10.4.2 Making Old-Fashioned Crucible Steel in Modern Times
|Old and New|
|After the advent of the blast furnace
around - roughly - 1500, the cast iron produced was the raw material for making
steel. Since it was liquid once, the steel produced from it was rather
homogeneous and without slag inclusions, just like crucible steel. Producing
steel from cast iron happens by taking some of the surplus carbon out. In
theory you can stop taking carbon out as soon as you have reached the
concentration you want. It is not an easy thing to do in practice but it has
been done. Stopping the carbon reducing process early produces ultra-high
carbon steel (UHCS) but nobody, it appears, made that kind of stuff during the
middle ages. The fact that it wasn't done most likely means that nobody wanted
Objects like swords made from cast-iron derived normal steel were still made by forging. Casting steel started wth Huntsman around 1750 on a small scale but only came into its own - roughly - around 1850. Cast steel is by definition crucible steel in the sense that it was liquid once, and that its carbon content was established while the steel was liquid. Once more it would have been no problem to make ultra-high carbon steel (UHCS) and thus a facsimile of ancient crucible steel, and once more, nobody made some. UHCS was not in large demand then, nor is it now.
|Why? Why did
the "West" not appreciate UHCS, considering that is is a very hard
steel? To answer in the words of J. Oleg D.
Sherby, one of the
researchers who worked most of his life on the topic: "Ultrahigh carbon steels (1.0 to 2.1%C), now designated as
UHCS, have been viewed for most of this (20th) century as belonging in the
no man's land of carbon steels being sandwiched between the
extensively-utilized high carbon steels (0.6 to 1.0%C) and the mass-produced
cast irons (2.1 to 4.3%C).
Ultrahigh carbon steels position in the no mans land of carbon steels is because UHCS have been considered to be brittle at room temperature and thus have been generally ignored commercially. The origin of this belief can be traced to the classic work of Howe, published in 1891, in which the tensile ductility of steel was studied as a function of carbon content."
Here is the classical curve of Howe, augmented by the new data of Sherby and colleagues:
|Good old Howe, whoever he was, thus pronounced
UHCS to be useless - but not lightly. He had a hell of a lot of data points as
you can see. However, all his data points were from UHCS where the grains were
completely enclosed in a brittle cementite shell and that cannot but render the
composite brittle, too. I have
dealt with that
What you can do to alter that deplorable state of being is to break up the cementite. Disperse it in the form of small lumps in a continuous or contiguous matrix of ferrite, and you should have some ductility. You know by now, I take it, that for UHCS the phase diagram predicts a mixture of ferrite and cementite. It only remains to produce this mixture by having cementite particles in a ferrite matrix instead of having pearlite grains encased in additional cementite lining the grain boundaries.
|How could one produce a
"nice" ferrite / cementite mixture that is stable at room
temperature? Sort of little spheres of cementite embedded in a continous
matrix. A "spheroidization of
the cementite", in other words, is what you want. There are three
major ways to get this:
possibility is not very attractive - high temperatures and time equals money.
Moreover, while making spherical particles or spheroids of cementite is advantageous for the
crystal in terms of energy, making plain
carbon (graphite) is even better. If you overdo it, you might end up
with a structure akin to white cast iron; totally useless
for making swords.
In fact, one of the first modern publications dealing analytically with wootz steel and swords, the remarkable 1962 paper "Damascus Steel in Legend and Reality" by Carlo Panseri 1), does report that 0.3 % of the 1.42 % total carbon in a magnificent wootz blade was present in graphite form.
The second treatment will always happen if a sword is forged - provided the smith keeps the temperature low. It then is more or less mixed up with the
third way: inducing a divorced eutectoid transformation.
Combining all three treatments in a smart way will produce "Damascus steel, defined as a hypereutectoid ferrocarbon alloy, with partially
|"How do you divorce an eutectoid
transformation?" you might now ask. "I wonder if I even want to know
about it. My own divorce was messy enough, after all", would also be a
perfectly normal reaction. Well - don't worry, or at least not too much. There
was no divorce; the term only means that there wasn't any cooperation either
between the cementite and the ferrite when both were formed from austenite.
A picture says more then thousand words:
|In the normal cooperativemode, ferrite and cementite form together
as shown and
discussed before. The result is the famous
"zebra" structure as seen upon etching in pretty much all metals
that underwent an eutectic / eutectoid transformation during cooling. With
increasing cooling rates the distance between the lamellae gets smaller because
the diffusion lengths
decreases. If you cool too fast at "normal" carbon levels, you first
start to produce bainite, no longer with a clearly
defined zebra structure, and finally
would be justified in calling the formation of bainite a "DET", too;
some people actually do this.
However, if we look at UHCS, a divorced transformation ideally produces spheroidized cementite particles in a ferrite matrix, just as shown above.
|What does it take to produce a nice
DET structure? Either a lot of luck and cunning or a very good understanding of
what is going on. In essence it boils down to:
|The only problem remaining is to supply those
nuclei. They must be very small cementite particles because in our
hypereutectoid steel we can only dissolve about 0.7 % carbon close to
A1, and the rest must exist as cementite. We can produce these
nuclei by doing whatever is needed, essentially annealing at high temperatures.
If these nuclei are randomly distributed,
we will have a random distribution of the large cementite particles found later
in the ferrite; if not, not.
We are inching towards pattern formation here!
|That all this works is shown here:|
|On the left we have fully spheroidized cementite in a UHCS-1.8 %C sample. The treatment included tempering around A1 and deformation by hot rolling. On the right hand side the sample has been annealed for a while at 840 oC, i.e. well in the austenite region, and the "zebra" pearlite now starts to form again during cooling. It goes without saying that spheroidized UHCS is no longer brittle but has some ductility; the red data points in the picture on top are from samples like the ones shown here.|
|Would there be a difference between a
modern UHCS and an antique wootz cake or bulad egg? Both are very hard but with
some ductility left.
Well, yes, of course. The modern "crucible" UHCS would contain some defined amounts of manganese, silicon, and some unspecified and possibly unknown traces of this and that - but no appreciable amount of phosphorous. It would also be rather uniform throughout.
The bulad egg would contain more or less random amounts of this and that, including possibly some (beneficial) manganese but quite possibly also some phosphorous. Moreover, it could be a "bad egg" if it was not completely molten but only partially. Then it was only a mix of a carbon rich liquid and some solid and relatively carbon-lean austenite, look at the phase diagram. It wasn't very uniform either in this case.
So what is better? It is not impossible that some UHSC containing some undefined impurities would be better with respect to basic mechanical properties in comparison to a well-defined modern UHSC that contains only what was put into it. It is not impossible - just very unlikely.
So is there anything that wootz can do that a modern UHSC cannot?
|Well - maybe the ability to arrange the spheroidized cementite particles in such a way that they form a nice pattern, the famous "water" or "damascus" pattern, is special? Good question, just a bit stupid. We know of course that some wootz cakes or bulad eggs do offer that possibility - there are swords with the "water" pattern, after all. The real question is|
|That is "the" question. The answer isn't in yet. Some people looking into the issue say "yes", some say "no". Searching for the answer proceeds like in the old days: the champions of the two factions fight it out in a jousting tournament:|
|The Great Verhoeven - Wadsworth Jousting Tournament|
|There is no shortage of people who have addressed the issue. Two guys, however, have not only addressed the issues but started a veritable jousting tournament, taking stabs at each other (with pens) in turn. I'm referring, of course, in alphabetical order to John. D. Verhoeven and Jeffrey Wadsworth and the men-at-arms who go along with them.|
|John D. Verhoeven is a distinguished Emeritus Professor of Materials Science and Engineering, College of Engineering of Iowa State University. He worked in areas like electrotransport, thermotransport and convection in liquid metals, directional solidification, superconductivity, cast irons, steels and Damascus Steel in his capacity as Professor at Iowa State; publishing a lot of papers.||
|Jeffrey Wadsworth we have met before. He is President and CEO of Battelle Memorial Institute since January 2009. Before that he was the deputy director for science and technology at Lawrence Livermore National Laboratory, the manager of the Metallurgy Department at the Lockheed Research and Development Division of Lockheed Missiles & Space Company Inc. and affiliated with Stanford University. He has authored or co-authored more than 260 papers on a wide range of materials science and metallurgical topics, too|
|These guys are heavyweights and their
jousting is fun to watch from the sideline. They both are limping a bit after
26 years of stabbing at each other (with pens), but none has yielded yet and
both seem to be still capable to mount a horse and charge. Here is (simplified)
what the fight is all about
|Mind you, the tousle is not about if you can or cannot make a pattern with any modern UHSC steel. It is about:|
|The emphasize is on "nice" because you can definitely get some pattern with any modern high-carbon steel. Verhoeven claims that you only get a "nice" pattern (he calls it, for example, "museum-quality" as opposed to "granular") if the original steel contained some carbide-forming trace impurities like vanadium (V) or molybdenum (Mo) that become distributed inhomogenously due to dendrite formation during solidification. More about pattern appreciation in this module.|
|So who is right? I'll tell you if you send me some money in unmarked large bills. Or maybe in the next chapter. Until then use the link above and enjoy the tournament.|
|Making Crucible Steel the Old-Fashioned Way|
|There seem to be very few old wootz cakes / bulat eggs around, and whatever there is cannot be used for experimenting. So let's make some, following the old recipes as far as we can. Maybe with a few shortcuts in the beginning. You don't need to build a kiln and burn charcoal anymore to get the temperature up - we have electricity for that. But that doesn't matter. Whatever is in the crucible doesn't care how it is made hot. It might care if the atmosphere is oxidizing or reducing, though.|
| Verhoeven began a
unique collaboration with master blade-smith Alfred H.
Pendray in 1988. The idea was to prove his
point by recreating blades just as nice as the ones we admire in museum from
crucible steel made for this purpose. Pendray was infected by the scientific
spirit and charged his crucible with high purity stuff and deep thoughts about
Richard (Ric) Furrer, however, did his crucible experiments under the influence of beer. Both guys produced crucible steel from which blades could be forged. Here are their recipes:
Pendray is a blacksmith and the former President of the American
Knifemaker's Guild. Not only is he an expert on "wootz" steel but as
a farrier has shoed some of the most famous (and valuable) thoroughbred race
horses in the world. He has
for more than 20 years with John D. Verhoeven.
Richard Furrer started as a black smith and is now running the "Door County Forgeworks". He is an internationally renowned expert for ancient metallurgy who works with scientists from several universities and teaches classes there.
Both produce wootz cakes of quality, and both can make blades with a definite water pattern:
|Pendray's piece show a "Mohammed's ladder with roses" pattern, the most complex pattern made in the old days.|
|Incidentally, Pendray confirms what I
suspected some time ago: Organic
matter as a source of carbon might be advantageous because it also supplies
gases like hydrogen, helping the process.
Meanwhile several smiths produce modern wootz blades with a pattern. You find an example in Calabrés paper, by looking for the work of a Russian smith named Mr. Ivan Kirpichev, or by looking at the work of Eric M. Taleff 3, a researcher who coauthored several papers with Wadsworth and Sherby. Or just search the Net, you will find more.
|But are those modern patterns
"nice" patterns? The one above
might qualify, I believe, even so it is not traditional. But that is a touchy
topic since it is a matter of taste and it is hard to define where
"nice" ends and "inferior" starts.
From what I have seen, Pendray's blades take the price. So far he seems to be the only one who recreated the "ladder and rose" pattern as shown above. Verhoeven attributes this to traces of vanadium that are contained in the Sorel iron used (without knowing this) for the making of the wootz. This might well be true and the question now is:
|I'll look into this in the next chapter where I finally will get to swords|
|1)||Carlo Panseri: "Damascus Steel in Legend and Reality" reprinted in Gladius, IV (1965), pp. 5-66; originally published in ARMI ANTICHE, Bulletin of the Accademia di Marclano, Turin, sole number for 1962, and translated to English by H. Bartlett Wells. Washington DC, in 1965|
et al.: "Traditional Forging of Swords and Knives with Legitimate Damascus
Steel", Prakt. Metallogr. 38/6 2001 . p. 325 - 337
(Note the "Legitimate")
|3)||Eric M. Taleff:
"Microstructural Characterization of a Knife with Damask Patterning",
Technical Report, Internet
Taleff investigated a "blade created from a commercial tool steel using a combination of thermal and mechanical processing steps." His conclusion concerning the pattern shown above is: "The knife blade examined clearly falls into the category of a genuine damascus steel. The damask pattern it exhibits results from microstructural bands in the distribution of very fine carbide particles."
© H. Föll (Iron, Steel and Swords script)