Test for nitrided case

[sandero]

Hi all there,
We are a small aluminum extrusion company in Argentina. Unlike other extruders in our country, we have our own furnace to gas-nitride our dies. Does anybody in the forum know how to test nitrided case integrity? I know that solutions containing Cu++ can be used for this purpose. Does anyone know how they are formulated?
Also; is the test still useful for treatments that produce low thickness white layers (e.g. plasma and low pressure gas nitriding)?
Thank you very much for replying,

Sandero

[Gordon]

Hi Sandero

to the Surface Engineering Forum.

I think you are referring to the copper sulphate test (copper sulphate solution acidified with sulphuric acid, various concentrations and exposure times). The primary use is for testing stainless steels ability to resist corrosion or to identify stainless steel from carbon steel. The idea being that carbon steel is readily attack by copper sulphate solutions forming a copper deposit, while a good passivated austenitic stainless steel should not.

The presents of a nitrided surface should reduce the degree of attack from copper sulphate and the presence of a white layer probably further. I can only see this being useful to determine whether a steel surface is nitrided or not. To be honest I think testing with a hard file to assess the hardness or grit/shot blasting to highlight soft areas would be far more informative, plus a lot less messy and hazardous to health. I assume you are wanting to test worn dies with this test. Testing nitrided cases after heat treatment really should involve hardness testing and metallographic evaluation.

[Gordon]

Hi Sandero

The following link may be worth reading
What is the Correct Test Method for Case Hardened Steel

[volf]

To check nitrided case on nitrided gear for example the standard control is Vickers Hardness Test by 5 kg load, applied on the surface. Required hardness is about 850 HV.
I suppose that after gas nitriding you will ground or polish the working surface of your die. Nital etching is useful to control surface integrity and to ensure that grinding does not damage nitrided surface, (abusive grinding).

[sandero]

Hi Gordon,

Thanks for your kind reply

I think you are referring to the copper sulphate test (copper sulphate solution acidified with sulphuric acid, various concentrations and exposure times).

I think you are right. In fact, I have used these solutions in the past but didn't know for sure how they where formulated.

The primary use is for testing stainless steels ability to resist corrosion or to identify stainless steel from carbon steel. The idea being that carbon steel is readily attack by copper sulphate solutions forming a copper deposit, while a good passivated austenitic stainless steel should not.

The presents of a nitrided surface should reduce the degree of attack from copper sulphate and the presence of a white layer probably further.

My experience tells that Cu++ solutions do not work for treatments producing rather thin (if any) white layer. I remember arguing with a supplier of low pressure gas nitriding service who asked me to try the dies they treated in the press even when they had failed to pass the CuSO4 test. In the end, he was right. Low pressure gas nitrided tools outperformed salt bath nitrided ones which had passed the test (the solution kept its blue-green color when poured on die surfaces).
Trying to think chemically I do not found any reason for Cu++ not reacting with iron except when a continuous compound layer (white layer) is present.
As you surely know, extruders actually try to keep white layer's thickness as low as possible since they are fragile and tend to spall.
All in all, my concern is that the test is probably telling that I have what I don't want to. Is this correct?

I can only see this being useful to determine whether a steel surface is nitrided or not. To be honest I think testing with a hard file to assess the hardness or grit/shot blasting to highlight soft areas would be far more informative, plus a lot less messy and hazardous to health. I assume you are wanting to test worn dies with this test.

Actually, I would like to develop a test for nitriding treatment quality

Testing nitrided cases after heat treatment really should involve hardness testing and metallographic evaluation.

I know this but our testing facilities and equipment for metallographic analysis are rather poor.

Thank you very much replying to my inquiry,

Sandero

Note: edited quotes to make reading easier

[Gordon]

Hi Sandero

Yes, I think any white layer will tend to give little or no reaction to the copper sulphate test (no copper deposit or reduction in blue colour of solution). Nitrided layers with no white layer I think will give some reaction, but less so than a non nitrided surface. I can only suggest experimenting with with a strictly controlled copper sulphate testing procedure on known test pieces.
Problem is you will really need to evaluate the test pieces using metallography and hardness testing to know what you have really got, so you can predict what the tests are telling you. I think these ASTM standards; A-967, A380 and A262 have reference to the copper sulphate test (mainly for stainless steel).

As mentioned by Volf, using a nital etch (nitric acid in alcohol) could also yield information on your surface. Again you will need to experiment a bit to get your procedure right. Try swabbing with a 4% nitric acid in ethanol or isopropanol and possibly follow this by swabbing with a 1% hydrochloric acid/alcohol mix to remove smut and develop etch. You may need to change concentrations to suit or use water (ideally with wetting agent or drop of washing-up liquid) instead of alcohol to make etch more aggressive. Not tried this on nitrided surfaces, but would expect no etching colouration on white layers, very slight etching of non white layer nitriding and heavy etching of non nitrided surface. This test is typically used to check for grinding burn and to show heat treatment patterns in surface hardened steels.

I am sure you are aware that these chemical tests only reveal the extreme surface properties and not what lies below. Also, I must stress the health and safety issues when using the above chemicals, some are corrosive, toxic, inflammable and may give off harmful vapour.