Steel is a material that is used in a wide variety of applications, from construction to cutlery. It is well-known for its strength and durability, but did you know that steel has three different phases? The phase of a material refers to its crystal structure and the phase of steel changes as the temperature increases.
At room temperature, steel has an alpha phase with a BCC (body-centered cubic) structure. This is what gives the steel its magnetic properties. However, above room temperature, the steel switches to the gamma phase, which also has an FCC (face-centered cubic) crystal structure. This phase is non-magnetic. Finally, at temperatures above 1135 degrees Celsius, the steel will change its crystal form again to delta with BCC structure. But in this form, the steel is no longer magnetic. So next time you’re looking at a piece of steel, keep in mind the different phases it goes through!!
Let’s start from the basics and learn step by step, the role of crystal structure and how different crystal structures of steel exhibit unique characteristics making it the most important structural alloy.
What is Crystal Structure?
A material’s crystal structure is the three-dimensional arrangement of atoms in its physical space. The type of crystal structure that a particular substance has depends on the chemical composition of that substance and how it cools from a liquid state to a solid-state.
There are seven different types of crystal structures: cubic, tetragonal, hexagonal, trigonal, orthorhombic, monoclinic, and triclinic. The most common crystal structures are cubic and hexagonal.
Introduction to Unit Cell
The structure of a crystal can be described in terms of its unit cell. The unit cell is the smallest repeating unit in a crystal. It is the basic building block of the crystal and contains all the information necessary to describe the crystal. The unit cell can be visualized as a cube, with each corner representing an atom.
Alpha Iron – BCC Structure in Steel
At room temperature, steel has a BCC (body-centered cubic) crystal structure. This is the same crystal structure as alpha iron, which is the pure form of iron. The BCC unit cell is shown in Figure 1. The BCC unit cell is made up of 8 atoms, with one atom at each corner of the cube and one atom in the center of the cube.
The BCC unit cell can be thought of as two interpenetrating FCC lattices, with one lattice shifted by a quarter of the unit cell size. This is known as the A2 structure. The BCC crystal structure is stable up to 723°C, at which point it transforms to the FCC crystal structure.
The BCC crystal structure gives alpha iron its unique properties. It is these properties that make it the most important structural alloy. The BCC crystal structure gives alpha iron:
- A high density (7.87 g/cm3)
- A high melting point (1538°C)
- A high hardness
- Good electrical and thermal conductivity
- A low coefficient of thermal expansion
- Good ductility and formability
The BCC structure also gives alpha iron its magnetic properties. The BCC unit cell has one unpaired electron, which is what gives alpha iron its ferromagnetism.
BCC Structure – Steel Formability
The BCC structure of steel also gives it poor formability. Because of the single slip system, the structure exhibits poor ductility and can not bear drastic changes in shape. If a change in shape is desired, steel is heated till it reaches a temperature in the FCC range, and then it can be pass through excessive rolling or drawing operations. Cold drawing and rolling is possible to limited extent.
Gamma Iron – FCC Structure of Steel
The BCC crystal structure is not stable at high temperatures. Above 723°C, the BCC structure begins to transform into the FCC crystal structure. The transformation is complete at 910°C. At this temperature, the steel changes from alpha iron (BCC) to delta iron (FCC). The FCC crystal structure is more energetically favorable than the BCC crystal structure at high temperatures.
Read more indepth in article, “Annealing of Steel” and ” TTT Diagram of Steel“.
The FCC crystal structure has a number of advantages over the BCC crystal structure. The FCC structure gives delta iron:
- Better Ductility
- Better Formability
- Greater mechanical strength
- Better wear resistance
- Better corrosion resistance
The FCC structure also gives delta iron its non-magnetic properties. The FCC unit cell has no unpaired electrons, which is what gives delta iron its paramagnetism.
FCC Structure – Steel Formability
The FCC structure of steel gives it better formability than the BCC structure. Because of the multiple slip systems, the structure exhibits good ductility and can bear drastic changes in shape. If a change in shape is desired, steel can be formed into the desired shape without heating.
Delta Iron – BCC (non-magnetic) in Steel
At high temperatures, the FCC structure of delta iron begins to transform into the BCC crystal structure. The transformation is complete at 1400°C. At this temperature, the steel changes from delta iron (FCC) to gamma iron (BCC).
At this temperature, the bcc structure is not magnetic. The cause of the non-magnetic nature of this BCC structure is an increase in atomic vibration, which causes disruption to the magnetic domains. As a result, the steel is not attracted to a magnet.
How can you tell if metal is BCC or FCC?
There are a few ways to tell if metal is body-centered cubic (BCC) or face-centered cubic (FCC).
One common way is to look at it under a transmission electron microscope. Another is to use x-ray diffraction (XRD). However, these methods can be expensive and may require special equipment.
A less precise but more accessible method is to check the metal’s ductility. BCC metals are usually less ductile than FCC metals, so if the metal is very difficult to bend or break, it’s probably BCC.
Overall, the best way to determine a metal’s crystal structure is to use TEM or XRD. However, these methods may not be available to everyone. If you can’t use TEM or XRD, checking the metal’s ductility is a good second choice.
Why FCC metals are more ductile than BCC?
One reason FCC metals are more ductile than BCC is that the FCC structure has more slip systems than the BCC structure. Slip systems are planes of atoms that can slide past each other without breaking the crystal lattice. The FCC structure has 12 slip systems, while the BCC structure has only 1. This means that FCC metals can deform more before breaking than BCC metals.
Another reason FCC metals are more ductile than BCC is that the atomic spacing in the same plane between two atoms is more incase of BCC than FCC. That’s why atmos require more force to move in a plane and cause slip if a structure is BCC.
Which is stronger BCC or FCC?
There is no definitive answer to this question because it depends on the specific metal. In general, however, BCC metals are stronger than FCC metals. This is because the BCC structure has a lower number of slip systems and requires more force compare to FCC for atoms to move in a plane and cause a slip.
There are some exceptions, however. For example, aluminum is a BCC metal but it is not as strong as iron, which is an FCC metal. This is because aluminum has a lower number of close-packed planes than iron.
Is carbon steel body-centered cubic?
Carbon steel is an alloy of iron and carbon, and it can have a variety of crystal structures depending on the amount of carbon present.
The most common crystal structure of carbon steel is ferrite, which is a form of alpha iron (BCC). At high temperatures, however, the BCC structure of delta iron begins to transform into the FCC crystal structure. The transformation is complete at 910°C.
Is steel FCC or BCC at room temperature?
At room temperature, steel exhibits BCC structure if there is no alloying addition causing a drop in phase transformation line. Additions of chromium, molybdenum and vanadium decrease the line so that at room temperature, steel can be a mixture of both FCC and BCC.
Is Alpha Iron BCC or FCC?
Alpha iron, also known as ferrite, is a form of BCC.
Is low carbon steel bcc?
Low carbon steel typically has a BCC crystal structure at room temperature. They exhibit poor corrosion resistance and high ductility because of a higher percentage of iron.
