White Cast Iron

 White cast iron derives its name from the fact that its fracture surface has a white or silvery appearance.

WHITE CAST IRON

1. What is Meant by White Cast Iron?

✓ White cast iron derives its name from the fact that its fracture surface has a white or silvery appearance.

✓ White iron has all the carbon in the combined form as cementite (i.e., iron carbide) in a pearlitic matrix. When the rate of cooling is fast, nearly all the carbon in a cast iron exists as cementite.

✓ Composition: The typical composition of a white cast iron is given below:

Carbon - 1.8 to 3%

Silicon - 0.5 to 1.9%

Manganese - 0.25 to 0.8%

Phosphorus - 0.05 to 0.2%

Sulphur - 0.10 to 0.30%

Remaining is iron.

2. Microstructure of White Cast Iron

✓ Since cementite is caused by quick cooling of molten iron, white cast iron is very hard and brittle.

✓ It is used only in applications where hardness and wear resistance are important. e.g., grinding and crushing machinery. 

✓ Most white cast irons are hypo-eutectic. A typical microstructure of a white cast iron is shown in Fig.3.6.

Note

Grey cast iron will form a white cast iron structure when rapidly cooled from the casting temperature. In this case, the cast iron is termed as 'chilled' iron.

3. Characteristics of White Cast Iron 

The important properties of white cast iron are given below:

1. White cast iron is very hard and brittle.

2. It possess high abrasion resistance.

3. It has a high tensile strength and a low compressive strength.

4. Since it is hard, it cannot be machined.

5. White iron castings can be made in sand moulds.

4. Applications of White Cast Iron

1. White cast iron is used as a raw material in the production of malleable cast iron.

2. The typical applications of white cast irons include rolls, wear plates, pump linings, balls, etc.

3. It is also used for inferior casting and in places where hard coating is required as in the outer surface of car wheels.

Malleable Cast Iron

 Malleable iron is a cast iron that has been heat treated so that it has significant ductility and malleability.↑

MALLEABLE CAST IRON

1. What is Malleable Cast Iron?

✓ Malleable iron is a cast iron that has been heat treated so that it has significant ductility and malleability.↑

✓ Composition: The composition of a typical malleable cast iron is given below:

Carbon - 2.0 to 3.0%

Silicon - 0.6 to 1.3%

Manganese - 0.2 to 0.6%

Phosphorus - 0.15%

Silicon - 0.10%

Remaining is iron

2. Microstructure of Malleable Cast Iron

✓ Malleable iron is produced by heat treating unalloyed (2.5% C, 1.5% Si) white iron. During the heat treatment process, the cementite in the white cast iron structure breaks down into ferrite and graphite clumps (or nodules). This graphite nodules, also called tempered carbon, appears like poncorn. This rounded graphite shape permits a good combination of strength and ductility.

† For the meaning of these terms, refer Unit 5, Section 5.1.

✓ The structure of a typical malleable iron is shown in Fig.3.7.

3. Types of Malleable Irons

Two types of malleable irons, depending on the type of heat treatment cycle used to produce, are:

1. Ferritic malleable iron, and

2. Pearlitic malleable iron.

1. Ferritic Malleable Cast Iron

✓ The white iron castings are heated beyond the upper critical temperature and held for a prolonged period of time so that carbon in the cementite converts to graphite. Subsequent low cooling through the eutectoid reaction results in a ferritic matrix (Fig.3.7). The cast iron so obtained is termed as ferritic malleable cast iron.

✓ The ferritic malleable cast iron has good toughness compared with that of other cast irons.

2. Pearlitic Malleable Cast Iron

✓ When the white cast iron is cooled from temperatures higher than a upper critical temperature more rapidly through the eutectoid transformation range, the carbon in the austenite will not have enough time to form additional graphite but is retained in the pearlite matrix. The irons so produced are called pearlitic malleable cast irons.

✓ Pearlitic malleable cast iron is characterised by higher strength and lower ductility than ferritic malleable cast iron.

4. Designation of Malleable Cast Iron

✓ The designation system for malleable cast iron given by ASTM, is a five-digit number.

✓ The first three digits represents the minimum yield strength in psi of the iron, and the last two digits represents the percent of elongation.

✓ For example, a grade 32510 malleable cast iron has a minimum yield strength of 32.5 × 103 (≈ 224 MPa) psi and 10% elonga- tion; and a grade 35018 has a minimum yield strength of 35 × 103 psi (≈ 242 MPa) and 18% elongation.

✓ Table 3.13 presents the typical compositions, and mechanical properties of some malleable cast irons.

5. Characteristics of Malleable Cast Iron

The important properties of malleable cast iron are given below:

1. The malleable cast iron possess good ductility and malleability properties than grey cast iron.

2. It exhibits high yield strength and tensile strength.

3. It is not brittle as grey cast iron.

4. It has high Young's modulus and low coefficient of thermal expansion.

5. It exhibits excellent impact strength and fatigue strength.

6. It has good wear resistance and vibration damping capacity.

7. It also has excellent machinability.

6. Applications of Malleable Cast Iron

✓ Malleable cast irons are widely used in the automobile industries, because of their combination of castability, shock- resistance, and good machinability. Typical components include brake-shoes, pedals, levers, wheel-hubs, axle-housings, connecting rods, transmission gears, and door hinges.

✓ They are also suitable for the manufacture of thin sections which require high ductility. Typical components include pipe fittings, parts for agricultural machinery, switchgear equipment, and fittings for bicycle and motorcycle frames.

Spheroidal Graphite or Nodular Cast Iron (or Ductile Cast Iron)

 Spheroidal graphite (SG) cast iron is also known as 'nodular iron' or as 'ductile iron'.

SPHEROIDAL GRAPHITE OR NODULAR CAST IRON (OR DUCTILE CAST IRON)

1. What is Meant by Spheroidal Graphite Cast Iron? 

✓ Spheroidal graphite (SG) cast iron is also known as 'nodular iron' or as 'ductile iron'.

✓ Composition: The composition of a typical SG cast iron is given below:

Carbon - 3.2 to 4%

Silicon. - 1.8 to 3%

Manganese - 0.2 to 0.5%

Phosphorus - 0.08% max

Sulphur - 0.01% max

Remaining is iron.

✓ The SG iron is the cast iron with nodular or spheroidal graphite. The nodules, also called spheroids, are about the same as those in malleable cast iron (temper carbon), except that they are more perfect spheres.

2. Microstructure of SG Cast Iron

✓ The nodular cast iron is produced by adding magnesium and/or cerium to molten cast iron (i.e., the grey iron before casting). The magnesium converts the graphite of cast iron from flake form into spheroidal or nodular form. The resulting alloy is called spheroidal or nodular cast iron.

✓ The typical microstructure of a spheroidal cast iron is shown in Fig.3.8.

✓ The presence of spheroidal graphite improves the ductility, strength, fracture toughness, and other mechanical properties.

✓ Ductile cast iron derives its name from the fact that its ductility is increased (by 20).

✓ Addition of magnesium gives good results and hence it is widely used. Magnesium is usually added in the form of a master alloy such as ferro-silicon-magnesium or nickel-magnesium alloy.

3. Designation of SG Iron

✓ The ASTM has designated SG irons by a six or seven digit number. The first two or three digits stand for the minimum tensile strength in psi, the middle two digits for the minimum yield strength in psi, and the last two digits for percent elongation in a tensile test.

✓ For example, a grade 60-40-18 SG cast iron has a minimum tensile strength of 60 × 103 psi, a minimum yield strength of 40 × 103 psi, and 18% elongation.

✓ Table 3.14 presents the typical compositions, and mechanical properties of some malleable cast irons.

4. Characteristics of SG Cast Iron

The important properties of the SG cast iron are given below:

1. S.G. cast iron has excellent ductility, tensile, and yield strengths than grey and malleable cast irons.

2. It has good toughness than the grey cast iron.

3. It has good fatigue strength.

4. It exhibits good impact strength.

5. It possesses good hardness and high modulus of elasticity.

6. It has corrosion resistance similar to that of grey iron.

7. It possesses excellent castability and wear resistance.

8. It has ability to resist oxidation at high temperatures.

9. It has good machinability.

Thus, one can say that ductile iron has mechanical characteristics approaching those of steel and hence it behaves like steel.

5. Applications of SG Cast Iron

The typical applications of SG cast iron include valves, pump bodies, crank shafts, gears, pinions, rollers, rocker arms, flanges, pipe fittings, power transmission equipments, earth inoving machineries, and other machine components.

Alloy Cast Iron

 The cast irons discussed so far are called plain cast irons, in the sense that the above cast irons contain only small amounts of impurities.

ALLOY CAST IRON

1. What are Alloy Cast Irons?

✓ The cast irons discussed so far are called plain cast irons, in the sense that the above cast irons contain only small amounts of impurities.

✓ The alloy cast irons, like alloy steels can be produced by adding alloying elements like Ni, Cr, Mo, Cu, Si, and Mn.

✓ In general, the effects of alloying elements on the properties of cast iron are similar to the effects of same elements on the properties of steel.

✓ Purpose: Alloy cast irons have been produced to give high- strength materials, hard and abrasion-resistant materials, corrosion resistant irons, and irons for high-temperature service.

2. Effects of Alloying Elements in Cast Iron

Table 3.15 presents some of the alloying elements and their primary effects on the cast iron.

3. Properties and Applications of Alloy Cast Irons

Table 3.16 presents compositions, typical mechanical properties, and applications of some of the commonly used alloy cast irons.

Non-Ferrous Materials

All the metallic elements other than iron are referred to as non- ferrous materials.

NON-FERROUS MATERIALS

1. Introduction

✓ All the metallic elements other than iron are referred to as non- ferrous materials.

✓ In other words, non-ferrous materials are those metals which contain elements other than iron as their chief constituent.

✓ Even though non-ferrous materials are not produced in as great tonnages and are more costly than ferrous materials (steel and cast iron), the non-ferrous materials are widely employed in current engineering industries due to the following characteristics↑:

1. Lighter in weight.

2. Higher electrical and thermal conductivity.

3. Better resistance to corrosion.

4. Ease of fabrication (casting, rolling, forging, welding, and machining).

5. Colour.

✓ Various non-ferrous materials: Of all the non-ferrous materials, the important non-ferrous materials used in engineering practice are :

1. Copper,

2. Aluminium,

3. Lead,

4. Magnesium,

5. Nickel,

6. Tin,

7. Titanium, and

8. Zinc.

Now, we shall discuss about copper and its alloys, and aluminium and its alloys, in detail, in the following sections.

† These characteristics are the distinct limitations of the ferrous materials. 

Copper

 Copper↑ is one of the oldest and the most widely used non-ferrous metals in industry.

COPPER

Copper↑ is one of the oldest and the most widely used non-ferrous metals in industry.

1. Properties of Copper

The copper possesses the following properties :

1. Copper possesses very high electrical conductivity. In fact, in this respect it is second only to silver.

2. It also has very high thermal conductivity.

3. It exhibits excellent resistance to corrosion.

4. It is very soft, ductile and malleable.

5. It becomes brittle just below its melting point.

6. It can be worked in hot or cold condition, but it cannot be welded.

Table 3.17 presents the principal properties of pure copper.

† How is copper extracted?

Copper is extracted almost entirely from ores based on copper pyrites. Copper pyrites refers a mineral in which copper is chemically combined with iron and sulphur. First the copper ore is concentrated. Then it is converted into a copper sulphide-iron sulphide matte. Oxygen introduced to the matte converts the iron sulphide into iron oxide and the copper sulphide to an impure copper called blister copper. This blister copper is then purified to obtain pure copper.

The pure copper so obtained is of the order of 99% pure and it may be refined to higher purities by electrolytic, or fire refining techniques.

2. Applications of Copper

✔ Copper↑ is extensively used for manufacturing power cables, telephone cables, cables for computer networks, printed circuit boards, connectors, etc.

✔ Other grades of pure copper are used in the form of sheet for architectural cladding and for shaping into articles such as domestic water tanks and vessels used in the food and chemical industries, and in the form of tubing for heat exchangers.

✔ It is mainly used in the manufacture of important alloys such as brass and bronze.