Materials
Properties of engineering Heat-treatment
processes Manufacture of steel
Materials
Iron mine
Iron ore enrichment
Coke ovens Coal mines Limestone quarry Dust collector Blast furnace Molten iron Molten slag Stove
Manufacture of steel
Extracting metals from ores
Iron ore: Iron is the most important element in steel. Most steel comprises
at least 98% iron. The remaining two percent is carbon, silicon, sulfur, manganese, nickel, tungsten or other elements. Iron comes from iron ore, which exists in nature as a solid or as a powder. Iron ore varies in colour from red to yellow to black and is mostly mined in open-pit mines.
Pig iron: Once mined, iron ore is put into a blast furnace. Heat in the blast
furnace separates the iron from oxygen and other materials in the ore. The molten iron that comes out of the blast furnace is either used immediately or cast into solid slabs or blocks, called pigs, and stored for future use.
Figure 3.1: Producing iron
There are several methods for extracting iron from the ore. Smelting is the most important method. In smelting, a chemical process, reduction, separates the iron from the oxygen. 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.
DID YOU KNOW?
The name ‘pigs’ has an interesting history. When iron production started, a trough, called a ‘runner’, carried molten metal from the tapping hole on the side of the blast furnace. The runner sloped downward and had runners branching off from it. The side runners were called ‘sows’, after the name for a female pig. Moulds along the side runners received the molten iron like ‘rows of suckling piglets’.
A blast furnace is a tall, round structure about 30 m high and 9 m in diameter. 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. 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.
Figure 3.2: Smelting iron in a blast furnace
Hopper
Small bell
Larger bell
Stack
Refractory brick lining
Melting zone
Hot air supply from stoves Hearth
Iron tap hole Slag tap hole
Bosch Hot air supply
from stoves Steel casing 250 °C 550 °C 850 °C 1150 °C 1500 °C
Steel
Different 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.
Open-hearth process
Open-hearth furnaces are large, rectangular basins. To make steel, the open- hearth furnace 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.
When 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.
After the impurities have been burnt 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 tall moulds to form ingots.
Figure 3.3: An open-hearth furnace
Scrap metal Steel Charging ladle Funnel Charging machine Charging boxes Tap hole Slag thimble Ladle
Bessemer process
Sir Henry Bessemer’s breakthrough in the 1850s, that contaminants and pollutants could be removed from molten iron by their oxidation with an air blast through the hot molten metal, was a key step forward for industry in his time. The oxidation raises the temperature of the molten iron and keeps it in a molten state. The Bessemer process was named after its inventor and patentee, Sir Henry Bessemer. This process was the first, cheap, engineering manufacturing process for mass production of steel from pig iron.
The Bessemer converter is a large, pear-shaped container in which molten pig iron is converted to steel by the Bessemer process. The Bessemer converters’ refractory lining were silica or ganister (hard rock containing silica that can resist high temperatures) bricks and this was the beginning of acid steel making. The operation of the Bessemer converter is illustrated in figure 3.4.
Figure 3.4: The sequence of operations in the Bessemer converter
Mining and the environment
Mining is not an environment-friendly industry. It causes severe scarring of the sensitive environment, e.g. big hole of Kimberley, as well as air and noise pollution, dust, and contaminated soil and water, e.g. ground water. Once ground water is contaminated, it is almost impossible to clean it.
On the other hand, mining for iron ore, metals (semi- and precious
metals) provides work for an estimated 600 000 South Africans and people from neighbouring states. Because of the nature of their work and living conditions, HIV/AIDS and lung diseases are the most important illnesses affecting mine workers. Accidents in foundries, factories, mines and work places are a great concern for trade unions and the Department of Labour.
ganister
hard rock containing silica that can resist high temperatures
PAUSE FOR THOUGHT
Scrap vendors, e.g. people pushing shopping trolleys filled with scrap metal, contribute to recycling, reducing our carbon-
footprint, reducing our mining activity and add to our GDP (Gross domestic product).
Refractory lining
Charging of the Bessemer Blowing of the Bessemer Pouring of the Bessemer converter converter converter
The Department of Environmental Affairs endeavours to control air and water pollution, global warming, and the release of greenhouse gases such as CFCs, carbon dioxide, water vapour, and nitrous oxides and methane.
Many mining and industrial companies have become socially aware of the need to rehabilitate land, such as mine dumps, to minimise dust. They have planted trees and greened areas to decrease carbon dioxide levels in the air. The production of smokeless coke also helps. We can play a part by participating in Arbour Day, through tree planting and by remembering the three Rs (reduce, recycle and reuse).
Assessment
This is a group activity. Discuss the following topics in groups of five and nominate a spokesperson to report back to the class.
• 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.)
How can we preserve our natural resources for future generations? Asian countries import scrap metal from South Africa and export highly sophisticated products back to South Africa. How can we, as South Africans, reuse our scrap metals and turn them profitably into usable products that will reduce our carbon foot print?
Read up on the COP 17 conference that was held in Durban, South Africa, in December 2011. Formulate a report on how the outcome of the conference will benefit future generations of the world (global warming).
Properties of metals
In engineering, metals must meet certain requirements and must, therefore, have certain characteristics. It is vital for the most suitable material to be used for the job. Often this will be specified in the drawing from which we are working, but sometimes we have to decide what to use.
The following properties have to be considered:
• 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.
• Plasticity refers to the material’s ability to change shape permanently – it is the reverse of elasticity.
• 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.
• 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.
• Toughness refers to the material’s ability to withstand shock loads and
remain intact after continual bending in opposite directions. • Strength refers to the material’s ability to withstand forces that are
applied to it, without breaking, bending, shattering or deforming in any way.
• 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.
Methods of enhancing the properties of steel
In Grade 10 you learnt about different materials and their uses and composition, as well as how to identify these different materials. In this chapter we will discuss heat treatments of metals, quenching media, and look at the different kinds of heat treatment furnaces.
DID YOU KNOW?
Heat treatment is a word used to describe a process during which the mechanical and physical properties of a metal are changed by heating and then cooling it down again.
Heat treatment is the heating and cooling of metals (under controlled conditions) in their solid state so as to change their properties. All metals have a crystalline grain structure while in their solid state and the kind of grain structure determines their properties. The size of grains in steel depends upon a number of factors, of which the main one is the furnace treatment the steel has received.
To bring about the required grain structure and so produce the most wanted properties, the metals are heated and then cooled down in a number of ways. To refresh your memory, these properties (discussed in Grade 10) are strength, elasticity, plasticity, ductility, brittleness, toughness, hardness, softness, stiffness and flexibility, to name but a few.
Figure 3.5: Grain structure and changes to the grain structure
Assessment
This is an engineering activity. Use a drill press to drill holes into sample pieces of wood and metal. Which of the two is easier to drill through? Can the same drill bit be used on both materials?
History of heat treatment
Metals become hardened when suddenly cooled from a heated condition. People have known about the process of hardening for many centuries, although the exact origin of metal hardening cannot be determined.
During the fifteenth and sixteenth centuries, the practice of hardening metal became an art. A great deal of superstition and secrecy developed amongst the skilled workers who practised this art. Process secrets were handed down from generation to generation.
crystalline
having a chemical formation of a crystal or to resemble a crystal
Grain structure Before heating
After heatiing
Small grains
Even though skilled workers were involved in the process, they did not understand why the metal hardens. During the last century, scientists attempted to discover the reasons for metallic behaviour. They have developed the science of metallurgy.
Heat treatment is part of metallurgy. It changes the structure and grain of metals by applying heat.
Heat treatment and the environment
Care for the environment includes concerns about how industry
manufactures products and the effects that such products and manufacturing processes could have on the environment. Codes of practice provide
companies and manufacturers with guidelines and requirements for adopting an environmental policy and implementing an environmentally responsible system. These codes help companies and manufacturers set their own environmental objectives.
Steelmakers have to ensure that they minimise pollution from fumes and gases produced by the steelmaking process. Modern steel mills are very efficient at controlling pollution. The fumes that come from heating the metal are collected in hoods above the furnaces. Other gases are reused as fuel for heating.
In the heat-treatment department of a metal treatment plant, a safe working environment is crucial. Hot metal is very dangerous and great care must be taken when using or handling it. The working area should be well ventilated and provided with exhaust hoods, because the fumes given off by the heated steel and the cyanide used for case-hardening are very toxic and hazardous. Goggles must be worn when you are working on lead, cyanide, or nitrate pots. Always ensure that nothing damp or wet is introduced into a heating pot since an explosion could occur.
A leather apron and gloves must be worn when you are working with tongs and hot metal. Never pick up a workpiece with bare hands, unless you are sure that it is not hot. Always use correctly shaped tongs that are in good working order to pick up hot workpieces and do not leave hot tongs where other people may accidentally be burnt by them.
metallurgy
the science of the production, purification, properties of metals and their application
toxic
The heat-treatment process
The rate of heating is very important in any heat-treatment operation. Heat flows from the outside to the inside of the metal at a definite maximum rate. If metal is heated too fast, the outside of the part becomes hotter than the inside, and a uniform structure is very difficult to achieve.
All heat treatment processes involve heating and cooling metal according to a time-temperature cycle that includes the following three steps:
1. heating the metal slowly to a certain temperature to ensure a uniform temperature
2. soaking the metal
3. cooling the metal at a certain rate to room temperature.
The hardness that can be achieved from a specific treatment depends on the following three factors:
• workpiece size • quenching rate • carbon content.
Water is normally used as a quenching medium for low carbon and medium carbon steels. Rapid quenching is required to harden these steels. An ideal quenching medium for high carbon and alloy steels is oil. The quenching rate of oil is not as harsh as that of water. Where extreme cooling is needed,
brine is used.
The temperature at which steel is normally quenched for hardening is known as the hardening temperature.
The hardening temperature depends mostly on the carbon content of the steel. The highest degree of hardness achievable in steel by means of direct hardening is determined mainly by the carbon content. Steel with low carbon content will not respond very much to the hardening process. Carbon steels in general are considered shallow hardening steels. The hardening temperature is, as a rule, 10 °C to 38 °C above the critical
temperature.
soaking
holding the metal at this predetermined, elevated temperature for a certain period to ensure uniform penetration of heat DID YOU KNOW? Room temperature is 20 °C to 25 °C quenching to cool rapidly in a quenching medium brine salted water DID YOU KNOW?
Brine is the result of dissolving common rock salt in water. It is a very effective quenching medium as its ionic structure conducts heat very easily.
Chisels and punches can be heated on an electric hot plate until the required colour shows, after which it can be cooled in water.
Assessment
1. What is meant by the term “heat treatment”?
2. What is the main factor determining the grain size of steel?
3. The hardness that can be obtained through a given heat treatment depends on which three factors?
4. Which safety precautions must be taken when conducting heat- treatments?
5. List the three groups of plain carbon steels.
6. What is meant by soaking during heat treatment?
7. List four kinds of quenching mediums. Which cools most rapidly? Which one cools the least rapidly?
Types of heat treatment
As we now know, heat treatment is any one of a number of controlled heating and cooling operations used to cause the desired change in the physical properties of metals. There are five basic heat-treatment processes to obtain characteristics like toughness, hardness and wear resistance. To obtain these characteristics, operations such as hardening, tempering, annealing, normalising and case-hardening are necessary.
Tempering
This is a follow-on process from hardening. After a material has been quench-hardened, it is not always ready for immediate use. Tempering is a process generally applied to steel to relieve the strains induced during the hardening process and to reduce brittleness. It consists of heating the hardened steel to a temperature below its critical temperature (tempering temperatures are normally much lower than the hardening temperature), soaking it at this temperature for a period, and then quenching in water, brine, air or oil.
During this process, the degrees of strength, hardness and ductility obtained depend directly upon the temperature to which the steel is heated. High tempering temperatures improve ductility at the expense of tensile, yield strength, and hardness.
DID YOU KNOW? Critical temperature is the temperature range in which steel undergoes structural change during heating and cooling.
Case-hardening
Case-hardening is a surface-hardening process. The objective is to produce a hard case over a tough core. Case-hardening is an ideal heat treatment for parts which require a wear-resistant surface and, at same time, must be tough enough internally at the core to withstand the applied loads, such as gears, cams, cylinder sleeves, etc.
The steels best suited to case-hardening are the low-carbon and low-alloy steels. If high-carbon steel is case-hardened, the hardness penetrates the core and causes brittleness. In case-hardening, the surface of the metal is changed chemically by inducing a high carbide or nitride content. The core is unaffected chemically. When heat-treated, the surface responds to hardening while the core toughens. The common methods of case-hardening