1. DEPARTMENT OF MECHANICAL AND MANUFACTURING ENGINEERING
UNIVERSITY OF NAIROBI
DEPARTMENT OF MECHANICAL AND MANUFACTURING ENGINEERING
ENGINEERING DESIGN II
FME 461
PART 2
GO NYANGASI
November 2008

2. MATERIALS IN DESIGN OF MACHINE PARTS
PROPERTIES OF CAST IRONS
SUMMARY

3. SELECTION OF MATERIAL FOR MACHINE PARTS
MANUFACTURING FACTORS
Available methods for producing metal parts are:
Cutting;
Machining;
Welding;
Casting;
Forging;
Heat treatment;
Rolling;
Extrusion, e.t.c.
The choice of material for part must consider the intended manufacturing method.
AVAILABILITY
In practice, the factor of availability should be considered first.
COST
The cost of a machine part is made up of the direct cost of input (raw) material, and the cost of processing it.
The choice of manufacturing process must therefore consider this cost of manufacturing.

4. SELECTION OF MATERIAL FOR MACHINE PARTS
CAST IRON
Cast iron can be produced in three forms, namely:
(a) Grey cast iron;
(b) Malleable cast iron;
(c) Ductile (nodular) cast iron;
(d) White iron.
All the three forms of cast iron are alloys of iron and carbon, with carbon content by weight ranging from 2 % to 4 %.
Cast irons also contain a high amount of silicon, often in excess of 1 %. Other elements that occur in small quantities ( often less than 1 %) are Sulphur, Manganese, and Phosphorus.

5. SELECTION OF MATERIAL FOR MACHINE PARTS
GREY CAST IRON
Grey cast iron is obtained when casting conditions combine with the chemical composition to yield a product in which the carbon occurs in the form of flakes of graphite.
These thin flakes of graphite are distributed evenly through the ferrite and cause the appearance of the micro-structure to darken, hence the name grey cast iron.
Grey cast iron is widely used.
In most national standards, it is classified according to its tensile strength, as shown in Table 6.2 with the example from British Standard specifications.

6. SELECTION OF MATERIAL FOR MACHINE PARTS
GREY CAST IRON
Table 6.2:Mechanical properties of grey cast iron (Tensile strength corresponds to the BS Grade)
Mechanical Properties
Grade Designation (British Standard Specification)
150
180
220
260
300
350
400
Tensile strength (Mpa.)
Compressive strength (Mpa.)
587
663
766
868
970
1097
1225
Shear strength (Mpa.)
176
222
284
346
407
484
562
Endurance limit (Mpa.) 71 82 96
111
125
143
161
Young's modulus (Gpa.)
71-96
79-104
89-114
Modulus of rigidity (Gpa.)
29-40
32-42
36-45
40-48
43-51
48-55
53-58
Hardness (HB)
160
196
216
236
261
286

7. SELECTION OF MATERIAL FOR MACHINE PARTS
GREY CAST IRON
Other national standards apply a classification scheme similiar to that shown in Table 6.2.
For example, grey cast iron Grade FG 150 in the Indian standards is equivalent to Grade 150 in the British standards.

8. SELECTION OF MATERIAL FOR MACHINE PARTS
WHITE CAST IRON
White cast iron is a product in which the carbon is combined with iron to form Cementite ( a compound of iron Fe, and carbon C), with no free graphite present.
The product is obtained by a specified combination of casting process, and chemical composition of the melt.
Cementite is very hard, and white cast iron therefore also displays this property.
White cast iron is therefore very hard, brittle, and difficult to machine.

9. SELECTION OF MATERIAL FOR MACHINE PARTS
MALLEABLE IRON
Malleable iron is made by the heat treatment of white cast iron.
Malleable cast iron is obtained, when white cast iron, within a certain composition range, is annealed.
The annealing process breaks down the Cementite, and frees the carbon from the Cementite.
The free carbon then reconstitutes itself into graphite.
However, instead of the free graphite taking the form of flakes, as in grey cast iron, the annealing process causes it to form rosettes of free graphite.

10. SELECTION OF MATERIAL FOR MACHINE PARTS
MALLEABLE IRON
This transformation of graphite into form of rosettes changes the properties of the resultant material from hard and brittle, to softer and ductile.
The tensile strength of the material is also increased.
Malleable iron therefore has properties somewhat similar to low carbon steel, but with the added advantage of easier casting.
The significant properties are the combination of ductility, strength, and castability.
The heat treatment process required to transform white cast iron, into malleable iron is however long, taking a period of days.
The product is therefore expensive to produce.

11. SELECTION OF MATERIAL FOR MACHINE PARTS
DUCTILE IRON
The microstructure of Ductile iron is similar to that of malleable iron, to the extent that it also contains free graphite, although in nodular form.
The material is therefore sometimes referred to as nodular iron.
The difference between malleable and ductile iron is in the process of production.
Instead of the lengthy and expensive heat treatment given to white cast iron to yield malleable iron, ductile iron is produced in the as cast condition, and thereafter it is given a simpler heat treatment of 1 hour annealing.

12. SELECTION OF MATERIAL FOR MACHINE PARTS
DUCTILE IRON
The transformation of free graphite from flakes to nodules is achieved by innoculating the cast iron melt with cesium and magnesium.
These additives cause the carbon in the melt to form nodules of free graphite during cooling and solidification.
Ductile iron, in the as cast condition, therefore exhibits properties similiar to malleable iron, after the ductile iron is given a simpler heat treatment of 1 hour annealing.