Materials technology - Ferrous Metals

Metals

What metals are made from: The building blocks for all metals are called atoms. Two or more atoms joined together is called a molecule.
All elements in the universe is made out of different combinations of atoms and molecules. Scientists have arranged these elements that they know into a table called the Periodic table.
The different properties of metals
Different metals have different properties which make them useful for different purposes. The following properties is important to know when deciding which metal to use for a specific task or project.
Malleability: Malleable means that you can hammer, stretch, press or work the metal into different shapes without breaking the metal. This means that malleable metals can be made into very thin sheets and the mold into different shapes like car bodies. Malleable metals are often also very ductile.
Ductility: Ductile means that you can stretch or chance the shape of the metal without breaking it. Ductile metal will also keep the new shape after it has been stretched, pulled or pressed. Ductile metals can be pulled into very thin wires. Think of the thin copper wires that are twisted together inside an electric cable.
Compressive Strength: Compression strength means that you can put a lot of squashing (pushing together) force on the metal and it will not break or bend.
Tensile Strength: Stretching strength means that metals which have a high tensile strength can withstand a stretching force without breaking.
Elasticity: Elastic means that you can apply a high load or force to a material which will either stretch or compress the material, but when you remove the load or force the material will return to its original shape. For example a motorcar spring.
Brittleness: Brittle means very hard and resistant to any change in shape. If any force or load is applied to a brittle material it will simply break without showing any (or very little) change in its shape. Ex. Glass and cast iron.
Conductivity: Most metals are good conductors which means that they allow heat and electricity to pass trough them very easily.
Mixing metals to change their properties.
When you mix two or more metals together the mixture is called an alloy. Alloys do not have the same properties as the metals that were used to form them but they will acquire totally new and sometimes unexpected properties of their own.
Material science that study the different properties and behavior metallic elements is called Metallurgy. The scientists or engineers that work in this field of science is called metallurgists.

Ferrous metals

Ferrous metals is grouped together by scientists because they are all metals containing IRON. Most ferrous metals are magnetic because they contain iron.
Characteristics of ferrous metals.
The characteristics of ferrous metals depend on the elements that it contains. Adding different elements to a metal will give it certain properties.

Name	Alloy of	Properties	Uses
Mild steel or low carbon steel.	Carbon: 0,1%-0,3% Iron: 99,9%-99,7%	Tough with high tensile strength, can be hardened and rust very easily	Most common metal used in school workshops and used in general metal products and engineering.
Medium carbon steel	Carbon: 0,3%-0,6% Iron: 99,7%-99,4%	Harder and stronger than mild steel, can be hardened and heated to strengthen and toughen it.	Machine parts
High carbon steel	Carbon: 0,6%-1,4% Iron: 99,4%-98,6% Plus small amounts of manganese, silicon and copper	Strongest and hardest of carbon steels, can be hardened and heated to strengthen and toughen it	Cutting tools such as drills
Stainless steel	Iron, nickel and at least 11% chromium	Tough, resistant to rust and stains	Cutlery and medical instruments
Tool and die steels	Usually high carbon steels with up to 10% molybdenum, up to 18% tungsten and/or small amounts of vanadium and other metals	Strong tough and wear resistant	Machine parts for cutting
Cast iron	Carbon: 2,5%-4% Iron: 89%-94%	Strong but brittle with high compressive strength	Castings, manhole covers and engines
Wrought iron	Almost 100% iron	Fibrous, tough, ductile and resistant to rusting	Decorative gates and railings and not very much used today

Types and grades of ferrous metals.
Because of the confusion of different numbering systems, the system that most people use today are the Unified Numbering System (UNS). This system uses a letter followed by five numbers.
Carbon steels and their uses:
Low carbon steel: Also called mild steel contains less than 0,3% carbon. This type of steel is often rolled into thin sheets and used as car bodies etc. You can do this because mild steel is very malleable.
Medium carbon steel: Contains between 0,3% and 0,6% carbon as well as between 0,6% and 1,65% manganese. The increase in manganese means that the steel can be toughen by heating it to certain temperatures. You can therefore use medium carbon steel where you need more strength like machine parts and drill bits.
High carbon steel: Contains more than 0,6% carbon. (0,6%-1,0%) High carbon steel are used where you need a lot more strength, hardness and resistance. Cutlery, cutting tools, springs and high strength wires. Parts are normally hardened after they were made into their different shapes.
Stainless Steel: Stainless steel is very easy to recognize because it is very shiny. Stainless steel are also very strong and ductile and it does not rust very easily. This is why it is called stainless. Stainless steel can resist rust because it contain at least 12% of the element called Chromium.
Tool and die steels: Tool and die steels are alloyed so that they are very strong, tough and wear and tear resistant at room temperature as well as very high temperatures.
The two main types of tool and die steels are high speed steels and die steels.
A good example of high speed steel product is an industrial drill bit.  High speed steels are the most alloyed of all the tool and die steels. That means they have the highest amount of other elements added to give them particular properties. They keep their hardness at very high temperatures.
There are two types of high speed steels. The molybdenum type and the tungsten type.
Molybdenum type steel contain 10% of the element called molybdenum and is cheaper as well as harder than the tungsten type steel. It also keep its shape better when heated.
Tungsten type steel contain between 12% and 18% of the element tungsten together with chromium, vanadium and cobalt. This type of steel is very wear resistant and is used for tools and hard-wearing machine parts
Die steels: There are three types of die steels:
o Hot worked steels: usually used in areas where the temperature can get very high, and when you need toughness and resistance to wear and tear.
o Cold worked steels usually used at normal temperatures and resistant to wear and tear and to cracking. Use to make cutting and punching tools.
o Shock resistant steels also make good cutting and punching tools. Shock resistant tools are good at resisting shocks and being hit quite hard. These steels are often used for the heads of hammers.
Cast iron: Contain between 2,5% and 4,5% carbon. Also small amounts of other alloying elements such as silicon, phosphorus and manganese. Cast iron is usually very brittle and not very ductile.
Cast iron has a low melting point and when melted, it can be poured into moulds or casts to make detailed shapes that are strong and hard. Cast iron is used to make pipes, machine parts, balustrades and gates that have lots of details.
How carbon affects ferrous metals:
The effect of carbon on a ferrous metal will depend on the amount of carbon that you add. When you add carbon to a ferrous metal, it increases the strength, hardness and resistance to wear. However, at the same time it decreases the ductility and toughness of a ferrous metal. So, when adding carbon, you have to try and find a middle point to get the properties that you need.
Carbon steel: Carbon steel contains low amounts of carbon and is very soft, but can be shaped easily. As the amount of carbon increases, the metal gets harder and stronger, but also less ductile.
Cast iron: The properties that different types of cast iron have, depend not only on the amount of carbon that the iron contains but also on the form and structure that the carbon has in the alloy. For example, white cast iron contains iron in the form of cementite, which is also called iron carbide.
Cementite is a chemical compound containing both iron and carbon. Its particular crystal structure is called orthorhombic. Cementite is a very hard and brittle and it breaks very easily. The following figures shows the four orthorhombic structures.

Grey iron (the original cast iron) contains carbon in the form of fine flakes of graphite. These flakes make the metal brittle. The reason for this is when you try to bend or shape the cast iron it will crack along the graphite flakes. Grey cast iron is used in machine housings and electrical boxes where you don’t need a lot of tensile strength.
A newer type of cast iron is called ductile iron.  This cast iron contains small amounts of magnesium allowing it to be more ductile. The magnesium changes the flakes to spheroids. Spheroids are shapes that are not perfectly round and also called nodules. It is this nodular shape of the graphite that allows the metal to be more ductile and shock resistant than grey iron.