Cast Irons

The steel phase diagram above is not actually the equilibrium phase diagram for the iron-carbon system, but due to kinetics Fe3C usually forms. For ferrous alloys with higher carbon contents graphite often (although not always) forms. These high C content alloys are referred to as cast irons, the three common types of which are &#;grey&#;, &#;spheroidal&#; and &#;white&#;.

You can find more information on our web, so please take a look.

Cast irons usually contain some Si or other alloying additions, which often stabilise the graphite phase, such that it precipitates out even when the wt% present is less than 4.3% (the eutectic composition for the Fe-C phase diagram).

Grey Cast Irons:

Usually contain more C or Si than white cast irons, and require a lower cooling rate.   They are called &#;grey&#; cast irons not because of their colour, but due to the appearance of a fractured surface.  Grey cast irons are quite ductile and have unreflective fracture surfaces.

Steps on cooling:

1. When the alloy falls below the liquidus, graphite begins to precipitate out. For a simple Fe-C system this means the composition must be hypereutectic, but the addition of Si moves the eutectic composition by stabilising the graphite phase. The graphite precipitates are flake-like with growth occurring in preferred crystallographic directions
2. At the eutectic temperature a cementite and

   γ

   (austenite) eutectic forms from the remaining liquid phase this is known as ledeburite.
3. As the temperature continues to decrease carbon diffuses out of solid solution to the graphite precipitates.
4. When the eutectoid temperature is reached, the remaining austenite transforms to pearlite (lamellar cementite (Fe3C) and ferrite (iron with some carbon in solid solution). Some alloying additions may modify this final transformation, for example if enough Ni is present the austenite will not transform to pearlite.

The final microstructure shows graphite flakes in a matrix of transformed ledeburite, see micrograph entry number 63 in the DoITPoMS micrograph library where many more examples may be found.

Spheroidal Cast Irons:

These are similar to grey cast irons, but they contain &#;inoculants&#; &#; alloying additions that change the form of the graphite precipitates. These inoculants are usually Mg or Ce (~0.1wt%) and they cause the graphite to grow in spheres rather than flakes.

greatlionparts are exported all over the world and different industries with quality first. Our belief is to provide our customers with more and better high value-added products. Let's create a better future together.

Related articles:
How to Choose the Right Baling Wire for Your Needs?
Maximize Efficiency: Top Metal Stamping Service FAQs Answered
What are the benefits of OEM black annealed wire?

There are two theories offering an explanation for this. The first describes the Mg or Ce impurities &#;poisoning&#; the graphite growth sites, attaching to them and slowing growth in that direction. The second suggests an increase in interfacial energy &#; the surface energy between the melt and the graphite, such that surface area per volume is minimised.

The cooling steps follow the same route as the grey cast irons with the graphite precipitates growing in spherical shapes.

White Cast Irons:

These contain less Si or C than grey cast irons and undergo faster cooling. This results in cementite forming in favour of graphite. Again the name &#;white&#; has little to do with the ordinary appearance of the alloy, but rather refers to the fracture surface. White cast irons are much more brittle than grey cast irons, and so their fracture surfaces are reflective, leading to their classification as &#;white&#;.

The cooling route depends on the composition of the melt, whether it is hyper &#; or hypo &#; eutectic (eutectic composition is at 4.3wt.%C). A hypereutectic composition leads to the cementite precipitating out first; a hypoeutectic composition leads to γ &#; austenite precipitating out first.

Note &#; &#;hypereutectic&#; has a higher carbon content than the eutectic composition.
            &#;hypoeutectic&#; has a lower carbon content than the eutectic composition.

The first phase to precipitate out forms dendrites due to non-equilibrium effects; the cooling melt does not always follow the predicted composition on the phase diagram (see the page on the lever rule in the Phase Diagrams and Solidification TLP and the dendritic growth page in the Solidification of Alloys TLP).

When the eutectic is crossed the remaining melt solidifies as an austenite, cementite eutectic (ledeburite).  The carbon continues to be ejected from the austenite as the alloy cools, diffusing to the cementite.   At the eutectoid temperature the final transformation takes place from austenite to pearlite.  In some very quickly cooled white cast irons the austenite may transform to martensite.

If you are looking for more details, kindly visit cast iron prototype.

9.5 Cast-Iron

9.5.1 General Remarks

Cast-iron seems to hold no interest for connoisseurs of sword blades. Well, yes - but wootz steel, with a carbon content around 2 %, is just at the edge of what one usually associates with cast-iron: steel with a carbon content of 2 % to about 4,5 %.
That's why I will give cast iron a quick look. Let's see what we can learn with respect to sword blades. Having made it so far, you are now chomping at the bit and ready for a quick look at the relevant part of the phase diagram. Here it is:   Iron - carbon phase diagram with the cast-iron part.       Let's make two things absolutely clear before we start:

1. Cast-iron is not (pure) iron that has been cast as a liquid into some mould but an iron alloy!
2. There is no such thing as plain carbon cast-iron! Carbon is always the major alloying element in cast-iron but there are always others, too.

Having made that clear I will from now on write cast iron without a hyphen, like everybody else. There are two basic kinds of cast iron and it is important to make this distinction:

1. The liquid stuff that comes out from a blast furnace (or any other furnace if it gets hot enough). This kind of cast iron is also know as "pig iron" and contains all kinds of "dirt" besides a high carbon concentration.
2. Some intentionally engineered iron alloy with a low melting point and thus always with > 2 % carbon as one of several alloying elements.

The first kind is the raw material we use for steel making since the - roughly - 15/16th century. The second kind is now a "High-Tech" material used for many demanding applications. In between these extremes is everything you can think of.
In order to get an idea of what the "cast iron" alloy family contains, let's start with the simple and purely theoretical situation of having iron and more than 2 % carbon in a state close to nirvana. That is what the phase diagram above describes. This fictitious material can serve as a starting point just as well as it did for the fictitious material we call "plain carbon steel" if the carbon concentration is lower than 2 %. First I need to make a confession: the phase diagram shown above is actually not showing the real nirvana states! All this time I was deceiving you! Sorry - but no real damage was done so far. So what is the truth and nothing but the truth about the iron-carbon phase diagram? Here it comes: True nirvana states do not call for mixtures of ferrite or austenite and the iron carbide (Fe3C) that was named cementite but for iron and pure carbon in the form of graphite.
An old science module actually contained all of that. It also shows the true iron-carbon phase diagram, and if you activate the link you see that it hardly differs from the "untrue" iron-cementite phase diagram.
The point of all this is:      
There is never graphite in steel.
There might be graphite in cast iron.

      As long as we have low carbon concentrations, or (plain carbon) steel in other words, it is always cementite that forms first. It is only metastable, yes, and should eventually decay into graphite and iron. But it will not do so in your or my lifetime and that of our descendants; just like our diamonds.
For high carbon concentrations, i.e. cast iron, this is different. We might find graphite instead of cementite in there, in particular if some other elements like silicon (Si) are also present.
Allright - now let's look at the phase diagram in the 2 % carbon - 6.67 % regime and go with cementite and not graphite for a first shot. The major features are. For a composition of 4.3 % carbon we have an eutectic point at 1.130 oC (2.066 oF). Use the link to refresh your memory about that. A melt with this composition will directly solidify into an "eutectic" mix of austenite and cementite. It is similar to what happens at the eutectoid point for steel, where we get ferrite and cementite. The resulting mix in this case we called pearlite and treated it as a (pseudo) phase in its own right.
Same thing here. Solidification at the eutectic point produces a mix of austenite and cementite, and we treat that as a (pseudo) phase. We call that pseudo-phase ledeburite , after Adolf Ledebur, an eminent German iron and steel engineer and scientist in the second half of the 19th century. Ledeburite might be expected to form the typical eutectic zebra pattern but that is not what it does. The zebra pattern forms because repartioning the carbon (or any other element) is difficult at lower temperatures. It is not all that difficult when a liquid solidifies. We might rather expect dendritic structures in this case. Whatever - nobody has ever seen ledeburite directly because it only exists at high temperatures. So far so easy. But now we decrease the temperature to below the transformation temperature at 723 oC ( oF). What will happen? Well - the phase diagram does tell: The austenite (g) and cementite mix that we call ledeburite transforms into a pearlite - cementite mix.
Only the austenite needs to transform into pearlite or ferrite plus cementite. The primary cementite already present at high temperatures in the ledeburite need not do anything. We still call the pearlite - cementite pseudo-phase that we get a low temperatures at 4.3 % carbon concentrations ledeburite or, if you are a stickler for details, transformed ledeburite or ledeburite II (the high-temperature variant is then ledeburite I). We might expect pearlite grains embedded in cementite since the mixture is cementite-rich. What happens if we are not at the precise eutectic concentration of 4.3 % carbon? What always happens: for lower carbon concentrations ("hypoeutectic") we have a decomposition into pearlite and ledeburite II, and for higher concentrations ("hypereutectic") the decomposition produces ledeburite II and cementite. That is a bit academic, however. We know already that what you get depends very much on how fast you cool. If we cool not all that slow, the transformation of the austenite into pearlite becomes difficult and we encounter all the phenomena we found for steel (and possibly some more). With increasing cooling rate the pearlite gets narrower and less well developed, and eventually we find bainite. For very large cooling rates we might even get martensite.
Contrariwise, for very slow cooling and high carbon concentrations we might now encounter the real nirvana structure, which is not cementite but graphite - see above. So what does ledeburite II look like at room temperature? If we cool slowly, we would expect pearlite grains, developed from the primary austenite and embedded in the primary cementite. Why should they be embedded? Look at the carbon concentration. A pure Fe - 4.3 wt% C alloy corresponds to about 18 at % carbon. Since every carbon atom binds three iron atoms to form cementite, 54 % of the remaining 82 % iron atoms are used up and we have only about 30 at % "free" iron left in a ledeburite eutectic. Most everything is cementite and that's why we expect the ferrite to be completely embedded in cementite.
Ledeburite types of cast iron, or essentially pearlite embedded in cementite, will fracture right through the cementite, giving the fracture surfaces a whitish appearance. That's why this kind of cast iron is called white cast iron. It is not easy to find good microstructure pictures of eutectic pearlite- cementite ledeburite II. The reason is simple and mentioned above: there is practically no such thing as a pure Fe - 4.3 wt% C alloy, slowly cooled.
Here is the best picture I could find:     Microstructure of white cast iron Source: Internet article of Miguel Angel Yescas-Gonzalez and H. K. D. H. Bhadeshia; from the PhD thesis work of Miguel Angel Yescas-Gonzalez. With friendly permission.       We see a cut through an original austenite dendrite that has turned to pearlite. It is embedded in whitish cementite. In addition we have small flecks of pearlite because the sample actually had a carbon content of 3.6 % carbon (and 0,1 % silicon) and thus was hypoeutectic and needed to form some more pearlite