Viking Swords and Wootz

[Chris Evans]

Hi Jeff (D),

I have no first hand experience with Wootz, so I was hoping that Jeff Pringle or someone else would help us out. So what follows is really based on reading the works of others and reasoning back from firsts principles. If I am in error, others can correct me.

Originally Posted by Jeff D

Thank you Chris and Jeff.
Excellent explanation guys but, I think I may be missing something here. Is the central core wootz or crucible steel (on the real deals)? Just so that I am on the same page, my understanding is that wootz is crucible steel with a surface pattern, correct?

My understanding is that the term Wootz and crucible steel are synonymous because Wootz was made in crucibles, so it is a crucible steel.

Why would tempering crucible steel be any different then forged or case hardened steel? Tempering wootz would obviously be a much bigger problem if the surface pattern is to be maintained.

I am not sure that I understand your question, but if you mean why is the hardening of Wootz by a process of quenching problematic then this explanation may be of help:

The microstructure of forged Wootz, a very high carbon steel, in the unhardened condition consists of pearlite (0.8%C) plus the rest of the carbon in the form of iron carbides. In this state, Wootz can be hard enough to render a sword serviceable, but IMO barely so. To attain a really hard edge, hardening by quenching is required, but this is problematic.

Conventional hardened steel consist of converting the pearlite to austenite by heating and then this austenite is rapidly cooled (quenched) to transform it into martensite (hardened steel). If we only had pearlite to deal with, as in the case of conventional steels, there would be no great problem. However with Wootz, once the pearlite is heated and converts into austenite, the iron carbides tend to dissolve in it, raising its carbon content beyond 0.8% C. Upon quenching the austenite with the now elevated carbon content transforms into a very brittle form of martensite plus iron carbide that precipitates out of solid solution, all intermixed with some of the austenite that failed to transform (weak and soft), known as retained austenite. Whilst hard this is a bad microstructure from the point of view of strength and toughness. There is more to it, but this is a basic summary.

I have posted the picture of what I believe to be a temper line with an intact (but altered) surface pattern, to show that it could and was done.

One way around the above described problem would be to harden only the edge, so that the unhardened spine of the blade would provide the necessary strength and toughness that the blade requires. This would counter the negative traits of the hardened section.

In all my readings on Wootz, the question of heat treatment seems have received little attention, so we are left wondering. Yet to justify the legendary fame of many Wootz blades, they would have had to be hardened in some way or another.

Cheers
Chris

[Rick]

I own this old wootz sword .
http://www.oriental-arms.com/photos.php?id=1048

The pattern disappears where it seems to have been hardened (picture 2).

[Jeff Pringle]

a rough translation of that most interesting article, just to get the gist of it

My guesstimate is at the end of this post. The sword was made in a very similar way to the one analyzed by Edge & Williams, the main difference is the steel edge consisting of pearlite in the Swedish sword, not martensite. This might be due to a lack of hardening at time of manufacture, but it could also indicate that the sword went through a cremation burial which erased its heat treatment. Since the swords are so similar, I’ll presume they are both genuine +ULFBERHT+s and go with the latter.

could you shed some light on its historical heat treatment as uncovered by modern research, if such information exists?

This has always been a problem, not enough data on the old swords, but this recent article is a good start – Heat treated wootz/crucible steel has a significant edge over regular steel in hardness…
The Metallurgy of some indian swords
Alan Williams, David Edge
Gladius, Vol XXVII (2007):149-176
http://gladius.revistas.csic.es/inde...e/view/102/103

There is the theory that there was no need to harden wootz, since you just wanted very tough pearlite carrying those extra-hard carbides to the target, but since all the contemporary descriptions of wootz sword making include a quench, and since many swords look like they have a hardened edge, I suspect that theory is another modern misinterpretation based on too little info. Current experimentation reveals that water quenching is risky (well, we knew that already! ), that you can erase none, some or all of the pattern depending on the specific alloy and how you austenitize (heat before the quench) the blade, and that hardened and unhardened wootz respond to the etch differently, so yes, those weird lines that show up in the old swords & knives are evidence of heat treating. I’ll attach some fotos of quenched blades, from 1% to 1.9% Carbon. Lower carbon and the martensite grabs all the carbon, banding disappears, higher carbon and you get martensite studded with banded carbides. I recently bought an Indian wootz sword that was hardened at the edge in the area of the center of percussion, but don’t have a photograph of it (yet! )

Little different direction in this article

Better than the Guardian, but I suspect there’s still some misinterpretation going on. Yesterday I received a confirmation that the Williams article is on its way, so soon I’ll be able to compare all three!

wootz is crucible steel with a surface pattern, correct?

Wootz is hypereutectoid crucible steel with a pattern, you need to be over 0.8% Carbon to get the carbides.

The guts of the Swedish article:
Metallographic analysis of inlays in a Viking Sword, inv. nr. SHM 907
The blade is made up of several layers of varying carbon content, an almost carbon-free central layer with several weld joints marked by slag streaks, surrounded by two outer layers with higher carbon content. The central layer, which is built of 10-12 layers, consisting of relatively coarse-grained ferrite with small pearlite at grain boundaries, carbon content of less than 0.1%. The side layers are also layered and consist of one side of pure pearlite (carbon 0.8%) that is very fine-grained and finely laminated. The second side has lower carbon content, 0,4-0,6%; and consists of a powdery mixture of ferrite and perlite. The edge is badly corroded but seems to be the layer with the highest carbon content.
The inlay is almost entirely carbon-free, with coarse grains of ferrite. The cross-section is nearly trapezoidal and divided by a corrosion streak, which is probably a slag line between two twisted wires (Figure 3). The two threads show in their internal structure traces of stratification. The inlay is likely to consist of two twisted iron wires, probably containing phosphorous, which were forged down the fuller in the blade prior to the final processing to finished shape.

[Chris Evans]

Hi Jeff (Pringle),

Great post and many thanks for that most informative article in gladius, which I speed-read and yet have to go over several times so as to digest its contents. It would seem that the better blades were hardened by heat treatment.

Cheers
Chris

[kisak]

A thought about the heat treating of these hypereutectoid steels. As changing the carbon content in such would only change the amount of carbides, not the composition of the carbides and matrix (well, in theory at least), could it be that the smiths who were good enough with keeping the austenitisation temperature just so where then presented with a steel which might have been more predictable in how it reacted to the heat treatment?

Such could, I guess, result in a slightly higher overall quality amongst the blades which weren't more or less obviously botched.

[Chris Evans]

Hi Kisak,

Originally Posted by kisak

A thought about the heat treating of these hypereutectoid steels. As changing the carbon content in such would only change the amount of carbides, not the composition of the carbides and matrix (well, in theory at least), could it be that the smiths who were good enough with keeping the austenitisation temperature just so where then presented with a steel which might have been more predictable in how it reacted to the heat treatment?

Such could, I guess, result in a slightly higher overall quality amongst the blades which weren't more or less obviously botched.

From a modern perspective and without having done any hands on experimentation myself, it seems to me that Wootz is a variation on the theme of carbided tool steels, many of which have been adapted to cutlery usage. Their heat treatment is not difficult, but this is with modern technology, especially temperature control (pyrometry), all backed with knowledge. The general idea is to heat the steel just sufficiently to austenitize the pearlite and leave the primary carbides undissolved, so that upon quenching a sound microstructure results. With these tool steels the role of the carbides is to provide abrasion resistance. If you would like to read up on this here is a very good work: http://www.feine-klingen.de/PDFs/verhoeven.pdf

Whether any of the ancient smiths knew about this I cannot say, but do find it plausible that every once in a while someone would have got the temperature just right by chance with a very gratifying end result.

Again from a modern perspective, we would not make a sword blade from high carbon (hypereutectoid) steel because of lack of toughness, even if given optimal heat treatment. For one the martensite that forms at the higher carbon contents is very brittle and the cementite (iron carbide) does nothing, save to provide unnecessary abrasion resistance, and undermines toughness further. However in knives the added abrasion resistance is welcome and toughness is much less of a requirement.

Something to keep in mind is that piano wire, which is work hardened (hard drawn) unquenched pearlitic (eutectoid) steel is very tough, surprisingly hard and is used in springs. I mention this because of the possibility of cold work hardened but unquenched Wootz edges being up to the task of cutting very well and at the same time retaining a high level of toughness and springiness.

But there was more to ancient swordmaking than the above simplistic considerations would suggest. Those smiths could come up with composite layering and heat treatments and thereby overcome the inherent limitations of the steels that they worked with. How good were these swords? We don't know as there is not enough published data. Some time ago there was this thread http://www.vikingsword.com/vb/showthread.php?t=3377 and the question posed in the first post remained unanswered.

Cheers
Chris

[Chris Evans]

Hi Folks,

Here is something I posted on that earlier thread on Wootz and may be of general interest:

By Dr. John Verhoeven:

There is a general myth in some of the popular literature that genuine Damascus steel blades possess outstanding mechanical properties, often thought superior to modern steels. This idea was shown to be incorrect as long ago as 1924. A famous Swiss collector, Henri Moser, donated 4 genuine Damascus steel swords, one with a non typical carbon content and microstructure, to B. Zschokke, who performed extensive careful experiments including metallographic and chemical analysis in addition to mechanical testing [15]. A series of bending tests compared samples from the swords to a pattern welded blade and a cast blade from the famous German knife center in Solingen. The 3 good Damascus blades showed significantly inferior bending deflection prior to breakage than the 2 Solingen blades in spite of the fact that the Brinell hardness of the 3 ranged from only 193 to 248, compared to 347 and 463 for the pattern welded and cast Solingen blade, respectively. This is not too surprising in view of the now well known fact that toughness of high carbon steels is inherently low; the Solingen blades had carbon levels of 0.5 to 0.6% compared to 1.3 to 1.9% for the 3 Damascus blades. The reputation of Damascus steel blades being superior to European blades was probably established prior to the 17th century when European blades were still being made by forge welding of carburized iron. It is hard to avoid embrittlement of such blades due to imperfect welding during the forging process as well as difficulty with the carburizing process.

The full article is here: http://bronksknifeworks.com/historical.htm

Cheers
Chris