Mechanical Technology Grade 12 Chapter 5 Materials

History of IRONIron has been used since prehistoric times. No one knows how humans discovered the use of iron or how they first learnt to extract iron from its ores.

It appears that humans were using iron as early as 4000 BC.

There is evidence that the Egyptians used iron tools as early as 3000 BC when they built the pyramids.

USES OF IRON It provided spears, arrowheads, axes, swords, daggers, maces, bayonets, guns and canons for attack, and helmets and shields for defence

It can be bent, stretched, twisted, folded, cast, riveted, welded, drilled and cut with precision

Giant projects like building bridges, ocean-going tankers and oilrigs at sea all use the tremendous strength of metals

Manufacture of steel :Iron ore

Iron is the most important element in steel and is mined in open-pit mines as a solid or powder

Most steel comprises at least 98% iron The other 2% is either carbon, silicon, sulphur, manganese, nickel, tungsten or other elements

: Pig iron the iron is separated from oxygen and other materials in the iron ore.

The molten iron is cast into solid slabs or blocks, called pigs and is stored for future use.

Manufacture of steel

: FurnaceCharged with coke and limestone that is ignited and used to extract iron from iron ore using a smelting process

:Smelting separates the iron from the oxygen known as reduction

IRON ORE MINING

IRON ORE

Extracting Iron

Iron oreSmelting is the most important method for extracting iron from the ore.

The ore is dumped into a blast furnace and heated with coke and limestone.

Oxygen escapes from the iron and combines with the carbon from the coke.

Other impurities from the iron ore and coke become trapped in the molten limestone.

Furnace Blast Furnace

a tall, round structure about 30 m high and 9 m in diameter

Cupola furnace a cylindrical blast furnace used in foundries for remelting iron or

other metals

- Open hearth Furnaces large, rectangular basins

Basic Oxygen Furnace large bottle-shaped container that holds about 80 tonnes of metal

Electric Furnace uses an electric current to reach the high temperature required for

melting

Blast Furnace

The blast furnace is charged before smelting begins. In charging, the blast furnace is filled with coke,

limestone, and iron ore and then ignited. Air is heated to 675 °C by smaller furnaces called stoves

and is forced in through the bottom of the blast furnace. The blast of hot air intensifies the burning of the charge

material. The temperature at the bottom of the furnace rises to well

above the melting point of iron, which is 1 535 °C.

Blast Furnace This high temperature causes chemical reactions to occur,

during which pure iron is released from the iron ore. The molten iron drops to the bottom of the blast furnace. The molten limestone traps the impurities from the iron ore and

coke. The mixture, called slag, floats on the top of the molten iron. The slag is then drawn off through a hole in the furnace called a

slag tap hole. The molten iron is drawn off near the bottom of the furnace and

is either used immediately for making steel or stored as pig iron.

Foundries make iron castings from re-melted pig iron.

Cupola Furnace A cupola furnace is similar to but smaller than a blast

furnace. Charges of pig iron, scrap iron, low-sulphur coke or

anthracite, and limestone go into the cupola furnace. Scrap steel is added to make certain kinds of cast iron. The iron is melted and poured into moulds usually made of

sand, in which it is allowed to solidify. This is a simple, convenient and relatively cheap process to

manufacture components of complicated shapes.

Cupola Furnace

Open-hearth Furnaces It is charged with limestone and steel scrap. Iron ore may also be added.Gas, oil or coal is burned as fuel, and hot air is directed over the charge in the furnace.

The temperature above the charge reaches about 1 650 °C and the charge melts.

Open-hearth FurnacesWhen the charge is nearly melted, molten pig iron from the blast furnace is added to the furnace.

Heating continues, and the impurities combine with the oxygen.

Some of the oxidised impurities bubble up through the molten metal as a gas.

Others float to the top and combine with the molten limestone to form slag

Open-hearth FurnacesAfter the impurities burn away, alloying elements are added to bring the steel to the required composition.

The steel is then drawn from the furnace into a ladle and poured into tallmoulds to formingots

Electric furnaceHigh voltage causes electricity to arc between the

carbon electrodes within the furnace.The electric furnace gives the operator precise

control of both the furnace atmosphere and the amount of alloying elements added during the process.

It is the only furnace that can remove all the sulfur from steel.

Tool steels, high-speed steels and other speciality steels are produced in electric furnaces.

Electric furnace

Electric furnaceThe finished steel from the furnace is poured into tall, rectangular moulds.

The steel solidifies in the moulds to form ingots weighing 10 tonnes.

When the mould is lifted off, the red-hot ingot is lowered into a heated pit called a soaking pit.

Ingots in the soaking pit stay hot while they wait to enter the rolling mill.

Basic Oxygen Furnace It is charged with molten pig iron.A water-cooled pipe called a lance is inserted into the furnace.

Pure oxygen is forced through the lance into the metal.

The oxygen combines with the contaminants and removes them from the iron.

Using this process takes about only one hour to convert 80 tonnes of iron into steel.

Basic Oxygen Furnace

AssessmentDiscuss the following topics:

1. How can you decrease noise pollution in urban areas? (Hint: Think, for instance, of revving car engines and loud radios.)

How can you improve sanitation, and thus health, in rural areas? (Hint: Think of VIP latrines [toilets], the function of bacteria in these toilets, and preventing ground water from being contaminated.)

2. What are the conditions of mine workers and why are certain diseases prevalent amongst them.

Properties of Metals Strength refers to the material’s ability to withstand

forces that are applied to it, without breaking, bending, shattering or deforming in any way.

Elasticity refers to the material’s ability to absorb forces and flex in different directions and return to its original shape when the load is removed.

Plasticity refers to the material’s ability to change in shape permanently – it is the reverse of elasticity.

Properties of Metals Ductility refers to the material’s ability to change

shape by stretching it along its length, or to be drawn into wire form.

Malleability refers to the material’s ability to be reshaped in all directions without cracking. Lead is a malleable material but lacks ductility because of low tensile strength.

Brittleness refers to the material’s behaviour when fractures occur with little or no deformation. Glass is a classic example of a material with this property.

Properties of Metals Toughness refers to the material’s ability to withstand

shock loads and remain intact after continual bending in opposite directions.

Softness is the opposite property to hardness. Soft materials may be easily shaped by filing, drilling or machining in a lathe, milling machine or shaping machine.

Stiffness is the ability to withstand bending. Flexibility refers to metals which remain bent after a

bending force has been removed.

Properties of Metals Hardness refers to the material’s ability to resist penetration, scratching, abrasion, indentation and wear. Unfortunately the harder carbon steel tools are made, the more brittle they become, so some hardness must be sacrificed for toughness in the tempering process.

Production of Metal Cast iron

Cast iron is produced in a cupola furnace in much the same way as the iron ore is smelted.

SteelDifferent processes produce different kinds of steel, each process requiring a special furnace.

Steel-making furnaces include open-hearth furnaces, basic oxygen furnaces and electric furnaces.

Cast IronCast iron is an alloy of iron and carbon with

, small amounts of manganese silicon, sulfur and phosphorus.

Cast iron is brittle and relatively weak in tension with low tensile strength and poor shock resistance, but strong in compression and is easily machined.

Cast iron is not ductile and cannot be bent without fracturing.

The main advantage of cast iron is that it is easily cast into various shapes.

Cast Iron It absorbs vibrations well, making it suitable for supporting machine tools.

Cast iron is used in the manufacturing of machine beds, marking-off tables, machine tables and internal combustion engines as well as in the production of pistons, piston rings and cylinders.

Covers on road drains are usually made from cast iron.

Carbon is present in cast iron, in the form of graphite.

Cast IronThe carbon flakes act as a lubricant, enabling the cast iron to be machined dry.

Drilling or tapping of cast iron components is fairly easy and no lubricant is required.

There is, however, a hard skin in which some of the moulding sand may still be present.

There are five types of cast iron, depending on the structure of the carbon in the iron.

These are grey cast iron, white cast iron, ductile cast iron, malleable cast iron and high-alloy cast iron

Ferrous Metals Ferrous metals contain iron as their parent metal. For instance, in iron, carbon is the most important supplement only present in small amounts of ‒approximately 0,05% to 1,7%, and seldom exceeding 1,5%.

The presence of carbon in steel causes big changes in the nature of the metal, and also determines the hardness of the metal.

Ferrous Metals If the carbon content is increased:

Greater hardness is obtained. Tensile strength is increased. Ductility is decreased. Welding ability is decreased.

There are three classes of plain carbon steels. low-carbon steel, medium-carbon steel and high-carbon steel.

Plain carbon steels - ( )Low carbon steel mild steel

commonly known as soft, mild or machinery steel Used where ductility and softness are important and a high tensile strength is required.

carbon content of between 0,15% and 0,3%. used for operations such as cold bending and riveting

easy to press into a new shape, machine, weld or forge

Low-carbon steel (mild steel) May be worked hot or cold cannot be hardened by

heating and , -quenching but can be case hardened Products like rivets, nuts, bolts, nails, washers, chains,

machine parts, wire fence, forged parts and shafting can be made from this type of steel.

available in sheets of varying thickness, squares, bar form with hexagon, round, wire, plates or flat sections in a ‘black’ or ‘bright’ form.

Medium-carbon steel Has a carbon content between 0,3% and 0,75% Is less ductile, harder and has greater tensile strength than low-

carbon steel. Its hardness and strength can be increased by quenching the

metal while it is red hot in water or oil. Also has better machining qualities and is suitable for many

general engineering purposes where the stresses applied are greater than could be withstood by mild steel.

These steels are used for shafts, rails, connecting rods, car axles, spindles, gears, heavy forgings and other machine parts requiring medium strength and wear resisting surfaces.

High-carbon steel High-carbon steel is frequently known as tool steel,

with a carbon content ranging between 0,75% and 1,7%.

High-carbon steel has a higher tensile strength and hardness than steels in the lower carbon range.

It responds readily to heat treatment and is used for most cutting tools especially with alloys after being hardened and tempered.

High-carbon steels are also used for chisels, files drills, reamers, taps, hammers and crowbars.

Alloying ElementsThese are mixed with metals:

to improve its mechanical properties so as to permit higher tempering temperature while maintaining high strength and improving ductility

to improve mechanical properties at low or elevated temperature

to increase strength and toughness to increase resistance to high temperatures to secure greater hardness for wear resistance to provide high impact resistance to secure better machinability.

Alloying ElementsAlloying can also lower the melting point of the metal, increase the resistance to corrosion and rust and change the colour and structure of the metal.

Alloying may also reduce the cost of a metal.

Ferrous Alloying ElementsThe most commonly used elements are:

Chromium (Cr)Vanadium (V)Manganese (Mn)Nickel (Ni)Tungsten (W)Molybdenum (Mo)

Chromium (Cr) Is essentially a hardening agent. Chrome steel is also known as stainless steel. Frequently used with nickel as a toughening element to

produce superior mechanical properties. Steels containing chromium are noted for wear and

abrasion resistance. Chrome steels are used in machine parts, races for

bearings, ball bearings, gears, journals, shafts, dies, coil springs, gauges, nuts and bolts, and flat springs.

Does not hold size as accurately as manganese steels

Vanadium (V) Improves the elasticity, strength and fatigue-resistance of the steel

Has a more powerful effect upon the properties of steel than any other element and gives: increased hardnesssecondary hardening upon tempering increased hardness at elevated temperatures.

Manganese (Mn) Normally present in all commercial steels It is essential to steel production

It is necessary not only in the melting process but also in rolling and other processing methods

In hot forging, the action of manganese on sulphur improves the hot-working characteristics

Manganese steels have a greater impact and yield strength than plain carbon steels

Manganese (Mn) It lowers the temperature to which the steel must be

heated for hardening Easily cast into any shape, although the cast condition

is weak and brittle Heat treatment can give this steel great wearing power

with much ductility. This makes it useful in steel crusher jaws, and

rammers for crushing ore.

Nickel (Ni) When added to steel nickel increases the ductility,

strength, hardness and toughness of the metal. Nickel steels are used for machine parts subject to

repeated shock and stress. They are easily heat-treated because nickel lowers the

critical cooling rate. This critical cooling rate is necessary to produce hardening

by quenching. These steels are used for axles, crankshafts, special gears,

scientific and measuring instruments, marine shafting and parts for earthmoving equipment

Tungsten (W)

One of the principal alloying elements found in many alloy tool steels is tungsten.

When added to steel it increases the strength and toughness at high temperatures and makes a dense fine grain structure in steel.

When added to high-carbon steel it is used for high-speed cutting tools, dies, shear blades and exhaust valves.

Molybdenum (Mo) Added to steel to improve the heat-treatment

properties It increases the hardness in steel, resists softening

upon heating and prevents steel from becoming brittle when tempered.

Machine parts such as propeller shafts and transmission shafts, bolts, differential gears, coil springs, stainless steel, roller bearings and leaf springs are made from this type of steel.

Non-ferrous MetalsNon-ferrous metals are those metals which do not contain iron

These are metals such as such as copper, tin, lead, zinc, aluminium and antinomy.

These metals may be mixed to give us the various alloys, which are of great importance.

Copper (Cu) Copper is red in colour and is tough, ductile and malleable. Copper is a pure metal. It bends and stretches without fracture. It is an excellent conductor of heat and electricity. It is usually drawn into wire. Pure copper is difficult to cast but has a very high tensile

strength when cold drawn. It is used for cables, switchboard parts, electrical bolts and

nuts, busbars, telephone wires, soldering irons, electrical wiring, tubing for water supply and sometimes for roofing.

Tin (Sn)Tin is a silvery-white shiny metal with a bluish shade.

It is corrosion resistant, soft and malleable, and is a poor conductor of electricity.

Tin is used in soft solder, the canning industry and the cladding of steel sheeting.

It is also used in brasses and bronzes.Tin provides a protective coating in copper wires and is the basis of white metal bearings.

Tin (Sn)

Lead (Pb) Lead is a soft, bluish-grey coloured metal. It is malleable, ductile, and tough and has very low

tensile strength. Lead has a very low melting point. It is a pure metal, which bends and stretches easily. It is often added to other metals to make them free-

cutting. It is used for soft solder, bullets, lead cables,

plumbing, on roofs, and as plates in car batteries

Lead (Pb)

Zinc (Zn)Zinc is a bluish-white colour and is hard, brittle and malleable.

Zinc is a pure metal which casts well and resists corrosion.

Zinc is seldom used alone but is alloyed with other metals to make brass and bronze.

It is used as a coating (galvanising) on steel sheets, water tanks and wire.

Aluminium (Al) - ( ) Aluminium bearing ore bauxite is ground into a powder and processed chemically to produce an , oxide alumina.

Aluminium has a bluish-white colour, is fairly hard, extremely light and resistant to corrosion.

It is a pure or base metal. It is the lightest of the commonly used metals.

Aluminium (Al)

A cast aluminium engine block

Aluminium (Al) It is too soft to use in its pure state but is alloyed with

copper, magnesium and manganese. It is widely used for many components. Aluminium is impossible to solder by the usual

methods. It is non-magnetic and a far better conductor of

electricity than copper. It is used for cooking utensils, foil (often called silver

paper) and electricity conductors.

AntimonyAntimony has a bluish-white colour and has a scaly, crystalline structure.

It is very brittle and is used only in alloys such as pewter, solder and anti-friction metals.

Non-ferrous Alloys Brass Bronze White metal Duralumin

Brass Brass is an alloy of copper and zinc, available in many

varying proportions of the two metals. Brass turns well, is easily cast and resists corrosion. The higher the zinc content, the lower the melting point,

which results in lower malleability and ductility. The lower the zinc content, the better the corrosion

resistance, and the higher the strength and ductility of the metal.

BrassBrass is easily machined.Brass is usually tougher than bronze and produces a stringy chip when machined.

Brass is widely used for its resistance to corrosion and also for the manufacture of condenser parts, tubes, rods and sheets.

It is also used in bolts and nuts, gears, bushes, electrical components, taps and other water fittings.

Bronze Bronze is found in many combinations of copper and

other metals, but copper and tin are its main elements. Bronze is sometimes alloyed with zinc, lead or

phosphorous, which is used in the production of phosphor bronze.

Bronze is usually harder than brass, and is easily machined with sharp tools.

The chip produced is often granular. Bronze is used for valves, valve seats, bearings and

gear wheels.

White metalWhite metal is an alloy with either a lead or tin base.

White metal is also known as Babbitt metal.The tin-based white metal is used in heavy duty bearings to withstand greater pressures and speed whilst the lead-based metals are used under less exacting conditions.

White metal is used for bearings and to reduce friction.

DuraluminDuralumin is an aluminium alloy, which contains magnesium, manganese, copper, and silicon in small percentages.

It is a light metal with a high tensile strength and good resistance to corrosion, even in seawater.

Duralumin is used in the manufacturing of bars, sheets, and rivets and in automobile and aircraft parts.

Identification of Metals All metals are normally marked or colour coded on the

ends If the marking is cut off and the piece of metal is

separated from its proper storage rack, it is very difficult to determine the carbon content and alloy group.

start cutting from the unmarked end and leave the marked end intact.

Identification of Metals By a process of elimination, they can determine which

of the several steel types in the workshop is most comparable to the sample.

These methods of workshop testing include: Visual test Scratch test Spark test File test Sound test Machinability test

Visual test Heat scale or black mill scale is found on all hot rolled

steels, that is low-carbon, medium-carbon, high-carbon and alloyed steels.

Cold-finished steel usually has a metallic lustre. Ground and polished steel have a shiny finish. Chromium, nickel and stainless steel which is austenitic

and non-magnetic, usually have a white appearance. When grey cast iron fractures, it appears dark grey and will

smear your finger with a grey graphite smudge when touched.

When white cast iron fractures it appears silvery or white.

Scratch testBe sure that all scale and other surface impurities have been removed before scratch testing.

Simply scratch one sample with another and the softer sample will be marked.

A variation of this method is to strike similar edges of two samples together.The one receiving the deeper indentation is the softer of the two.

Spark testTests for carbon content in many steels.Always wear safety goggles or a face shield.When held against the grinding wheel, the metal

tested will display a particular spark pattern depending on the carbon content.

Adjust the grinding wheel so that the sparks will fly outward and downward, and away from you.

Use a coarse grit wheel which has been freshly dressed to remove contaminants.

Spark test- : High carbon steel

short, very white or light yellow carrier lines with considerable forking with many star-like bursts

Spark test - : Low carbon steel

straight carrier lines with a yellowish colour with a very small amount of branching and very little carbon burst

Spark test : Cast iron

short carrier lines with many bursts, which are red near the grinder andorange-yellow farther out; considerable pressure is required on cast iron to make sparks

File testFiles can establish the relative hardness between two samples, as in the scratch test.

This method, however, requires skill.Take care not to damage the file, since filing on hard materials may ruin the file.

Testing should be done on the tip or near the edge

Sound testThe metal type can also be determined by the sound it makes when it is tapped with a hammer or when it is dropped on the floor.

If the sound is loud and clear, the metal is a high-carbon steel (hard) but if it is a dull sound, the metal is a low-carbon steel (soft).

Machinability testMachinability can be ‘sample-tested’. For example, two unknown samples identical in appearance and size can be cut in a machine tool, using the same speed and feed for both.

The ease of cutting should be compared and the chips observed for heating colour and curl.

Composites Nowadays there is an ever-growing number of synthetic

materials available on the market. These materials become plastic above certain

temperatures and while plastic, they can be squeezed into dies and moulds to give them the required shape which is retained on cooling.

These materials hardly ever show signs of plastic properties in their finished state.

Two main types of plastic materials are thermoplastics and thermosetting plastics (also referred to as thermosets).

Thermoplastics These plastics can be re-heated and therefore shaped

in various ways. They cannot be used at temperatures much above

100 °C although they harden again on cooling. They tend to be tougher than thermosetting plastic but

not as firm. This material can be recycled. It is tough, of low density, low cost and can be formed

in intricate shapes with ease

Thermoplastics Some thermoplastics are transparent, for example

celluloid and Perspex, and can be coloured by adding pigment.

Nylon is one of the best-known and earliest plastics and is used for a variety of purposes including gear wheels and pulleys.

Polyvinyl chloride (PVC) is a member of this group and is a flexible, rubber-like substance which makes a dull sound when dropped.

It is commonly used for insulating electrical cables.

Thermosetting plasticsOnce these plastics set they cannot be re-heated to soften, shape and mould.

They are firm, hard and relatively fragile.They are very durable and of great strength.Bakelite falls within this category.Thermosetting plastics are mostly used for electrical equipment and components, melamine dinnerware, connectors and surface coating.

Reinforced plasticLaminated plastics like Tufnol® consist of a fibrous material such as woven cloth or paper saturated with phenolic resin.

The fabric sheets are laid up in a hydraulic press and squeezed and heated so that they become solid sheets, cylinders or shafts.