Investigating Extra Carbon in Steel Using SEM

[lancashire liuyu]

when i was using Scanning Electron Microscope to investigate fracture surface of a metal sample,the results of composition of it appeared so different to what the composition should be, that sample contains 2% carbon comparing to normally 0.1%. i thought there are 3 ways of doing that:
1.hand--i need to use my hand to handle the operation during the process.
scurf and sweat may pollute the sample surface.
2.environment--instrument;air with high content of carbon dioxide;
residual grease.
3.Chemical reaction--with all possible source.
what is the major reason of that happened? any possibility else?
it's about my project,thank you!

 

[PerennialII]

... the 1st question that comes to my mind is what were you actually analyzing from the fracture surface? Fracture surfaces often have concentrations of various alloying, impurity etc. elements (which contribute to the fracture process and as a result often reside there, and are the focal point of analysis), so is there a chance you are analyzing something which shouldn't have the nominal composition of the steel?

 

[Astronuc]

It might be possible that the facture is along the grain boundaries (intergranular fracture) which have chromium carbide (Cr₂₃C₆) precipitates.

 

[lancashire liuyu]

... the 1st question that comes to my mind is what were you actually analyzing from the fracture surface? Fracture surfaces often have concentrations of various alloying, impurity etc. elements (which contribute to the fracture process and as a result often reside there, and are the focal point of analysis), so is there a chance you are analyzing something which shouldn't have the nominal composition of the steel?

i was using the normal type of steel,which contains normally 0.1%carbon in itself. and the same situation happened in other materials which should not have any carbon inside, like aluminum,copper.

 

[lancashire liuyu]

I might be possible that the facture is along the grain boundaries (intergranular fracture) which have chromium carbide (Cr₂₃C₆) precipitates.

and why there is chromium carbide precipitated inside?? theoretically i used pure materials,like copper,aluminum...

 

[PerennialII]

i was using the normal type of steel,which contains normally 0.1%carbon in itself. and the same situation happened in other materials which should not have any carbon inside, like aluminum,copper.

What kind of an analyzer were you using in your SEM (and with what sort of analysis parameters if there is something unusual or so in them)? Sample preparation might be one, how was it done and were there any steps which might result in carbon 'contamination'? If you're getting the result (with the analyzer) consistently irrespective of the sample, might the analyzer / SEM unit have gotten contaminated itself (like by some other user)? Sometimes we have problems when someone studies unclean samples, especially samples which have been in corrosive etc. environments.

 

[Gokul43201]

An SEM itself will not give you a composition analysis - it takes an EDAX attachment to get that info.

Now really, unless the OP provides more specifics on the measurement, any discussion is quite a waste of time.

First, it was said that the high C content was measured in a steel (what grade??). Later, we find out that this was also seen in "theoretically" pure metals (again, no description of grade). There is no mention of sample history (notably, type of fracture), preparation or instrumentation.

 

[Astronuc]

and why there is chromium carbide precipitated inside??

I was thinking of Stainless Steel with about 0.08%C or higher, and chromium carbides will precipitate based on thermal treatment. It is one reason to develop low carbon SS, with =< 0.03% C.

What does one mean by 'normal' steel? There are structural steels or various types, and stainless steels of various types.

e.g. http://www.keytosteel.com/default.aspx?ID=CheckArticle&NM=64

As PerennialII and Gokul questioned, what sample prep did on use. Any grinding with SiC, for example, and grinding liquid? Any type of etchant, or cleaning with alcohol or other carbon-bearing material? If one is sampling 'pure' materials, maybe there is cross-contamination in one's procedures.

Also, as Gokul alluded, does the SEM process involve another analysis? XRF? or EDAX? See for example - http://www.edax.com/

One find this useful in the future:
http://www.keytosteel.com/default.aspx?ID=Articles

 

[brewnog]

The few times I've used EDX compositional analysis on fracture surfaces using an SEM (failed combustion engine components), I've generally found false carbon peaks to be down to oil contamination.

 

[lancashire liuyu]

for instance,i broke using tensile mechine steel sample A first, and i did not do any preparation,i.e coating, before place it into SEM,the result of its composition comes out by EDAX on carbon is 2.74% comparing to theoretical value 0.1%...basically same things happened to copper and aluminum...