Cast irons
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Abstract:
In this paper is presentation flake graphite iron. Finds use due to: its cheapness and
ease of machining; low-melting temperature (1140-1200°C); ability to take good casting
impressions; wear resistance; high damping capacity; a reasonable tensile strength of
108-340 MPa associated with a very high compressive strength, making it very suitable for
applications requiring rigidity and resistance to wear. The different types vary from
grey iron which is machinable to either mottled or white iron which is not easily
machinable. The white irons of suitable composition can be annealed to give malleable
cast iron. During the last thirty years much development work has taken place and it has
been found worth while to add even expensive elements to the cheap metal because vastly
improved properties result. The new irons formed by alloying or by special melting and
casting methods are becoming competitors to steel.
Flake graphite iron finds use due to:
1. its cheapness and ease of machining;
2. low-melting temperature (1140-1200°C);
3. ability to take good casting impressions;
4. wear resistance;
5. high damping capacity;
6. a reasonable tensile strength of 108-340 MPa associated with a
very high compressive strength, making it very suitable for
applications requiring rigidity and resistance to wear.
The different types vary from grey iron which is machinable to either
mottled or white iron which is not easily machinable. The white irons
of suitable composition can be annealed to give malleable cast iron.
During the last thirty years much development work has taken place and
it has been found worth while to add even expensive elements to the
cheap metal because vastly improved properties result. The new irons
formed by alloying or by special melting and casting methods are
becoming competitors to steel.
The various irons can be classified as shown in Fig. 1 based on the
form of graphite and the type of matrix structure in which it is
embedded. The metallurgical structure, composition and section of the
casting largely govern the engineering properties. One of the
differences between cast iron and steel is the presence of a large
quantity of carbon, generally 2-4%, and frequently high silicon
contents. While carbon in ordinary steel exists as cementite (Fe3C), in
cast iron it occurs in two forms:
• stable form-graphite;
• unstable form-cementite, analysed as combined carbon.
CAST IRON
Grey machinable iron White, unmachinable iron no
graphite
Flake Graphite Spheroidal Graphite Pearlitic Martensitic
Ferritic Pearlitic Austenitic Martensitic
Malleable iron temper carbon
graphite
Ferritic Pearlitic
Blackheart Thin Whiteheart Whiteheart Special
Malleable
Figure 1. Classification of cast iron (Pearce)
Graphite is grey, soft, and occupies a large bulk, hence counteracting
shrinkage; while cementite is intensely hard, with a density of the
same order as iron. On the relative amounts, shape and the distribution
of these two forms of carbon largely depend the general properties of
the iron.
The factors mainly influencing the character of the carbon are:
1. The rate of cooling.
2. The chemical composition.
3. The presence of nuclei of graphite and other substances.
1. Rate of cooling. A high rate of cooling tends to prevent the
formation of graphite, hence maintains the iron in a hard, unmachinable
condition. If the casting consists of varying sections then the thin
ones will cool at a much greater rate than the thick. Consequently, the
slowly cooled sections will be grey and the rapidly cooled material
will be chilled. These points are illustrated in Fig. 2, which shows
the variation in hardness of a step casting.
Figure 2. The
relation between the
rate of cooling and
hardness as indicated
by sections of
varying thickness
2. The effect of chemical composition.
1. Carbon lowers the melting-point of the metal and produces more
graphite. Hence it favours, a soft, weak iron.
2. Silicon slightly strengthens the ferrite but raises the brittle
transition temperature, Indirectly, however, it acts as a
softener by increasing the tendency of the cementite to slip up
into graphite and ferrite. Fig. 3 shows the relation between the
carbon and silicon contents in producing the different irons for
one rate of cooling. It will be noted that either a high carbon
and low silicon or low carbon and high silicon content give grey
iron; the fracture can, therefore, be misleading as to analysis,
especially if the rate of cooling is not considered. The amounts
of silicon, giving the maximum values for various properties, are
also shown in Fig. 3. The percentage of silicon is varied
according to the thickness of the casting.
3. Sulphur and manganese. Sulphur can exist in iron, as either iron
sulphide, FeS, or manganese sulphide, MnS. Sulphur as FeS tends
to promote cementite producing a harder iron. When manganese is
added, MnS is formed which rapidly coalesces and rises to the top
of the melt. The first effect of the manganese is, therefore, to
cause the formation of graphite due to its effect on the sulphur.
The direct effect of manganese is to harden the iron, and this it
will do when it exists in amounts greater than that required to
combine with the sulphur-1 part sulphur to 1,72 part manganese.
4. Phosphorus has a little effect on the graphite-cementite ratio;
but renders the metal very fluid indirectly through the
production of a low-melting constituent, which is readily
recognised in the micro-structure (Fig. 4). In the production of
sound castings of heavy section, phosphorus should be reduced to
about 0,3% in order to avoid shrinkage porosity.
5. Trace elements not normally considered in routine analyses can
exert a profound influence upon the characteristics of cast iron.
Examples are 0,1% of aluminium graphitises, antimony embrittles,
lead, tellurium promotes carbide but reduces strength of iron;
0,003% of hydrogen can greatly affect soundness of castings and
tends to coarsen graphite. Nitrogen behaves as a carbide
stabiliser; oxygen has no specific effect.
Figure 3. Diagram indicating the
structures of iron resulting from
variation of silicon and carbon
contents
Figure 4. Common grey iron showing
ferrite (F), pearlite (P) and
phosphide eutectic (PH) (x250).
Ferrite is associated with the
graphite. Note banded structure in
the phosphide eutectic
The carbon equivalent value. From Fig. 5 it will be seen that the
eutectic E is at 4,3% carbon and irons with a greater carbon content
will (under suitable conditions) start freezing by throwing out kish
graphite of large size. With carbon contents progressively less than
4,3% normal graphite is formed in diminishing quantities until a
mottled or white iron range is reached. Naturally other elements,
especially silicon and phosphorus, affect the composition of the
eutectic point in a complex alloy and a carbon equivalent value is
suggested as an index which converts the amount of these elements into
carbon replacement values.
Figure 5. Iron-cementite equilibrium diagram
Carbon equivalent value (CE) = Total C% + 1/3 (Si% + P%)
For a given cooling rate the carbon equivalent value, therefore,
determines how close a given composition of iron is to the eutectic (CE
4,3) and therefore how much free graphite is likely to be present, and
consequently the probable strength in a given section: the carbon
equivalent value is also a useful guide to chilling tendency of a given
section, although it must be borne in mind that pouring temperature,
cooling rate and alloying elements have a marked influence