Dear Amalia (and all),



I encountered the same issue last year. I hope the following information is broadly correct.



Plain carbon steels (or iron carbon alloys) can range in their carbon content, up to 2% carbon (though this is rare).



Carbon steels are often divided into the following categories:

Low:            <0.3%           (sometimes called mild steel)

Medium:         0.30-0.45%

High:   0.45-0.75%

Very High:      0.75-1.50%



Hypoeutectic steel:     <0.8%

Hypereutectic steel:    >0.8%

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Visual Estimation:

You can 'estimate' carbon content visually when looking at an etched specimen (polished block) using a reflected light microscope. Petzo (1978) suggests a selection of different etchants for iron, steel and cast iron. I used 2% Nital and find that works quite well. Once you have etched the sample, you should be able to see the microstructure of the metal and see the ferrite and pearlite. In order to judge the carbon content, I compared my samples to images from publications where the carbon content was given. There are some images in Avner (1974) and Bailey (1972). Samuels (1999) is by far the most helpful text with some good images. I found Scott's (1990) work extremely helpful, as there are some fantastic images of the microstructure of iron-carbon alloys, with captions describing the microstructure and the carbon content. There are some good papers in the Journal of Historical Metallurgy over the years. Agrawal et al's (1990) paper contains some nice images, with a particularly useful image displaying the full range of different carbon content's (p. 21).



Judging carbon content this way is a fairly qualitative method, though you can see broadly whether the steel is hypo- or hypereutectic.

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Mechanical analysis:

You can also test your specimen for its hardness (Vicker's diamond test) to gauge the carbon content. There are some conversion tables in Samuels, though there must be some well used reference tables used generally in physical engineering/materials science. I am not as familiar with this method. I think you have to be aware of where you are testing in the microstructure, and several tests throughout the metal are advisable.

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Chemical analysis:

I am even less clear about the established protocol for measuring carbon content using chemical analysis, though I did find a paper worth looking at, where they use an electron probe micro analyser (EPMA) to measure carbon content (Robaut et al 2006). In Robaut et al's work, they stress the importance of basic metallography (optical microscopy and hardness testing), as well as making sure the sample is not contaminated.



The problem with SEM-EDS method for measuring carbon is the affect carbon coating and contaminants can have on the analysis.

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Overall, I think it is best to look at the microstructure and visually assess the carbon content. I hope this information helps, as I found (and still do) the same issue very challenging last year. The best place to start is with Samuels (1999).



I would be very grateful also to hear of standard methods/texts used for measuring carbon content in steels, as I know that other (archaeo)metallurgists have been doing this for years!

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References



Bailey, A. R. (1972). The Role of Microstructure in Metals: An Introductory Volume. Annotated metallographic specimens: special series-exercises in materials science. Betchworth, Surrey: Metallurgical Services Laboratories Ltd.



Avner, S. H. (1974). Introduction to Physical Metallurgy. Tokyo: McGraw-Hill Kogakusha, Ltg.



Agrawal, O. P., H. Narain, J. Prakesh & S. K. Bhatia (1990). Lamination technique in iron artifacts in ancient India. Historical Metallurgy, 24(1), 12-32.



Petzo, G. (1978). Metallographic Etching: Metallographic and Ceramographic Methods for Revealing Microstructure. Metals Park, Ohio: American Society for Metals.



Robaut, F., Crisci, A., Durand-Charre, M. & Jouanne, D. (2006). Practical Aspects of Carbon Content Determination in Carburized Steels by EPMA. Microscopy and Microanalysis 12, 331-334.



Scott, B. G. (1990). Early Irish Ironworking. Ulster: Ulster Museum.



Samuels, L. E. (1999). Light Microscopy of Carbon Steels. Materials Park, Ohio: ASM International. - you can also find older versions titled 'Optical Microscopy of...'

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Best wishes,



Tom



________________________

Thomas Birch

Department of Archaeology

School of Geosciences

University of Aberdeen

St. Mary's, Elphinstone Road

Aberdeen AB24 3UF

Scotland, UK

Room G16A



+44 (0) 1224 274138

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-----Original Message-----

From: Arch-Metals Group [mailto:[log in to unmask]] On Behalf Of Amalia Siatou

Sent: 27 May 2010 10:27

To: [log in to unmask]

Subject: requesting information on iron analysis



dear all,



I'm a metals conservator from Greece doing a master in materials science. Part of my thesis is the study of archaeological iron and bronze. I came across some difficutlies in measuring the carbon content of archaeological iron (roman nails). Using SEM-EDS the percentage measured was particularly high (almost 10%) which doesn't follow the dating of the objects. Furthermore, I was anable to find adequate equipment to perform chemical analysis.

Do you know of a different method of estimating the carbon percentage in iron?



thank you in advance



sincerely

Amalia Siatou





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