Partial nitiriding in part (within part variation) 2

[smurali1]

Hello,

We are doing nitriding of large internal gear (appx. 1.5m in dia) We are finding that some areas are as bright as original steel material before nitriding. There is high level of variation within a part.

We tried 5% copper sulphate solution to check the effectiveness of nitriding. (apply a dot of copper sulphate solution on the area to be checked. Wait for few minutes to see if it turns to copper color. If yes, then the area is not nitrided)

My questions are :
1. How effective is the above copper sulphate test method for detecting partial or no nitriding?

2. Is there any other method? We tried portable vickers but being gear flanks are not easy to reach.

3. What if I apply copper sulphte on the gear flanks? Will it lead to any pitting (immediate or at a later time). The gear flank take heavy load. So, I do not want any adverse effect due to copper sulphate test.

4. What could be reason for partial/no nitriding in certain areas of same part? We are washing with 90%white spirit +10% DM water before loading into nitriding furnace.

 

[mfgenggear]

what type of nitride process?
gas, Ion, salt bath etc etc.
was there white layer removal?

My vendor uses copper sulphate to verify for white layer removal. there may be nitride present but once the white layer is removed then there will not be copper present.

clean in solvent
then bead blast to remove surface oxide or other.
then clean again & air dry
use white gloves to handle the parts.
rack rings flat
then nitride

if the parts nitrided correctly there should be a darker appearance to the surface. not bright in appearance.
if the nitrided surface has been post bead blasted it will be bright.

do you use a metlab test sample to verify your results?

you may try to nital etch a sample part. it will turn
dark black. if nitride is not present, it will not change.
do not do this for production parts.

 

[smurali1]

Hello mfgenggear,

Thanks for your feedback. Here are answers for some of your queries and my follow up questions.

1. Nitriding is by gas nitriding.

2. No white layer removal is done. Grinding done on only very minimal area for bearing fitment. In gear area and most of the areas of the part, there is no grinding.

3. I googled on copper sulphate test. Copper sulphate reacts with rich iron nitride layer (white layer). Copper precipitates out of the solution. Can it be a test to check white layer presence as well nitriding?

4. Why bead blasting to be done after cleaning in solvent? Can you please elaborate?
Also, what is clean again - how? can you please elaborate?

5. Yes. We do metlab test sample and they are perfectly OK.

6. Why nital etch cannot be done on production parts?

(Incidentally, on different set of gears which are carburised, we do nital etching of production parts to check for grinding burns.)

 

[mfgenggear]

I am out of the office to day but I will try to help.

for # 3 Yes the copper sulphate will only adhere to the white layer.and there is a set time that the parts must be in the sulphate copper solution.

for #4 it is to ensure that the parts have no surface impurities. it prevents the copper stop off from stripping off & aids with case hardening.(bettter case hardening results) "no softter spots"
standarded clean for removal of forein debri, there is all types of approved solutions, & methods.
maybe some of the other forum associates camn help you with this.

for #6 It is my understanding nital etching is very bad for nitrided parts. "embrittlement, 9310 vs nitraloy or nitride vs carburize, nitride is posibly more prone to IGA, intergranual attack. " maybe the specialist here can elaborate.

Nital etch of carburized parts is standard, depending on the solutions, a post bake is generally required.

 

[defdef]

Huh??

mfgenggear - depending on surface prep, concentration and time on surface, the copper sulphate may or may not deposit Cu on a white layer. A smooth, low porosity white layer will RESIST copper sulphate to a high degree.

Also, if you are spot testing a 1.5m dia gear, nital won't hurt. the copper sulfate solution can lead to pitting if you do not remove it soon after applying, same with nital. I recomend smoothing the surface before applying either.

Your irregulatity is probably resulting from either un-uniform temperature or gas flow.

smurali1 - are you cycling the temperature? are you purging during cool down? that can affect the results of the white layer....

 

[mfgenggear]

defdef

review specification AMS2759/8 Ion Nitriding

Para 8.9 Test for compound layer presents
composition
40 grams copper sulfate
1000ml of water
5 ml wetting agent
3.9 to 4.1 PH

application

areas to be tested shall be cleaned & then swabbed with the solution of the above solution to be left for 30 seconds or less,

if no copper is color occurs in 30 seconds or less, non compound is present.

however as stated in my previous statement
my experience it is better to dip the parts in the solution.

I work with this stuff all the time.
this is standard procedure for testing for white layer removal.

Our customers even specify this type of test.

 

[Pressed]

>Q 2. Is there any other method? We tried portable vickers

You can perform hardness tests to the questionable areas, do a hardness map to find variations. Or you can make a similar sacrificial part, maybe one with no teeth, that you can cut and check hardness at will.

Remove a surface section and mount it for microanalysis. Also perform a micro-hardness test from the surface down, to determine depth of hardness, hence depth of nitriding.

If you could, tell us how long the parts are nitrided?

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Forging

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[defdef]

mfgenggear - Impossible... If you are talking about copper sulphate (CuSO4).

Fe+CuSO4 --> FeSO4+Cu therefore only bare Fe will result in Cu deposition.

You must be thinking of cupric ammonium-chloride
Cu(NH4Cl)2

I prefer to use copper(II) chloride dihydrate power mixed to saturation in DI water... but this is not helping smurali1

**I would recommend the following book** since we can't all agree...
Practical nitriding and ferritic nitrocarburizing By David Pye

That will help you fix the problem your having.

 

[mfgenggear]

not impossible but agree

using cupric cholride technical grade

bright pink layer absence of white layer
slightly gray color indicates presense of white layer
brick red color indicates absense of nitride case

on my previous post copper sulfate is as specified by
the AMS2759/8

I totally agree with Practical nitriding & ferritic nitrocarburizing by David Pye

 

[smurali1]

Hello all,

Thanks for your inputs.

Here are some feedback and queries from me:

1. We maintain nitriding temperature at 515 deg. C conctant.
2. Also purging is done during cool off.

3. I tried cupric chloride test on sample pc. It exactly behaves like copper sulphate test, except for the color.
(Please see the attached photos). i.e. it helps in identifying presence of white layer. (if no white layer, it changes to red, irrespective of if it is on diffusion zone or much inside the core where original steel remains)

4. I am looking for a test which will tell me if nitriding is done or not? The copper sulphate and cupric chloride test check for presence of white layer only.

5. Is it possible that nitriding on some areas of the part is possible, even though we do not see any white layer in that particular area?
Or is it that white layer MUST be formed to have nitriding?

Thanks

SPM

 

[defdef]

White layer is not a necessity of nitriding, the qualification of "good" nitriding is with testing the hardness (unless you get into ion nitriding, but that is different from your situation I think).

You might want to try Ultrasonic Contact Impedance (UCI) probes, which can read in vickers. For example, the General Electric Inspection Technologies MIC10 unit. A good reading depends on what you are doing and the material you are starting with, but will generally range 600-900HV with a 1kg load UCI probe. You can test on 3mm centers over the entire gear if you wanted. For large peices, the MIC10 was my favorite tool....

Now, the reason for the varied effect is related to one of two things:
1. the temperature (and remeber uniformity counts too!)
2. the present of Ammonia at the surface (again, uniformity counts, the gas is being consumed as it flows over the surface)

Hope that helps,
def