Block

Ductile Iron Handle

Ductile Iron Handle

 

Malleable iron is ideal for thin-sectioned components that require ductility. Ferritic malleable iron is produced to a lower strength range than pearlitic malleable iron but with higher ductility. It is the most machinable of cast irons, and it can be die-strengthened or coined to bring key dimensions to close tolerance limits. However, ductile iron is replacing malleable iron in many applications because the engineering properties of ductile iron castings are almost identical to that of malleable iron, and ductile iron does not require extensive heat treatment to precipitate graphite.

Grey Iron

Standard Specifications

• ASTM A48: gray iron castings
• ASTM A74: cast iron soil & pipe fittings
• ASTM A126: gray iron castings for valves, flanges & pipe fittings
• ASTM A159: automotive gray iron castings
• SAE J431: automotive gray iron castings
• ASTM A278 & ASME SA278: gray iron castings for pressure-containing parts
for temperatures up to 650F (343C)
• ASTM A319: gray iron castings for elevated temperatures for
non-pressure-containing parts
• ASTM A823: statically cast permanent mold castings
• ASTM A834: common requirements for iron castings for general industrial use

Characteristics

Several strength grades;
vibration damping; low
rate of thermal expansion &
resistance to thermal fatigue; lubrication retention; and good machinability.

Applications

heads; manifolds for internal combustion engines; gas burners; machine tool bases;
dimensionally stable tooling subjected to temperature variations, such as gear blanks & forming
die covers; cylinder liners for internal combustion engines; intake manifolds; soil pipes; counterweights; and enclosures & housings.

Ductile Iron

Standard Specifications

• ASTM A395 & ASME SA395: ferritic ductile iron pressure-retaining castings for
use at elevated temperatures
• ASTM A439: austenitic ductile iron castings
• ASTM A476 & ASME SA476: ductile iron castings for paper mill dryer rolls
• ASTM A536 & SAE J434: ductile iron castings
• ASTM A571 & ASME SA571: austenitic ductile iron castings for
pressure-containing parts suitable for low-temperature service
• ASTM A874: ferritic ductile iron castings suitable for low-temperature service
• ASTM A897: austempered ductile iron castings

Characteristics

Several grades for both
strength & ductility; high
strength, ductility & wear
resistance; contact fatigue
resistance; ability to
withstand thermal cycling;
and production of fracturecritical
components.

Applications

Steering knuckles; plow
shares; gears; automotive &
truck suspension components;
brake components; valves;
pumps; linkages; hydraulic
components; and wind
turbine housings.

Malleable Iron

Standard Specifications

• ASTM A47 & ASME SA47: ferritic malleable iron castings
• ASTM A197: cupola malleable iron
• ASTM A220: pearlitic malleable iron
• ASTM A338: malleable iron flanges, pipe fittings & valve parts for railroad,
marine & other heavy-duty service up to 650F (343C)
• ASTM A602 & SAE J158: automotive malleable iron castings

Characteristics

Soft & extremely ductile.

Applications

Chains; sprockets; tool parts & hardware; connecting rods; drive train & axle components;
and spring suspensions.

There are no generally accepted standards for the surface finish, machining allowances or dimensional tolerances. Although some production iron foundry facilities have established guidelines for their own capability in dimensional control, these controls are typically established through engineering based on the requirements of the application and the needs of the end user.

Material Properties vs. Casting Processes

The properties of all cast metals are influenced by the manner in which they solidify and cool. The individual design of a casting (the molding process, the way the molten metal is introduced into the cavity and the pouring temperature) determines the rate of cooling in the various parts of an iron casting. The cooling rate in any particular section factors heavily into the mechanical properties of the iron.

Types of Specifications

When mechanical properties are important, the most common procedure to qualify gray or ductile iron castings is to use a standard test bar poured separately with the specified lot of general engineered castings. Most specifications of the American Society for Testing and Materials (ASTM) apply this method to qualify the iron used to pour the castings. The actual properties of the metal in the casting will depend upon its characteristics and the cooling rate of the metal in its various sections. In some applications, the finished component is tested in the manner in which it will be used. For example, pressure-containing parts can be hydraulically proof tested. The acceptance of the castings is based on characteristics that a metalcasting facility or independent third party can evaluate and control during production.

Gray Iron Properties

Gray iron’s high damping capacity, combined with its excellent machinability and high hardness, is unique to this material and makes it ideally suited for machine bases and supports, engine cylinder blocks and brake components. Excessive vibration causes inaccuracies in precision machinery and excessive wear on gear teeth and bearings. The damping capacity of gray iron is considerably greater than that of steel and other iron types. For example, if gray iron, CGI and ductile iron have a similar composition, the relative damping capacity of gray iron is 1, CGI is 0.35 and ductile iron is 0.14. The damping capacity of gray iron is about 20-25 times higher than steel. For comparison, aluminum’s damping capacity is one-tenth that of steel.

Gray iron’s compressive strength is typically three to four times more than its tensile strength. The lack of graphite-associated volume changes allows for a similar Poisson’s ratio to other engineering metals but different tension properties. Poisson’s ratio remains constant at 0.25 over a large compressive stress range and increases at higher stress levels.

To classify gray iron in accordance to its tensile strength, ASTM Standard A48 and Society of Automotive Engineers (SAE) Standard J431 provide the best details. The two specifications approach the task from different standpoints, but the concept essentially remains the same. For example, the number in a Class 30 gray iron refers to the minimum tensile strength in ksi. In ASTM A48, a standard size test bar is added to the class. Class 30A indicates that the iron must have a minimum 30 ksi (207 MPa) tensile strength in an “A” bar (0.875-in. as-cast diameter).

In SAE Standard J431, tensile strength is not required, but hardness and a minimum tensile strength to hardness ratio are required. The class then is identified as a grade. A Class 30B iron for ASTM A48 would be comparable to a grade G3000 in SAE Standard J431. The other gray iron specifications build off of these two primary specifications.

Table 3. Property Comparisons for Gray Iron Classes

Property

Class 25
(as-cast)

Class 30
(as-cast)

Class 30
(annealed)

Class 35
(as-cast)

Class 40
(as-cast)

Brinell Hardness

187

207

109

212

235

Tensile Strength

29.9 ksi (206 MPa)

33.7 ksi (232 MPa)

20.6 ksi (142 MPa)

34.8 ksi (240 MPa)

41.9 ksi (289 MPa)

Modulus of Elasticity

16.6 Msi (114 GPa)

17.0 Msi (117 GPa)

14.5 Msi (100 GPa)

18.0 Msi (124 GPa)

18.2 Msi (126 GPa)

Tensile Poisson’s Ratio

0.29

0.19

0.21

0.22

0.24

Compression Poisson’s Ratio

0.27

0.28

0.26

0.28

0.23

Compression-to-Tensile Strength Ratio

3.68

3.84

4.05

3.63

3.71

Ductile Iron Properties

Five grades of ductile iron are classified by their tensile properties in ASTM Standard A536 (Table 4). SAE Standard J434c (for automotive castings and similar applications) identifies these five grades of ductile iron only by Brinell hardness. However, the appropriate microstructure for the indicated hardness also is a requirement.

Table 4. Property Comparisons for Ductile Iron Grades (ASTM A536)

Grade

Heat
Treatment

Tensile
Strength

Yield
Strength

% Elongation
(min. 2 in.)

Brinell
Hardness

Poisson’s
Ratio

Tensile Elastic
Modulus

60-40-18

1

60,000 psi (413 MPa)

40,000 psi
(276 MPa)

18

130-170

0.28

24.5 Msi
(169 GPa)

65-45-12

2

65,000 psi (448 MPa)

45,000 psi
(310 MPa)

12

150-220

0.28

24.5 Msi
(169 GPa)

80-50-06

2

80,000 psi (551 MPa)

55,000 psi
(379 MPa)

6

170-250

0.28

24.5 Msi
(169 GPa)

100-70-03

3

100,000 psi (689 MPa)

70,000 psi
(482 MPa)

3

241-300

0.28

25.5 Msi
(176 GPa)

120-90-02

4

120,000 psi (827 MPa)

90,000 psi
(620 MPa)

2

240-300

0.28

25.5 Msi
(176 Gpa)

ADI Properties

Austempering of ductile iron castings at higher temperatures produces ADI with lower strength and hardness, as well as higher ductility and toughness. The property combinations available for ADI list five standard grades in ASTM A897. These specifications give the minimum tensile and impact levels along with typical Brinell hardness values (Table 5).

Compared to the best forged steel, ASTM 897 Grade 5 ADI Charpy V-notch impact toughness is low, but the fracture toughness is approximately equal. Ausferrite microstructures respond to shot peening, fillet rolling and grinding and lead to an increase in bending fatigue strength with favorable compressive residual stresses impacted on the surfaces.

 

Table 5. Property Comparisons for ADI Grades (ASTM A897)

Grade

Tensile
Strength

Yield
Strength

%
Elongation

Brinell
Hardness

Fracture Toughness
(ksi-sq.in./MPa-sq. m)

1

140 ksi (966 MPa)

110 ksi (759 MPa)

11

302

100/109

2

165 ksi (1,139 MPa)

130 ksi (897 MPa)

10

340

78/85

3

190 ksi (1,311 MPa)

160 ksi (1,104 MPa)

7

387

55/60

4

220 ksi (1,518 MPa)

180 ksi (1,242 MPa)

5

418

48/52

5

240 ksi (1,656 MPa)

210 ksi (1,449 MPa)

3

460

40/44

Machining Cast Iron

To efficiently machine cast irons, the appropriate tool material of the correct grade and shape must be selected. Coated carbides, particularly those with aluminum oxide exteriors, are effective in production machining applications. New tool materials for economical machining of iron castings are in a continual state of improvement and development.