Often we have oversize of massive elements of casting. This oversize can reach 5 % (look at the picture). This problem looks like material grows (ductile iron). We know that increase of metal volume while it cooling, is impossible. Bat we have it.
One more, when we pour grey iron (without the spheroidising and modifying master alloy in a reaction chamber), we will have normal sizes of our casting.
May be someone had similar problem and knows decisions or materials of it?
you do not give any details of your mold. I assume this is a greensand mold, and probably not particularly hard rammed. Even so, having said this, little is to be gained unless the design of the feeder and feeder neck is correct. Frankly this is not easy.
During the expansion of the graphite, the corresponding volume increase has to be allowed to escape from the mold cavity, otherwise the pressurized residual liquid will expand the cavity. The large expansions you note arise simply from the sum of volumes from other parts of the casting, which will concentrate in the hottest, weakest regions of the casting.
The feeder neck is the only outlet for the pressurized liquid. Whereas most feeders simply provide feed metal to the mold cavity to feed the contraction of the liquid metal on freezing, in the case of irons, especially ductile irons, the feeder also has to work in reverse, acting as a overflow receiver.
However, the design of your feeders ensures that they will have outer surfaces which are too cool. They will be unable to move to provide extra volume, so the pressure is unfortunately relieved by expanding the casting rather than reversing the volume into the feeder.
If the feeder is designed as a tall cone, pointed at the top, this curious design means that all the early feeding from the feeder is concentrated on this top point, which therefore collapses quickly as the metal level in the feeder falls, being sucked down and into the casting. In this way the feeder maintains a liquid upper surface, which can reverse, and refill with displaced liquid as necessary, to avoid the generation of pressure inside the mold cavity.
The last caution about this complicated design is that the ingate into the system must freeze early, to ensure that the feeding action, the suction on the top points of the feeders, occurs early. If the ingate is too large, some feeding will occur through this gate, and will therefore delay the feeding action of the pointed feeders. This is disaster. So Beware!
I do not know the origin of this clever but difficult technique. I imagine it may have descended from the revered Steven Karsay.
For information on other casting techniques and other matters such as the mechanisms of graphite formation, I recommend my book "Complete Casting Handbook" 2nd Edition 2015.
Best of luck!
John Campbell
Dear Sir. Ductile iron have dual behaviour, this may be contractive or expansive. The ideal is to obtain the last behaviour because the yield of the process improve. For more information I attach to you a paper make by an world especialist in this casting material.
May be, if you read spanish, Sir, I have more interesting information about this issue.
Thank you for answer. Your article is very interesting for me.
Unfortunately I can`t read Spanish.
But about my question.
I know about expansion of iron it happen because of graphitization.
But I think that value of expansion is not more than 1 %.
The cross section of massive part on the picture is 40 X 36 mm (W/H). Sometimes we have 42 X 37,5 mm.
As for me, this is look like appearance of some pressure after formation solid shell.
We use modification agent with 6-7 % Mg.
Here is another interesting work.
I take contact with expert Norberto Rizzo to know your comments about your question.
Dear Sir. I talk with Norberto Rizzo about your question and he said that in the better conditions can be expand until 4%. The % of expand depend of the quality of liquid, the module of the part and the casting system.
Dear Sir. I read last article very intently. This is that I need.
I got a confirmation of my guess.
Usually we have hyper-eutectic iron, so I must check something.
About casting system I am no sure but have some ideas.
Thank you very much and thanks to Norberto Rizzo.
If you are interest I will send you what we will get or not.
But it will be no so soon.
you do not give any details of your mold. I assume this is a greensand mold, and probably not particularly hard rammed. Even so, having said this, little is to be gained unless the design of the feeder and feeder neck is correct. Frankly this is not easy.
During the expansion of the graphite, the corresponding volume increase has to be allowed to escape from the mold cavity, otherwise the pressurized residual liquid will expand the cavity. The large expansions you note arise simply from the sum of volumes from other parts of the casting, which will concentrate in the hottest, weakest regions of the casting.
The feeder neck is the only outlet for the pressurized liquid. Whereas most feeders simply provide feed metal to the mold cavity to feed the contraction of the liquid metal on freezing, in the case of irons, especially ductile irons, the feeder also has to work in reverse, acting as a overflow receiver.
However, the design of your feeders ensures that they will have outer surfaces which are too cool. They will be unable to move to provide extra volume, so the pressure is unfortunately relieved by expanding the casting rather than reversing the volume into the feeder.
If the feeder is designed as a tall cone, pointed at the top, this curious design means that all the early feeding from the feeder is concentrated on this top point, which therefore collapses quickly as the metal level in the feeder falls, being sucked down and into the casting. In this way the feeder maintains a liquid upper surface, which can reverse, and refill with displaced liquid as necessary, to avoid the generation of pressure inside the mold cavity.
The last caution about this complicated design is that the ingate into the system must freeze early, to ensure that the feeding action, the suction on the top points of the feeders, occurs early. If the ingate is too large, some feeding will occur through this gate, and will therefore delay the feeding action of the pointed feeders. This is disaster. So Beware!
I do not know the origin of this clever but difficult technique. I imagine it may have descended from the revered Steven Karsay.
For information on other casting techniques and other matters such as the mechanisms of graphite formation, I recommend my book "Complete Casting Handbook" 2nd Edition 2015.
Best of luck!
John Campbell
Fascinating answer from John Campbell. Free stuff on ResearchGate!
David
Thanks for your answer Mr. Campbell!
You are absolutely right about mold, feeders and feeder neck.
Existing casting is on first picture.
Feeder neck in this case is to cold, so month ago I decided to change it (second picture).
My logic was simple. In first case casting volume was isolated from feeder and this could generate some pressure that changed geometry of casing. Changing of feeder neck will connect casting and feeder volumes and possible reduce the pressure. Now i know how to change feeders and ingate. Thank you.
I will write results.
Good day, colleagues!
We decided problem with the oversize. In last controlling pouring we produced around 500 details without any oversize and shrinkage defects.
We changed the feeder neck as I showed in last message, and we enhance pressurization of sand.
I think that increase of pressurization was more important in this case, but I can`t say that it was main problem.
We have other detail where oversize problem is take place. I will continue to change a technology and observe a changes.
One more moment, sometimes after several second after pouring, metal in pouring cup starts to oscillate. We observe metal emissions. It is unfinished reaction in reaction chamber, but I don’t know why it happens. It may be cause of impulse of high pressure that may change casting form.
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