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Decarb Testing

Decarb Testing

Decarb Testing

Does SAE J121 cover the optical method of performing a decarb test?
Both the micro-hardness and the optical method are referenced in ASTM A-574 and I am looking for a Standard that details how the optical decarb test is performed.
- Is the Nital solution warm, hot or ambient ?
- how long is the item kept in the Nital ?
- etc. etc.

When I find the Standard (SAE, ASTM, MIL etc.) that details the optical test (as a minimum) I'll purchase it.

Thanks in advance for your time.

RE: Decarb Testing

Interesting question.  First, SAE J121 and J121M describe both the microscopic (optical) and hardness methods for measuring decarburization of threaded fasteners.  The content is essentially identical to ISO 898-1.  Second, here is an excerpt from SAE J121M, which is the entire content related to etching located in this standard:


...c. Etching in a 3% nital (concentrated nitric acid) or picral (saturated picric acid) is usually suitable for
showing changes in microstructure caused by decarburization.

As you can see, there is no discussion of temperature, immersion time, or anything else related to etching.  In fact, none of the fastener standards discuss these details, which includes ASTM, SAE, ISO, etc.  The most comprehensive standard related to microetching is ASTM E 407 Standard Practice for Microetching Metals and Alloys, and even this will not answer all of your questions.  Here is an excerpt from the section describing nital etching of Fe-C alloys:

1–5 mL HNO3, 100 mL ethanol (95 %) or methanol (95 %)
Etching rate is increased, sensitivity decreased with increased percentage of HNO3.
(a) Immerse few seconds to a minute.

It is understood, based on the format and content of ASTM E 407, that unless a specified temperature range is given, all of the etchants are to be used at room temperature.

I think if your laboratory procedure referenced ASTM E 407 as the source for microetching practice, and ASTM E 3 as the source for metallographic sample preparation, then you should have no problems certifying decarburization results using the microscopic method.  Keep in mind that standards like ISO 898-1 and SAE J121M contain important details describing where the measurements must be taken, what are the allowable limits, etc., and therefore must be part of your standards collection if you are certifying or testing fasteners.

RE: Decarb Testing

Are ambient laboratory conditions assumed when not specifically mentioned?

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