Microsegregation question

I have a question about the chemical analysis of an alloyed metal, one
that had such elements as copper, antimony and arsenic added as
hardening agents. My understanding is that if one examines very small
samples, the grain boundaries that accumulate those elements (Cu, Sb,
etc.) could skew the results: in other words, using a small sample
would amplify your chances of mischaracterizing the chemical content of
the overall piece of metal. This is a result of microsegregation.
My question is this: within each sample, would the "spike" in antimony
or copper correlate with each other? If there was a jump in copper
in one sample, should there be a close/promixate jump in antimony in
the same sample, or would their relative accumulation in the grain
boundaries have nothing to do with each other?
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Some of this will depend on the elements added, but sampling is and has been a problem for many years.
There are a number of approaches to gaining a representative sample, some of which are more effective than others. There are methods that allow for cleaning the surface of the furnace prior to dipping a (maybe coated) sampling ladle or a vacuum operated sampler, different approaches to moulding, and different ways to cut or prepare a face to analyse. These change depending on the type of metal you are trying to test. Hard metals like iron and steel are normally prepared with a grinding operation, while softer metals are often cut on a lathe or milling machine to prevent smearing of "stuff" (*technical term) from one part of the sample to another.
There are also a range of matrix effects that occur simply because of the way the sample has been taken or prepared. For example, MnS inclusions will occur at varying depths from the face of a sample simply due to the way the sample has cooled. Reading this sample with say an optical emission spectrometer (spark machine or spectrograph depending on how old you are!) will give different readings if varying amounts of metal are removed from the face of the sample.
For spectrometers, which are the principal machine used in typical foundries for metal analysis, the samples are normally taken such that a thin sample that cools rapidly on a copper chill block or water cooled mould allows for very little crystal or plate or grain or whatever growth, and the sample is as homogonous as practically possible.
Some additions are particularly problematic, such as lead in free cutting steel. The Pb form into little balls that provide a chip breaker function for machining, like on a lathe. Of course, this makes it difficult to obtain stable results.
Hope this helps, Peter
Reply to
Bushy Pete
Thank you for your response. This is more of an after-the-fact assessment. What I'm trying to get at is this: in trying to assess whether or not the "damage has been done", would it be fair to look for correlations between the elements added (in this specific case, antimony, copper) in specific grain boundaries? Or am I making an unwarranted assumption: could antimony accumulate in the grain boundaries in levels completely unrelated to the accumulation of copper? If someone came to me and said-- "since there is no correlation between the alleged spike in antimony and the alleged spike in copper in each of the given samples, we can place faith in our analysis"--- would that person be making a fatal assumption?
Thanks, Stu
Bushy Pete wrote:
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G'day Stu, How has your analysis been done? And how have the samples been taken?
I'm not up to date with SEM or similar techniques, the type of instruments I'm used to are OES/spark spectrometers which will analyse an area of a sample some 10mm in diameter and up to about 0.1mm deep, so testing a particular grain boundary is out of the question.
If you are testing just a particular grain boundary, then a number of test sites across the sample and averaging the results would seem to be in order.
Reply to
Bushy Pete
It's not so much testing the grain boundary as testing small samples. There aren't a tremendous amount per piece of metal from the same lot. There are some discrepancies in the readings. There has been a thought that if a one looks at the key elements over a set of samples, IF there is no correlation between a rise in antimony and a rise in copper, then any problems with the readings are not due to microsegregation but something else. I'm wondering if that premise is accurate: that mischaracterization due to microsegregation must imply that the added hardeners (antimony, copper) accumulated in the grain boundaries in some relation to each other... any increase in copper must also see an increase in antimony. What does your experience tell you in that regard? Thanks.
Bushy Pete wrote:
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I'll try to stick to my area of "expertise"? I fix spectrometers to stop the bank manager sending me dirty letters, so I'm not up to date with the metallurgy, but can give you a bit of knowledge based on my experience.
Samples that are tested by spectrometer in a range of labs are normally samples with a single face surface larger that 16mm, if you think of a coin shaped sample, it would normally want to be at least 1 mm thick. This sample size will cover the hole in the sample table where it is analysed, and provide ample thickness that will prevent the sample from melting from the current used to spark the sample such that up to about 10mm by 0,1mm becomes the sample section that is actually melted and vaporised. (and tested!)
Samples prepared from molten metal are normally chilled as poured so there is not much chance for the metal to for a complex grain structure, and to try to make the sample as homogonous as possible. Samples are routinely tested that have been cut from a larger casting and there may be quite some difference through the casting, but the sample is taken from a part that may be higher in alloying constituents than other sections of the casting.
For example, out of a continuous cast 7" diameter 6063 alloy aluminium log, a 2" thick piece was cut. Then with a hole saw several 2" diameter pieces were cut and the pieces tested to see if they could be used as setup samples. The levels of several elements were higher in the pieces cut from the outer area of the log, and different levels were found in the piece cut from the centre of the log. There was also difference from one side of the sample to the other side of the sample, which corresponds to a greater distance from either the centre (slowest cooling!) and the outer edge (fastest cooling).
Another example, with a manganese steel sample poured into a paper cup shaped sand mould, with a bottom diameter around 40mm and a top diameter of about 60mm and a height of about 70mm, there is significant difference in reading for both manganese and sulphur from the bottom surface through the next 20 mm as the sample is re-prepared by grinding more metal off the bottom test face. This could be referenced back to tests reported in Karl Slicker's book on Spectrometry. It has a chapter on sampling of metal that describes this in some detail. This once again comes from the cooling rate allowing MnS inclusions to occur at different depths. Also note that as some elements are solidifying at different depths, there is a corresponding difference in the concentration in other elements that do not exhibit this behaviour, due to dilution from the elements that do. It's sort of like a can of worms, if all the big ones fight their way to the top, the little ones are pushed down to the bottom!
I believe that there is quite some difference in pieces cut from larger chunks of metal, compared to samples taken from molten metal and cooled such that this segregation is kept to a minimum. The design of the sample mould has quite a bearing on the stability of results, and varies from one metal type to another. Maybe we are talking on different scales, and the segregation that I can refer to is on a larger scale that the microsegregation you are talking about, and I am thinking in terms of the larger levels obtained by averaging several hundred grain boundaries that are melted and vaporised in my machines and you are thinking on the levels between any two adjacent grains. The SEM (scanning electron microscope) can analyse down towards this sort of sample size.
Smaller samples that me be tested regularly range from turnings, filings or drillings, which are often remelted and recast, ideally under an inert atmosphere, there are sample remelt machines that can do this. Wires down to 3mm diameter can be held in a chuck and held over the sample stage, there is reduced stability in the results when thinner wires are tested, but wires over about 10mm can give quite reasonable results. Foils can be folded a number of time (and compressed?) to obtain a thick enough sample. Other chunks can be cast in epoxy, and as long as a sample clamp can make electrical contact with the sample, normally by leaving part of the sample exposed at the rear of the epoxy casting, the epoxy will seal the sample table's hole and keep the argon in. With the exception of the smaller diameter wires, the same cross-section of the sample is exposed to the spark, and similar results can be obtained. There may be a bit of playing around with the spark's electrical conditions, and the length of time taken to flush the sample chamber before sparking, before good consistent results are obtained, but this method development helps to keep my bank manager happy.
What sort of metal, and how was it made, are you having problems with, and how large a piece are you able to obtain from each lot? Is each "lot" from the same melt, plant, company, or are these "scrap" samples?
There are a lot of reasons for segregation, and I'm happy to let someone else jump in with their experience, but remember, there are lots times that you will wish you could get a representative sample when you won't be able to. However, there are ways that are accepted as suitable to get a representative sample, that the powers that be, deem to be reasonable methods, and some of these, well, there are three types of lies: lies, damn lies and statistics!
Hope this helps, Peter
Reply to
Bushy Pete
Bushy Pete,
This question is addressed to you. Since you say you are associated with OES and more particularly in servicing them. Do you also deal in used spectrometrs or refurbished ones. I am looking for a very basic model around 10kUSD to analyse steel and stainless steels inhouse.
Arun Rao
Bushy Pete wrote:
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G'day Arun, Suspect you are stateside! I would love to help you, but I am based near Brisbane, Australia. I fear the shipping costs, and the cost of getting me over to install it for you may blow the budget! If I heard the news right this morning, the "Aussie Peso" is not worth much......
I do aftermarket service, and have worked with ARL spectrometers for over twenty years, and a number of years before as a tech in the Aussie Army. I now live on a small farm and have cut a hole in the side of my two bedroom garage where I currently have two spectrometers in my modified second bedroom. Out of the two, I could get one (a 34000) set up for steels, but this could take some time as one is currently set up for aluminium base, and the other (a 3580) is an ICP for liquid samples which has been substantially raided for parts. The 34000 is about 600Kg and the 3580 is 860Kg, both are top heavy machines that want a substantial shipping pallet, and are regularly dropped in transport as some fork lift drivers don't allow for the high CofG.
Keep an eye out for a "local" second hand ARL 34000, or 3460 or 3560 model in "working?" order, and I can help you and your local sparky pull it down for transport, set it up, and advise on it's use and servicing for the next twenty years. If it needs extra channels fitted, I can do this, and have a good range of parts. The earlier (and much cheaper) 34000 model is over twenty years old, but can still be maintained, the later 3460 and 3560 are still current instruments and most parts are available for both, direct from Thermo, which is the company that owns ARL, or from local electrical supply outlets. There is also a 3360 model that was made as a low budget foundry machine that would do a good job as well, these could be around your budget, as would a 34000. The 34xx instruments will go for more at foundry auctions and so on, depending on if they have new or old series electronics, and what channel set up they have. A new machine would set you back about US$80,000 to US$100,000, depending on options.
Laboratory instruments in general, are not highly desirable to the majority of second hand dealers, due to the high "skill levels?" that can be required to remove and reinstall them. Consequently, they can often go for ridiculously low prices. If you have the time to wait, you can get bargains. Try to obtain the paperwork from the lab as well, as the instruments had a heap of circuit diagrams shipped with them, and there would be notes about the calibration that may make later work easier. Many will also have a range of spare parts on-site, and these box lots will also go for a song. If you are dealing with a foundry that has upgraded with a replacement machine, the value would depend on their perceived need for a back-up machine, and the amount of floor space they have.
Your requirement for ferrous metals is common, and there are more instruments out there for steel base than any other type. Most of these would not require much modification, if any, to suit your steel and stainless combination.
Problems you may find with the older 34000 machines are often related to temperature stability, and these can be addressed by having a better room airconditioner and a bit of insulation, the later ones are more stable. The room that you use can be cheaply set up, and also makes for a nice warm in winter and cool in summer office. I have a couple of insulated 20 foot shipping containers, that will in future, make ideal onsite labs with the only external requirement being power. They are easy to keep clean and seal against foundry dust quite well.
Maybe there is someone else that would like to share travel expenses, for a bit of a look over their lab?
Hope this helps, Peter
Reply to
Bushy Pete

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