Chemistry in Industry and Technology Option C

Making Steel

C.1.3 Describe and explain the conversion of iron into steel using the basic oxygen converter. C.1.4 Describe alloys as a homogeneous mixture of metals or a mixture of a metal and non-metal. C.1.5 Explain how alloying can modify the properties of metals. C.1.6 Describe the effects of heat treatment of steel. (Effects should include tempering, annealing and quenching) C.1.7 Describe the properties and uses of iron and steel.

The molten iron produced in the blast furnace contains impurities. The main impurity is carbon, but there are also small amounts of other elements. Element Approx. % Carbon 4 - 5 % Silicon 1 – 2 % Manganese Trace Sulphur Phosphorous

These impurities make the resulting iron very brittle when it cools down. Some is used to make cast iron goods (e.g. cylinder blocks for cheaper cars)

This impure iron is known as “pig iron” Which entirely coincidentally is the name of an album by The Anti Nowhere League. You will not be tested on the album!

To turn the pig iron into steel , it is added to a basic oxygen converter. Hot oxygen is injected into the vessel at high pressure.

This causes impurities in the pig iron to oxidise. C + O2  CO2 (escapes as a gas) Si + O2  SiO2 S + O2  SO2 (escapes as a gas) 4P + O2  P4O10

Quicklime is also added. This reacts with the impurities to form “slag” – e.g. Calcium Silicate CaSiO3 and Calcium Phosphate Ca3(PO4)2 6CaO + P4O10  2Ca3(PO4)2

Some additional notes: The crude steel formed is around 1.5% carbon. It also contains dissolved oxygen. This can be removed by adding controlled amounts of silicon and aluminium. Other elements can be added to the steel to modify its properties. More about this later. . . The oxidation is exothermic. Old scrap steel is added cold to control the temperature.

Alloys Pure iron is a shiny, soft, ductile malleable metal of high strength (typical transition metal properties!) It rusts easily to form hydrated iron(III) oxide. 4Fe + 3O2 + xH2O  2Fe2O3.xH2O This crumbles easily, and allows oxygen and water to penetrate it and continue the corrosion process. Iron is easily shaped (“wrought iron”) and is used for many ornamental purposes (gates, fences etc.) It is also used for bridges and as a catalyst in . . . The Haber process!

Most iron is used to make steel – which is an alloy. An alloy is a homogeneous mixture of metals, or a mixture of a metal and a non-metal. Or in English . . . An alloy must be 2 or more elements – at least one of which must be a metal. If a non-metal is involved, then the metal must be the main component. Alloys are made by mixing the required element with the molten metal and then cooling the mixture.

How do alloys work? Remember metallic structures?

They are a regular lattice where the atoms can slide past each other. Adding different sized atoms or atoms with different numbers of valence electrons, disrupts the lattice. This alters properties such as hardness, melting point and conductivity.

Low carbon (mild steel) Steel is not just one material. There are many different types of steel: Type of steel % Carbon Properties Uses Low carbon (mild steel) 0.07 – 0.25 Easily cold worked Car bodies High carbon (tool steel) 0.85 – 1.2 Wear resistant Cutting tools, railway lines

Properties given to steel Alloying element Properties given to steel Uses Cobalt Easily magnetised Magnets Molybdenum High strength at high temperature High speed drills Manganese (10 – 18% manganese) Tough Safes, earth moving machinery Titanium Withstands high temperatures Turbines, spacecraft, aircraft Vanadium Strong, hard High speed tools Stainless steel (20% chromium, 10% nickel) Resists corrosion Cutlery, surgical instruments, car accessories

The properties of steel can also be modified by heating and cooling it under carefully controlled conditions. You need to be aware of annealing, quenching and tempering.

Annealing The steel is slowly heated to about half its melting point (1040 °C ?) and then let cool slowly. This modifies the metallic crystals in the steel and makes it more malleable and ductile.

Artificially coloured scanning electron microscope picture of tungsten crystals at magnification x 2275

Quenching If a very hard steel is needed, the steel is quenched. The hot steel is cooled rapidly using cold water or oil. This traps the crystal structure which was present at high temperatures. The steel is hard but brittle and is used in cutting tools.

Water quenched steel etched to show crystal structure Water quenched steel etched to show crystal structure. Magnification x 400

Tempering Quenched steel can be made more malleable by tempering. The quenched steel is heated to about 300°C and cooled slowly (or 600 °C – depending on which book you read!) The resulting steel is still quite hard, but is now more malleable and ductile.

High impact toughness means that the metal is less brittle High impact toughness means that the metal is less brittle. NB You don’t need to know this graph!

Review C.1.3 Describe and explain the conversion of iron into steel using the basic oxygen converter. Reasons for getting rid of carbon Draw the basic oxygen converter Equations for all the main reactions

Review C.1.7 Describe the properties and uses of iron and steel. Be able to name several uses of steels

Review C.1.4 Describe alloys as a homogeneous mixture of metals or a mixture of a metal and non-metal. Or even describe them in English!

Review C.1.5 Explain how alloying can modify the properties of metals. Draw the pictures and talk the talk!

Review C.1.6 Describe the effects of heat treatment of steel. (Effects should include tempering, annealing and quenching) Describe each one. No equations needed; no pictures need!