You would like to know the actual structure of metals and what is important in iron production?°

Here you can find out what materials actually are and why materials technology is important. You will learn step by step about the structure of metals, see how the combination of atoms forms the crystal lattice and how the structure is created from this. You will also learn which properties metals have. Finally, you will go through the process of extracting iron and producing steel and see what needs to be taken into account.

Material subdivisions

Materials can be split into metals or non-metals. Steel and cast iron are among the most commonly used materials worldwide.

Materials technology and its importance

The purpose for which a particular material is used depends on its properties. These can be divided into different categories.

°°

Various scientific material technology methods are used to place materials into different categories and properties in the first place.

°°

Materials technology deals with extraction and use of materials and examines their various properties. This enables materials to be used properly, new materials to be developed and existing ones to be improved.

°°

Knowledge of the internal structure of materials is a prerequisite for analysing their properties. Only when you know what components a material is made up of and how these work are you able to gain new insights for further development.

Metals under the microscope

The positive charged particles of metal atoms are always arranged at fixed distances to one another, forming the crystalline lattice. They emit negative electrons, which surround the crystalline lattice as a so-called electron cloud. The electrons can move freely within this cloud, but cannot leave it.

The attraction of the positive and negative charges creates a strong metal bond. This ensures an extremely stable bond of the metal atoms and in turn the strength of the metal.

Summary of materials

You now know that materials have always played an important role and have characterised many eras and cultures. You have learnt that materials can generally be divided into metals and non-metals and can have different properties.°

You have also have learnt that materials technology is used to investigate the properties of materials so that new ones can be developed and existing ones improved. Finally, you have seen that metals are made up of a structure, crystal lattice and finally metal atoms, whose composition influences the metallic properties.

Structure of metals

Now we will take a look at the exact structure of metals and their properties. To do this, you will first find out how metals conduct and can be deformed. You will get to know different types of crystalline lattice and see what kind of lattice defects can occur. Then we will take a closer look at the structure of metals and get to understand what happens with alloys. Finally, you will learn what types of alloys there are and how to read corresponding parametric diagrams.  

Conductivity of metals

As metals are conductive, they find use for various purposes in everyday life. The structure of the crystal lattice makes it easier to understand the electrical conductivity.

Deformation of metals

Besides being electrically conductive, metals can also be deformed. They can be deformed°elastically or plastically.

Let's take another look at the atomic level in the crystal lattice. When force is applied to sheet metal, the metal atoms are displaced slightly in the lattice, but then spring back. By a°lead rod°however, the displaced positions in the crystal lattice remain after the force is removed, thus causing a permanent deformation.

Three key crystal lattice types

BODY CENTRED CUBIC (BCC)°

The cube has an enclosed metal atom at the centre. Since it is only loosely packed, there is no stacking sequence.°

Relatively wide spacing between the atoms permits additional foreign atoms to be incorporated.°

° °°

FACE-CENTRED CUBIC (FCC)°

The cube consists of of 8 atoms, one in each corner of the cube and one atom in the centre of each side.°

The atoms are arranged in three stacks (ABC) and thus form the densest spherical packing.°

A disturbed stacking sequence leads to stacking faults into which alloying elements can be deposited.°This results in other properties. 

HEXAGONAL-CLOSE-PACKED (HCP)°

The metal atoms are arranged in an hexagonal prism, with one atom at the centre of each base surface.°3 atoms are inside the prism.°

The stacking sequence is ABA and thus forms the densest spherical packing.°

Wide lattice gaps provide space for foreign atoms.°

Less cold formable than metals with fcc lattices. 

Grid structure errors

Metals are not perfect crystals, they have lattice defects. Lattice defects can be assigned to three dimensions, depending on type and size. The first dimension is the zero-dimensional defect. These correspond to a lattice point and can be further subdivided.

The zero-dimensional defects generally have no negative effects on the properties of metallic materials, but even enable important heat treatments.

One-dimensional error

There are also one-dimensional defects, so-called line defects. If a half-plane is inserted into the regular crystalline lattice, the line is faulty and a stage dislocation forms. Dislocations can move and consequently cause plastic deformability of metals.

Two-dimensional errors 

Two-dimensional errors result in stacking errors° ° °of the regular layer sequence. Stacking faults are caused by° °crystallisation or the collapse of a° °void cluster. Two-dimensional lattice defects have an° °influence on the tensile strength of a metal. 

Structure

You have already learned that metals consist of many regularly shaped grains, which form the structure. The structure is not visible to the naked eye. But with the aid of a metallographic micrograph, the structure can be made visible under a metal microscope.°

The micrograph shows the grain size and the grain boundaries of a metal. The grain size ranges from fine to coarse and can be adjusted by targeted treatments. Grain boundaries represent interruptions in the atomic arrangement of the grains and are part of the two-dimensional lattice defects mentioned. These form during crystallisation for example. 

Fine-grained metals have better mechanical properties compared to coarse-grained metals, as there are more grain boundaries. An increase in grain boundary improves toughness. °However, undesirable creep processes occur at increased temperatures.

Grain forms

In addition to the commonality of grain size and grain boundaries, grain forms vary depending on the metal and the type of crystal lattice.

GLOBULAR GRAINS Round grains, e.g. pure iron°	POLYHEDRAL GRAINS  Grains in polygonal shape, e.g. iron with°austenite structure
DENDRITIC GRAINS Needle-like grains, e.g. hardened steel°	LAMELLAR-LIKE STRUCTURE Lamellar crystals, e.g. lamellar graphite in grey cast iron

Status diagrams of°alloys

To research and further develop the properties of alloys, their state diagrams are analysed. In general, status diagrams show the aggregate status of pure metals. For this purpose the inflexion point in the cooling or heating curve is used. 

By alloys, the temperature of the second metal and the mixing ratio within the alloy is also taken into account. When all inflection points are transferred and the temperature points connected, the status diagram for the alloy is given.°

Solid solutions and crystal mixtures have different status diagrams. Solid solutions such as copper and nickel are characterised by complete solubility in the liquid and solid state.

The status diagram for crystal mixtures such as lead and tin looks different. This is characterised by the complete solubility in the liquid state and insolubility in the solid state.

Winning of pig iron

In this last section, we first look at how pig iron is actually won and which steps are important in the production process. You will also learn more about the various processes used to produce steel from pig iron. Finally, you will see the different ways of post-treatment and casting of steel.°

As you have learnt, steel is one of the most widely used materials in the world. Its main component is iron. Iron occurs naturally as iron ore, a compound of iron and oxygen. To produce steel, the iron must first be extracted using a reduction process.°

Two reduction processes are used in the production of pig iron. Here you can see an overview of both processes. 

Production of steel

Once the pig iron, or solid sponge iron, has been won, so-called 'refining processes' are used to produce the steel. Can you imagine what happens in the refining process?°

In the oxygen blast process , the liquid pig iron is filled into a converter together with scrap steel and additives. A water-cooled pipe blows oxygen into the vessel, causing a chemical reaction with the iron compounds. The carbon in the pig iron burns away and lime binds the iron compounds. Steel and slag are then poured off.

The electric arc furnace process is used to produce high-alloy steel grades. A smelting vessel is filled with pig iron and other components. Carbon electrodes are lowered onto the filling and an electric arc is ignited. During the smelting period, any remaining carbon and accompanying substances are burnt off. Steel and slag are then poured off.

After the production of steel, undesirable components often remain. To produce quality steels, these are removed by further treatments. Here you can see the key processes and their descriptions. Check the procedures for more details.