Making steel strong – the inside story
The secret to making steel strong involves controlling a number of critical factors – such as cooling, microstructure and chemical position.
To make our steel strong, Ovako takes precise control of three key factors:
- How the steel is cooled from its molten state
- Its microstructure – the very small scale structure seen under a microscope
- Its chemical composition – both the carbon content and other alloying elements
Steel consists mainly of the element Iron (Fe). In its molten or liquid state, at temperatures above 1,500 degrees Celsius, the iron atoms are able to move freely. As the liquid metal cools the movement of the iron atoms slows down. They start to pack closely together in a regular arrangement to form crystals.
These crystals start in many different places in the melt and then grow until they meet other crystals growing in different directions. The resulting solid steel has a polycrystalline structure. The areas where the crystals meet each other are known as grain boundaries. These grains are very small and usually only visible under a microscope.
Two crystalline forms
A very important aspect of iron is that it can exist in two different crystalline forms according to how its atoms are arranged - face centred cubic (FCC), known as austenite and body centred cubic (BCC), known as ferrite. At high temperatures iron exists as austenite. When it cools below a critical temperature it transforms to ferrite.
Carbon adds strength
The transformation of iron crystals between austenite and ferrite becomes vital when we add carbon atoms to create a strong steel. At high temperatures, austenite allows the carbon atoms to fit easily into some of the spaces between the iron atoms. But after cooling and changing into ferrite the spaces between the iron atoms are smaller. If we quench a hot steel, which means cooling it very quickly, the carbon atoms become trapped in the ferrite structure. This forces the creation of a distorted crystal structure known as martensite.
Martensite is very hard and is the key reason why steel can be made so strong.
Why dislocations are important
At the atomic level, no crystal has a perfectly structure. Instead, we find extra half-planes of atoms that create dislocations. They create fault lines in the crystal structure. And when the steel is under stress the dislocations can move easily through the atomic structure, resulting in plastic deformation.
Blocking dislocation movement
Plastic deformation makes steel versatile and useful. But to make steel extra strong we need to block the movement of dislocations. There are several ways we can do this.
- By reducing the grain size of the steel. Because grain boundaries are effective barriers to dislocation movement. Smaller grains provide more barriers.
- By introducing carbon in solid solution to create internal stresses that resist dislocation movement. You can think of this as the internal friction of steel.
- By using alloying elements help form small precipitated particles in the steel that can pin dislocations in place. These particles may be carbides or intermetallic compounds.
This is how steel becomes strong by a precise control of a number of critical factors, involving cooling, microstructure and chemical position.