In our business, we talk a lot about nodularity, and for good reason — it is a prime indicator of quality in ductile iron castings. But what is it, why is it beneficial, and how do we optimize our ductile iron casting process to achieve it?
What is nodularity?
In short, nodularity refers to the prevalence of spheroidal graphite nodules within a ductile iron casting's microstructure. We can measure that by how round the nodules are (graphite nodularity percentage) and how much of a casting's surface area they occupy (graphite area percentage).
Nodularity is more than desirable — it's the whole reason ductile iron is ductile (able to stretch and bend without breaking)! Because of its paramount importance, ductile iron is often alternatively referred to as nodular iron or spheroidal graphite (SG) iron.
Achieving more nodular iron
To achieve superior nodularity in our ductile iron casting process, there are a few things we must pay attention to:
- What we start with — the base iron and accompanying metals a melting furnace is "charged" with
- What we put in — the makeup of the magnesium treatment that encourages nodulization
- How we put it in — i.e. inoculation method
- Temperature phasing of the casting as it cools
Let's discuss these considerations one by one.
Starting off strong with purer pig iron
Pig iron (aka crude iron) forms the foundation of a ductile cast iron casting and the overwhelming percentage of its composition, so it's imperative we start off right.
We do not want to spend a lot of time and energy "rolling around in the mud" with our pig iron — filtering out residual elements that will interfere with nodule formation ( e.g. titanium, arsenic, lead, tin, antimony, bismuth, and especially sulfur) and/or dirty our metal matrix structure with carbides (chromium, molybdenum, vanadium, manganese).
Beginning with a cleaner canvas provides a clearer path to casting with desirable mechanical properties, without incurring high costs due to inefficiency and processing defects.
The magic of magnesium
You may have heard of "influencers" in social media — well, in a ductile iron foundry, magnesium typically plays that role. In nature, it's never not affiliated with other elements, and it enjoys causing reactions. When carefully incorporated into our molten iron, it promotes a phenomenon known as diffusion, pulling carbon and nitrogen ions away from the carbon that is needed to form our spheroidal graphite nodules, freeing it up for other engagements.
As we just mentioned, introducing elemental magnesium to the melt is not all that practical — given its proclivity to form compounds, it's difficult to isolate. Furthermore, its low density and boiling point would pose a significant safety risk (vapor pressure buildup > exploding gas bubbles), since they are so out of equilibrium with those of iron.
To circumvent this, we introduce a small amount of magnesium to the melt through a master alloy (magnesium plus another metal such as nickel, silicon, or copper) with thermodynamic properties more consistent with iron.
The intricacies of inoculation
As molten iron is poured over the master alloy, fine magnesium particles are infused into the melt, a process known as inoculation. There are multiple ways to do this, but at Urick we favor the vertical in-mold method, in which the magnesium-containing alloy is positioned in a reaction chamber built into the inlet of the casting mold. This encourages an even distribution of inoculants throughout the casting, and in turn, eutectic solidification as the part cools — meaning the graphite and iron firm up at the same rate (and conversely would melt again at a single temperature lower than the constituent parts — eutectic is Greek for "well-melting").
The size of the inoculation chamber, the rate at which we pour the molten iron, and the dissolution properties of the magnesium alloy can be adjusted depending on the properties we want in the finished casting.
If getting the graphite molecules together is step one to a more nodular casting; step two is getting strong networks (crystalline matrices of iron) to form around them. We can manipulate which microstructures form by phasing the casting through various temperature ranges as the part cools. We also might apply any of the heat treatments of ductile iron, such as austempering, after production.
The ultimate objective of the ductile iron casting process is to achieve a 100% nodularity rating — meaning the graphite nodules are perfectly spheroidal. When a graphite particle's length is more than twice its diameter, it is classified as a floccule and disqualified from the nodule count. Urick is proud to produce castings that regularly exceed the industry standard of 80% nodularity, regularly achieving 90% nodularity or higher.
Urick does ductile differently
Nodular iron requires nodules, and our proprietary ductile iron processes beget higher nodule counts and nodularity than the competition. For consistency and capability, look no further than Urick.