Ductile Iron casting process 4

[awol]

True or False? A536 castings that are shaken out of their molds before the test coupon is shaken out may have different mechanical properties than the coupon. (the coupon has more "cool time" in the molds)
Is the only analytical method to determine stength is to hardness check each casting? Is the casting company following ASTM code?

Also, I was informed I cannot use standard brinell conversion charts that are based on "steel" to convert brinell to aproximate tensile strength of ductile iron.

Is there a "special" hardness conversion chart for ductile iron?

Thanks for the help.

AWOL

 

[TVP]

Answer to Question 1: Yes, it is possible that castings and coupons will have different mechanical properties, depending on the cooling rate, which affects final microstructure.

Answer to Question 2: No, hardness is not the only method for determining "strength". In fact, hardness is not specified as a requirement in ASTM A 536-- yield strength, tensile strength, and elongation are the requirements. Hardness is a specified requirement for SAE J434, which is another specification for ductile/nodular iron.

If you have actually specified ASTM A 536 on your Engineering Drawing, then your casting vendor should be providing you will tensile test data, as required by section 4, Tensile Requirements. This testing will be performed on separately cast test specimens only, unless you have also specified that test bars be machined from actual castings. Also, section 6 of ASTM A 536 provides details on how the test specimens should be prepared.

Answer to Question 3: You did not provide enough data for me to be sure. I recommend that you discuss the following with your casting vendor: creating a test matrix that includes all of the pertinent variables related to strength. That is, perform tensile tests on separately cast specimens, as well as on specimens obtained from representative area(s) on the casting. Also perform hardness tests on both of these different types of specimens. The final testing to correlate all of this would be to analyze the microsructure of the separately cast test specimens with that of the actual castings. The goal is to obtain similar microstructure and properties in both the test specimens and the actual castings.

Answer to Question 4: Correlating hardness to tensile strength does differ from material to material, so you should only use data that is appropriate to cast iron. You should definitely NOT use the standard data obtained on steel specimens that appears in ASTM E 140, SAE J417, Machinery's Handbook, etc. There does exist some good data on strength (yield & tensile) vs. hardness and elongation vs. hardness in ASM HANDBOOK Volume 1, Properties and Selection: Irons, Steel, and High-Performance Alloys. The section on ductile iron is approximately 20 pages, and would give you a lot of background on the material, microstructure, properties, etc.

The entire book is somewhat expensive (~ $200 nonmember, $160 member), but it is well worth the price, as is the entire ASM HANDBOOK collection. ASM also sells a volume called ASM SPECIALTY HANDBOOK, Cast Irons, which is 494 pages, and sells for ~ $210 nonmember, $170 member. It contains this info, as well as all of the information on cast iron that is printed in the 21 different volumes of the ASM HANDBOOK series (Vol 15 is Casting, Volume 16 is Machining, Volume 8 is Mechanical Testing, etc.)

A lot of this information is also available in the METALS HANDBOOK, which is a condensed version of the 21 separate volumes of the ASM HANDBOOK series. The price of this book is ~ $210 nonmember, $170 member. If you or your someone that works at your casting vendor is already a member of ASM, the METALS HANDBOOK information is available free over their website:

http://www.asminternational.com.

You can also obtain more information on how to become a memeber, how to purchase books, etc.

If you have any questions, feel free to post another message. Good luck.

 

[tedg]

Further to TVP's excellent post, you might note that copies of the ASM Metals Handbook are available in the reference section of most technical libraries, and in quite a few municipal libraries as well. The 10th edition is the latest (the first edition appeared in 1930), but an earlier one will do almost as well.

 

[Mike450]

Testing a separate coupon (test bar) to the ductile iron casting has to be demonstrated to be equivalent to one of the key sections of the casting, since it will be either a keel block or y block or some diameter test bar.
The foundry has to make sure the test bar is as good as the casting or vice versa, and it is probably better if the test bar understates the mechanical properties of the casting.
The shakeout time of the test bar can be different to the casting but has to be consistent in both cases, otherwise the matrix will be different shades of ferrite or pearlite content, influencing UTS and %El.
Hardness testing will not necessarily predict good or bad tensile results. The tensile results are not only affected by the matrix (ferrite or pearlite ) but also whether the graphite is 90-95% spheroidal (nodularity) or somewhat compacted or worse, flake. Hardness testing will not find changes in graphite nodularity unless it has promoted more or less ferrite.
The test lab can also affect what results you see on a test bar, especially % El, and strain rate will affect the result.
So, a test bar can represent a casting, as long as both parties accept the correlation. The molding, cooling conditions can be acceptably different.
One other event that can be different is if the casting is inoculated as the mold is filling (late inoculation) and the test bar isn't - the final structures would be worse in the test bar Vs the casting.

 

[awol]

To MIKE450,
Thanks for responding. This is still an open issue. We have DI castings using NPT threads. We use the DI because of the anti- or reduced galling properties of the material.

Our heat treat vendor was using the wrong process and the castings were very hard; hard to machine. And the galled repeatedly. I was under the impression the "flakes" were the good form of graphite (carbon) and the nodules were the bad form. Pearlite is what I assumed we were getting and causing the most trouble.

Since we have the correct HT process, we have been checking the BHN and it has been acceptable. Out casting vendor is still, in my opinion, "out-of-sync" with the timing of the shaking out the casting and the shaking out of the test bar.

Per your recommendation, I should specify a timeframe for both to be released from the mold. Should I take cross-section area into account? I have suspected the bars didn't represent the castings if not processed the same way.
With your information I will investigate further.
Thanks,
AWOL