Carbon quant in steel

[Probeman]

I have a question on low carbon (e.g., 0.1, 0.2, 0.4 wt% C) steel standards as utilized by some people as a multi-standard calibration curve.

I know that carbon in steel is meta stable, but at room temperature does the carbide segregation continue to occur on the micro or nano scales?

I once heard from someone that for homogeneity one needs to re-manufacture ones low carbon steel standards every 10 years or so even
when stored at room temperature. Does this make any sense at all to anyone?
john

[Mike Jercinovic]

Sorry John, I am not able to address this, but I have a related question.. I have a facility user who is now working on nitriding and carburizing of steel.  There ends up being iron nitride phases, and they want to know something of the possible carbon content of these phases.  As we are not really setup (at least yet) to do these ultra-light elements well, she would like to know if anyone out there might take on this sort of tricky analysis?  So, desperately seeking some expertise in C and N in steel!  I will be happy to pass along contact information.

[Probeman]

Hi Mike,
I got a response to my question from Philippe Pinard on the room temperature stability of these carbides in steel, and he says yes, it is an issue.  Hopefully he will post his response here for all to see.

On your question, Philippe is definitely the expert, but he is at Oxford Instruments now.  I have a little experience with this sort of thing and as long as she doesn't require ultra high spatial resolution (we just have a W gun), I would be pleased to give it a try in our lab. 

Some years ago I did some trace carbon and nitrogen analyses for a customer that I thought turned out very nice. They are here:

http://probesoftware.com/smf/index.php?topic=48.msg270#msg270

Now maybe I've learned a thing or two since then, or maybe not.  But today I would probably apply the TDI scanning method to get best results form trace carbon as seen here:

http://probesoftware.com/smf/index.php?topic=933.msg5990#msg5990

Trace nitrogen is actually much easier than one would think.

[Mike Jercinovic]

Thanks John,
I will let her know you are willing to give it a try.  That is interesting to know that nitrogen is easier.  In this case, nitrogen will be major, with minor carbon.  I don't know anything about the dimensions of these phases, but hopefully this is something not too unreasonable.

[Probeman]

I will let her know you are willing to give it a try.  That is interesting to know that nitrogen is easier.  In this case, nitrogen will be major, with minor carbon.  I don't know anything about the dimensions of these phases, but hopefully this is something not too unreasonable.

I should add that while trace nitrogen is relatively easy (because it's merely the background determination that is critical for traces), major nitrogen is considerably harder to do accurately, since it depends more on the matrix and peak shape corrections. Haven't we all at one time or another attempted to analyze TiN compositions and found the Ti L absorption edge to be in the worst place possible...?   

At least with trace nitrogen analysis we don't deposit nitrogen during the acquisition as we see with carbon.  I still want an "in situ" ultra-violet laser in my instrument for surface cleaning during data acquisition:

http://probesoftware.com/smf/index.php?topic=140.msg643#msg643

Even just a small UV LED with focusing optics might work.

[Les Moore]

The Nitriding of steel is done at a temperature where the steel is Ferritic or in the BCC crystal form.  In this phase it is relatively insoluble and eventually reaches the activity where it forms iron nitride.
The iron nitride is often present as a very hard phase at the surface which can be revealed by etching.  It is also often evident within the diffusion gradient as needles of iron nitride.  These needles should not be confused with Iron carbides which may also be present. The following online ref is OK.

https://vacaero.com/information-resources/metallography-with-george-vander-voort/1138-microstructure-of-nitrided-steels.html

If the temperature is too high, the steel becomes austenitic or FCC and the nitrogen rapidly dissolves into the structure. In terms of a wear scenario this is a problem as wear may generate locally high temperatures which can locally austenitize the surface (form the FCC structure) and the coating diffuses away.

Note: if you examine the Fe-C phase diagram, you will see that C solubility in ferrite is very very low at nitriding temps.  What this means is that the C will be tied up in carbides or in pearlite.  Unless there is a free energy driving force for the carbides to dissolve and enter the iron nitride, I would imagine that iron nitride would be relatively devoid of carbon.  The increase in nitrogen content in the steel below any nitride layer is also likely to displace the carbon and it would be 'pushed into the coating (see formation of carbides on grain boundaries in above ref).   This may be a load of rubbish if the nitriding process is also carburising at the same time.

Note2: Do not do the analyses on an Nital etched sample as the nitric acid etch products swamp any Nitrogen in solution.  Been there done that.... The question was "is it nitride?".  Analysis on an etched sampled said "yes it was", analysis on a polished sample said "no it wasn't".

Note3. The microstructures in the ref are mostly martensitic in which the C is frozen in a BCT distorted transformation from the high temp FCC form.  This adds another layer of complexity as the C is relatively homogeneous instead of essentially zero in ferrite and 0.8 in pearlite. 

Lotsa luck and be careful. 

[Les Moore]

Regarding the wandering of standards for microanalysis over time....

Hi John,

Carbon "should" be quite stable at room temperature.

 Do you know if the standards are martensitic or ferrite/pearlite structures?

Theoretically if they are as-quenched or fully hard martensitic structures, the carbon could diffuse and form Epsilon carbide which are just localised clusters that require atom probes to see.

This typically forms at temperatures of 100-200 C over an hour or so but I imagine could perhaps be detectable with an extended time at room temp.

Wikipedia suggests that Epsilon carbides will form 100 C to 200 C and decompose above that (this meets my understanding)
https://en.wikipedia.org/wiki/Cementite

This might give some useful background:
https://en.wikipedia.org/wiki/Martensite

Good Old Harry Bhadeshia has something to say here:
https://www.phase-trans.msm.cam.ac.uk/2007/Epsilon/Epsilon.html

The following ref suggests that these Epsilon carbides can form at room temp:
https://www.researchgate.net/publication/248138123_An_interpretation_of_the_carbon_redistribution
I haven't seen inside the article.

What is probably perceived to be going on is a peak shift from the location for C caught in the metastable BCT structures in the interstitial positions to the location (or shape) in Epsilon carbides.
Whether it is really going on, I cannot say.
I'll have a bit of a hunt.

If the structures are ferrite and pearlite, they are probably not good for microanalysis anyway as they are locally variable (0.008 in the ferrite and 0.8 in the pearlite) but should be stable for millions of years until the Fe3C decomposes to graphite.

An interesting page with a heap of steel metallurgy in an interesting context.
http://products.asminternational.org/fach/data/fullDisplay.do%3Fdatabase%3Dfaco%26record%3D1910%26search%3D

This also may give some insight into my comments on the Carbon and Oxygen analysis requests in your other problem.

Cheers

Les

PS. I work in the steel industry and never do (kicking and screaming) Quant C or N in steel.
These elements are best analysed by a bulk technique such as LECO gasses in metal analyser.
Locally, the C distribution is driven by variations in the alloying elements distribution.
I am more concerned with interaction volume, contamination, localised sub micron carbide formation within the steel; all these render the relatively bulk analysis techniques nonsensical.

PPS if you really want a challenge, try finding out if submicron boron entities in steel are boron carbides, boron nitrides or a mixture.  Never got that one sorted.   

[Probeman]

PS. I work in the steel industry and never do (kicking and screaming) Quant C or N in steel.
These elements are best analysed by a bulk technique such as LECO gasses in metal analyser.
Locally, the C distribution is driven by variations in the alloying elements distribution.
I am more concerned with interaction volume, contamination, localised sub micron carbide formation within the steel; all these render the relatively bulk analysis techniques nonsensical.

Hi Les,
I find this all very interesting.  But I have to proceed with EPMA as some clients seem to require micron spatial resolution of these elements.

Trace nitrogen by EPMA is actually pretty easy.  Trace carbon of course challenging due to the carbon contamination issue, but I think our TDI correction takes are of this quite well. The carbon background is the hardest part, but MAN backgrounds plus a blank correction as shown here, seems to work well I think:

http://probesoftware.com/smf/index.php?topic=48.msg270#msg270

For the MAN background on carbon one has to choose standards that don't interfere with the carbon emission line.  Interestingly I just had an academic, studying heat treatment of surfaces (in cross section), contact my lab yesterday, asking for trace nitrogen and carbon analysis on steels again on micron scales.  So I guess I will give it a go again...

[Philippe Pinard]

John, Les, Mike,

As far as I know, the calibration steel reference materials are martensitic and martensite is metastable at room temperature. I attached a figure from my thesis, where I performed a line scan on 4 steel reference materials. Besides the overall shape due to contamination (large increase in the first 4um), I think the small spikes in the line scans confirm the carbon segregation and the formation of small carbides. All the quant work in my thesis was done using Fe3C standard, where the Fe3C "islands" are artificially enlarged to several micrometers.

Philippe

[jon_wade]

Phil - at room temp (and from recollection) its a slow reaction. The temp rise in a conductor must be very low and I suspect that the the cracking of carbon in this case is not due to temperature rise on the surface but some reaction in the vacuum and  the presence of a cool surface. 

I suspect that a lot of the bean damage on an insulator is not related primarily to temp rise, but related to the dielectric strength of the material and the imposed electric field. This would explain, for instance, the very different behaviour of quartz and amorphous silica under the beam.

[Probeman]

The figure that Philippe attached in the previous post, is very similar to what I saw in my own efforts:

http://probesoftware.com/smf/index.php?topic=48.msg339#msg339

The idea he and Sylvia came up with some time ago was to acquire the sample and also the standard for the (MAN) background calibration, using the exact same acquisition parameters. So if you scan 10 seconds per pixel on your sample, also acquire 10 seconds per pixel for the background standard.  This is the basis of the correlated pixel quantification method (CPQ) in CalcImage.  The idea being that carbon contamination might normalize out between the sample and the background standard. 

The main issue I saw with this method was that the carbon contamination reproducibility in the beginning of the CPQ scan isn't as consistent from sample to background standard) as we hoped for.  However, Phillipe soon realized that if they started the scan acquisition well before the area of interest, it seems the carbon contamination eventually "settles down" and is much more reproducible after the initial pixels.

Basically there are several possible methods for quant of carbon.  I am not sure which is the best.  Philippe's method is a single line scan that relies on starting the scan far enough before the area of interest so that the carbon contamination from the previous pixels is allowed to "equilibrate" to a stable level on the subsequent pixels. For example, a single scan that utilizes 5 seconds per pixel will deposit carbon on each pixel, and as the scan proceeds, the subsequent pixels will "encroach" on the carbon contamination from the previous pixels.  The difficulty then is quantifying the carbon when there is carbon being added by beam contamination from the previous pixels.

The other method, is to use what I call TDI scanning, where multiple scans over the same area are performed, but using a much shorter pixel dwell time as seen here:

http://probesoftware.com/smf/index.php?topic=933.msg5990#msg5990

For example, 10 separate 500 msec per pixel scans on the same area (for a total of 5 per pixel dwell time). The idea being that the carbon contamination is deposited in a more even and controlled manner by using multiple fast scans, and because we acquired multiple scans, we can extrapolate the carbon signal back to zero time on a pixel by pixel basis.  Aside from the tests linked to above, I haven't performed enough analyses to fully test this scanning TDI method for trace carbon but it looks very promising.

I will be using this method on the heat treated samples I get next week and will post results here.  On the issue of carbide stability, I should also quote Philippe who responded with this addition info:

Yes the carbon segregates at room temperature. Very slowly but over several years it is measurable.

So the average carbon concentration doesn't change, but it does continue to segregate over time apparently. Does any one have measurements showing how this carbon segregation progresses and at what size/time scales?   Also what effect does this carbon segregation over time have on the mechanical properties of the carbon steel?  I'm sure someone has looked into this...  on a TEM perhaps?

[jon_wade]

Les
Thanks for your post - we've done a lot of ATP analyses on 'odd' high pressure irons and have been trying to correlate these to chemical maps derived from the FEG EPMA.  Its interesting - these things have small oxide blebs in them, on the order of around 100nm's which are mappable.  Whats interesting is that they appear to be correlated to C content on the probe.  Except the ATP says they aren't.  There is a small, atoms thick, 'rim' of carbon around the oxide blobs, but the signal from the probe is significantly bigger.
It transpires that C deposition from the FEG is significant (even with LN2 / Cryocooler) and this together with the change in substrate (from iron to oxide) causes C-estimates derived from the probe to be excessive.   
Les - do you know of any good ATP papers that have looked at this Martensite phase transformation as this seems to be, analytically at least, analogous?

Im also stumped as to why our probe is so filthy with Carbon.  Its dry scroll pumped, grubby finger free,  but the C build up rate makes even quick maps impossible without LN2 and even this ain't great.  Its almost like its bleeding P10 into the column.....any clues folks? 

[Probeman]

Im also stumped as to why our probe is so filthy with Carbon.  Its dry scroll pumped, grubby finger free,  but the C build up rate makes even quick maps impossible without LN2 and even this ain't great.  Its almost like its bleeding P10 into the column.....any clues folks?

I looked at carbon contamination a bit since we couldn't afford a turbo pumped system when we bought our SX100 in 2006, but we did specify a 100K cryo pumped baffle which has worked great since then.

The tests I did some time ago, showed that this system wasn't much worse than a normal turbo, dry scroll pump system:

http://probesoftware.com/smf/index.php?topic=140.0

I don't know where the carbon comes from either but it could be from P-10 (another reason to replace flow detectors), native hydrocarbons on the sample and/or vacuum system.

I still like the idea of an "in situ" UV LED laser to clean continuously during acquisition.

[Probeman]

I suspect that a lot of the bean damage on an insulator is not related primarily to temp rise, but related to the dielectric strength of the material and the imposed electric field. This would explain, for instance, the very different behaviour of quartz and amorphous silica under the beam.

I don't want to get off-topic but I've always thought that SiO2 glass is more robust under the beam than SiO2 crystal simply because the glass is already at a lower state of entropy.  Here is a discussion on silica glass and crystal:

http://probesoftware.com/smf/index.php?topic=130.msg610#msg610

My thinking is that the crystal can only get more disorganized due to phonon excitation by the electron beam, but the glass not so much.
john

[jon_wade]

I suspect that a lot of the bean damage on an insulator is not related primarily to temp rise, but related to the dielectric strength of the material and the imposed electric field. This would explain, for instance, the very different behaviour of quartz and amorphous silica under the beam.

I don't want to get off-topic but I've always thought that SiO2 glass is more robust under the beam than SiO2 crystal simply because the glass is already at a lower state of entropy.  Here is a discussion on silica glass and crystal:

http://probesoftware.com/smf/index.php?topic=130.msg610#msg610

My thinking is that the crystal can only get more disorganized due to phonon excitation by the electron beam, but the glass not so much.
john

I haven't got any pics but when we use high purity Silica capsules in experiments, they are a pig to analyse - you can see a little crater develop as the analysis progresses.  Sorry - off topic.