11.5.2 Structure by Dendrites?
|Verhoeven's Dendrite Model Step by Step|
|Let's start by looking closely at
Verhoeven's "nice wootz
structure by dendrites" hypothesis. A short version of the
"structure by dendrites" hypothesis a given
in this module;
I will repeat it here for easier reading:
According to Verhoeven, what you need for making a wootz sword with a "nice" pattern that is based on a striated distribution of cementite as discussed before, are the following ingredients:
|Verhoeven and "his" master smith Al
Pendray have succeeded in producing a very good
kirk nardeban and
rose pattern from scratch. So far nobody else seems to have been able to do
that. That is certainly a strong point in the favor of Verhoeven's
"dendrite" hypothesis, if not an outright proof? Well - not quite.
Lots of ancient smiths have also produced "kirk nardeban" patterns
from scratch, and their theories, if they had any, were certainly wrong.
In what follows I'll discuss the "structure by dendrites" hypothesis of Verhoeven in some detail and put it into perspective. His 2007 paper, accessible by this link, provides the base.
|First, let me point out that
Verhoeven's hypothesis could easily work - provided a few "minor"
points are met. It could easily work because most of it is based on sound and
well-known scientific ingredients.
The first point to look at is the solidification of a liquid iron - carbon mix, that also contains traces of some key impurities, via formation of dendrites.. That is not an easy thing to do. I have given you an "easy" backbone module for the topic and you might want to refresh your memory now by looking at it.
|If you're not easily scared, you may also want to
look at the following science modules dealing with solidification, segregation
and dendritic growth:
|Now let's look at Verhoeven's
"nice wootz structure by dendrites" hypothesis step by step. Before I
start I need to point out two things:
|First step: Have
Strong Carbide Formers
For starters, the high-carbon crucible steel material must contain some impurities that are strong carbide formers like (Nb), titanium (Ti) or vanadium. In the "science of alloying" module I have written:
"Very strong carbide forming elements, such as niobium (Nb), titanium (Ti) and vanadium (V), always form alloy carbides, preferentially at alloying concentrations less than
Note that these elements form niobium, titanium or vanadium carbides and not iron carbide or cementite in the first step.
|Verhoeven claims a vanadium (V) level as low as
0.004 % is sufficient to cause nice wootz patterns later. Other just
"strong" carbide formers like molybdenum (Mo), chromium (Cr) and
tungsten (W) might also be good.
A concentration of 0.004 % or 40 ppm is extremely low for iron / steel. Plenty of ancient crucible steels might have contained these impurities at such low levels. There is no particular problem yet with Verhoeven's hypothesis. However, it does sort UHCS samples in two camps: those with the proper impurities and those without. Only the first group is suitable for making blades with nice wootz pattern.
Wadsworth would disagree
|Second step: Have
large dendrites during solidification
Solidify in such a way that large dendrites are produced at the solid-liquid interface. Note that it is not good enough to have dendrites. They must be rather large because in Verhoeven's model their average distance determines the distance between the cementite bands that form the pattern in the blade. Since forging reduces this distance (akin to what is shown here), it needs to be in the 0.5 mm region.
|To achieve that is quite possible. I have
provided already several
pictures that show
large dendrites in steel. You need a slowly moving solid-liquid interface and a
suitable temperature gradient or just the "right" slow cooling
conditions during the solidification.
Note that it doesn't matter for this how fast or slow you cool after everything has solidified.
It is certainly possible to achieve this. Chances for getting large dendrites / grain structures tend to be better if the steel was liquid well above its melting temperature or well superheated and not just barely so; look at this picture.
Verhoeven illustrates this (and a bit) more with the following picture:
|There is nothing wrong with that picture, except perhaps that it insinuates, just like this picture shown in the famous "Scientific American" article, that all dendrites, everywhere and at all times, are the same size are growing parallel to each other all over the place. That is not the case!|
Realize that an inhomogeneous distribution of the carbide formers
During solidification the carbide forming impurity atoms (and the carbon) strongly "segregate" into the region between the dendrites. This means that the iron between the dendrites, the iron that solidifies after the dendrites have formed, is enriched with carbide formers (and carbon), i.e. the concentration there us much higher than the nominal concentration. Inside the dendrites themselves the concentration is much lower, they are denuded of carbide formers (and carbon).
It is thus far more likely that carbides like vanadium-carbide forms between the dendrites than in the dendrites when the steel cools down. Since these metal carbides act as nucleation centers for the cementite that also needs to form during cooling but somewhat later, the primary cementite forming in hypereutectoid steels a high temperatures will also predominantly be found in the interdendritic areas, especially if you don't cool down very fast. This may also help to spheroidize the carbide.
|All of this is absolutely correct; it just makes ingenious use of basic segregation theory. One only needs to add that other defects like grain boundaries might also nucleate some primary cementite in competition with the metal carbides.|
|The big question:
Does Step 3 produce sheets or bands of cementite in the blade?
Now it gets interesting. In the figure above there are dotted lines labeled "sheet geometry". The text going with it reads:
"As shown in view B of Fig. 4 (the figure above), the interdendritic arrays possess a geometry that displays sets of discrete sheets or bands".
|It is not! Or only up to a point. There are several reasons for this:|
|First of all, there simply aren't planes of high vanadium or whatever concentration. There are lines! This pictures makes that clear.|
|Second, even if one accepts the "sheet geometry" in Verhoeven's picture shown above for the sake of the argument, it does not exist like this in real materials, even within just one grain. Just look a the following pictures and imagine the black dots to be the top of the dendrites:|
|You are looking at pores in indium phosphide (InP; left) and silicon (right); a bit more to that is here. But it doesn't matter what exactly is shown in the picture above. Think of the dark spots as tips of dendrites coming straight up at you. What you see is that the area between the dendrites can be described as three overlapping bands, indeed, but only as long as the dendrites are arranged in a rather perfect repetitive pattern (forming a "crystal", in other words). Real dendrites won't do that; the pictures in this link show that. So arrange them a bit disorderly like in the right-hand picture - and where are your bands?|
|Third, the steel is poly-crystalline. The dendrite orientation changes from one grain to the next one, and so do the "sheets" or "planes". Modifying Verhoeven's picture to take this into account yields this:|
|The drawing shows a schematic structure with perfectly aligned dendrites. An actual structure can be seen in this link or below|
|Ironically, this picture is from Verhoeven's book: "Steel Metallurgy for the Non-Metallurgist". It just makes clear that there no sheets aligned for long distances in poly crystals.|
|Enough! It is absolutely clear by now
that there are no parallel sheets or bands of high impurity concentration
because of dendrites that run nicely
parallel to each other through the whole wootz ingot right after it solidified
or after it cooled down to room temperature. This is not to say that there
isn't an inhomogeneous distribution of impurities - it is just not banded in an
orderly fashion because of dendrites!
To make that very clear:
|Verhoeven knows this, of course. In
he writes: "The ingots (of wootz crucible steel made
by him) are flat across the top displaying large radially directed surface
dendrites. Their microstructure displays a combination of grain boundary and
Widmanstätten cementite with no apparent
relationship to the prior dendrite arrays.".
So let's look at the crucial next step:
Mechanically deform the ingot / wootz cake into a blade shape
That is the step where the necessary banded structure of the metal carbides and thus the banded structure of the cementite originates. Interestingly, this step is not prominently mentioned in Verhoeven's papers. Verhoeven, however, is quite aware of the importance of the mechanical deformation discussed here as a necessary ingredient for making wootz swords with a nice pattern. Here are some relevant quotes:
|In plain words: Whatever distribution of carbide
formers / cementite particles you have in the UHCS that you use for forging a
blade, it is the massive mechanical deformation (by hammer in the old times, by
a roller mill today, by whatever) that might align cementite particles in bands.
That is - in a small way - what I have shown to happen for purely geometric reasons before.
That's what Wadsworth more or less claimed all along. However, he did not invoke carbide formers, and they will make a difference as we shall see.
It is important to note:
|Also note that after solidification is complete, the dendrites do not exist any more. All that exists is a crystal like austenite that contains some dirt and possibly already some primary cementite. Only the distribution of those things in the crystal supplies a kind of vague memory of the former dendritic structure. In addition, there might be a few small-angle grain boundaries in places where the dendrite alignment was a bit off.|
|Now comes a
surprise: Running a (hot) piece of any
steel through a roller mill, reducing its thickness, often produces banded
structures with a nicely staggered sequence of carbon-lean, carbon-rich,
carbon-lean, .... , layers. Consult the advanced module for details. Verhoeven
has actually written a big review paper1) about banding as that
phenomena is called, full of complex stuff. It is entitled: "A Review of
Microsegregation Induced Banding Phenomena in Steels".
However, Verhoeven (and everybody else) is rather vague about exactly how this mechanical alignment process works. He goes through great lengths to elucidate the various (and rather complex) mechanisms of cementite formation in various kinds of hypo- and hypereutectoid steel, and how it depends on cooling rates and God knows what else. Here is what he has to say about the mechanisms that are responsible for forming the bands during the heavy plastic deformation occurring during milling (or hammering):
|In other words: Verhoeven (and
everybody else) does not know exactly how this
process happens. Nobody knows. I have studied a lot of papers and books and
found no convincing explanation. Pretty much all authors weasel a lot because
this is acutely embarrassing. Banding is a major (and mostly very unwanted)
effect that occurs in much of steel processing - and we don't know exactly what
However, Verhoeven concludes that pronounced banding is easier to obtain if the material already has "pre-banded" regions. He might well be right about this because banding does not always occur in structures containing mixes of hard and soft regions. The "grain boundary wootz" of Wadsworth or Harnecker's wootz is observed in steel that seems not to have produced banding upon deformation. I'm not sure if I subscribe wholesale to Verhoeven's point of view but Verhoeven knows a lot more than I do about banding and making wootz structures so let's go with it for the present.
Let's just look at a simple picture to illustrate what happens:
|it is a very simple picture but shows the
essentials (and I am repeating myself to some extent on purpose here):
|Banding due to rolling occurs in
hypo- and hypereutectoid steel, including steel with no known addition of
strong carbide formers. This might be seen as implying that carbide formers are
not all that important for the formation of a nice pattern, in contrast to what
Verhoeven claims. That is not quite true, however. The difference shows in
three situations; two of which are (related) :
Thermal cycling / Ostwald ripening
We now have a banded distribution of at least the carbide formers if not the cementite. It might be sufficient for producing a pattern but not yet for producing a nice pattern on wootz blades. Verhoeven lists as one of his 7 conditions for nice patterns: "There are no significant differences in particle size or shape on transverse versus longitudinal sections".
In other words; the cementite particles should all have about the same (and not too small) size.
|Well, we already know how to achieve that. Either straight Ostwald ripening at high temperatures but still below A1 or thermal cycling between just a bit above and below A1. I have described that that in detail before; here I will let Verhoeven describe what happens:|
|These results show that
during the thermal cycling the microsegregated impurities are causing the
cementite particles to gradually increase in size and density along the remnant
IRs (interdentritic regions) of the forged ingot
as the number of thermal cycles increases.
During the heat-up part of each thermal cycle the smaller cementite particles will be removed by dissolution while the larger particles will remain at a reduced size. During the cool-down portion of the cycles the larger particles will grow. It is not likely, however, that the smaller particles will reform at adequate rates to replace themselves during cool-down because this requires nucleation, and the presence of the nearby larger particles provides sites for cementite growth without need of nucleation. Hence if bands of larger particles once form, the cycling process will cause them to grow at the expense of the smaller particles.
Hence it seems most likely that the cementite is simply nucleating on austenite/ austenite grain boundaries during the thermal cycling and then forming into aligned bands of c!ustered particIes by the coarsening processes such as that discussed above along with some coarsening due the classical Ostwald ripening mechanism.
|Now we are almost done - as far as
"Structure by Dendrites" is concerned. I only need to mention that in
some of what I have written you might replace the A1
temperature (where secondary cementite forms) by the
temperature (where primary cementite forms) and look at the austenite
formation instead of ferrite formation for getting some important
inhomogeneities. But things are already complicated enough, so let's stop here.
Whatever happens in detail, the steps described above, if done right, make sure that the blade now contains sheets or bands of proper cementite particles that are roughly parallel to the blade surface. We can now produce nice patterns by using some forging tricks.
That will be covered in the next sub-chapter. Here it only remains to state two things. First:
|Second: I still need to announce the winner of the Great Verhoeven - Wadsworth Jousting Tournament. So:|
|The Winner is...|
|Me, of course, for figuring out what
this was all about. But seriously now: Verhoeven does come out ahead!.
He and his co-workers did shed a lot of light on the specifics needed to
progress from a pattern to a nice pattern. Moreover, they actually made nice
wootz structures, including kirk nardaban with roses, somebody nobody else so
far has achieved as far as I know.
My only criticism is that some crucial issues are a bit underdeveloped in some publications (on both sides) and that it is not always stated outright that there are still some open questions.
It should also be clear by now that Verhoeven's work leads right into some topics of modern iron an steel research that are not yet fully understood. And that allows me to make a suggestion of my own:
|All the problems around producing parallel bands
of first carbide formers and later cementite precipitates disappear if that
happens right during solidification. It is clear by now that this does not
happen by a simple "dendrite mechanism". What about some other
The wanted structure is actually known from the solidification of many other liquid-solid systems and called striations. I have already written a lot about that so I just give you the links:
|1)||John D. Verhoeven: "A Review of Microsegregation Induced Banding Phenomena in Steels", Journal of Materials Engineering and Performance (JMEPEG), Vol. 9/3 (2000) p. 286 - 296|
© H. Föll (Iron, Steel and Swords script)