Steel has very widely ranging applications, and a number of different commonly used forms. Many of these forms can be seen on the Fe-Fe3C phase diagram showing the equilibrium between iron and the cementite (Fe3C) phase. Many steels are described by the carbon content, they have less than 1.5% carbon, but most contain other additions. The interactive phase diagram below shows different regions with varying carbon concentrations, hover over them for more information.
The phase diagram above shows a very important example of a solid-solid transformation: the eutectoid transformation. The type of microstucture that forms is dependant on the cooling rate, due to diffusion limitations in the lattice.
One of these microstructures is the eutectoid lamellae structure. As a eutectoid steel cools below the eutectoid temperature pearlite lamellae form. Due to kinetic limitations carbon rejected from the ferrite phase cannot diffuse far away from the boundaries. This results in cooperative growth of the lower and higher carbon phases: ferrite and cementite respectively.
Increasing the cooling rate, taking the alloy further from thermodynamic equilibrium can result in shear transformations rather than diffusive. The transformation to martensite is an example of this.
Note the lenticular deformation twins that minimise strain energy. See the Micrograph library entry number 45 for more information.
Additions such as Cr, Si, Ni or Mn stabilise different phases. The diagrams below show how increasing concentrations of additions can change the stable phase at low and high temperatures. This is how we can have austenitic steel at room temperature, where ferrite and cementite are more stable and a diffusionless martensitic transformation would take place if the system (without additions) was quenched.
These show temperature vs concentration of stabilisers. The diagram on the left shows ferrite is stable at low and high temperatures for high concentrations of stabilisers. The one on the right shows that austenite can be stable at lower temperatures with C, Ni or Mn additions.