×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Carbides in Ductile Iron

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

RE: Carbides in Ductile Iron

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%20Embrittlement

RE: Carbides in Ductile Iron

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.

RE: Carbides in Ductile Iron

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.

RE: Carbides in Ductile Iron

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)

RE: Carbides in Ductile Iron

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).

RE: Carbides in Ductile Iron

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources

White Paper – Your Construction ERP RFP Checklist
Selecting business software for a medium to enterprise-sized construction concern is extremely challenging in large part because most enterprise resource planning (ERP) suites originated in the world of repetitive manufacturing and are therefore a poor fit for a project and asset-centric business. However, midsize to large contractors need the predictable, auditable processes that ERP delivers. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close