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Jeroen Zuiderwijk

Shallow hardening of ancient steels, deepening my knowledge

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When discussing the hardening of ancient steel blades, I often mention that ancient steels produced in bloomeries were much more shallow hardening then modern steels. But I find that de depth of my knowledge is lacking, and I want to prevent spreading nonsense. So I hope that some with more in depth knowledge can help correct me where I might be wrong, as well as answer some additional questions. Is the following correct:

 

"Modern steels usually have manganese and other element added to increase the hardenability, which usually were lacking in ancient steels. Because of that, producing a through hardened sword or sax using in the old days was difficult if not impossible, even if they were made entirely of hardenable steel. When they were quenched, they usually got a so-called auto-hamon, meaning that only the thinner edge hardens in a quench, and the rest stays soft. That's probably one of the reasons why valuable hardenable steel was only used for the cutting edge, while the rest was constructed of iron, possibly with a high phosphor amount which is at least somewhat harder and stronger then just plain iron. That also means that you have to pay attention to where you measure hardness on a blade, to see if quench hardening was performed. It can sometimes only be a few mm of the edge that is hardened, which can easily be missed if not measured close to the edge, or if the edge is lost through corrosion."

 

Further questions:

- the higher the carbon level, the more easy it is to through harden a blade, is that correct?

- the quenching media that were available had a strong influence, including the temperature. Theophilus prescribes using urine. Would that be because the salt and possible other substances increase the cooling rate, allowing a deeper hardening compared to just plain water?

- were through hardened ancient swords, how common were they? Did they include a high manganese content?

- I believe manganese does occur in iron ores, but frequently most is lost in the slag during the smelting process. Are there ways to keep it in the iron, and is it likely that some smelters were doing this on purpose to create deep hardening steel?

 

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The little I can add here is that the depth of hardening of a shallow-hardening steel can be enough to through-harden a very thin sword blade if you assume a hardening depth of 2mm from every surface ( a big if, I know...) That 2mm figure is for 1095, W-1, and W-2. Which leads me to point #2:

 

The amount of carbon does not influence the depth of hardening as long as the carbon is between around 0.4% and 1.1%. Below 0.35% C and it won't harden much at all, and above 1.1% you start getting iron carbides that act like deep hardening but are not. A simple low-Mn steel will be shallow-hardening in that range.

 

There was a discussion about quenchants a while back under the heading "quenching in a living slave" or something like that. Urine is a little faster than plain water, not as fast as brine. Bloom steels also do not respond the same as homogeneous steels, often requiring a higher temperature of austenitization and a faster quench.

 

I'll have to look through my copy of a copy of Tylecote and Gilmour, but I think most of the pattern-welded sword blades they tested were barely hardened on the edges. I remember one of them was far harder in the core than the edges because of phosphorus in the iron. Only one blade, a late Saxon one, showed anything nearly like the level of edge hardness we expect from a modern heat-treated steel. I do not remember the analysis.

 

Finally, Mn does occur WITH iron ores, and sometimes in. Brown ore (limonite/goethite) is often found in two distinct zones in the same deposit; one of nearly pure iron ore and a lower one of nearly pure manganese ore (psilomelane). The two are very distinct in appearance, with the brown iron ore being somewhat earthy and red/brown/black in color, and the manganese ore being purple to blackish, and either very hard and compact small nodules or a puttylike mass. The region where I live has had both an iron industry and a manganese industry based on the same orebanks, just using different parts as the iron ran out. I do know the blast furnace cast iron they were producing was high in Mn, making it in great demand for railroad car wheels. I suspect the bloom iron and shear steel was not so high due to the difference in production methods, time at heat, and temperatures.

 

I'll be pondering this one for a while...

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Manganese doe come through the smelting process, I have some in my Uk siderite. and there is manganese in the steel produced from the swedish magnetite I have smelted. about 0.2% in the maganatite steel...

 

Manganese also plays a role combating sulphur and will form preferential sulphides that are not as disastrous to the iron as iron sulphide.....

 

in my experience the 2mm depth of hardening does not work like alan has mentioned above......as its more about locaL surface area vs volume.

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in my experience the 2mm depth of hardening does not work like alan has mentioned above......as its more about locaL surface area vs volume.

This is why you can see auto-hamon line without removing 2mm from the flats of the blade. Volume retains heat and that is the "problem".

 

I would bet Theophilus recommended urine not because it was better, but because it was more exotic than plain old water. As bad as smithing myths are today, they were much worse back then because nobody could scientifically prove/disprove any of them. This is just a gut feeling on my part though.

 

Here are the TTT diagrams for 1060, 1060 with high Mn, and 1080. If you look at the bottom of each one you can see the hardenability chart from an end-quench test, where the x-axis represents the depth of hardness. Note that with these 3 charts you can see what kind of effect both C and Mn have. It is kind of hard to read, as the charts got a little fuzzy, but the first major vertical line on the chart represents 8/16" (and the smaller lines are 1/16").

 

1060_TTT.jpg

 

1060Mn_TTT.jpg

 

1080_TTT.jpg

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I do a LOT of quenching of very ( ancient like) steels.

 

Steely bloom, from limonite ore, will nearly always take brine ( salt water) to harden. Plus, usually, a fair high temp 1600+, (at least in my tank)
And, will usually be quite shallow hardening. I have ground away the hardening more then once. ;(

I will often do the brine, followed by oil (Parks) with hearth-refined bloom steel. Just to be safe, and cracking something that has that kind of time in it, is a real bitch !
This steel, will often harden quite well, but not always on the first try. Again, it likes to be hot.

I have only had a couple finished metals tested. Not enough to be sure of anything. But, if you get even very low C steely bloom hot enough, it will harden some. Just need to take care about grain growth. :)

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All the bloomery steels that I've worked with, are very shallow hardening. Like was said, they usually require higher temps, and a brine or water quench, to get hard enough. I've got a few pics of auto hamon's on bloom steel blades here somewhere, I'll try to locate them and post them. Working with Mark's ore, every time I've had the steel anaylized, there was nothing for manganese content. I've always wanted to add some manganese dioxide to a smelt to see if it would add manganese to the steel, this is an experiment for sometime in the future. Comparing chemistry between "old" steel and modern, IMHO, could be problematic though, as there is always a slag component in the "old" steels, which could be mistaken for being in the steel and actually be a component of the slag, depending on the method of measurement.

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I have always wondered if the Leidenfrost Effect has any bearing on quanchants.

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I have always wondered if the Leidenfrost Effect has any bearing on quanchants.

Yes, very much so.

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This is good stuff! Thanks! I will go into research mode to dig up what I can find from analyzed ancient blades.

 

P.s. one of the reasons I'm diving into this is because I get to discuss this topic with archeologists more often these days. Earlier this week I spoke to someone who is going to perform metallurgical analysis on early iron age swords, which will be extremely interesting, as so far I only have metallurgical data on middle to late iron age swords. With that of course it would be very interesting if they would detect a hardened steel edge in one of them. That could flip the whole discussion on the transition from bronze to iron age upside down. I don't expect there will be, if there is they really need to know what to look for to make sure it's not missed. Or that if no evidence of hardening is found in a badly corroded blade missing the edge, it does not prove the sword was not hardened.

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After going through Tylecote and Gilmour's 1986 The Metallography of Early Ferrous Tools and Edged Weapons, (B.A.R. research series 155) I found no through-hardening on anything. Most of the early stuff not only showed little evidence of quenching, some of it showed evidence of deliberate annealing, presumably to reduce brittleness in a high-P iron. The highest level of Mn listed in any of the blades was 0.2%, consistent with Owen's analysis of his Swedish ore steel.

 

Much of the pattern-welding was indeed getting contrast by mixing high and low-P irons, but intriguingly a couple of samples showed the presence of Nickel! No mention of where that came from, but I suspect it would almost have to be in the ore rather than a deliberate alloy. The other thing that struck me was the presence of bands of arsenic enrichment along weld boundaries. No explanation why, but it's there. Was it in the ore? The flux?

 

And yes, I was overgeneralizing with the 2mm depth of hardening. That is on a round bar held in the quench until cold. Blade sections tend not to harden at all in thicker parts for reasons Jerrod mentions. Tylecote and Gilmour's sections show hardening depths of a few mm of edge only, consistent with what we know about bloom steel behavior.

 

One of my favorite observations in that book is that knives tend to be as hardened as possible, barely tempered for enhanced edge holding; and swords tend to be treated for maximum toughness, in some cases being basically normalized even if % C is high enough to harden. I feel better about giving my swords a full spring temper now. B)

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Just found "The Celtic Sword" online: https://www.academia.edu/16271211/The_Celtic_Sword_Pleiner_1993_

It's got data on a lot of iron age swords. I just skimmed through it, and it's all in text, very little tables/pictures. So that's going to take a lot of reading to take a lot of reading. Saves you the $200 of getting a hard copy anyway :)

 

Another book I just came across:

http://www.getty.edu/conservation/publications_resources/pdf_publications/pdf/metallography.pdf

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Very good discussion here and thank you for the links, Jeroen.

 

I would personally be interested in seeing a study tracing back the hardening process as to where and when was it introduced historically. To be done correctly many blades would have to be examined from many eras and locations and the study would need to include micro-structure as well as hardness testing from edge (when not corroded) to spine.

I know, I am being an idealist and that type of study would be almost impossible to carry out.

 

On the matter of alloying elements, both Mn and Ni can be found in ores where iron ores are. Noticed how close those elements are in the periodic table? What I have seen in Ni containing ores is that there is segregation to a bright band in the bloom. Mn ore has a very distinct look and by being so it could be readily identified. I imagine that back in time they would have chosen to include it or exclude it in the ore fed in the furnace if they would have noticed a distinct advantage to doing so. Those guys did not know about elements but they could test the properties of the metal made with different ores for their final purpose/use and determine if it was worth using that ore or not. If they did not consistently use a quench/hardening process the benefit of having Mn in the ore would have not been noticed but it might have had other effects such as better in-furnace bloom consolidation. BTW, have you notice also that other two elements that we deal with in smelting are very close in the periodic table but separated from the Fe group. I am talking about S and P.

 

My experience with hardening ferrous metals obtained in a process similar to those used in antiquity is similar to what has been described here. I harden in warm water and try to make sure that I have a homogeneous metal with around 0.6% carbon in it. That seems to be the magic number. I first heard of that number as referenced in the Japanese literature and metallurgical textbooks seem to point to the same number. 0.6% carbon allows for the hardest martensite possible and its true independently of whether it is a plain carbon steel or alloyed steel. Higher carbon levels results in retained austenite which lowers the hardness level and may cause other issues (cracks). Higher carbon would also require lower quenching temperatures. Mn and Ni work by delaying the transformation of austenite to pearlite/ferrite. In my experience, the thickness of the edge seems to be the most influential factor in determining the depth of hardening all other factors being equal (which is never the case when using bloom metal). Interestingly enough, my un-clayed blades quenched in water show hardening at the corners (vertices of triangles). That means the edge and the two (or three) corners of the spine. The flat side of the blade seems to behave similarly to the core of the blade but that is a whole different discussion and I am starting to ramble away from Jeroen's starting question.

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P.s. I must add that I absolutely love this forum, with so much available knowledge amongst members. There are few places where I can go very deep into a subject, and not being all on my own in having to figure things out.

 

 

I would personally be interested in seeing a study tracing back the hardening process as to where and when was it introduced historically. To be done correctly many blades would have to be examined from many eras and locations and the study would need to include micro-structure as well as hardness testing from edge (when not corroded) to spine.

I know, I am being an idealist and that type of study would be almost impossible to carry out.

 

I'm trying to gather what data is available there. But there are big gaps, and one of them is early iron age iron in (northern) Europe. But at least some swords from this period are scheduled to be researched later this year, and I'm in contact with the archeologist who is initiating the research. However, I've seen the swords they are going to do first, and they are reduced to just flakes of metal, very little original metal left (which is why she can cut out samples). So I'm hoping there will be some edge material left to investigate.

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What I find interesting is that one thing that's almost never mentioned in the search for when it was first learned about quenching steel, is fire strikers. A fire striker needs to be high carbon and properly hardened in order to even work. IIRC there are documented finds of these in the early iron age, which would seem to suggest that they did know how to select steel that could be hardened and harden it

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What I find interesting is that one thing that's almost never mentioned in the search for when it was first learned about quenching steel, is fire strikers. A fire striker needs to be high carbon and properly hardened in order to even work. IIRC there are documented finds of these in the early iron age, which would seem to suggest that they did know how to select steel that could be hardened and harden it

 

Good point! The oldest AFAIK are from Belgium, dating to 400BC. I think there is evidence of earlier hardened steel, but more in terms of punches or things like that. For fire strikers you don't have to know tempering yet though. Finding out that some steels will harden when quenched is something likely to have been identified early on by accident. But they were probably aware of it's brittleness, so avoided that for a long time, until someone figured out tempering. And I can imagine that those in the know were very careful to keep that to themselves.

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Going back to Tylecote and Gilmour 1986, some of the iron age Celtic tools ca. 200BC and a few of the Roman ones from ca.100BC - 300AD they tested appeared to be carburized rather than made of steel to begin with or having had a steel edge welded on. Something else to think about.

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Going back to Tylecote and Gilmour 1986, some of the iron age Celtic tools ca. 200BC and a few of the Roman ones from ca.100BC - 300AD they tested appeared to be carburized rather than made of steel to begin with or having had a steel edge welded on. Something else to think about.

Did they mention how they found it to be case hardened? I have seen some use shallow depth of hardness as an indication of case hardening.

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They sectioned the blades and looked at the microstructure, then did microhardness testing on the Vickers scale. They base it on a lack of weld lines and microstructural carbon content and diffusion boundaries.

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