11

METALS AND ALLOYS

11.1 INTRODUCTION

We use metals daily in one form or the other. Metals are also used for various engineering works such as structural members, roofing materials, reinforcing material, pipes, tanks, etc. Among the metals, iron is the most commonly used one. It is also available in abundance. It is stated that about 5% of iron is present in the crust of the earth.

Iron is extracted from solid naturally occurring mineral aggregates called ores. About five important iron ores are used to extract the metal iron.

Metals are grouped under two broad groups, viz.,

1. Ferrous metals and
2. Non-ferrous metals.

Iron is the main constituent of ferrous metals. The important ferrous metals are, viz., pig iron, cast-iron, wrought iron and steel. The non-ferrous metals, viz., aluminium, copper, etc., do not contain iron as their main constituent. Alloy is an intimate compound of two or more metals.

11.2 FERROUS METALS

11.2.1 Iron Ores

Iron ores are compounds of iron with non-metallic elements. They also contain impurities such as carbon, manganese, phosphorous, silicon and sulphur. The iron ores are extracted from the earth by mining operations.

While selecting iron ores, the following aspects should be considered:

1. The iron ores should be rich in metallic iron content and have less impurities.
2. The location of mines should not have constraints which may lead to complications during quarrying operations.
3. The location of mines should possibly be linked to the surrounding towns by suitable transport facilities.
4. The impurities should not contaminate the rich iron ore. However, there are some substances that increase the value of lean iron ore, which are to be considered.
5. Any initial treatment made on the iron ore should be in a position to increase the output of iron.

Important varieties of iron ores are haematite, limonite, magnetite, pyrite and siderite.

11.2.2 Pig Iron

Pig iron is the basic material from which wrought iron and steel are manufactured. They are extracted from iron ores, viz., magnetite, haematite, limonite and siderite. The percentage of iron content, and their chemical formula are given in Table 11.1. Pig iron is extracted from these ores in a continuous working furnace called the blast furnace. The product obtained from the blast furnace is crude and is an impure form of iron.

1. Manufacture of Pig Iron

Three stages are involved in the manufacture of pig iron, viz.,

1. Dressing
2. Calcination and roasting and
3. Smelting

Dressing is the process in which the iron ores obtained from mines are crushed into pieces of about 25 cm diameter sizes by rock crushers. If the ores contain impurities like clay, loam and other earthy materials they are removed by washing the ores.

After dressing, the ores are calcinated and roasted. Calcination is the process where the ores are heated in the presence of air to oxidise them. By this action carbon dioxide and water are removed. Roasting the ores enables the dissipation of the volatile parts such as sulphur.

Smelting is the process of melting so as to separate metal from the ore. It is carried out in a special furnace called the blast furnace. The blast furnaces of the old type had a capacity of producing 15,000 KN of pig iron per day, whereas the more modern types have a capacity of 25,000 KN.

2. Properties of Pig Iron

The properties of pig iron are as follows:

1. Pig iron can be hardened, but tempering is not possible.
2. It is not possible to magnetise pig iron.
3. Riveting or welding is not possible.
4. Unlike steel it does not rust.
5. Bending is not possible.
6. It is neither ductile nor malleable.
7. It has high compressive strength with less tensile strength.
8. It can be melted easily.
9. It is hard and brittle.

11.2.3 Wrought Iron

It is the oldest form of iron made by man. It was originally produced from iron ore in a huge fire. This gives a very impure iron which needs further refining by mechanical working. Wrought iron contains high-purity iron and iron silicate. It contains a very low percentage of carbon, and the iron silicate is distributed throughout the base metal. When fractured it shows a woody or fibrous appearance.

11.2.4 Cast Iron

Cast iron is an alloy of iron and carbon with the content of carbon being more than 2%. In addition, it contains impurities such as manganese, phosphorous, silicon and sulphur. The presence of manganese makes it brittle when it is more than 0.75%. The fluidity of cast iron is increased by the presence of phosphorous. Silicon decreases shrinkage and ensures softer and better casting when it is less than 2.5%.

1. Manufacture of Cast Iron

Cast iron is manufactured by re-melting pig iron with coke and limestone. This process of re-melting is done in a furnace called the cupola furnace. Except for the size, it is almost the same as that of blast furnace. To some extent, the impurities in the pig iron are removed by this process and comparatively pure iron is obtained in the molten stage from the bottom of the furnace. The slag is removed at regular intervals from the top of the cast iron. The molten cast iron is led into the moulds called cast-iron castings.

2. Properties of Cast Iron

As an engineering material, cast iron has the following properties:

1. It has high compressive strength.
2. It has high machinable qualities based on the composition.
3. It has a fairly low melting point.
4. It has high fluidity which helps in making good casting impressions.
5. It has fairly good corrosion resistance.
6. It lacks plasticity and is not suitable for forging.
7. It is hard and brittle but not ductile.
8. It can not be magnetised.

11.2.5 Steel

Steel is a very ductile alloy consisting of iron and carbon. Based on the percentage of carbon content, steel is classified as follows:

1. Low-carbon or mild steel: carbon content up to 0.25%
2. Medium-carbon or medium hard steel: carbon content from 0.25 to 0.70%
3. High-carbon or hard steel: carbon content from 0.70 to 1.5%

1. Manufacturing of Steel

There are five processes by which steel is manufactured:

(i) Bessemer Process

A Bessemer convertor (a furnace) is used in this process, which is wide at the bottom and narrow at the top and rotates about the horizontal axis. Pig iron is filled in the convertor and hot blast air is forced into it. The impurities get oxidised by the blast air and a reddish-yellow flame is seen at the mouth of the convertor, and the flame diminishes gradually. Now the supply of air is stopped and adequate quantities of materials like ferromanganese is added to make steel of desired quality. The blast is again started for a few minutes and the molten metal is poured into moulds to form blocks called ingots.

(ii) Cementation Process

Here a dome-shaped furnace called a cementation furnace is used. In this process the pig iron is first converted into pure wrought iron. After adjusting the carbon content, steel is obtained.

(iii) Open-hearth Process

Here a mixture of pre-heated air and coal gas is used as a fuel. The steel produced by this process is homogeneous and is of high reliability.

(iv) Electric Process

Here the raw material is melted using electricity. The other procedure is the same as that of the Bessemer process and more control of temperature is feasible here. It is a quick process and special-grade steels are produced by this process.

(v) Duplex Process

It is a combination of the Bessemer and the basic open-hearth process.

Figure 11.1 shows a flow chart for the production of steel.

Figure 11.1 Flow diagram for the production of steel.

2. Uses of Steel

1. Mild steel is used to manufacture tools, machine parts, tubes, sheet metal, tin plate, structural steel, etc.
2. Medium hard steel is used to manufacture machine/engine components, boiler plates, rails, hammers, pressing dies, structural steel, agricultural implements, aero-engine, cylinders, springs, etc.
3. Hard steel is used in the manufacture of plates, cutlery, springs, miner’s drills, heavy tools, sledge hammers, axes, planning and slotting machines, lathes, etc.

3. Properties of Mild Steel

The properties of mild steel are as follows:

1. It has a high tensile strength.
2. It has a high elastic property reflected by a high modulus of rigidity.
3. It develops an effective bond with concrete.
4. Because of a good bond, it is capable of transferring stresses.
5. As it has an almost equal coefficient of thermal expansion and contraction, there is no room for the development of thermal stresses.
6. It is readily available and is comparatively cheap.

4. Properties of Medium Hard Steel

The properties of medium hard steel are as follows:

1. It has higher strength than mild steel.
2. It can be hardened to a certain extent.
3. It is tougher and harder than mild steel.
4. It can not easily be forged or welded.
5. It has a high resistance to shocks and vibrations.

5. Properties of Hard Steel

The properties of hard steel are as follows:

1. It has a granular structure.
2. It is very hard and is tougher than other steels.
3. It is possible to magnetise permanently.
4. It has very high compressive and tensile strengths.
5. It can be tempered and hardened.
6. It has a very high resistance to shocks and vibrations.
7. It is difficult to forge.

6. Suitability of Steel

The suitability of steel for different works as per Indian Standards is given in Table 11.2.

11.2.6 Alloy Steels

To obtain special properties, some elements such as nickel, chromium, manganese, etc., are added to the carbon steel. Steels thus formed are called alloy steels.

The addition of alloying elements is effected to attain one or more of the following properties:

1. To improve the fine-grain size content of steel.
2. To improve case-hardening properties.
3. To improve the elastic properties of steel.
4. To improve corrosion and fatigue resistance.
5. To improve hardness and toughness.
6. To improve machinability.
7. To improve tensile strength.

Alloying materials have the following properties:

1. Nickel: It increases toughness and improves responses to heat treatment. When added in large quantities, it provides special electrical and magnetic properties.
2. Chromium: Helps to provide stainless properties in steel. It is used in making tool steels and electrical plates.
3. Manganese: Improves heat treatment properties and is used in high-speed tool steels.
4. Tungsten: It retains the hardness property of steel and toughness at a high temperature. It is used exhaustively in the manufacture of tools, dies, valves, magnets, etc.
5. Silicon: It provides high electrical resistance and magnetic permeability in electrical machinery.
6. Copper: It improves corrosive resistance even when added in minute quantities.

Some of the alloy steels are discussed below:

1. Silicon Steel

Silicon steel has high electrical properties and has very low magnetic hysteresis when present in about 3.5%.

Silicon steel is used in the manufacture of different materials in combination with other metals with a lesser percentage. The various materials made include springs, internal combustion engines, laminations of electrical machines, etc.

2. Nickel Steel

The percentage of nickel added varies from 2 to 40. Elastic limit and toughness are increased with about 4% of nickel. The tensile strength is highly increased with 20% nickel. To make the steel non-corrosive and non-magnetic, the nickel percentage should be about 27%. If the nickel content is 3.5% and about 0.3% of carbon, the nickel steel formed is highly ductile and such a material is used for long span bridges.

3. Manganese Steel

The steel attains very high tensile strength with toughness and non-magnetic properties when the presence of manganese is around 12.5%. Manganese steel is weldable and forgeable. Heat-treated cast manganese steel in bar form is extremely ductile that it can bent when cold without fracturing. Manganese steel has a high percentage of elongation. It is used in the jaws of crushers, rails, helmets, shields, etc.

4. Tungsten Steel

The hardening and magnetic properties are improved with the addition of tungsten to steel. The addition of a high percentage of tungsten makes the steel retain its hardness even at a high temperature. It is used for making permanent magnet and high-speed cutting tools.

5. Stainless Steel

Stainless steel is of chromium-based steel. They form two groups, viz., plain chromium and high-chromium low-nickel steel and chromium nickel steel. The first type is used for dies, valves and cutlery and can be heat-treated. These steels can be heat-treated chromium nickel steels, which are non-magnetic and can not be hardened. They have high resistance to corrosion. Further, they may be cold or hot worked, pressed, welded, barbed or soldered.

11.3 NON-FERROUS METALS

Metals which do not contain any percentage of iron is generally called non-ferrous metals. The important metals that fall under the non-ferrous metal category are discussed below.

11.3.1 Aluminium

Aluminium is abundantly available in the ore called bauxite. Bauxite is hydrated aluminium oxide. The manufacturing process involves the bauxite first being purified and then dissolving in molten cryolite. From this solution, aluminium is separated by electrolysis at about 900°C.

Aluminium has the following properties:

1. It offers high resistance to corrosion.
2. It is ductile and malleable.
3. It is quite strong.
4. It has tensile strength varying from 95 to 160 MN/m².
5. It easily forms alloys with iron, copper, zinc and other metals.
6. It is electron positive.
7. It is unaffected under normal atmospheric conditions.

11.3.2 Copper

Copper is one of the metals directly available in the metallic state. It is found in minerals like copper glance malachite and azurite It is extracted by different methods from ores. Depending on the process by which copper is extracted, they are designated as:

1. Tough-pitch copper
2. Oxygen-free copper
3. Deoxidised copper
4. Arsenic copper
5. Free-cutting copper

Copper has the following properties:

1. It is one of the best conductors of heat and electricity.
2. It is highly resistant to corrosion by liquids.
3. It forms important alloys, viz., bronze and gun metal.
4. Its tensile strength varies from 300 to
5. It is strongly attacked by nitric acid.

11.3.3 Lead

The lead ore is galena or galenite consists of lead sulphide. It is widely distributed. In the manufacturing process, the roasted ore, silica, coke, metallic iron and lime are smelted together in a blast furnace. Lead oxide and sulphate react with iron to form ferrous oxide and sulphate, resulting in the formation of lead. Ferrous oxide in combination with silica forms slag.

Further, the heavy metal formed at the bottom of the furnace is further oxidised in Bessemer’s converter to get pure lead after the removal of impurities.

The properties of lead are as follows:

1. It has high metallic lustre when freshly cut.
2. Among the common metals, it is the softest and heaviest metal.
3. It is highly malleable and can be formed into foils.
4. It is least tenacious.
5. In hot conditions it may be extruded by a hydraulic press into tubes, rods and wires.

11.3.4 Tin

The chief source of tin is tinstone (or cassiterite) SnO₂. Tin is a bright shining white metal. It is soft and malleable and can be made into the form of thin foil. As it has high resistance to corrosion, it is used for coating purposes on metals and alloys. It is particularly used in low melting point alloys as a substitute for bismuth. It is also widely used for moisture-proof packing.

11.3.5 Zinc

Common ores of zinc are zincite (ZnO), calamine (ZnCO₃) and zinc blende (ZnS). It is a bluish grey non-ferrous metal. It becomes brittle at a high temperature of 200°C and can also be powdered at this temperature. It is used in the form of a sheet as a corrosion-resistant surface. Further, it is also used as a protective coating on iron and steel in the form of a galvanized or sprayed surface. It is also used in making brass and other alloys and in electric cells.

11.3.6 Magnesium

It occurs in nature in several minerals such as magnesite (MgCO₃), dolomite (CaCO₃, MgCO₃), kieserite (MgSO₄ · H₂O) and carnalite (MgCl₂ · KCl · 6H₂O). The metal is prepared by the chloride or oxide process. The oxide process is not in use anymore.

In the chloride process, a melted mixture of chlorides of sodium, potassium and magnesium is electrolysed. To prevent decomposition of magnesium chloride during the heating process, alkali chloride should be present. When the magnesium rises during liberation, it rises to the surface and has to be ladled out. If there is a need for pure magnesium, it is prepared by distillation.

The properties of magnesium are as follows:

1. It is the lightest material.
2. It can be readily machined.
3. It hardens very rapidly with cold working.
4. It is easily attacked by weak acids and even by saline solution.
5. It ignites easily.
6. It can be welded.

11.3.7 Nickel

Nickel is found combined with iron sulphides in deposits.

The properties of nickel are as follows:

1. It is like silver, which can take high polish.
2. Its hardness is equivalent to that of steel.
3. It is malleable with a small amount of carbon.
4. It is resistant against many acids except nitric acid.
5. It retains its surface upon exposure to the atmosphere.
6. It is somewhat less ductile than soft steel. But with a small amount of magnesium, ductility improves remarkably.

11.4 NON-FERROUS ALLOYS

Non-ferrous alloys may be defined as a coherent non-ferrous metallic mass produced by combining two or more non-ferrous metals. Some important non-ferrous alloys are discussed below.

11.4.1 Copper Alloys

Two important principal classes of copper alloys are brass and bronze. Brass is primarily an alloy of copper and zinc, whereas bronze is a combination of copper and tin.

Brasses are formed with 5–45% of zinc and copper. Brasses possess excellent mechanical properties. They are corrosion resistant and are readily machinable.

The mechanical properties of brass and the corrosion resistance of the brasses are further improved by the addition of one or more non-ferrous metals. Thus, manganese brass, iron brass, tin brass, lead brass and aluminium brass are modified brasses.

Bronze has varied physical properties. An increase in the percentage of tin increases the tensile strength of bronze. Bronze is most ductile when it contains about 5% of tin, but it decreases with increase in the percentage of tin. Like brass, bronze in modified forms is available as phosphor bronze, silicon bronze and aluminium bronze.

11.4.2 Aluminium Alloys

Principal elements that are alloyed with pure aluminium to improve its tensile strength and hardness are copper, silicon, manganese, zinc, magnesium and nickel.

Copper is the main hardening element for aluminium alloy by the addition of a small percentage of magnesium, which improves the hardness and strength after heat treatment. Similarly, the addition of a small percentage of manganese and nickel to an aluminium alloy further improves the qualities.

Silicon is the next alloying element which in combination with magnesium forms a hard compound that is largely responsible for the hardness.

The addition of manganese prevents the formation of coarse crystalline structure during heat treatment.

Popular aluminium alloys are duralumin, hindalium, magnalium and Y-alloy.

11.4.3 Magnesium Alloys

Magnesium alloys comprise 3–10% aluminium, 1–3.8% zinc and 0.4% manganese. The two important magnesium alloys are dow metal and electron metal. Dow metal comprises 9.1% magnesium and 9% aluminium. Electron metal, a trade name of magnesium, is a base alloy that contains 4% zinc and small percentages of copper, iron and silicon.

The tensile strength of magnesium alloy is low. Heat treatment increases the resistance to shock. On exposure to the atmosphere, it develops a dark oxide film which resists corrosion. Magnesium alloy, dow metal, can be forged, welded and drawn as wires.

11.4.4 Nickel Alloys

Important nickel alloys are iconel and monel metals. The composition of iconel is 75% nickel, 15% chromium and 9% iron. Monel metal comprises two-thirds nickel, one-thirds copper with a small percentage of elements like iron, silicon, manganese and carbon.

Iconel can be cast, forged, rolled and cold drawn. It has brittle behaviour when the temperature is between 650 and 950°C. Both at ordinary and high temperatures, it has high corrosion resistance. It can be soft soldered. It is used to manufacture springs that can withstand high temperature.

Monel metal is superior to brass or bronze in resisting corrosion. At ordinary temperature it is magnetic and becomes non-magnetic at temperatures between 100 and 150°C. It can be heat treated when aluminium and beryllium are added. Because of its peculiar flow properties, it can be welded only with special techniques. Because of its excellent corrosion resistance characteristic, it is widely used for parts of water pumps, propellers, domestic water storage tanks and parts subjected to high temperatures like intense combustion engines.

1. Iron is extracted from solid naturally occurring mineral aggregates called ores.
2. Metals are grouped under two broad groups, viz., ferrous metals and non-ferrous metals.
3. Ferrous metals contain iron as their main constituent. The important ferrous metals are pig iron, cast iron, wrought iron and steel.
4. Non-ferrous metals, viz., aluminium, copper, etc., do not contain iron as their main constituent.
5. Alloy is an intimate compound of two or more metals.
6. Iron ores are compounds of iron with non-metallic elements. Important varieties of iron ores are haematite, limonite, magnetite, pyrite and siderite.
7. Pig iron is the basic material from which wrought iron and steel are manufactured.
8. Three stages involved in the manufacture of pig iron are dressing, calcination and roasting and smelting.
9. Dressing is the process in which the iron ores as obtained from mines are crushed.
10. Calcination is the process by which the ores are heated in the presence of air and are oxidised.
11. Roasting the ores enables the dissipation of the volatile parts such as sulphur.
12. Smelting is the process of melting so as to separate metal from the ore. It is carried out in a special furnace called a blast furnace.
13. Wrought iron contains high pure iron and iron silicate.
14. Cast iron is an alloy of iron and carbon, with the carbon content being more than 2%. In addition, it contains impurities such as manganese, phosphorous, silicon and sulphur.
15. Cast iron is manufactured by re-melting pig iron with coke and limestone. The re-melting is done in a furnace called the cupola furnace.
16. Steel is a very ductile alloy consisting of iron and carbon.
17. Based on the percentage of carbon content, steel is classified into low carbon or mild steel, medium carbon or medium hard steel and high carbon or hard steel.
18. Manufacturing of steel may be done by five processors, viz., Bessemer process, Cementation process, Open-hearth process, Electric process and Duplex process.
19. To obtain special properties, elements such as nickel, chromium, manganese, etc., are added to the carbon steel. The steel thus formed is called alloy steel.
20. Metals that do not contain any percentage of iron are generally called non-ferrous metals. The important non-ferrous metals are aluminium, copper, lead, tin, zinc, magnesium and nickel.
21. Non-ferrous alloys may be defined as a coherent non-ferrous metallic mass produced by combining two or more non-ferrous metals. Non-ferrous alloys are brass, bronze, aluminium alloys (duralumin, hindalium, magnalium), magnesium alloys and nickel alloys.

1. Define an ore. Discuss the important varieties of iron ore.
2. What is the composition of pig iron? How is pig iron produced?
3. What is cast iron? How is cast iron produced?
4. How is steel classified based on the carbon content?
5. Discuss various methods of manufacturing steel.
6. Draw a flow diagram explaining the production of steel.
7. Enumerate the uses of steel.
8. Differentiate between the properties of medium hard steel and hard steel.
9. What are the various properties of mild steel?
10. State the effects of adding nickel, chromium and manganese to steel. How are the products named?
11. What are non-ferrous metals?
12. Explain the properties of aluminium.
13. What is copper? Explain their properties.
14. What is the chief source of tin, and what are the uses of tin?
15. Differentiate between brass and bronze.
16. What are aluminium alloys?
17. Discuss the two forms of nickel alloys.
18. Compare the properties of Cast iron, wrought iron and steel and enumerate their uses.
19. Mention the properties and uses of

    (i) Aluminium, lead and copper

    (ii) Alloys of Copper.

20. Explain the properties of magnesium.