Viking Swords and Wootz

[Jeff Pringle]

Whoa, you guys type faster than I do!
This repeats some of what Chris just stated, with a different spin...now I got to go think about heat treating...
Wrought iron and steel are very different animals, I do not think it is advisable to extrapolate from one to the other. What might be a welcome crack arrestor in iron could be an unwelcome stress riser in steel.
The slag stringers are a natural outcome of the method used to refine freshly smelted bloomery/tatara iron and steel, repeated folding, which has the primary purpose of removing the slag and homogenizing the metal. The more folding done, the less slag there is and the grain of the slag inclusions becomes finer. The edge of a sword would be severely compromised by a large slag stringer crossing there, or in the metal just behind the edge, so the metal used there is more refined. In the body of the sword, supported by more surrounding mass and not at the stressful cutting edge of the blade, such a degree of refinement is not needed so they used material that had less labor and materials invested in it. The slag stringers are in the middle because it is a more efficient way to make a blade and they can do no harm there, no other argument is required. This Moro spear nicely illustrates the technique.
The wootz/crucible steel method trades in the ‘repeated folding’ refinement for a liquid separation, but the goal is the same - homogenous metal and little or no slag.
It is entirely possible that the Ulfberht Co. got a nice price on imported ‘pre-refined’ steel for a couple years and when the economic situation shifted they continued on with the usual stuff, but I have yet to read the Williams article and see what it actually says. The Guardian article is goofy, but a good conversation starter!
Here’s another +Ulfberht+ (as opposed to +Ulfberh+t) analysis -
http://fornvannen.se/pdf/1980talet/1981_024.pdf
Metallografisk analys av inläggningar i vikingatida svärdsklinga,
inv. nr SHM 907 Go, Hogrän sn, Ålands
Av Mille Törnblom

[Chris Evans]

Hi Jeff (Pringle),

Thanks for your very informative post, to which I can add nothing of significance. Any chance of a rough translation of that most interesting article, just to get the gist of it? And as someone who has had a fair bit to do with Wootz, could you shed some light on its historical heat treatment as uncovered by modern research, is such information exists?

Cheers
Chris

[ward]

Happy new year

Little different direction in this article
http://www.physorg.com/news150373962.html

[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? 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 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.

Thanks guys!
Jeff

[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.