What is normalising steel? Normalising is a heat treatment of steel to change the grain size to improve the physical properties of the steel
To explain normalising you have to know a little bit about iron. In steel the iron atoms can form different arrangements and these have a huge impact on the physical properties of the metal.
Diamond and pencil lead are both forms of carbon.
One is hard and the other is soft. They have the same chemical properties butdifferent physical properties.
In the same way there are a number of different types of iron structures. Some are soft and some are hard.
When you make steel you get a mix of these different types of structure in the steel. Now depending on what you want to use the steel for you can change these to something more useful.
When the steel is originally cast and rolled a lot of the steel consists of large ferritic grains.
To get rid of these you have to heat it hot enough so that all the ferrite grains turns into small grains of austenite (named after Mr Roberts-Austen who discovered it). At this point all the alloying elements have dissolved and the steel has a homogenous structure.
Then by cooling it down at just the right rate you can control the size and structure of the grains that form. These grains are ferrite again – but now much smaller and they often mix with a compound of iron and carbon called cementite. This process is called normalising steel.
If you cool it quickly – by quenching in oil or water for example you get martensite which gives you steels which are very hard (up to 700HB). If you cool it in air you get pearlite which is a form of steel made from layers of ferrite and cementite.
The steel is now composed of many small round grains of steel with a very different appearance. And this is the reason why it is called pearlite – because it looks like tiny pearls.
Needless to say this isn’t simple stuff.
What matters to us, as engineers, is that once we’ve heated the steel and then slowly air-cooled it again it has wonderful fine grains of steel throughout. This means that it is stronger with a higher yield strength, it’s slightly harder and slightly less ductile.
The other thing to bear in mind is that how the air cooling is done is important to the eventual physical properties of the steel. This again is one of the areas where the technical ability of German mills, such as Dillinger Hütte, helps to deliver a higher quality steel.
So, it is better for many engineering processes. The reason why it is more expensive is because you have to heat the steel up to over 1340F – and there’s the corresponding electricity bill to pay for that.