Carbon into cementite – a question

[Keith Ducklin]

Hello everyone,

Question from a new member and metallurgical autodidact, something I can’t quite grasp even after a lot of studying. If anyone can help, that would be splendid.

I’m trying to understand the effects of cooling on a late Iron Age or early Medieval sword steel, say with around 6%C. I believe I’m right in saying that when austenite cools and the phase change reverts to ferrite, the rejected carbon compounds into cementite, which can then form pearlite. Thus, most blades of this period, assuming there’s no quenching involved, have pearlitic/ferritic structures. What still eludes me is whether the amounts of cementite and pearlite formed are directly related to the amount of carbon rejected during the downward transformation. I’m assuming that a higher level of carbon in the blade directly equates to a higher level of cementite/pearlite formation during the cool, but am I right about that?

I may be sounding very dim, but I’ve yet to read or hear anything that answers this question in so many words. Of course, “so many words” may not be possible if I’m completely missing the point.

Thank you,

KD.

[Rchard Williams]

Yes, Keith, in a normally cooled plain carbon steel, the pearlite content at room temperature is a function of carbon content, since all the carbon has to be redirected when austenite changes to ferrite, (the solubility of carbon in ferrite is very low). However, a microstructure which is 100% pearlite only exists at a carbon content around 0.8%, the eutectoid value. Up to that point ferrite occurs in the microstructure roughly pro rata with the carbon content, whilst beyond 0.8% carbon, cementite is increasingly the phase in equilibrium with the pearlite. You presumably mean that your sword contained 0.6% carbon. (6% is not possible, even in saturated cast irons.) Very crudely, the sword’s microstructure would contain a weight ratio of 2/8 ferrite to 6/8 pearlite, but of course it is a volume ratio which you observe..

If it had been cooled more quickly, perhaps quenched in water, then all sorts of tortured intermediate structures could result, of which martensite is the most famous. Surely, by the era that you are talking about, iron smiths would have known that to get the best performance out of of a steel sword, some sort of heat treatment was necessary and the material would not have been left in what is known as the normalised state?

[Gerry McDonnell]

As a 0.6% C steel cools the ferrite precipitates out, and the remaining austenite phase become richer in carbon, until at 727oC the remaining austenite contains 0.8% carbon and forms pearlite defined in Samuels (2003, p25) as ” eutectoid transformation of ferrite and cementite with an ideally lamellar structure”. Samuels, L.E 2003 Light Microscopy of Carbon Steels. ASM International. The definitive text for carbon steels, an essential book for the metallography of steel (and ferritic iron). A quick look on Abe books, shows plenty of copies available but in USA/Canada

[Keith Ducklin]

Richard, Gerry,

Thanks so much for your prompt and helpful replies.

I’m coming at this sideways, from a lifetime of practical sword use, so I’m aware of just how much I have to learn.

Gerry, is Samuels’ “Light Microscopy” an updated version of his earlier “Optical Microscopy”? I only ask because, if it’s still useful, “Optical” is much cheaper and there’s a UK seller! (Besides, I’ve just spent a fortune on Alan Williams’ “The Sword and the Crucible”.)

Richard, are you familiar with your namesake’s book and sundry papers/articles? In “TS&TC” he talks in great detail about how widespread the use of heat treatment was (less than one might expect): how the Vikings, for instance, ran the gamut from utilising hypereutectoid (probably crucible) steel to very mild steel for their blades. Some blades are heat-treated, others aren’t. Some have heat-treated edges welded onto mild steel cores etc.. One of the overall conclusions of Williams’ work is that heat treatment was not universally employed, either for swords or armour, before the Western adoption of the blast furnace and finery. It’s an engrossing read.

Thanks again,

KD.

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