Compound layer of gasnitrided steel spalls ? 1

[Kennethhp]

Hello !

Where i work we have an issue i been involved in (It has been seen maybe 5 times over the last 5 year)

We do a pretty standard gas nitriding process at 520 degrees celcius with NH3.
We pre oxidize the parts at 400 degrees before nitriding the parts.

For some reason after treatment, it looks like the nitrided layer lays on top of the part, like paint bubbling up on a rusty car. When we rub the part the nitrided layer falls of easily. When we measure the hardness underneath the nitreded layer we get the required hardness. So the hardness in the steel is ok. But the compound layer is gone, and it looks awefull!

We are clueless whats going on, and why it appears. like mentioned this only happends like, 1/40 times and we cant find any faults at all.

Has anyone seen this before who can point me in a good direction ?

/Kenneth

 

[WKTaylor]

Pre-cleaning process, alloy, temper, form/geometry [raw stock and finished part], nitriding process, records of any process deviations, metallurgy on the damage part/coating, etc??



Regards, Wil Taylor

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[tbuelna]

Sounds kinda like there was a decarburized surface that was not removed prior to nitriding.

 

[mfgenggear]

Kenneth when the parts are core harden prior to the nitriding is it in a protective atmosphere or copper plated?
what type of steel?

 

[Kennethhp]

Hello.
Sorry for the late reply, i have been away from my pc the last 2 weeks

@WKTayler

Precleaning is the same as we do on any of our treatments, Nothing differs from treatment to treatment, it just appears out of no where.
Geometry is random, the treatment consisted of 10 different type of parts in very different shape and size.
From what we can momentarily see under the microscope is that there is a compound layer underneath the spalling. Which may point towards a fine oxide on top (Have to do more inspections to be sure)

@tbuelna
I have talked with David Pye about decarburizing, and he says it doesnt look or sounds like it, since its only, what appears to be, the compound layer spalling.

@mfgenggear
That i have to look into. The steel is
34CrNiMo6
C=0,34 - Si=0,25 - Mn=0,5 - Cr=1,5 - Mo=0,25 - Ni=1,55
We have treated the same grade of steel several times with no issue. Like i said, it just randomly happends. and it happends to all the different parts in the oven. (As above, there where 10 different costumer parts"

 

[metengr]

For some reason after treatment, it looks like the nitrided layer lays on top of the part, like paint bubbling up on a rusty car. When we rub the part the nitrided layer falls of easily.

This sounds like the white (compound) layer that results from gas nitriding. There are numerous articles on reducing the compound layer formation during gas nitriding.

 

[Kennethhp]

As you can see in the bottom it looks like the compound layer is just laying on top, ready to fall of with a touch of a feather

@Metengr
I am aware of different techniques to reduce the compound layer, this is not our which as some of our costumers want the aesthetic from the pure "grey ish" color.
And from what i am aware, a compound layer, no matter how thin, should always be a part of the base material since a compound layer is roughly going 2/3 of thickness into the base material and only adding 1/3 out from the original surface.
I might actually now think i didnt make my self 100% clear in my question and we are taking in each direction But the picture should explain my issue.

 

[Kennethhp]

More info. From what it looks under the microscope (500x) there is a fine compound layer all the way through the "black" spots where the unknown layer has fallen of. My 1000x microscope isn't working at the moment, so its hard to tell under 500x, but it looks like there might be a really fine Oxide layer on top of the compound layer. This layer doesn't appear on the whole surface which might indicate it could be a fine oxide layer spalling off ? What is still wondering me is the color of the oxide layer if it supposely was that. If i remember correct an insufficient oxidation results in hematite and will most likely spall off unlike magmetite which is the durable oxidation.

Hope it can help you help me

 

[mfgenggear]

Hi KennethHP

since the above steel is not Nitralloy 135 (with Aluminum element) it is very difficult to obtain a deep case.

look at AMS2759/6 look at using Class 1 (Two stage) to reduce the amount of white layer.
white layer on parts is not a desired on parts. in fact most of our customer want it removed.
because it is to hard and brittle. and flakes off during service.

a one stage process creates more white layer. an educated guess it is used for parts with out
bearing surfaces. just needs to be hard for wear.

now the case depth is an other issue. most important . and the surfaces is very hard. 70 HRc at surface ref. (94 15-N )
don't mix up the white layer with the case depth. ( Gas Nitride and not Tufftride)

the preparation of the steel is most important, and the gas mixtures as well the case depth required.
the longer the cycle time the more white layer is produced.

 

[Kennethhp]

@mfgenggear

Hello again

Our compound layer thickness is really thin already so we dont want to change the process type, (yet) (We get an avg 5 microns compound layer)

The case depth isn't a problem. (dont know where you see that being an issue )

I have researched alot and we have come to a conclusion. In the microscope, like i mentioned, i can see a secondary extremely thin compound layer. An oxide layer occuring during the process would after exposure to ammonia turn into a compound'ish layer. We are 95% sure it is due to a leak or some other exposure to either h2o or some oxidizing element during the process. This would most likely give the bad results we are seeing.

We suspect our oven fixtures (specially the old porous ones) to contain oxidizing elements, from treatments with post oxidation, which are realease during nitriding temperature and there for oxidizing the surface slighty and giving the spalling surface.