Chapter 2

Types of cast iron

Introduction

The first iron castings to be made were cast directly from the blast furnace. Liquid iron from a blast furnace contains around 4%C and up to 2%Si, together with other chemical elements derived from the ore and other constituents of the furnace charge. The presence of so much dissolved carbon etc. lowers the melt point of the iron from 1536°C (pure iron) to a eutectic temperature of about 1150°C (Fig. 2.1) so that blast furnace iron is fully liquid and highly fluid at temperatures around 1200°C. When the iron solidifies, most of the carbon is thrown out of solution in the form either of graphite or of iron carbide, Fe3C, depending on the composition of the iron, the rate of cooling from liquid to solid and the presence of nucleants.

If the carbon is precipitated as flake graphite, the casting is called 'grey iron', because the fractured surface has a dull grey appearance due to the presence of about 12% by volume of graphite. If the carbon precipitates as carbide, the casting is said to be 'white iron' because the fracture has a shiny white appearance. In the early days of cast iron technology, white iron was of little value, being extremely brittle and so hard that it was unmachinable. Grey iron, on the other hand, was soft and readily machined and although it had little ductility, it was less brittle than white iron.

Iron castings were made as long ago as 500 BC (in China) and from the 15th century in Europe, when the blast furnace was developed. The great merits of grey iron as a casting alloy, which still remain true today, are its low cost, its high fluidity at modest temperatures and the fact that it freezes with little volume change, since the volume expansion of the carbon precipitating as graphite compensates for the shrinkage of the liquid iron. This means that complex shapes can be cast without shrinkage defects. These factors, together with its free-machining properties, account for the continuing popularity of grey cast iron, which dominates world tonnages of casting production (Table 2.1).

Greater understanding of the effect of chemical composition and of nucleation of suitable forms of graphite through inoculation of liquid iron, has vastly improved the reliability of grey iron as an engineering material. Even so, the inherent lack of ductility due to the presence of so much graphite precipitated in flake form (Fig. 2.2) limits the applications to which grey iron can be put.

A malleable, or ductile form of cast iron was first made by casting 'white

24 Foseco Ferrous Foundryman's Handbook

1600

1200 0 o

E

800

400

6 + liquid

8

G + liquid ,o • z \ ~ + ..... liquid / Fe~C + liquid

/

- . . . . . . / - . . . . . C

j t . . . . . 6 - -

a + 7 / , " or ,-, ,,,,,," 7 + G

\ - - D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..1 3~

a + Fe3C or a + G

I I I

0 2 4 6 wt. %C

Fe - G system Fe - Fe3C system A B C D

( - - ) Fe-G 2.09 4.25 0.68 %C 1154 1154 739 °C

(---) Fe-Fe3C 2.12 4.31 6.68 0.76 %C 1148 1148 1226 727 °C

F igu re 2.1 The iron-carbon phase diagram. (From Elliott, R., Cast Iron Technology, 1988, Butterworth-Heinemann, reproduced by permission of the publishers.)

Table 2.1 Breakdown of iron casting tonnages 1996 (1000s tonnes)

Total iron Grey iron Ductile i r o n Malleable iron

Germany } France 6127 3669 (59.9%) 2368 (38.6%) 84 (1.37%) UK USA 10 314 6048 (58.6%) 4034 (39.1%) 232 (2.25%)

Data from CAEF report The European Foundry Industry 1996 US data from Modern Castings

iron' and then by a long heat treatment process, converting the iron carbide to graphite. Under the right conditions the graphite developed in discrete, roughly spherical aggregates (Fig. 2.3) so that the casting became ductile with elongation of 10% or more. The first malleable iron, 'whiteheart iron'

Types of cast iron 25

Figure 2.2 Random flake graphite, 4% picral, x lO0. (From BCIRA Broadsheet 138, reproduced by courtesy of CDC.)

was made by R6aumur in France in 1720. The more useful 'blackheart' malleable iron was developed in the USA by Boyden around 1830. Malleable cast iron became a widely used casting alloy wherever resistance to shock loading was required. It was particularly suitable for transmission components for railways and automotive applications.

A major new development occurred in the late 1940s with the discovery that iron having a nodular form of graphite could be cast directly from the

f

P

/

-, x i .

I ~ . ,

_ . . . . . .

Figure 2.3 Malleable cast iron, 4% picral, x 100. (From BCIRA Broadsheet 138, reproduced by courtesy of CDC.)

26 Foseco Ferrous Foundryman's Handbook

melt after treatment of liquid iron of suitable composition with magnesium. (Fig. 2.4). The use of 'spheroidal graphite' or 'nodular ' iron castings has since grown rapidly as the technology became understood and 'ductile iron', as it is now generally known, has gained a large and still growing, sector of total cast iron production (Table 2.1).

Figure 2.4 Nodular graphite, 4% picral x lO0. (From BClRA Broadsheet 138, reproduced by courtesy of CDC.)

The great hardness and abrasion resistance of white iron has also been exploited. The strength of white iron has been improved through alloying and heat treatment, and white iron castings are widely used in applications such as mineral processing, shot blasting etc. where the excellent wear resistance can be fully used.

Finally there are a number of special cast irons designed to have properties of heat resistance, or acid resistance etc. In the following chapters, each type of iron will be considered separately and its method of production described.

The mechanical properties of cast iron are derived mainly from the matrix and irons are frequently described in terms of their matrix structure, that is, ferritic or pearlitic:

Ferrite is a Fe-C solid solution in which some Si, Mn, Cu etc. may also dissolve. It is soft and has relatively low strength. Ferritic irons can be produced as-cast or by annealing. PearIite is a mixture of lamellae of ferrite and FeBC formed from austenite by a eutectoid reaction. It is relatively hard and the mechanical properties of a pearlitic iron are affected by the spacing of the pearlite lamellae, which is affected by the rate of cooling of the iron from the eutectoid temperature of around 730°C.

Types of cast iron 27

Ferrite-Pearlite mixed structures are often present in iron castings. Bainite is usually formed by an austempering heat treatment (normally on spheroidal graphite irons) and produces high tensile strength with toughness and good fatigue resistance. Austenite is retained when iron of high alloy (nickel and chromium) content cools. Heat and corrosion resistance are characteristics of austenitic irons.

Physical properties of cast irons

The physical properties of cast irons are affected by the amount and form of the graphite and the microstructure of the matrix. Tables 2.2, 2.3, 2.4, 2.5 and 2.6 show, respectively, the density, electrical resistivity, thermal expansion, specific heat capacity and thermal conductivity of cast irons. The figures in the tables should be regarded as approximate.

Table 2.2 Density of cast irons

Tensile strength 150 180 (N/mm 2) Density at 20°C 7.05 7.10 (g/cm 3)

Grade 350/22 Density at 20°C 7.10 (g/cm 3)

Grade Density at 20°C (g/cm 3)

Type

Density at 20°C (g/cm 3)

Grey iron

220 260 300 350 400

7.15 7 . 2 0 7 . 2 5 7 . 3 0 7.30

Ductile iron

400/12 7.10

500/7 600/3 700/2 7.10-7.17 7.17-7.20 7.20

Malleable iron

350 / 10 450 / 6 550 / 4 600 / 3 700 / 2 7.35 7.30 7.30 7.30 7.30

Other cast irons

White cast irons Unalloyed 15-30%Cr Ni-Cr 7.6-7.8 7.3-7.5 7.6-7.8

Austenitic Grey (Ni-hard) high-Si (6%) 7.4-7.6 6.9-7.2

28 Foseco Ferrous Foundryman's Handbook

Table 2.3 Electrical resist ivi ty of cast i rons

Tensile s t reng th 150 180 ( N / m m 2)

Resis t ivi ty at 20°C 0.80 0.78 ( m i c r o - o h m s . m a / m )

G r a d e 350/22 Resis t ivi ty at 20°C 0.50 (m ic ro -ohms .m2 /m)

G rade 350/10 Resis t ivi ty at 20°C 0.37 (m ic ro -ohms .m2 /m)

Grey iron

220 260 300 350 400

0.76 0.73 0.70 0.67 0.64

Duct i le iron

400/12 0.50

500 /7 600/3 700 /2 0.51 0.53 0.54

Malleable iron

450 /6 550 /4 0.40 0.40

600/3 700 /2 0.41 0.41

Table 2.4 Coefficient of linear thermal expansion for cast irons

Type of iron Typical coefficient of linear expansion for temperature ranges (10 -6 per °C)

20-100°C 20-200°C 20-300°C 20-400°C 20-500°C

Ferritic flake or nodular 11.2 11.9 12.5 13.0 13.4 Pearlitic flake or nodular 11.1 11.7 12.3 12.8 13.2 Ferritic malleable 12.0 12.5 12.9 13.3 13.7 Pearlitic malleable 11.7 12.2 12.7 13.1 13.5 White iron 8.1 9.5 10.6 11.6 12.5 14-22% Ni austenitic 16.1 17.3 18.3 19.1 19.6 36% Ni austenitic 4.7 7.0 9.2 10.9 12.1

Table 2.5 Specific hea t capaci ty of cast i rons

Typical mean values for grey, nodular and malleable irons, from room temperature to 1000°C

Mean value for each temperature range (J/kg.K)

20-100°C 20-200°C 20-300°C 20--400°C 20-500°C 20-600°C 20-700°C 20-800°C 20-900°C 20-1000°C 515 530 550 570 595 625 655 695 705 720

Typical mean values for grey, nodular and malleable irons, for 100 °C ranges

Mean value for each temperature range, (J/kg.K)

100-200°C 200-300°C 300-400°C 400-500°C 500-600°C 600-700°C 700-800°C 800-900°C 900-1000°C 540 585 635 690 765 820 995 750 850

Iron casting processes

T h e m a j o r i t y of r e p e t i t i o n i r o n c a s t i n g s a r e m a d e in g r e e n s a n d m o u l d s w i t h

r e s i n - b o n d e d co res . T h e C r o n i n g r e s i n s h e l l m o u l d i n g p r o c e s s is u s e d w h e r e

Types of cast iron 29

Table 2.6 Thermal conductivity of cast irons

Tensile strength 150 (N/mm 2) Thermal conductivity (W/m.K) 100°C 65.6 500°C 40.9

Grade Thermal conductivity (W/m.K) 100°C 500°C

Grade Thermal conductivity (W/m.K) 100°C 500°C

Grey iron

180 220 260 300 350 400

59.5 5 3 . 6 50.2 47.7 45.3 45.3 40.0 3 8 . 9 3 8 . 0 37.4 36.7 36.0

Ductile iron

350/22 400/12 500/7 600/3 700/2

40.2 38.5 36.0 32.9 29.8 36.0 35.0 33.5 31.6 29.8

Malleable iron

350/10 450/6 550/4 600/3 700/2

40.4 38.1 35.2 34.3 30.8 34.6 34.1 32.0 31.4 28.9

high precision and good surface finish are needed. The Lost Foam Process is also used for repetit ion castings. Castings made in smaller numbers are made in chemically bonded sand moulds.

Special sand processes such as Vacuum Mould ing and Full-Mould are used for certain iron castings and there are a few permanent mould (diecasting) foundries making iron castings, but the short die-life of only a few thousand components has restricted the use of ferrous diecasting.