As foundries, we always try and produce the best product possible for our customers. However, as anyone familiar with foundries knows, there is an inherent amount of variation in foundry processes that sometimes leads to less than desirable outcomes. Most of the time, we can catch these issues before they leave the facility. Sometimes it’s by visual inspection, other times by non-destructive testing, and also by doing tension and impact tests on test specimens cast from and processed with the same batch of metal as the casting. Unfortunately, despite all of these measures, some bad products inevitably pass through the screen of tests and make it out into the field, and though it is uncommon, failures in the field do happen.
What causes a “Rock Candy” fracture
In steel castings, one of the most common failures that we see in the field is called a “rock candy” fracture. The reason it is called a “rock candy” fracture is because the fracture face itself tends to have a jagged and granular appearance, similar to what rock candy looks like. Under a microscope rock candy fractures will have a shiny appearance and the cracking will be almost curved in some areas. Rock candy fracturing has been a persistent issue for steel casting manufacturers for over 100 years, and even though we have learned a lot about it in the past 20-30 years, it is still a common issue in the industry. The cause of a rock candy fracture is a defect that is called aluminum nitride embrittlement. Though “rock candy fracture” is still a well-recognized definition of this defect in the community, a more technical term for this failure would probably be something like “aluminum nitride fracture”.
A closer look at aluminum nitride embrittlement
To begin to understand what aluminum nitride embrittlement is, lets start at the molecular level and consider what an aluminum nitride is. An aluminum nitride is of the formula (AlN), it is a single aluminum atom and a single nitrogen atom that are bonded together to form the compound. A cluster of aluminum nitrides forms a crystal-like structure that is rigid and brittle.
How aluminum nitrides form in a steel foundry
Another important area of intrigue is how and why aluminum nitrides form in a steel foundry. Aluminum nitrides in a steel foundry typically form during the tapping of the metal from the furnace into the ladle. During tapping, steel foundries conduct a process called “de-oxidation”, where certain elements are pitched into the stream of metal coming from the furnace into the ladle to react with or “tie up” the free oxygen that is present in the melt. Elements used for de-oxidation are typically those with the highest affinity for oxygen, and near the top of that list is aluminum, which makes it a very effective de-oxidizer. In addition to frequently bonding with oxygen, aluminum also tends to bond with nitrogen, and in an air melt process, there is always some amount of free nitrogen in the melt, so it is very easy for aluminum nitrides to form in a steel foundry.
In a process like the one that Temperform uses, following the tapping of the metal from the furnace into the ladle, the metal is then poured directly into the prepared air set molds to make castings. When aluminum nitrides are formed during the tapping of the metal into the ladle, they get poured into the mold along with the rest of the volume of the molten metal. As the metal solidifies, the aluminum nitrides cluster together at the grain boundaries in the steel, and when the metal is all the way cool, they are still there at the grain boundaries. As we discussed previously, aluminum nitrides are brittle, so having them at the grain boundaries can be a big problem, because you are essentially making the weakest point of your metal more susceptible to a brittle fracture. Aluminum nitride fractures also tend to grow very quickly and easily, so once they begin, they can very easily break all the way through even a very heavy section of a casting.
Monitoring the quantity of aluminum nitrides is the key
The story of aluminum nitrides largely revolves around the quantity of them in a given metal casting. One thing that is important to consider is that in some small quantities, aluminum nitrides are acceptable, and to some degree, can actually be considered a good thing. Aluminum nitrides, while they are brittle, also do something called “pinning” in which their location at the grain boundaries stops the grains from growing during heat treatment, resulting in a finer grain steel, which leads to better strength and toughness properties. However, if you pass a certain quantity of aluminum nitrides, depending on the section thickness, that is when embrittlement occurs, and the strength and toughness of the material begins to become severely compromised. The exact point at which embrittlement starts to become a problem is somewhat debated among researchers, but it can be reasonably interpolated based upon the nitrogen and the aluminum content of the material.
Preventing and detecting aluminum nitride embrittlement
Preventing and detecting aluminum nitride embrittlement largely comes down to understanding the composition of the metal that is being poured. Knowing the amount of residual nitrogen and aluminum in your furnace allows you to determine your risk. It lets you plan to combat that risk by choosing your charge materials carefully, controlling your melt, and selecting the right type and quantities of materials for de-oxidation. Unfortunately, there is no direct way to test for aluminum nitride quantities in metal. They are pretty much entirely undetectable on a micrograph and there is no way to chemically measure for them. However, once you de-oxidize the metal, verifying the composition of the metal in the ladle is a good way to get a gauge on whether or not there might be a problem with aluminum nitride embrittlement. Another method to check for aluminum nitride embrittlement is to look at an impact fracture surface and see if it exhibits the telltale signs of an aluminum nitride fracture. A good way to check this is to look at a test bar that has been subjected to a Charpy impact test, or to just notch a piece of metal and hit it with a hammer, forcing it to break, and inspecting that surface.
Foundries should take steps to mitigate “Rock Candy” fracturing
Overall, rock candy fracturing, or aluminum nitride fracturing, is a common failure made in steel castings that foundries should be aware of and take steps to mitigate. In general, this follows the common theme in foundries, and anyone who has listened to a presentation of mine, or spoke to me in person, has probably heard me say some version of the following: “In a foundry, understanding how all of the input variables effect your final product is key to achieving success.” Aluminum nitride embrittlement is an output that can be controlled by understanding what we are putting into our metal and how it could lead to this defect.
Contact Lawton Standard for your casting needs
Lawton Standard is committed to the highest quality castings. We can meet your steel and iron casting needs with our 6 foundries in the Lawton Standard family of companies. Reach out to us today to talk to a foundry expert or to get a quote.