Story of Steel Part 5 “What is Ferrite”?

Through the last four parts of our story, we have come to understand that steel began its journey in the stars and was eventually deposited on Earth in its iron oxide form. 

To date, we have looked at iron and steel, its creation and its weldability strictly from a macro viewpoint. This has been as follows

• Creation of Iron in the stars

• Deposition on Earth and the bounty we have in Canada

• Making steel, now and in the “green” future

• The creation of weldable steel alloys

Now it’s time to look at iron and steel from a micro viewpoint, that is to describe what is going as your liquid weld pool cools and then solidifies before your eyes. We will describe what the steel may look like under magnification and, eventually, what may be the ramifications of this. 

The first thing we need to examine is that iron exits in more than one form (crystallographic form) and the ones we need to discuss are termed as “body centered cubic” and “face centered cubic”. These terms just describe how the atoms of iron form and how they eventually line up. At this point we don’t need to delve into this in any greater detail, we just need to know the following:  

• The type of iron that is described as Ferritic, and which is magnetic, is Body Centered Cubic (BCC) 

• The type of iron that is described as Austenitic, and which is non-magnetic, is Face Centred Cubic (FCC)  

Ferrite and Austenite are metallurgical terms that we have already mentioned in Part 4 of our story. 

Pure iron is molten above 1536 Degrees C and when it starts to solidify it does so first in the BCC form and then, at 1394 C, it rearranges its atomic structure to the FCC form. As it continues to cool, at 912 C, it flips back to the original BCC form which completes its changes. So, the result is that, at room temperature, our iron is ferrite, magnetic and in the BCC form.  

This is illustrated in the diagram shown in Figure 1 illustrates what happens as pure iron cools.

Figure 1 Cooling of Iron from its Melting Point showing changes from BCC to CCC and back to BCC

If we were to look down a microscope at a sample of pure iron at room temperature it would be similar to that shown in Figure 2. This is a so called single phase and, in this case its 100% BCC and exhibits grains of ferrite.

Figure 2. Typical Microstructure of Pure Iron at Room Temperature and at Magnification

However, in reality we are not welding iron, we are welding steel, an alloy of iron, which has a specified level of carbon in its makeup. This carbon changes what occurs in the cooling weld pool into a more complex picture than that containing only ferrite at ambient temepearture.  

As a result of the introduction of carbon we produce another phase defined as pearlite. Figure 3 defines the phases that occur as we add carbon to the iron along the horizontal axis of the graph to a mix of 0.8% Carbon and 99.2% iron.

Figure 3. Phase diagram adding Carbon to the Iron up to 0.8%

Since we already know that weldable steels are usually 0.35% carbon/carbon equivalent and less, this diagram is all we need to study to explain what mix should occur at room temperature. If we consider the cooling of a steel containing 0.15% carbon, depicted by the blue vertical line in Figure 3, then above X at, 875C, the steel is austenitic. If we continue to cool then the following will occur as we move toward room temperature.

1. At a temperature just below point X, the austenite begins to change into ferrite

2. As we continue to cool toward Y, at 723C, more ferrite is formed and the amount of austenite reduces accordingly. 

3. At and around Y, the remaining austenite changes to another constituent we define as pearlite.

Pearlite consist of bands of ferrite and iron carbide and it is where most of the carbon ends up as the ferrite phase itself cannot accommodate much carbon. Consequently, under the microscope, the room temperature single phase structure of ferrite we saw earlier, is now one of a mixture of ferrite and pearlite as shown in Figure 4. The ferrite and pearlite amounts will be related to the percentage of carbon in our alloy. The right-hand figure below shows a slowly cooled 0.15% steel, the white areas are ferrite and the darker areas are pearlite.

Figure 4. Pure Iron at Room Temperature (left) and Steel containing 0.15% Carbon (right)

Pearlite itself is a lath structure composed of parcels of ferrite between parcels of iron carbide and the iron carbide contains most of the carbon of this 0.15% carbon steel. This is shown in Figure 5 below

So, this micro view we have taken may seem a little fancy but its critical to the understanding of what gives steel it’s properties. The thing you must remember is that these kinds of changes are going on in the molten and cooling weld pool, right before your very eyes as you manipulate your arc. 

This is not something that’s remote and only appears in books, you are producing these things in your welds and what you do influences the outcome.

The story above, however, only describes this amazing microstructural dance it in terms of a relatively slow cooled, thin piece of carbon steel. In the next chapter of “our story”, we will see what happens to particular steels when the thickness is increased or when the steel is alloyed and its ability to harden (hardenability) increases. 

What is happening is relatively complex and you are playing a large part in it. So “stay tuned” for Part 6 of our Story of Steel.