Carbides in Ductile Iron

[habu2112]

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

 

[metengr]

According to several of my technical references, the ASTM A 536 ductile iron 80-55-06 is specified in the as-cast condition, and consists of a ferrite/pearlite microstructure. Pearlite is a lamellar structure that consists of two phases – ferrite and iron carbide.

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;

http://www.ductile.org/didata/Section7/7intro.htm#Temper Embrittlement

 

[arunmrao]

Ductile iron castings are not as section sensitive as gray iron castings. In the case of gray iron castings this problem is acute. However, a well inoculated ductile iron casting should not pose any carbide related problem.

This alloy is predominantly pearlitic . If you can normalise and use these castings any traces of carbides present will have dissolved in the matrix.

 

[Flesh]

Yes, it is a concern. What you are referring to is known as "primary" carbides, different from the lamellar carbide metengr discussed. Formation is dependant on chemistry, cooling rate, and mold design. What you will find is that your percent elongation will fail to meet your minimum requirement if the amount of primary carbides is too high. There are several tech papers that investigate your percentage question. I have several of them I can post back with their authors when I find them.

 

[metengr]

The primary carbides that flesh refers to are called eutectic carbides. As anrunmrao stated, a properly inoculated ductile iron casting that promotes graphite spheroids will not result in eutectic carbides. The bottom line is that large section thickness in a casting would tend to adversely effect graphite shape. Thinner sections would cool more rapidly, and would tend to develop a higher percentage of graphite spheroids of the required shape.

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)

 

[Flesh]

habu2112:

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).

 

[swall]

Years ago, I dealt with a foundry that had problems with high primary carbides (up to 40% level)in 5505 ductile iron in sections .25" and smaller.The cause was high residual levels of magnesium from their Fischer converter process used for nodulizing.