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dho (Mechanical) (OP)
3 May 12 11:50
we have sent 440C to a lab to find the % of the retaining austenite. the lab asked us the % of carbides. they said it is used in calculation of the retained austenite.
1) is there a book value in some military spec, or else to specify the carbides % for 440C material?
2) or, the % of carbides varies from lot to lot, so, it needs to be determined by some "test".
anyone can help me?
thanks a lot.
Helpful Member!  cloa (Petroleum)
4 May 12 8:47
Is he asking for carbides or carbon content %?  
dho (Mechanical) (OP)
4 May 12 9:14
cloa, i was asking carbides, NOT carbon.
Maui (Materials)
4 May 12 14:48
The percent retained austenite can be determined using X-ray diffraction techniques, as you stated. 440C is a crystalline polyphase material with the atoms in each phase arranged in a periodic 3-dimensional pattern. If the material was processed using standard forging and rolling techniques, there may be some preferred orientation of the grains. But for the purposes of this type of test, the approximation of randomly oriented grains is usually made. The spacing between the planes of atoms in each phase is distinct, and results in very clean-cut peaks in the X-ray diffraction data. By using the relative intensities of the peaks, they should be able to determine the amount of retained austenite that is present. This is assuming they can identify which peak represents austenite, which should be relatively easy for them.

Provided they are using proper calibration techniques, why would they need to know the weight percent (or volume percent) of carbides to make this determination? I suggest that you contact them and pose this question. Let us know what they say.

Maui

www.EngineeringMetallurgy.com

dho (Mechanical) (OP)
4 May 12 15:21
The requirement is the retained austenite has to be less than 3%.
A year ago, the test lab cert said "Note, 15% carbides assumed in calculation".
This time, the lab asked us what percent carbides should be used. I asked why ask us, it should be either a book value or result of a test.
The test lab says "This is based on a NADCAP requirement and had to be applied since May 2011"
I asked last time your report said 1.1% retained austenite and assumed 15% carbides. what the retained austenite would be if carbides at 0 and 30%"
The lab says "0% would be 1.3%; 30% would be 0.9 (which we would report as <1.0%)"
The lab then said a micro analysis to determine the carbides %, $140.14 per sample.
Maui (Materials)
4 May 12 15:37
Due to a variety of assumptions that go into performing the calculations and the inherent material inhomogeneities that may not be properly accounted for in this test, retained austenite values that are reported below 2% should be taken with a grain of salt. Instead of reporting a numerical value, such as the 1.1% you provided above, they should simply be reported as "retained austenite levels were detected to be below 2%".

Maui  

www.EngineeringMetallurgy.com

EdStainless (Materials)
4 May 12 16:21
The price for carbide determination does not sound out of line.
Polish and etch samples and then look at enough fields to get a statistically meaningful value.
Of course for this application you don't need super good statistics since the austenite value isn't that sensitive to the carbide content.
But that begs the question, if you polish and etch samples for micros why not just do the retained austenite that way instead of XRD?

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

Helpful Member!  Maui (Materials)
4 May 12 17:50
Ed, a reliable method does not exist for quantitatively determining the amount of retained austenite at the levels required by dho using standard metallographic techniques. The following paper by George Vander Voort gives some good examples that support this:

http://www.industrialheating.com/Articles/Feature_Article/BNP_GUID_9-5-2006_A_10000000000000564292


Maui

 

www.EngineeringMetallurgy.com

EdStainless (Materials)
5 May 12 10:17
Good link, I learned something today.
Now I see the issues.
My experience has been with lower C, higher alloy grades where the austenite is distinct enough to measure optically.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

Metalhead97 (Materials)
7 May 12 21:06
Am I missing something? Wouldn't it be easier to determine the relative proportions of BCT/FCC phases by XRD and measure the amount of carbides using microscopy? I mean carbides stick out like a sore thumb.

metalhead
TVP (Materials)
8 May 12 9:37
All,

Here is a reference specific to retained austenite determination in Type 440C that mentions the effect of carbide concentration:

http://camet-lab.com/retained_austenite.html

 

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