Once solidified, metals can undergo further mechanical working to enhance their properties for intended use.
For more information on this, visit the NDT resource centre.
Mechanical working can include:
- Thermal treatments
An example of this is aluminium. Forming and cold working aluminium will double its tensile strength. As well as this, an alloy can be introduced to the aluminium such as silicon, copper magnesium and zinc. Aluminium can also be made stronger with heat treatmebnt
Strength and Hardening
To make a metal stronger, there are mainly three mechanisms to do this:
- Alloying – This involves adding another element to the crystalline (being a point defect which you can find more information about here).
- Managing Grain Size – This involves decreasing the continuity of atomic planes.
- Introducing Strain – A metal can be hardened by introducing many dislocations which become tangled against each other.
The grain sizes are important in hardening a metal. The larger the grain size, the further the distance a dislocation can move until it hits a grain boundary. This means that the smaller the grain size, the less distance a dislocation can move, therefore, making the metal stronger. The sizes of grains can be controlled by the rate at which the liquid is solidified.
Strain hardening is when a metal is made harder and stronger by undergoing plastic deformation. When the metal plastically deforms, dislocations are moved and new dislocations are created. These dislocations become tangled and weave in and out of each other decreasing the mobility of each dislocation and, therefore, hardens the metal. It is called cold working because the metal plastically deforms at a temperature that is low enough so that the atoms cannot rearrange themselves and remove the dislocation. Any temperature higher than that, the atoms can rearrange themselves back to normal and little strength is achieved.
It is important to know that too many dislocations is one area will cause the metal to weaken. These areas where there are many dislocations at one point weaken the structure of the metal rather than strengthen it. These areas are known as persistent slip bands.
Of course, if a metal is strengthened, the ductility of that metal will decrease.
Heat treatment is when a metal is held at a elevated temperature. Below are the following steps to heat treatment:
- Grain Growth
The reason why heat treatment is used is to further enhance the properties (e.g. Grains), to release some of the strain hardening (e.g. improve ductility and condition for service).
When a metal is held at an elevated temperature, the atoms start to move more freer and break their own bonds. This movement of the atoms removes any of the tangled dislocations and produces a lattice structure again. The internal residual stresses are lowered because the density of dislocations has decreased. The atoms that were once tangled with dislocations are recovering back to their lattice structure. After heat treatment, the ductility of the metal is high and the strength is low.
At a new higher temperature, strain-free grains nucleate inside the old distorted gains and their grain boundaries. This new grains replace the own deformed grains which were produced from cold-working. With recrystallization, the mechanical properties of the metal return to their original weak and ductile state.
Properties of alloys can be engineered though varying the composition (i.e. the weight usually shown as a % of the whole). Here is a list of the typical elements used in the below alloys:
- Brass – Copper and Zinc.
- Bronze – Copper, Zinc and Tin.
- Pewter – Tin, Copper, Bismuth and Antimony.
- Cast Iron – Iron, Carbon, Manganese and Silicon.
- Steel – Iron and Carbon (plus small amounts of other elements).
There are two changes that can occur in the microstructure when a metal is alloyed:
- The alloy is formed as a single phase or a homogeneous structure of consisting of identical crystals.
- The alloy is formed of two (or more) separate types of crystals creating a heterogeneous microstructure of two or more phases.
Iron is a polymorphic material because it can exist in more than one crystal-form. Below 910 degrees Celsius, Iron is BCC. Above 910 degrees Celsius, Iron’s structure changes to FCC. FCC iron tends to take quite a lot of carbon (up to 2%) where BCC iron will only dissolve a maximum of 0.02% carbon.
With steel, the carbon is so small it can fit through the gaps between the iron atoms. Therefore, it is known as a interstitial solid solution: the carbon is located in the interstices of the iron crystal.
Any solution of carbon up to a maximum of 2% in FCC iron is called austenite; whilst the very dilute solid solution formed when up to 0.02% carbon dissolves in BCC iron is called ferrite (which is regarded as being more or less pure iron). Carbon steel at 1,000 degrees Celsius will have all the carbon dissolve into the solid austenite. When the steel cools, the austenite changes to ferrite, which will retain practically no carbon in solid solution. Assuming that the cooling has taken fairly slowly, the carbon will be precipitated as the hard compound cementite.
When the steel is austenite and slowly cools so that some of the iron atoms change from FCC to BCC, the amount of carbon dissolved in the BCC carbons is very low. Therefore, the carbon that is left over is concentrated into the residual austenite.
- Any solid solution of carbon up to 2% in FCC iron is called austenite.
- The very dilute solution up to 0.02% carbon in BCC iron is called ferrite (although we generally consider ferrite as pure iron).
- Below 723 degrees Celsius, the austenite transforms by forming alternate layers of ferrite and cementite (known as pearlite).
- Austenite begins to solidify at 1500 degrees Celsius and is completely solid at 1450 degrees Celsius.
- Austenite begins to change to ferrite as new small crystals at the austenite grain boundaries (upper critical point).
- Remembering that ferrite will hold very little carbon, the bulk of the carbon must remain in the shrinking crystals of the austenite.
- At 723 degrees Celsius, there is a mixture of ferrite and austenite crystals (at the lower critical point).
Decreasing the size of the grains and decreasing the amount of pearlite in mild steel will improve the strength, ductility, and the toughness of the steel.
Mild steel is a very versatile and useful material. It can be machined and worked into complex shapes, has low cost and good mechanical properties. Mild steel has around 0.1% Carbon by weight and consists of pearlite and ferrite with small mild inclusions or impurities such as oxides and sulphides.
In 0.8% carbon with iron, the mixture is 100% pearlite. Since pearlite has a very fine structure, the steel is hard. However, this also makes it quite brittle and much less ductile than mild steel.