Cast Iron – Microstructure and Properties

Cast Iron is an alloy of iron and carbon with a carbon percentage of between 2wt% to 6.67 wt%. Cast Iron is considered as a metal-matric composite material with carbon particles embedded in an iron matrix. Distribution of precipitates and morphology of precipitates determines properties and usability of cast iron. Based on precipitates distribution and morphology, there is a total of four types of cast iron which will be discussed.

Cast Iron microstructure development

For better understanding of cast iron microstructure development, it’s better to understand the phase diagram of Fe-Fe₃C. We mentioned earlier, Cast Iron has carbon percentage in between 2wt% to 6.67 wt%. This region of 4.67 wt% carbon is divided into two sections divided by eutectic reaction line observable at 4.3 wt % carbon. At this carbon percentage, eutectic reaction takes place which result in conversion of liquid into two solids as reaction explain;

Now, for cast iron microstructure development study, we will follow S1 line as given in phase diagram and will observe microstructural chanes which will occue during cooling of cast iron from melting temperature. As cast iron solidifies following S1 line, transformations like Eutectic and eutectoid take place which results in several transformations as discussed below;

Liquid Region

In liquid region, solution is homogenous with iron and carbon completely miscible in each other. In phase diagram, this can be observed as no appearance of phases in liquid region.

Solid plus Liquid region

When we cross the solidus line in S1 region, we can observe the formation of delta iron. Delta Iron is a non-magnetic BCC iron. BCC iron has only one interstitial atom in the center of the lattice. That’s why it has a low capacity for carbon atoms. Maximum carbon that can be incorporated in the perfect single lattice as per phase diagram is 0.025wt %.

When tempertaure decreases, transformations follow two important line as shown in figure. In figure, one line is showing carbon increase in liquid region. And, other is showing increase of carbon in delta iron.

When delta iron forms, it will consume very small amount of carbon. So, neigboring region becomes rich in carbon. As temperature goes down, more and more delta iron is nucleated which causes in increase in carbon percentage. This increase in carbon percentage continues till eutectic line when carbon in liquid reaches 4.30 wt% and temperature is 1147^o C. At this temperature, eutectic reaction occurs and all liquid gives us coarse globules of iron carbide and gamma iron.

This eutectic mixture of gamma iron and iron carbide is called Ledeburite.

We also observe increase in carbon percentage of delta iron when temperature is decreasing. This indicates structure is transforming from BCC iron to FCC iron which has more capacity for carbon. Gamma FCC iron can absorb carbon in intersticial space upto 2wt%. That’s why below eutectic line, all delta iron formed before eutectic rection trasforms into gamma iron.

Gamma plus euctectic region

This region determines the properties of cast iron for a specific composition. There are a large number of variations for single compositions. For a specific application, heat treatment in this region is carried out to achieve specific properties. We denoted some points in S1 line as given in the below picture to reveal some microstructural features occurring in the conventional furnace cooling process.

As you can see in the picture, there are four important things to remember i.e. Eutectic line, Eutectoid line, K1 line depicting the decrease in carbon, K2 depicting eutectoid transformation.

This transformation is similar to martensitic transformation and widmansttaten transformation in steel.

We seen in previous region, after crossing eutectoid line, we have following phases;

• Pre-eutectic gamma iron coarse grains
• Eutectic globules of gamma iron (fine grains)
• Euectic iron carbide structure

The optimization of this region determines the properties said material.

Gamma iron grains will transform into pearlite after crossing the eutectoid line. This structural transformation happens by following K1 line. As the temperature is decreasing, and we have a composition in S1 line, gamma iron will follow K1 line which is depicting the loss of carbon as the temperature is decreasing. Basically, gamma iron forms at a temperature of 768^o C.

When temperature increases from this region to 1147^o C, gamma iron expansion occurs resultantly absorbing more carbon. It is clear from the figure that, at temperature 1147^o C, the carbon in gamma iron is 2wt % while, at a temperature 768^o C, it is just 0.8 wt%. So, similar to the heating mechanism, when we cool cast iron using furnace cooling mechanism, K1 line is being followed.

Carbon in gamma iron is getting decreased ultimately reaching 0.8 wt%. At 0.8wt% carbon and 768^o C, gamma iron undergoes a eutectoid transformation. In eutectoid transformation, gamma iron forms colony type pearlite containing plates of Fe₃C and alpha iron.

Now, as we mentioned earlier, we had two gamma iron or austenite phases. One is pre-eutectic coarse austenite grains. The other is gamma iron or austenite globules formed as a result of the eutectic reaction. Both austenite grains transform in the result of the eutectoid reaction into pearlite. Pearlite forms from lediburtie austenite are called transformed lediburite.

While pearlite forms from pre-eutectic austenite is called simple pearlite. This difference can be observed clearly in microstructure and, also, the main reason for a variety of cast iron microstructure variations.

We shared the mechanism of transformed lediburite and eutectoid transformation in cast iron. There is one more important concept i.e. nucleation of graphite flakes. If you look at K1 line, you will clearly see a reduction of carbon with a decrease of temperature. When the temperature lowers from 1147^o C, carbon diffuses out of austenite in form of graphite flakes. Carbon nucleation and morpology of flakes determines cast iron type. Types of flakes formation and their effects on mechanical properties will be discussed later.

Following cast iron microstructure appears after complete transformation using furnace cooling;

Properties of cast iron

There are various grades of cast iron with little variation of the heat treatment cycle. For information, we have enlisted grade A48 gray cast iron properties. For details, follow ASTM standard for cast iron.