Non-destructive measurement of case depth

[Bob_3]

Has anyone had any success using ultrasonic (ASTM A1038) or Leeb hardness (ASTM A956) measurements for case hardened steel? Not just the hardness measurements, but we're looking for an indication of case depth.

Thanks...

 

[Dhurjati Sen]

Dear Bob_3,

You can get the case depth using any micro-hardness machine.

Regards.

DHURJATI SEN

https://www.nace.org/people/dhurjatisen

 

[Bob_3]

Thank you for the reply, but I should have clarified our need. We need to sort 1500 parts, so a handheld device would make that so much easier.

 

[dbooker630]

Have you looked into eddy current testing? You will have to section a few parts but based on what you are trying to sort it may be worth looking into.

 

[mrfailure]

Leeb and ultrasonic methods that you list will give you surface hardness, but they won't tell you anything about case depth. If you have an accessible end surface that you could grind and polish in the field, then I guess you could etch and measure either visually from the surface or off a replica. Otherwise, you will need to perform destructive analysis to get the interior of the part.

 

I think the OP is asking for a non-destructive method.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[mrfailure]

I consider replication nondestructive - it does not affect part performance. You just have to have the right geometry to use it.

 

[Bob_3]

My bad for not giving complete information, so here goes…

Yes, non-destructive is required.
We have 1500 thin (1mm) sheetmetal parts we think are suspect as they have been cracking in use. They are supposed to be low carbon steel and nitrocarburized, or some other case hardening treatment, but we think the case is too deep. We tried communication with the offshore supplier, but that’s been less than desired, so I thought I’d come here for help.

We have an exemplar sample to compare to, so that’s been the baseline. Measurements with the equipment we have (Ra, Rb, and R15N) are all over the map, I’m guessing because of variation in case depth, so I’m either getting the case hardness or I’m hitting the core.
We’d like to be able to sort parts but I’m not sure I have a lot of confidence in the measurement method for just hardness, so I’m hoping to find some other method that also gives an indication of case depth. I came across the Leeb and Ultrasonic methods, but it seems like the Eddy current has possibilities too.

Thanks for all the reply's thus far...

 

[MagBen]

what is the depth? if very shallow, say .02'', eddy current might be valid method. The most popular/economic method for a large quantity is ultrasonic, but better with a depth >.08''. It can be very tricky, so better to have a level 3 build the setup for you.

 

[mrfailure]

But doesn't eddy current only identify discontinuities like cracks? I did not think it would be able to identify case, which I would not think would affect the flow.

 

[Bob_3]

For the 1mm thickness the nitrocarb should be about .03-.04mm.
I was thinking the ultrasonic would help sort the part-to-part consistency, but are hand-held units very accurate for this level work?

 

[MagBen]

Forget about ultrasonic for such a small depth. Anything different in conductivity could be identified by eddy current. No idea how accurate can EC tell the case depth, but it could serve as a comparative test: yes or no, go or no-go.

 

[bobjustbob]

There are eddy current case depth sorters. It won't necessarily tell you what the case depth is but it will allow you to compare it against a known sample. I have seen it used as a sorter for case carburized rings. I doubt you will find a handheld unit that will do this but I don't make these units. You would likely get a better answer by talking to a company that makes the eddy current sorters.

King Hardness testers has a case depth measurement tester that works by using an indenter like a Rockwell. It was released in the last year I believe. I have not used one but they do sell it.

Have you had the case depth measured on one of the cracked parts to see what the case depth is? It doesn't really help being able to sort if that is not the root cause of the problem.

Bob

 

[mrfailure]

Bob_3, have you had a metallurgical failure evaluation done on any of the cracked parts and compared with what you identified as the good exemplar? Have you compared case depth and chemistry?

 

[Maui]

Your 1mm thick sheet metal parts may have been through-hardened. Since you have 1500 of them, I would suggest taking 10 random pieces and destructively testing them in a metallurgical lab to evaluate the hardness as a function of depth from the surface using a microhardness testing machine. You should also examine the microstructure. If they all show the same out of specification condition, that should be sufficient evidence to provide to your supplier that there is a quality problem with their product. Then you can ask them to address it.

How did you perform the surface hardness check? If you did this using a standard setup on a Rockwell C hardness testing machine, a 1mm thick case carburized or nitrocarburized piece may be too thin to accurately measure the surface hardness using this method. A superficial Rockwell hardness test may be more appropriate in this particular case.

Maui

 

[Bob_3]

Thanks all for the replies. You've given me some things to look into.

Have you had the case depth measured on one of the cracked parts to see what the case depth is? It doesn't really help being able to sort if that is not the root cause of the problem.
We are having this done as I write, but my initial thinking was leaning towards how do we sort parts and compare to an exemplar sample? Knowing what the problem is is only half the solution.


Bob_3, have you had a metallurgical failure evaluation done on any of the cracked parts and compared with what you identified as the good exemplar? Have you compared case depth and chemistry?
See above...

How did you perform the surface hardness check? If you did this using a standard setup on a Rockwell C hardness testing machine, a 1mm thick case carburized or nitrocarburized piece may be too thin to accurately measure the surface hardness using this method. A superficial Rockwell hardness test may be more appropriate in this particular case.
We have an exemplar sample to compare to, so that’s been the baseline. Measurements with the equipment we have (Ra, Rb, and R15N) are all over the map, I’m guessing because of variation in case depth, so I’m either getting the case hardness or I’m hitting the core.


We have a lab looking at the parts, but if it turns out some were hardened too deep, we have a load of scrap and need to deal with our supplier. That still leaves me with my main focus being how to sort parts and salvage what we can to tide us over until replacements can be had. The handheld units are probably our best option for comparing to a known sample. so the method chosen will probably depend on the cost of the unit.

 

[Maui]

What country were these sourced from?

 

[mrfailure]

I think this is a great example of trying to fix the problem before you understand what is wrong. Be interested to find out the result of the metallurgical examination.

 

[Bob_3]

I think this is a great example of trying to fix the problem before you understand what is wrong. Be interested to find out the result of the metallurgical examination.

I never said I was trying to fix. I said I was trying to sort.

 

[mrfailure]

Bob_3,

I stand by my statement: you were trying to fix your problem by sorting out samples that had too deep a case without performing an evaluation to understand why you were getting cracking. It is great if your theory is right, but you run the risk of sending out defective parts if you don't first understand the cause.