Carbides in Ductile Iron
Carbides in Ductile Iron
(OP)
Hello,
I am not a metallurgist but do have a question. I have a casting that has thick (1.50”) and thin sections (.250"). It is produced from A536-84 Ductile 80-55-06. With the difference in thickness, is it prone to carbide buildup in the thin sections? Is there a maximum allowable percentage before the carbide would have a detrimental effect to the mechanical properties of the casting? (breaking at the thin section)
Thanks in advance
Steve
I am not a metallurgist but do have a question. I have a casting that has thick (1.50”) and thin sections (.250"). It is produced from A536-84 Ductile 80-55-06. With the difference in thickness, is it prone to carbide buildup in the thin sections? Is there a maximum allowable percentage before the carbide would have a detrimental effect to the mechanical properties of the casting? (breaking at the thin section)
Thanks in advance
Steve





RE: Carbides in Ductile Iron
The thinner sections of the casting would tend to cool quicker in comparison to the thicker sections. However, the faster cooling rate would tend to produce a finer pearlitic structure, which is desirable. The carbides that you refer to would still be lamellar in shape, and would not tend to segregate at preferred locations and adversely effect mechanical properties.
Considering the thickness variations that you described, I would recommend a stress relief of the casting between 1000 to 1100 deg F. This would remove harmful residual stresses that could result in failure at thickness transitions in service, or distortion during machining.
If you want more information regarding ductile iron castings, I would highly recommend the web site below for further reading;
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RE: Carbides in Ductile Iron
This alloy is predominantly pearlitic . If you can normalise and use these castings any traces of carbides present will have dissolved in the matrix.
RE: Carbides in Ductile Iron
RE: Carbides in Ductile Iron
The reference book titled "Principles of Metal Casting" by Heine, Loper and Rosenthal, states that prevention of eutectic carbides during solidification is dependent on a sufficiently high-base carbon equivalent and the development of an adequate number of graphite spheroids. An estimate of the minimum number of graphite nodules required in carbide free ductile iron is indicated below;
for bar 1/2" in diameter - 465 nodules per square inch
for bar 1" in diameter - 80 nodules per square inch
for bar 1.5" in diameter - 59 nodules per square inch
(this data was extracted from Principles of Metal Casting)
RE: Carbides in Ductile Iron
Check out these AFS Transactions for data that describes the effect of percent primary carbides on mechanical properties.
1. #01-082, A.Javiad.
2. #02-028, A.Javiad.
3. 97-30, Cast iron microstructure anomalies and their causes.
Nodule count is also a very key measurable, as arunmrao stated above. Proper inoculation throughout a production run is sometimes easier said than done. You may require in-mold inoculation. Paper #2 gives an equation for critical nodule count (up to 4.5mm).
RE: Carbides in Ductile Iron