Ferritic Nitrocarburising 2

[karlmet]

Attached is a micrograph of a tufftrided sample from a new H/T supplier. I can see there is a good white layer of 10 microns although no nitrogen diffusion zone. Is this because the test piece used is a predominantly pearlitic microstucture and the pearlite is acting as a barrier to the nitrogen?
Any input would be great.

 

[Metalhead97]

bkarlfound,

Is this a cast iron? It looks like it with those nodules. You typically have two nitided zones: porous near the surface and compact right below it. I can see both. Did you do an EDS analysis to confirm your presumption that there isn't a diffusion zone. For your materials to form gamma-prime/epsilon nitides nitrogen needs to penetrate into the cementite and structural changes occur in the following order: cementite -> gamma-prime -> epsilon. It is possible that this could be a slow process since in cementite nitrogen can only penetrate by vacancy assisted diffusion or limited interstitial diffusion.

Metalhead

 

[karlmet]

Metalhead,
700/3 grade I'm lead to believe, I can clearly see the visible layer but I was expecting to see somewhat of a diffusion zone below that. Without the facility to do EDS analysis I can't be sure but would I not expect to see a darker etching layer of around 50-100microns?

PS sample is etched in 3% nital

Karl

 

[Metalhead97]

Karl,

Are you sure that the darker etching layer isn't the diffusion zone? Those nodular ferrite 'bull-eyes' appear to be removed as a result of nitriding. I have used nital before and I can tell you that it tends to attack carbon saturated areas aggressively. It's likely that the high nitrogen, carbon levels are causing the diffusion zone to etch dark. Remember that carbon and nitrogen are interstitials in ferrite, which means they will substitute for each other. In fact, if you take a higher magnification image you might be able to see martensite.

 

[Guest102023]

" ... carbon and nitrogen are interstitials in ferrite, which means they will substitute for each other."

You might consider rephrasing that ;-)

 

[Metalhead97]

brimstoner,

Carbon and nitrogen are most certainly interstitials in BCC ferrite. Where else would they be? Certainly not on lattice points!

Metalhead

 

[karlmet]

I will try etching them in 5% nital and see if I achieve any further etching in the ferrite you suggest may be the diffusion zone, although could it be possible it has only diffused in areas with ferrite close to the surface and have a lot less/no diffusion in the pearlite? If this is the case it would seem pointless nitrocarburising a predominantly pearlitic iron?

Karl

 

[mcguire]

The solubility of nitrogen in ferrite is essentially zero. That means you change virtually immediately from nitride phase to nitrogen-saturated ferrite.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[Metalhead97]

mcguire,

That is true for equilibrium conditions, but in a similar manner as Fe-C, Fe-N also forms a supersaturated martensitic phase, which readily decomposes into nitrides. Gavriljuk's book: High Nitrogen Steels: Structure, Properties, Manufacture, Applications was a book that I referred to often while doing my PhD research. I was ion-nitriding steel, and I needed to understand the Fe-N system.

http://www.amazon.com/High-Nitrogen-Steels-Manufacture-Applications/dp/3540664114

karl,

I think if the nitrogen only diffused into the ferrite you would still get a pearlite-like laminar etching pattern. You obviously don't have that in the dark region. You could also try a vickers indentation in the dark region. My bet is that it is a lot harder than pearlite.

Metalhead

 

[karlmet]

Vickers indentation across (what should be) the diffusion zone Av397Hv in the etched ferrite and as low as 320Hv in the pearlite suggesting its not a uniform diffusion. Core is around 300Hv

I have also etched in 5% nital and can see further etching of the carbon saturated areas close to surface.

Q: As nitrogen does not diffuse as readily in pearlite why would a heat treatment company use such a grade for trials?!

Karl

 

[Metalhead97]

karl,

Cast irons are routinely nitrided, which is probably why they gave you that test piece. What material do you want to nitride? Perhaps you can supply them your own.

MH

 

[karlmet]

Just for a wear resistant surface layer I presume? It's for a range of materials. The trials are due to change of H/T company and it is my first experience with such a grade being used.

Karl

 

[Metalhead97]

Karl,

I can't think of another reason to nitride a cast iron. You might want to ask if you can send your own test piece because in this case you just want to make sure the new HT company on par with the previous. Is there a particular specification that your doing this to?

MH

 

[karlmet]

Just our own procedure metalhead 10-20micron surface layer plus 100 micron diffusion zone

 

[CoryPad]

The answer to the original question is yes.