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

MATERIALS IN DESIGN OF MACHINE PARTS PROPERTIES OF CAST IRONS SUMMARY

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.

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.

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.

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 89-114 100-124 110-135 124-147 137-160 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

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.

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.

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.

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.

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.

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.