Checking Hardness of Nitrided 4140/4340 Parts 3

[Tigerdawg]

Can anyone recommend a testing method for nitrided parts? The tester our QC is using is a Hartip 3000, and we seem to always get lower values than expected. It is possible to 'break through' the case and read a lower value than actually present? The case depth is supposed to be >.015". Also, generally how hard a case can you expect with these matrials?

 

[swall]

Assuming that the white layer has been removed, you can expect a surface hardness in excess of Rc 62. File hardness checking, with standard test files, is a good method for checking surface hardness. If your case really is .015" deep, you can perform a microhardness survey on a cross section and also get a near surface hardness. You can use a scrap part or a test slug of the same material so you don't have to cut a good part. Can't comment on the Hartip 3000, as I am not familiar with it.

 

[Carburize]

A test sample of the same material as the parts or a sacrificial part is the best method. Mount and prepare a section and test using a low load Knoop machine to give the most accurate measure of hardness and case depth.
As you note - on thin cases it is very easy to affect the apparent surface hardness if you try to test directly on the part and start to deform the substrate material.

 

[rorschach]

I agree with Carburize, a knoop survey is best. our friends across the pond are in love with vickers indenters but they are harder to stack the indentions vertically IMHO.

 

[TVP]

Some of us on this side of the pond are in love with Vickers hardness as well. It is a continuous scale from low hardness metals like cast zinc and magnesium all the way to high hardness ceramics like boron nitride and diamond.

With regards to nitriding, a microhardness traverse is definitely the way to go. You want to measure both hardness and depth. Knoop or Vickers with an indentation load of 0.025-0.050 kgf is best, with the indentations at least 3 diagonals apart. As swall mentioned, the hardness will likely be 62 HRC maximum (~ 750 HV or ~ 775 HK).

Nitrex has a nice description of this type of testing in Issue 25 (September 2003, page 3) of their newsletter called Nitrex Communicator. Use the following link to obtain more information:

http://www.nitrex.com/library_archives.ch2

 

[TVP]

I forgot to include a link to Issue 26 as well, which continues the discussion on the topic of hardness measurements. Also, indentation load up to 0.1 kgf (100 gf) is appropriate depending on the exact type of equipment used. The following two links lead directly to the .pdf files.

http://www.nitrex.com/iactivefiles/Nwsl026.pdf

http://www.nitrex.com/iactivefiles/Nwsl025.pdf

 

[CoryPad]

[blue]rorschach[/blue]'s comment is a double-edged sword. I have found that many operators do not follow the requirements of ASTM E 384 when performing microhardness testing with the Knoop indentor. The indents are spaced much too closely and give false information.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Carburize]

I'm actually a supporter of both methods - Knoop for thin cases less than about 0.015 to 0.020-inch and Vickers for thicker cases.

 

[metman]

On thin layers such as this it is good practice to cut through the sample surface at an angle to increase the apparent length of the the thin surface layer then use trigonometry to calculate the actual distance. This will allow you to space your microhardness indentations furhter apart and obtain a more accurate determination of depth.

 

[unclesyd]

metman,
You gave up an old trade secret of the metallurgical technician. I was shown this trick by my boss in about my third week on the job in 1952. I could even do the math. Used it many, many times.

Another one you can do if the part will allow it is to zig-zag, offset each reading.