Why Primary Graphite is Bad!
Author: Rudolf Sill้n
CEO, NovaCast Foundry Solutions AB

Primary graphite can easily occur in ductile iron if the carbon equivalent is too high in relation to the cooling rate. Primary graphite can be the cause of casting defects such as shrinkages and carbon flotation. Let us compare two cases:

 Case A: Assume a hypereutectic alloy with C=3.9 and Si=2.8 giving CEL=4.6% (see illustration).

During solidification primary graphite will precipitate from the liquidus temperature until the eutectic temperature is reached. Dissolved carbon in the liquid is thereby reduced by 0.3% (4.6 - 4.3).  The dissolved carbon in the liquid is thus reduced to 3.6% (3.9 - 0.3). If the austenite can dissolve 1.8% carbon then the amount of carbon available to create eutectic graphite is 1.8% (3.6 -1.8). This eutectic graphite expands and can reduce, or in optimal cases, eliminate shrinkages. Note that the primary graphite is precipitated too early and is therefore not effective for reducing shrinkages that occur at a later stage in the solidification process.

Case B: Assume another alloy with C=3.9 and Si=1.6 giving CEL=4.3%. That alloy will start to solidify as eutectic (no primary austenite, no primary graphite). Therefore the carbon in the liquid remains at 3.9% during the eutectic solidification. The amount of eutectic graphite is therefore 3.9 - 1.8=2.1%.

Thus this alloy has almost 20% more eutectic graphite than the hypereutectic alloy which is a quite substantial improvement!

If the amount of eutectic graphite and its expansion is not sufficient to compensate for the contraction of the austenite and the remaining liquid then micro shrinkages are created. Therefore it is very important to avoid primary graphite.

Where is the eutectic point?
The traditional iron-carbon phase diagram shows that the eutectic point is around 4.3%. However, that is during equilibrium conditions with a slow cooling rate.

Therefore, if the chemical composition of an alloy is higher than 4.3 e.g. 4.5 but the cooling rate is high, the solidification will probably NOT be hypereutectic as expected but eutectic! An alloy that is hypereutectic as calculated with the formula C+Si/4+P/2, will solidify with primary graphite in a casting only if the cooling rate does not exceed a certain level. Especially for ductile iron it is important to use an alloy that does not solidify with primary graphite in order to avoid defects.  Therefore the optimal active carbon equivalent (ACEL) value must be chosen depending on the cooling rate of the castings. In order to be able to keep a certain target for ACEL, thermal analysis using grey solidification as in ATAS should be used for maximum accuracy.

The diagram above illustrates the approximate location of the eutectic point as a function of the casting modulus which is related to the cooling rate.  The illustration below shows the occurrence of shrinkages both for hypo- and hypereutectic compositions. For ductile iron the eutectic “point” seems to be more like a plateau.

 Expansion of primary graphite
If the alloy solidifies as hypereutectic then the primary graphite grows directly in the contact with the liquid phase. Its expansion offsets the contraction of the liquid phase. An alloy with ACEL=4.5 will result in an expansion of about 0.8% between the liquidus temperature (about 1200 C) and the eutectic temperature. The contraction of the liquid phase is about 0.75% which means that the total volume change from liquidus to the eutectic temperature is more or less zero. If the casting uses a feeder then the effect might be that the feeder does not start to pipe as there is no need for feed metal initially. Instead shrinkages might develop close to the feeder neck when feed metal is required later in the solidification process.

Below is a picture of a real case where the feeder did not pipe. The micro to the right of the photo shows many large nodules which proves that the solidification was hypereutectic. When the solidification was eutectic then the feeder piped and the shrinkage disappeared.

If the alloy solidifies slightly hypoeutectic (ACEL 4.2 – 4.25) then some primary austenite will be the first phase. Then there is a contraction in liquid state and the feeders will start piping more easily. Another effect might be that the graphite spheroids, precipitated at the eutectic temperature, are surrounded by austenite at an earlier stage and their growth rate is thereby reduced. The graphite precipitation pattern might be more gradual with more expansion at the end of freezing.

The picture illustrates the density changes for a sample with hypereutectic composition during solidification. Initially the density increases resulting in a contraction in liquid state. When liquidus is reached (TL) then primary graphite is precipitated and expands. At the same time the liquid contracts. The expansion and contraction during this phase (S1) are almost the same which means that the density does not change. When the low eutectic is reached (TElow), eutectic graphite is precipitated which causes an expansion. The first part (S2) until TEhigh is reached is the recalescence. Then the second phase of eutectic freezing commences (S3), often with a slight density increase especially close to the solidus temperature (TS) also called end of freezing. Then the density increases further in solid state (pattern mans shrinkage). The picture below shows a typical hypereutectic curve analyzed by ATAS.

Conclusion:
Avoid precipitation of primary graphite! For maximum eutectic graphite adjust the chemical composition to fit the solidification rate in the casting! Using chemical analysis to calculate the carbon equivalent is not suitable due to the limited accuracy (+/- 0.05 or more). The foundryman might believe that the carbon equivalent is within range when it is not!   Use ATAS and the ACEL method in order to achieve the desired target for ACEL with high accuracy (+/- 0.01).