It Depends!: Why this is the most popular answer when you ask what is causing a particular centrifugal casting defect.
We often get asked about what could be causing a specific defect on a centrifugal casting. Our answer is often “It Depends”.
Successful Centrifugal Casting is like a puzzle. All the pieces must fit together to make a clear picture. You must start with a quality piece of spinning machinery that works to eliminate vibration. Once you have the machine you must control your process.
“It Depends” is the reason we always ask for the following information when our customers require process help:
- Size of casting.
- Location of defect. For example is it located all over the casting or specifically in one place.
- Alloy being poured.
- Degassing process being used.
- Coating type and application method.
- Pouring temperature.
- Initial spin speed and changes to spin speed once pouring is complete.
If you don’t have all or most of above items 1-7 documented when producing either castings of good quality or castings with defects then you need better control of your process!
“It Depends” is why when our customers tell us they are experiencing a porosity defect, we ask to see a picture. Porosity is not a detailed enough descriptor when talking about a centrifugal casting defect.
To Quote the on-line AFS article, Understanding Porosity,
The term porosity is used extensively by both casting suppliers and customers when talking about casting defects, but it does little to describe the actual problem. “Porosity is such an all-encompassing word,” said Vadim Pikhovich, materials engineer for specialty heavy truck manufacturer Oshkosh Corp., Oshkosh, Wis. “It can be shrink in the form of micropores, sponge-type voids, large macro-voids, inclusions. It all gets bundled up in one big word: porosity.”
In Centrifugal Casting, the process pieces we are most commonly working with are:
- Pouring
- Rotation
- Temperature
- Material
- Coating
Several common defects can be caused by issues with one or more of the above mentioned pieces of the process. It’s our job to figure out which one.
Longitudinal Hot Tear
For example, a Longitudinal Hot Tear can be caused by:
- High Spinning Speeds
- Excessive Machine Vibration
- Very High Pour rates.
- Very low Pour Temperature
- Very High Pour Temperature
This is why when you ask us why it’s happening, we say “It Depends”. So, if it depends, then how should you begin in determining what the cause is?
First, document your process in as much detail as you can in a consistent manner.
Second, we suggest you begin with what you can determine see or hear. For example, if you can see vibration in your machine, or in the rotating die, then it is likely causing a casting defect. Work to eliminate causes of vibration.
And, be sure to check that there is nothing preventing water cooling from effectively cooling your casting and affecting the solidification rate. Are your water cooling nozzles free of debris and unblocked? Do you have enough water pressure to supply the appropriate amount of water to the spinning die? If the answer to these questions is “no” then remedy this problem.
If the vibration and cooling processes check out, try to determine when you are getting the defect. Every casting? Only in the morning? Documentation of this kind of information can help determine if you are seeing a defect that can be attributed to one machine operation but not another. If you suspect that your machine operators might have different process control criteria, check things such as temperature of the metal when pouring and the pour rate. These are things that may vary from one operator to the next.
When researching the “why” of a defect we use as much data as is available to us to solve the problem. Having casting data can help us change our answer from “It Depends” to successful defect elimination.